passive solar system

POWER GENERATION THROUGH PASSIVE SOLAR SYSTEM

PROJECT SUPERVISOR

Engr. Muhammad Tufail & Sir. Shahid Zaman

SESSION: 2009 – 2012

SUBMITTED BY

AHMAD HAMAD: SP09-EPE-111

MUHAMMAD ALI: SP09-EPE-062

ADEEL SHAHZAD: SP09-EPE-101

DEPARTMENT OF ELECTRICAL ENGINEERING

COMSATS INSTITUTE OF INFORMATION TECHNOLOGY

ABBOTTABAD

January, 2013

IN THE NAME OF ALLAH

THE MOST GRACIOUS, THE MOST MERCIFUL

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A report is submitted to

COMSATS Institute of Information Technology, Abbottabad

as a partial fulfillment of requirement for the award of the degree of

Bachelor of Science in Electrical Engineering

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Department of Electrical EngineeringCOMSATS Institute of Information Technology Abbottabad

FINAL APROVAL

This is to certify that we have read the project submitted by: AHMAD HAMAD:SP09-EPE-111, MUHAMMAD ALI:SP09-EPE-062, ADEEL SHAHZAD:SP09-EPE-101, It is our judgment that this project is of sufficient standard to warrant it acceptances by the COMSATS Institute of Information Technology, Abbottabad for the BS Degree in Electrical Engineering.

COMMITTEE

1. External Examiner

Mr._________________________________Signature:________________________

2. Internal Examiner

Mr.________________________________Signature:_________________________

Department of Electrical Engineering,COMSATS Institute of Information Technology, Abbottabad

3. Supervisor

Mr.________________________________Signature:_________________________


4. Head of Department

Associate Professor,Signature:_________________________


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Table of Contents

LIST OF ABBREVIATIONS.................................................................................................................9

DEDICATION.......................................................................................................................................11

ACKNOWLEDGEMENT.....................................................................................................................12

ABSTRACT............................................................................................................................................13

CHAPTER # 1........................................................................................................................................14

INTRODUCTION.................................................................................................................................14

1.1 OVERVIEW..............................................................................................................................151.2 MOTIVATION...........................................................................................................................161.3 LITERATURE REVIEW..................................................................................................................17

1.3.1 Brief History of Solar Thermal Power.............................................................................171.3.2 Solar Steam Generation...................................................................................................20

1.4 PROJECT GOAL.........................................................................................................................25

CHAPTER # 2........................................................................................................................................26

MODES OF POWER GENERATION...........................................................................................26

2.1 POWER GENERATION................................................................................................................272.2 METHODS OF GENERATING ELECTRICITY........................................................................................27

2.2.1 Turbine............................................................................................................................282.2.2 Reciprocating engines......................................................................................................292.2.3 Photovoltaic panels.........................................................................................................292.2.4 Other generation methods................................................................................................302.2.5 Characteristics of Different generation methods.............................................................30

CHAPTER # 3........................................................................................................................................32

PASSIVE SOLAR SYSTEM................................................................................................................32

3.1 TECHNIQUE.............................................................................................................................333.2 TECHNICAL PRINCIPLE................................................................................................................333.3 CONCENTRATING SOLAR COLLECTOR............................................................................................33

3.3.1 Parabolic trough collector system...................................................................................343.3.2 Power Tower System........................................................................................................343.3.3 Parabolic Dish System.....................................................................................................353.3.4 Linear Fresnel Reflectors(LFR).......................................................................................37

3.4 PARABOLIC TROUGH COLLECTOR STRUCTURE.................................................................................383.5 PARABOLA...............................................................................................................................393.6 RIM ANGLE.............................................................................................................................403.7 ANGLE OF INCIDENT CALCULATION...............................................................................................41

3.7.1 Basic Angles....................................................................................................................413.7.1.1 Hour angle (ω)..................................................................................................................413.7.1.2 Latitude Angle (ɸ).............................................................................................................433.7.1.3 Declination Angle (δ)........................................................................................................43

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3.7.2 Solar Angles.....................................................................................................................433.7.2.1 Solar altitude angle (α).....................................................................................................443.7.2.2 Solar azimuth angle (ß).....................................................................................................443.7.3 Angle of incidence (θ)......................................................................................................45

3.8 REFLECTIVE SURFACE.................................................................................................................473.8.1 Silver Polymer Sheets......................................................................................................473.8.2 Different Reflective Sheets...............................................................................................48

3.9 ABSORBER...............................................................................................................................493.10 SOLAR FIELD............................................................................................................................51

3.10.1 Optical Losses.............................................................................................................523.10.2 Heat Losses.................................................................................................................53

3.11 HEAT EXCHANGER.....................................................................................................................543.11.1 Heat Exchanger Classification....................................................................................543.11.2 Flow Arrangement.......................................................................................................553.11.3 Types of Heat Exchanger.............................................................................................573.11.3.1 Shell and Tube Heat Exchanger......................................................................................573.11.3.2 Plate Heat Exchanger.....................................................................................................593.11.3.3 Plate and shell heat exchanger.......................................................................................603.11.4 Heat Transfer Rate......................................................................................................60

3.12 THERMAL STORAGE...................................................................................................................623.12.1 Heat Transfer..............................................................................................................633.12.2 Molten Salt Thermal Storage.......................................................................................64

3.13 STEAM TURBINE.......................................................................................................................653.14 DC GENERATOR.......................................................................................................................67

3.14.1 How DC Generators Work..........................................................................................67

CHAPTER # 4........................................................................................................................................68

COMPARISON BETWEEN ACTIVE AND PASSIVE SOLAR SYSTEM....................................68

4.1 SOLAR THERMAL VS. PHOTOVOLTAIC............................................................................................694.2 PHOTOVOLTAIC ELECTRICITY GENERATION......................................................................................694.3 PHOTOVOLTAIC APPLICATION......................................................................................................704.4 SOLAR THERMAL SYSTEM CHARACTERISTICS...................................................................................71

4.4.1 Application......................................................................................................................714.4.2 Benefits............................................................................................................................734.4.3 Impacts............................................................................................................................74

CHAPTER # 5........................................................................................................................................75

EXPERIMENTAL TESTS, RESULTS, CONCLUSIONS AND FUTURE WORK.......................75

5.1 DATA.....................................................................................................................................765.1.1 Dimensions of Parabolic trough Prototype......................................................................765.1.2 Generator Specification...................................................................................................76

5.2 EXPERIMENT RESULTS................................................................................................................775.2.1 Average Calculation Depending on Above Experimental Results....................................785.2.2 Parabolic Trough Prototype............................................................................................78

5.3 SYSTEM CHARACTERIZATION.......................................................................................................795.3.1 Mirror Loss......................................................................................................................795.3.2 Concentrator Geometry Loss...........................................................................................795.3.3 Absorption Loss...............................................................................................................79

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5.3.4 Radiation Loss.................................................................................................................795.3.5 Convection Loss...............................................................................................................805.3.6 Remaining Thermal Loss.................................................................................................815.3.7 Reducing Losses..............................................................................................................81

5.4 CONCLUSION...........................................................................................................................835.5 FUTURE WORK.........................................................................................................................83

REFERENCES.......................................................................................................................................84

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LIST OF FIGURES

FIGURE 1.1: WORLD HDI VS. ELECTRICITY CONSUMPTION...................................................................16FIGURE 1.2: PIFRE'S 1878 SUN-POWER PLANT DRIVING A PRINTING PRESS.........................................18FIGURE 1.3: DISH STIRLING SYSTEM....................................................................................................19FIGURE 1.4: (A) PROCESS FLOW AND (B) CORRESPONDING THERMODYNAMIC STATE FOR SHENANDOAH

TOTAL ENERGY PROJECT..............................................................................................................22FIGURE 1.5: (A) PROCESS FLOW AND (B) THERMODYNAMIC STATE FOR JOHNSON AND JOHNSON SOLAR

FACILITY.......................................................................................................................................23FIGURE 1.6: OVERALL VIEW OF SOLAR 1, AT BARSTOW, CA................................................................24FIGURE 1.7: SOLAR CONFIGURATION OF INDITEP PROJECT SEVILLE, SPAIN.......................................24FIGURE 1.8: SCHEMATIC OF PARABOLIC TROUGH..................................................................................25FIGURE 3.1: PARABOLIC TROUGH SYSTEM............................................................................................34FIGURE 3.2: TOWER SYSTEM................................................................................................................35FIGURE 3.3: PARABOLIC DISH SYSTEM.................................................................................................36FIGURE 3.4: BASIC CONFIGURATION OF LFR SYSTEM...........................................................................37FIGURE 3.5: COMPACT LFR SYSTEM.....................................................................................................38FIGURE 3.6: STRUCTURE OF PARABOLIC TROUGH.............................................................................39FIGURE 3.7: PARABOLA WITH FIXED POINT AND FIXED LINE.................................................................39FIGURE 3.8: PARABOLA WITH RIM ANGLE..............................................................................................40FIGURE 3.9: BASIC ANGLES FOR THE LOCATION Q............................................................................42FIGURE 3.10: VARIATION OF DECLINATION ANGLE................................................................................43FIGURE 3.11: EARTH SURFACE COORDINATE SYSTEM FOR OBSERVER AT POINT Q SHOWING THE SOLAR

AZIMUTH ANGLE ẞ, THE SOLAR ALTITUDE ANGLE Α......................................................................44FIGURE 3.12: A FIXED APERTURE WITH ITS ORIENTATION DEFINED BY THE TILT ANGLE (ʎ) AND THE

APERTURE AZIMUTH ANGLE (Ω)....................................................................................................45FIGURE 3.13: A SINGLE-AXIS TRACKING APERTURE ROTATING ABOUT THE AXIS R..............................45FIGURE 3.14: SINGLE AXIS TRACKING SYSTEM COORDINATES..............................................................46FIGURE 3.15: ROTATION OF THE U, B, R COORDINATES FROM THE Z, E, N (FIG. 3.10) COORDINATES

ABOUT THE Z-AXIS (DIAGRAM SHOWS VIEW LOOKING DOWNWARD ON THE SURFACE OF THE EARTH)..........................................................................................................................................46

FIGURE 3.16: ABSORBER TUBE OF A PARABOLIC TROUGH COLLECTOR [REB].......................................49FIGURE 3.17: PARALLEL FLOW ARRANGEMENT....................................................................................55FIGURE 3.18: COUNTER CURRENT FLOW ARRANGEMENT.....................................................................55FIGURE 3.19: CROSS FLOW ARRANGEMENT..........................................................................................56FIGURE 3.20: SHELL AND TUBE EXCHANGER........................................................................................57FIGURE 3.21: CONCEPTUAL DIAGRAM OF A PLATE AND FRAME HEAT EXCHANGER..............................59FIGURE 3.22: PLATE AND SHELL HEAT EXCHANGER.............................................................................60FIGURE 3.23: GRAPH OF HEAT RATE TRANSFER..................................................................................61FIGURE 3.24: SCHEMATIC DRAWING OF A HIGH-PRESSURE TURBINE [KWT]........................................65FIGURE 3.25: WORKING PRINCIPLE OF DC GENERATOR.......................................................................67FIGURE 4.1: GENERIC SCHEMATIC CROSS-SECTION ILLUSTRATING THE OPERATION OF AN ILLUMINATED

SOLAR CELL...................................................................................................................................70FIGURE 4.2: ON-PEAK CAPACITY FACTORS FOR FIVE 30 MW SEGS PLANTS DURING...........................72

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LIST OF ABBREVIATIONS

HDI: Human Development Index

PV: Photovoltaic

CSP: concentrated solar power

LCOE: levelized costs of electricity

LFR: Linear Fresnel Reflectors

STE: Solar Thermal Electric

HTF: Heat Transfer Fluid

SES: Stirling Energy Systems

SAIC: Applications International Corporation

CLFR: Compact Linear Fresnel Reflector

PTSC: Parabolic Trough Solar Collector

OTEC: Ocean Thermal Energy Conversion

TE: Thermoelectric

TI: Thermionic

TPV: Thermophotovoltaic

MHD: Magnetohydrodynamic

RPM: Revolution per minute

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DEDICATION

We dedicate this effort to our beloved parents who gave us support and encouragement on every step, to our teachers who helped us whenever we faced any difficulty and to everyone who was concerned to our project.

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ACKNOWLEDGEMENT

We have no words to express our deepest sense of gratitude to Almighty ALLAH who blessed us with the knowledge, gave us courage and strength to complete this project. We pay our tribute to our loving parents with the prayers of whom we were able to reach this milestone. We can’t forget the efforts for saving us being dead in childhood and their sleepless nights especially when we were fighting against diseases several times.

We would like to acknowledge the contributions of the faculty and students, both at COMSATS (Abbottabad), whose valuable suggestions throughout the project have made a positive impact on this task. We particularly want to thank Engr. Muhammad Tufail for his support and encouragement which was vital in this regard.

We also say thanks to Sir. Shahid Zaman our co-supervisor and whenever we needed any help, he was always there to help us every time.

And again at last we say thanks O-GOD for your love and mercy

AHMAD HAMAD:SP09-EPE-111

MUHAMMAD ALI:SP09-EPE-062

ADEEL SHAHZAD:SP09-EPE-052

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ABSTRACT

Pakistan’s thirst for electric power has been constantly rising over the years because of population growth, increase in industrial activity and failure of other resources for producing enough energy to meet its growing energy demand, particularly in the remote areas where energy is most needed. Pakistan is basically an energy deficient society and now going towards extreme energy crisis. Moreover, with current demand growth at 8 % annually, Pakistan will have to add 4000 MW to its existing capacity by the year 2018. Pakistan is rich in renewable energy resources; particularly solar energy has a special relevance in Pakistan due to high availability of Sun radiations at an average rate of 4.5-6 kwh / m2 / day.

The optical principle of a reflecting parabola is that all rays of light parallel to its axis are reflected to a point. A parabolic trough is simply a linear translation of a two-dimensional parabolic reflector where, as a result of the linear translation, the focal point becomes a line. These are often called line-focus concentrators.

Passive solar design refers to the use of the sun’s energy for the heating the water and generating the steam. This generated steam is use for generation of power energy. This is also called solar thermal power plants. In solar thermal power plants the incoming radiation is tracked by large mirror fields which concentrate the energy towards absorbers. They, in turn, receive the concentrated radiation and transfer it thermally to the working medium. The heated fluid operates as in conventional power stations directly (if steam or air is used as medium) or indirectly through a heat exchanging steam generator on the turbine unit which then drives the generator. It does not damage the environment in any way, renewable energy source, eco friendly, save energy and money, easy to use.

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Chapter # 1

Introduction

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1.1 Overview

Solar thermal power plants, also known as concentrating solar plants (CSP), use large arrays of mirrors to concentrate solar energy and produce high temperature heat. The heat is then used in a turbine or engine to generate electricity. The three solar thermal technologies in use today are parabolic trough, parabolic dish-engine, and central receiver. Parabolic troughs use long lines of u-shaped mirrors to focus solar energy on a tube containing a heat transfer fluid, which is typically oil. The heat transfer fluid is then used to produce steam and generate electricity in a traditional steam turbine and generator. Parabolic dish-engine plants arrange mirrors in a configuration similar to a satellite dish and concentrate solar energy on a power conversion unit. In this unit, solar energy heats a working fluid, which is usually hydrogen gas, and powers an engine with an electricity generator. High efficiency Stirling engines are typically used in these plants and the units are modular with capacities ranging from 10 to 25 KW. Central receiver plants use a large field of mirrors, known as heliostats, to focus energy on a centrally located tower. The tower contains a heat transfer fluid, which is usually molten salt, and the heat is then used to produce steam and generate electricity.

A limited number of power plants have been built and operated using each of three technologies, and the technologies are at different stages of commercial readiness. Currently, the solar thermal industry is growing rapidly and numerous proposals are in varying stages of development in the Southwest United States, Europe, Africa, and the Middle East that would significantly expand the installed capacity of these technologies. Because solar thermal plants utilize direct normal insulation and require large tracts of flat land for the mirrors, they have been primarily developed in desert areas. Solar thermal plants are currently less expensive than electricity produced from photovoltaic arrays and have estimated levelized costs of electricity (LCOE) that is competitive with electricity produced from natural gas-fueled combustion turbines (approximately 12-15 cents per KWh) (Stoddard et al., 2006). With ongoing expansions in installed capacity, the LCOE is expected to decline.

Many of the current solar thermal projects are proposing to include heat storage or “hybridize” with fossil fuel plants, and these options offer advantages over many other renewable technologies. Solar thermal plants that store heat for several hours can mitigate some of the intermittency problems associated with the variable solar resource. Heat storage can also help the power plant operator match plant output more closely with system peak demand in locations where the peak solar resource and electricity demand may differ by a few hours. Another option with solar thermal plants is to “hybridize” with fossil fuel plants to provide a more reliable power source in comparison to other renewable technologies. In current plants, hybrid plants use fossil fuel (typically natural gas) as a backup to produce heat during periods with low or no solar resource. New plants are proposing to add the steam from solar thermal plants directly into the steam cycle of a combined cycle plant. Current research is also

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analyzing the opportunities to add steam generated from solar thermal plants directly into the steam cycle of existing coal plants.

1.2 Motivation

The world is dependent upon energy. People’s energy use directly correlates to their grade of health care, life expectancy and education. These are important factors that determine a person’s quality of life. One quantities measure of life quality is the Human Development Index (HDI). The HDI combines life expectancy, literacy, education and GDP per capita for different countries. Figure 1.1 displays the HDI versus electricity use for different countries and one can see the clear correlation between electricity consumption and HDI.

Figure 1.1: World HDI vs. Electricity Consumption

Electricity allows people access to refrigeration for food and medicine, energy for cooking and cleaning water, and allows people to read and study at night when there is little work that can be done outside. A small amount of electricity can dramatically

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change the life of a person who has had none. It is estimated that the power requirement for basic healthy functioning in rural communities is about 0.08 kWh/day/person [1]. This is less than 1% of an average person's usage in the United States, yet many people cannot afford or do not have access to even this small amount.

Nearly two billion people live in rural areas without access to electrical grids. Developing an infrastructure in these remote areas is usually not feasible due to the extreme distance from existing electric grids. Building new power plants in these areas is not cost effective due to the relatively low electricity consumption. Economical, small-scale, distributed energy systems can fulfill the need and renewable energy is ideally suited for this purpose.

Many under-developed areas around the world receive large amounts of sunlight. Northern Africa and Central Asia receive as much as 7.5 kWh/m2/day. There is great opportunity to use solar power to provide basic energy needs in these regions. The two most prominent solar energy technologies are photovoltaic (PV) and concentrated solar power (CSP). PV systems are beneficial because they can be scaled to any size, but they are costly and solely produce electricity. CSP systems can provide electricity as well as thermal power. This thermal power can be efficiently used for cooking, water distillation and absorption refrigeration cycles. The drawback to these systems is that the most efficient solar thermal systems currently have an installation cost of $10,000/kW [2]. An economic CSP system could provide rural areas with electricity and energy needs to dramatically improve their quality of life.

1.3 Literature Review

1.3.1 Brief History of Solar Thermal Power

Concentrating solar power is a method of increasing solar power density. Using a magnifying glass to set a piece of paper on fire demonstrates the basic principle of CSP. Sunlight shining on the curved glass is concentrated to a small point. When all the heat energy that was spread across the surface of the magnifying glass is focused to a single point, the result is a dramatic rise in temperature. The paper will reach temperatures above 451ºF and combust. CSP has been theorized and contemplated by inventors for thousands of years. It is possible that as far back as ancient Mesopotamia, priestesses used polished golden vessels to ignite altar fires. The first documented use of concentrated power comes from the great Greek scientist Archimedes (287 212 B.C.). Stories of Archimedes repelling the invading Roman fleet of Marcellus in 212 B.C. by burning their ships with concentrated solar rays were told by Galen (A.D. 130-220) [3]. In the seventeenth century, Athanasius Kircher (1601-1680) set fire to a woodpile at a distance in order to prove the story of Archimedes [3]. This is considered the beginning of modern solar concentration. Solar concentrators then began being used as furnaces in chemical and metallurgical

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experiments [4]. They were preferred because of the high temperatures they could reach without the need for any fuel. Further applications opened for concentrated power when August Mouchot pioneered generating low-pressure steam to operate steam engines between 1864 and 1878.

Figure 1.2: Pifre's 1878 Sun-Power Plant Driving a Printing Press

Abel Pifre made one of his solar engines operate a printing press in 1878 at the Paris exhibition [3], but after extensive testing he declared the system too expensive to be feasible. His press is shown in Figure 1.2.

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Figure 1.3: Dish Stirling System

Pifre's and Mouchot's research began a burst of growth for solar concentrators. The early twentieth century brought many new concentrating projects varying from solar pumps to steam power generators to water distillation. A 50 kW solar pump made by Shuman and Boys in 1912 was used to pump irrigation water from the Nile. Mirrored troughs were used in a 1200 m² collector field to provide the needed steam [5]. In 1920 J.A. Harrington used a solar-powered steam engine to pump water up 5 m into a raised tank. This was the first documented use of solar storage. The water was stored for continual use as power for a turbine inside a small mine. Concentrating technology had made a huge leap from the nineteenth century but was halted by World War II and the resulting explosion of cheap fossil fuels. The advantages of solar power lost their luster and the technology would merely inch forward for nearly five decades. Starting in the late seventies and early eighties, solar power came back to the forefront of researchers' agendas with oil and gas shortages. In 1977 in Shenandoah, GA, 114 7-meter parabolic dishes were used to heat a silicon-based fluid for a steam Rankine cycle. The plant also supplied waste heat to a lithium bromide absorption chiller. The plants total thermal efficiency was 44%, making it one of the most efficient systems ever implemented [6]. More modern systems like the Department of Energy's Dish

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Engine Critical Components (DECC) project, which was built at the National Solar Thermal Test Facility, consisted of a 89 m2 dish with a peak system electrical efficiency of 29.4% [7]. This system utilizes the high 4 efficiency of the sterling engine to convert the heat generated into electricity. This efficiency is unmatched by any concentrator that utilizes a steam cycle, with one or two working fluids.

1.3.2 Solar Steam Generation

Solar thermal systems can utilize the Rankine cycle to produce electricity. This is done by creating steam using solar energy and passing it through a turbine. The most common modern technique to produce steam for electricity production or industrial needs has been to utilize a heat transfer fluid. Usually a salt or oil is pumped through a solar field to heat up the fluid. The hot oil is then passed through a heat exchanger with water to generate steam. The benefit of a heat transfer fluid is that it remains liquid at high temperatures, which increases heat transfer and ease of pumping. Oils have a liquid working range up to 400ºC, salts up to 600ºC and liquid metals can reach much higher temperatures.

The Solar Total Energy Project in Shenandoah, Georgia used a heat transfer fluid to create electricity and process steam for a textile factory. The heat transfer fluid used was Syltherm 800, a silicon-based fluid produced by Dow-Corning Corporation. The Syltherm carried 2.6 MW of heat from 1147 m diameter concentrating parabolic dishes. The fluid was pumped to cavity receivers that were placed at the focal point of each dish. The collector efficiency of the concentrators was 76.9%. Three heat exchangers were then utilized to preheat, boil and superheat water before it was passed through a steam turbine. A portion of the steam generated was used directly by the factory. The remaining hot water was used as a heat sink for a lithium bromide absorption chiller. The electricity generation efficiency was 11.8%. The total heat work efficiency for all of the processes was 44.6%. Figure 1.4 shows the process flow and corresponding state for the system.

Even though using an oil or salt as a heat transfer fluid is the most popular choice, water is still used in some cases. The Johnson and Johnson Solar Process Heat System used pressurized water as the heat transfer fluid in a parabolic trough plant. The facility had 1070 m² of parabolic troughs which heated water pressurized at 310 psi to 200 ºC. The water was stored in a tank where ash steam was created 4 times a day for industrial processes. To create steam, the pressurized water was throttled down to 125 psi where it vaporized additional feed water. The saturated steam that was produced was sent to do work. Thermal collection efficiency for the system was 30%. Figure 1.5 shows the process flow and corresponding thermodynamic state for this facility.

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(a)

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(b)

Figure 1.4: (a) Process Flow and (b) Corresponding Thermodynamic State for Shenandoah Total Energy Project

(a)

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(b)

Figure 1.5: (a) Process Flow and (b) Thermodynamic State for Johnson and Johnson Solar Facility

Direct steam generation (DSG) can be a more efficient and economic way of producing steam from solar collectors. Eliminating storage and heat exchangers decreases losses, capital investment and maintenance. The most famous case of a solar facility producing direct steam was the Solar 1 plant built in Barstow, CA. Solar 1 was a solar power tower that produced 10 MW of electricity in 1982. The receiver sat nearly 100 m above ground and was powered by 1818 39 m² collectors. The 13.7 m high and 7 m diameter receiver was made of 69 mm alloy tubes. The tubes were placed vertically, welded together and coated with an absorptive paint. Figure 1.6 shows an overall view of the Solar 1 facility. The surface of the receiver reached temperatures up to 620 ºC. Water was pumped through the tubes where it was vaporized and superheated to 516º C. The steam was then passed through a turbine to produce electricity. The maximum net monthly electrical efficiency was 15%.

A more modern example of DSG comes from the INDITEP project built near Seville, Spain in 2003. The 5 MW parabolic trough power plant uses DSG to run steam turbines. The pilot plant is testing the efficiency of both saturated and superheated steam production. For the saturated case, water is pumped at 77 bar though a solar collector field. The steam/water mixture exits the collectors with a quality of 0.85 at 285 ºC. A steam separator collects the steam and sends it to the turbine. The liquid water is recycled and passed back through the collector field. For the superheated case, the collected steam is passed through an additional set of collectors where it is superheated to 400 ºC. Superheating the steam increases the turbine efficiency.

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Figure 1.6: Overall View of Solar 1, at Barstow, CA

(a) Saturated Steam Cycle

(b) Superheated Steam Cycle

Figure 1.7: Solar Configuration of INDITEP Project Seville, Spain

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Figure 1.7 shows the two configurations. Collection efficiency for the saturated steam configuration was 66.9% and 65.1% for the superheated case [8]. The reduced efficiency of the superheated case can be attributed to the high temperature of the receiver line, which increases thermal losses. The net electrical efficiency of the plant was 16.4% for the superheated case and 16.2% for the saturated case. The increased turbine efficiency of the superheated case is offset by the increased thermal losses during collection. The net increase in efficiency is only 0.2% more for the superheated case than the saturated case.

Figure 1.8: Schematic of Parabolic trough

1.4 Project Goal

The first concentrator system built at our university, Parabolic trough, attempted to satisfy all of the research objectives for this project. Mirror inefficiencies held the system from producing enough steam to continuously run a micro-steam (small) turbine. The minimum thermal input to run a micro-steam turbine is near 5 W, 5 times what was produced by Parabolic trough. Furthermore, a micro-steam turbine with 15% thermal efficiency requires 6.67 W of thermal energy to produce 1 W of electricity. For the project discussed in this thesis, it was decided to focus on the steam generation and electrical generation. Basically our main focus is to design the prime mover for power generation. Once enough thermal energy is being produced, a micro-steam turbine will then by implemented. Figure 1.8 shows the schematic diagram of parabolic trough solar system.

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Chapter # 2

Modes of Power Generation

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2.1 Power Generation

Power generation is the process of generating electric power from sources of energy. The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday. His basic method is still used today: electricity is generated by the movement of a loop of wire, or disc of copper between the poles of a magnet.

For electric utilities, it is the first process in the delivery of electricity to consumers. The other processes, electricity transmission, distribution, and electrical power storage and recovery using pumped-storage methods are normally carried out by the electric power industry.

Electricity is most often generated at a power station by electromechanical generators, primarily driven by heat engines fueled by chemical combustion or nuclear fission but also by other means such as the kinetic energy of flowing water and wind. There are many other technologies that can be and are used to generate electricity such as solar photovoltaic and geothermal power.

2.2 Methods of Generating Electricity

There are seven fundamental methods of directly transforming other forms of energy into electrical energy:

Static electricity: From the physical separation and transport of charge (examples: triboelectric effect and lightning)

Electromagnetic induction: Where an electrical generator, dynamo or alternator transforms kinetic energy (energy of motion) into electricity. This is the most used form for generating electricity and is based on Faraday's law. It can be experimented by simply rotating a magnet within closed loops of a conducting material (e.g. copper wire)

Electrochemistry, the direct transformation of chemical energy into electricity, as in a battery, fuel cell or nerve impulse

Photoelectric effect, the transformation of light into electrical energy, as in solar cells

Thermoelectric effect, the direct conversion of temperature differences to electricity, as in thermocouples, thermopiles, and thermionic converters.

Piezoelectric effect, from the mechanical strain of electrically anisotropic molecules or crystals. Researchers at the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a piezoelectric generator sufficient to operate a liquid crystal display using thin films of M13 bacteriophage.

Nuclear transformation, the creation and acceleration of charged particles (examples: betavoltaics or alpha particle emission)

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http://en.wikipedia.org/wiki/Electric_power

Static electricity was the first form discovered and investigated, and the electrostatic generator is still used even in modern devices such as the Van de Graaff generator and MHD generators. Charge carriers are separated and physically transported to a position of increased electric potential.

Almost all commercial electrical generation is done using electromagnetic induction, in which mechanical energy forces an electrical generator to rotate. There are many different methods of developing the mechanical energy, including heat engines, hydro, wind and tidal power.

The direct conversion of nuclear potential energy to electricity by beta decay is used only on a small scale. In a full-size nuclear power plant, the heat of a nuclear reaction is used to run a heat engine. This drives a generator, which converts mechanical energy into electricity by magnetic induction.

Most electric generation is driven by heat engines. The combustion of fossil fuels supplies most of the heat to these engines, with a significant fraction from nuclear fission and some from renewable sources. The modern steam turbine (invented by Sir Charles Parsons in 1884) currently generates about 80% of the electric power in the world using a variety of heat sources.

2.2.1 Turbine

All turbines are driven by a fluid acting as an intermediate energy carrier. Many of the heat engines just mentioned are turbines. Other types of turbines can be driven by wind or falling water.

Sources include:

Steam - Water is boiled by:

1. Nuclear fission2. The burning of fossil fuels (coal, natural gas, or petroleum). In hot gas

(gas turbine), turbines are driven directly by gases produced by the combustion of natural gas or oil. Combined cycle gas turbine plants are driven by both steam and natural gas. They generate power by burning natural gas in a gas turbine and use residual heat to generate additional electricity from steam. These plants offer efficiencies of up to 60%.

3. Renewable. The steam is generated by:

I. BiomassII. Solar thermal energy (the sun as the heat source): solar

parabolic troughs and solar power towers concentrate sunlight

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to heat a heat transfer fluid, which is then used to produce steam.

III. Geothermal power. Either steam under pressure emerges from the ground and drives a turbine or hot water evaporates a low boiling liquid to create vapour to drive a turbine.

IV. Ocean thermal energy conversion (OTEC): uses the small difference between cooler deep and warmer surface ocean waters to run a heat engine (usually a turbine).

Water (hydroelectric) - Turbine blades are acted upon by flowing water, produced by hydroelectric dams or tidal forces.

Wind - Most wind turbines generate electricity from naturally occurring wind. Solar updraft towers use wind that is artificially produced inside the chimney by heating it with sunlight, and are more properly seen as forms of solar thermal energy.

2.2.2 Reciprocating engines

Small electricity generators are often powered by reciprocating engines burning diesel, biogas or natural gas. Diesel engines are often used for back up generation, usually at low voltages. However most large power grids also use diesel generators originally provided as emergency back up for a specific facility such as a hospital, to feed power into the grid during certain circumstances. Biogas is often combusted where it is produced, such as a landfill or wastewater treatment plant, with a reciprocating engine or a micro turbine, which is a small gas turbine.

2.2.3 Photovoltaic panels

Unlike the solar heat concentrators mentioned above, photovoltaic panels convert sunlight directly to electricity. Although sunlight is free and abundant, solar electricity is still usually more expensive to produce than large-scale mechanically generated power due to the cost of the panels. Low-efficiency silicon solar cells have been decreasing in cost and multifunction cells with close to 30% conversion efficiency are now commercially available. Over 40% efficiency has been demonstrated in experimental systems. Until recently, photovoltaic were most commonly used in remote sites where there is no access to a commercial power grid or as a supplemental electricity source for individual homes and businesses. Recent advances in manufacturing efficiency and photovoltaic technology, combined with subsidies driven by environmental concerns, have dramatically accelerated the deployment of solar panels. Installed capacity is growing by 40% per year led by increases in Germany, Japan, California and New Jersey.

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2.2.4 Other generation methods

Various other technologies have been studied and developed for power generation. Solid-state generation (without moving parts) is of particular interest in portable applications. This area is largely dominated by thermoelectric (TE) devices, though thermionic (TI) and thermophotovoltaic (TPV) systems have been developed as well. Typically, TE devices are used at lower temperatures than TI and TPV systems. Piezoelectric devices are used for power generation from mechanical strain, particularly in power harvesting. Betavoltaics are another type of solid-state power generator which produces electricity from radioactive decay. Fluid-based magnetohydrodynamic (MHD) power generation has been studied as a method for extracting electrical power from nuclear reactors and also from more conventional fuel combustion systems. Osmotic power finally is another possibility at places where salt and sweet water merges (e.g. deltas, ...)

Electrochemical electricity generation is also important in portable and mobile applications. Currently, most electrochemical power comes from closed electrochemical cells ("batteries"), which are arguably utilized more as storage systems than generation systems; but open electrochemical systems, known as fuel cells, have been undergoing a great deal of research and development in the last few years. Fuel cells can be used to extract power either from natural fuels or from synthesized fuels (mainly electrolytic hydrogen) and so can be viewed as either generation systems or storage systems depending on their use.

2.2.5 Characteristics of Different generation methods

The best way to describe the characteristics of these generation methods is by comparing them all together, as it is done below

Modes Advantages Disadvantages

Hydro-electric Plant

No fuel required

Neat and clean energy

Required less maintenance

Not required long starting

High capital cost

High cost in transmission line

Dam is required

Huge amount of water is required

Huge space required

Thermal Less initial cost Running cost is high

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Plant(coal..)

Installed at any place

Less space required

Lower initial cost

Pollutes the atmosphere

Thermal Plant(nuclear)

Amount of fuel is small

Less space required

Huge amount of heat

Fuel is expensive

Capital cost is high

Radioactive pollution

Solar plant

No fuel required

Neat and clean energy

Required less maintenance

High capital cost

Huge area required

Table 2.1: Comparison between Different generation methods

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Chapter # 3

Passive Solar System

34

3.1 Technique

Solar-thermal technologies are, more or less, a traditional electricity generating technology. They use the sun's heat to create steam to drive an electric generator. Use reflectors to concentrate sunlight to heat the water which is used to generate steam to drive a standard turbine.

3.2 Technical Principle

In general, solar thermal technologies are based on the concept of concentrating solar radiation to produce steam or hot air, which can then be used for electricity generation using conventional power cycles. Collecting the solar energy, which has relatively low density, is one of the main engineering tasks in solar thermal power plant development. For concentration, most systems use glass mirrors because of their very high reflectivity. Other materials are under development to meet the needs of solar thermal power systems. Point focusing and line focusing systems are used. These systems can use only direct radiation and not the diffuse part of sunlight because this cannot be concentrated. Line focusing systems are easier to handle, but have a lower concentration factor and hence achieve lower temperatures than point focusing systems.

3.3 Concentrating Solar Collector

Solar collectors are used to produce heat from solar radiation. High temperature solar energy collectors are basically of three types;

a. Parabolic trough system: at the receiver can reach 400° C and produce steam for generating electricity.

b. Power tower system: The reflected rays of the sun are always aimed at the receiver, where temperatures well above 1000° C can be reached.

c. Parabolic dish systems: Parabolic dish systems can reach 1000° C at the receiver, and achieve the highest efficiencies for converting solar energy to electricity.

d. Linear Fresnel Reflectors (LFR): It is a line-focusing system. The classical LFR uses modular, flat mirrors that track the sun to focus the sun's heat onto long, elevated tubular receiver (absorber) through which water flows. It can be reach 1000º C at receiver.

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3.3.1 Parabolic trough collector system

Parabolic trough power plants are line-focusing STE (solar thermal electric) power plants. Trough systems use the mirrored surface of a linear parabolic concentrator to focus direct solar radiation on an absorber pipe running along the focal line of the parabola. The HTF (heat transfer fluid) inside the absorber pipe is heated and pumped to the steam generator, which, in turn, is connected to a steam turbine. A natural gas burner is normally used to produce steam at times of insufficient insulation. The collectors rotate about horizontal north–south axes, an arrangement which results in slightly less energy incident on them over the year but favors summertime operation when peak power is needed.

The major components in the system are collectors, fluid transfer pumps, power generation system and the controls. This power generation system usually consists of a conventional Rankine cycle reheat turbine with feed water heaters, etc. and the condenser cooling water is cooled in forced draft cooling towers. These types of power plants can have energy storage system comprising these collectors usually have the energy storage facilities. Instead they are couple to natural gas fired back up systems. A typical configuration of such systems is shown in Figure 3.1.

Figure 3.9: Parabolic Trough System

3.3.2 Power Tower System

In power tower systems, heliostats (A Heliostat is a device that tracks the movement of the sun which is used to orient a mirror of field of mirrors, throughout the day, to

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reflect sunlight onto a target-receiver) reflect and concentrate sunlight onto a central tower-mounted receiver where the energy is transferred to a HTF. This energy is then passed either to the storage or to power-conversion systems, which convert the thermal energy into electricity. Heliostat field, the heliostat controls, the receiver, the storage system, and the heat engine, which drives the generator, are the major components of the system.

For a large heliostat field a cylindrical receiver has advantages when used with Rankine cycle engines, particularly for radiation from heliostats at the far edges of the field. Cavity receivers with larger tower height to heliostat field area ratios are used for higher temperatures required for the operation of Brayton cycle turbines Figure 3.2.

Figure 3.10: Tower System

3.3.3 Parabolic Dish System

The parabolic dish system uses a parabolic dish shaped mirror or a modular mirror system that approximates a parabola and incorporates two-axis tracking to focus the sunlight onto receivers located at the focal point of the dish, which absorbs the energy and converts it into thermal energy. This can be used directly as heat for thermal application or for power generation. The thermal energy can either be transported to a central generator for conversion, or it can be converted directly into electricity at a local generator coupled to the receiver (Figure 3.3).

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Figure 3.11: Parabolic Dish System

The mirror system typically is made from a number of mirror facets, either glass or polymer mirror, or can consist of a single stretched membrane using a polymer mirror of thin metal stretched membrane.

The PDCs (parabolic dish collector) track the sun on two axes, and thus they are the most efficient collector systems. Their concentration ratios usually range from 600 to

2000, and they can achieve temperatures in excess of 1500o C. Rankine-cycle engines, Brayton-cycle engines, and sodium-heat engines have been considered for systems using dish-mounted engines the greatest attention though was given to Stirling-engine systems.

The main challenge facing distributed-dish systems is developing a power-conversion unit, which would have low capital and maintenance costs, long life, high conversion efficiency, and the ability to operate automatically. Several different engines, such as gas turbines, reciprocating steam engines, and organic Rankine engines, have been explored, but in recent years, most attention has been focused on Stirling-cycle engines. These are externally heated piston engines in which heat is continuously added to a gas (normally hydrogen or helium at high pressure) that is contained in a closed system.

The Stirling Energy Systems (SES) and Science Applications International Corporation (SAIC) dishes at UNLV and the Big Dish in Canberra, Australia are representatives of this technology. Annexure–I presents the technical details of some existing solar thermal power plants globally.

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3.3.4 Linear Fresnel Reflectors(LFR)

It is a line-focusing system. The classical LFR uses modular, flat mirrors that track the sun to focus the sun's heat onto long, elevated tubular receiver (absorber) through which water flows. The concentrated sunlight boils the water in the tubes, generating hot water, saturated or superheated steam for use in power generation in steam Rankin cycle for instance or for process heat in industrial applications. Figure below illustrates the basic configuration of the system.

Figure 3.12: Basic configuration of LFR system

One fundamental difficulty with the LFR technology is the avoidance of shading of incoming solar radiation and blocking of reflected solar radiation by adjacent reflectors. Shading and blocking can be reduced by using higher absorber tower which increases cost or by increasing absorber size which allows increased spacing between reflectors remote from the absorber which leads to more ground usage and more thermal losses and shading by the absorber.A development of the traditional system is the Compact Linear Fresnel Reflector (CLFR) system. CLFR utilizes multiple absorbers within the vicinity of the reflectors as shown in figure below. This development makes the technology capable of supplying electricity in the multi megawatt range.

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Figure 3.13: Compact LFR system

The arrangement utilized in CLFR allows for minimum blocking of reflected radiation between adjacent mirrors which means the avoidance of large mirror spacing. It also allows for lower absorber height which eventually means lower investment cost.One of the basic advantages of the LFR which makes the system less expensive than other CSP systems is the use of water as the heat transfer fluid which eliminates the expensive and hazardous thermal oil as heat transfer fluid and eliminates the extra cost of oil/water heat exchanger. The use of water as HTF in other technologies like parabolic trough is now under development. An additional advantageous feature of the technology is that the moving parts are accessible which allows for easier maintenance of the system. Also, the absorber/heat transfer loop is isolated from the reflector field and does not move, thus avoiding the high cost of flexible high pressure lines or rotating joints.

3.4 Parabolic Trough Collector Structure

The equipment tested in this study consisted of a locally developed parabolic trough solar collector. The PTSC (parabolic trough solar collector) has a torque-tube structure with a length of 2.2 m, aperture width of 1.5 m and a rim angle of 82º as shown in figure. The reflective surface consists of stainless steel sheets covered with silver polymer reflective film and clamped into the profile formed by parabolic ribs.

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Figure 3.14: Structure of Parabolic Trough

3.5 Parabola

Parabola is the set of all point in the plane whose distance from fixed point is equal to its distance from a fixed line.

Figure 3.15: Parabola with fixed point and fixed line

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√(x−0)2+( y−f )2=√(x−x )2+( y−(−f ))2

x2+ y2+ f 2−2 fy= y2+ f 2+2 fy y= x2

4 f

Y is the depth (d), x is the radius(x=D/2) and f is the focal point

d=

D2

2

4 f

f = D2

16 d

3.6 Rim Angle

The problem presented is to determine the ideal width and focal length of a parabolic trough. In order to do this, we must first develop the mathematical characterization of the trough and rays incident upon it, shown in Figure 3.7 provided the parabolic equation y = x²/4f where f is the focal length of the parabola. The rays incident upon the trough are assumed to be parallel due to the sun's approximately infinite distance, and at perfect alignment, these rays are parallel to the y-axis.

Figure 3.16: Parabola with rim angle

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tanθ=

D2y

tanθ=

D2

f − y

y=d

tanθ=

D2

f −d

tanθ=8.3 .57

θ=83.2°

3.7 Angle of Incident Calculation

This section introduces the equations for the calculation of the angle of incidence (θ) defined as the angle between solar rays and the surface normal. The angle of incidence has a major significance in evaluating the performance of the solar collectors. Let us first define the three basic angles (see figure 3.9): hour angle (ω), latitude angle (ɸ), and the declination angle (δ).

3.7.1 Basic Angles

3.7.1.1 Hour angle (ω)

It is the angular distance between the meridian of the observer and the meridian whose plane contains the sun. The hour angle can be calculated by:

ω=15 (t s−12 ) (degrees )(3.7 .1)

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Figure 3.17: Basic angles for the location Q

where t s is the solar time (equals 12 at solar noon) which is different from the local clock time (LCT). For any point on earth, the solar noon occurs when the point faces the sun and its meridian is in line with the solar rays. Solar time can be calculated (in hours) by:

t s=LCT + EOT60

– LC – DLS (hours )(3.7 .2)

where EOT (equation of time) is a correction in minutes for the true solar time, LC is the longitude correction in hours, and DLS is the correction for daylight saving time which is one hour if the daylight saving time is in effect.The equation of time varies over the year and can reach seventeen minutes. An approximation that is accurate within 30 seconds[13 ] of the equation of time is:

Eot=0.258 cos x−7.416 sin x−3.648 cos 2 x−9.228sin 2 x (min )(3.7 .3)

Where x is an angle which is function of the day number N, starting from January

1st (N = 1)

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x=360(N−1)365.242

(degrees )(3.7 .4)

the longitude correction accounts for the difference between local time and standard meridian time.

LC=(longitude of strd time zonemeridian−local longitude )

15(hour )(3.7 .5)

For our system, the local longitude is 73.13º E and the longitude of standard time zone is 70º E.

3.7.1.2 Latitude Angle (ɸ)

The latitude angle is the angle between a line drawn from a point on the earth’s surface to the center of the earth, and the earth’s equatorial plane. Locations south of the equator have negative latitude angles and those to the north have positive latitude angles. The latitude angle of the system under study in this study is 34.71º.

3.7.1.3 Declination Angle (δ)

It is the angle that solar orbit makes with the plane of the earth's equator. The angle varies as the earth rotates around the sun. The yearly variation of the declination angle is shown in figure 3.10.

Figure 3.18: Variation of declination angle

Equation 3.7.6 is an approximation accurate within one degree for calculating the declination angle

sin δ=0.39795 cos [0.98563 (N−173 ) ](3.7 .6)

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3.7.2 Solar Angles

For solar power applications, we need to define the position of the sun relative to the site (any point on earth) in order to calculate the amount of solar energy that will be received by the solar system. The position of the sun can be described by two angles: the solar altitude angle (α) and the solar azimuth angle (ß). These angles are shown in figure 3.11.

Figure 3.19: Earth surface coordinate system for observer at point Q showing the solar azimuth angle ß, the solar altitude angle α

It is convenient to express the solar angles in terms of the three basic angles ω, ɸ and δ in order to identify the sun’s position for any location at any time.

3.7.2.1 Solar altitude angle (α)

The solar altitude angle is the angle between the direction of the geometric center of the sun's apparent disk and a horizontal plane containing the observer. It is given by

α=sin−1 (sin δ sin ϕ+cosδ cosϕ cosω ) (de grees )(3.7 .7)

3.7.2.2 Solar azimuth angle (ß)

It is the angle, measured clockwise on the horizontal plane, from the north pointing coordinate axis to the projection of the sun’s central ray. It is calculates as

β '=cos−1¿¿¿

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Where if: sin ω>0 , then β=360°−β '

sin ω≤ 0 , then β=β '

3.7.3 Angle of incidence (θ)

As previously defined, the angle of incidence is the angle between the solar rays and the surface normal. Calculating θ is of great importance for solar system design and performance evaluation since the amount of collected solar energy is reduced by the cosine of this angle. Figure 3.12 depicts a fixed aperture that is oriented at angle Ω (aperture azimuth angle) and tilted at angle ʎ (tilt angle). The angle of incidence for this aperture is given by

cosθ=sin α cos λ+cos α sin λ cos (Ω−β) (degrees )(3.7 .9)

Figure 3.20: A fixed aperture with its orientation defined by the tilt angle (ʎ) and the aperture azimuth angle (Ω)

For an aperture with a single-axis tracking system (such as parabolic trough), the aperture is rotated until the solar rays and the aperture normal are coplanar as illustrated in figure 3.13.

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Figure 3.21: A single-axis tracking aperture rotating about the axis r

Figure 3.22: Single axis tracking system coordinates

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Figure 3.23: Rotation of the u, b, r coordinates from the z, e, n (fig. 3.10) coordinates about the z-axis (diagram shows view looking downward on the

surface of the earth)

Figure 3.14 depicts a horizontal single-axis tracking aperture. The tracking angle (ρ) measures rotation about the tracking axis r with (ρ=0) when N is vertical. To describe this tracking scheme in terms of solar angles, the coordinates u, b, and r must be rotated by an angle Ω (see figure 3.15) from the z, e, and n coordinates that were used to describe the sun angles. The tracking angle can be calculated by

tan ρ=sin ( β−Ω ) / tan α (3.7 .10)

the value of the angle of incidence for this aperture is given by

cosθ=[1−cos2 (α )cos2 (β 0Ω )]12 (3.7 .11)

3.8 Reflective Surface

The reflective material used for this project is a silver polymer film produced by Reflec Tech. This film reflects 94% of the solar spectrum and lasts 10 years in an outdoor environment. This particular material was chosen for its extremely high reflectivity and flexibility of application. The film has an adhesive backing that can be applied easily to any at surface. One drawback of the product is that the film is very thin and extremely susceptible to “print-through”. This occurs when the film contours to imperfections on the applied surface and results in decreased optical clarity. This is a major problem when using fiberglass as the concentrator structure. Fiberglass resin hardens to the cloth, leaving a crisscross pattern of hardened fiberglass strands, which is a less than ideal surface for the Reflec Tech. Therefore, great care was given to

49

surface preparation of the fiberglass. Initially, all of the panels were covered and sanded with Bondo surface prep. This smoothed out the print pattern of the fiberglass panels. However, the durability of the surface prep was questioned during the painting process, and the panels were ultimately sanded down with an industrial sander. A base paint was then sprayed directly to the sanded fiberglass. This still left a small amount of `print-through' on the reflective surface. The optical error due to this effect is believed to be the same order of magnitude as the error in the curvature of the dish surface. Because the print through error is random it did not greatly amplify the surface error. This situation is acceptable given the advantages that the material brings. A better surface preparation solution needs to be found if the curvature of the surface slope was corrected.

In 2009, scientists at the National Renewable Energy Laboratory (NREL) and SkyFuel teamed to develop large curved sheets of metal that have the potential to be 30% less expensive than today's best collectors of concentrated solar power by replacing glass-based models with a silver polymer sheet that has the same performance as the heavy glass mirrors, but at a much lower cost and much lower weight. It also is much easier to deploy and install. The glossy film uses several layers of polymers, with an inner layer of pure silver.

3.8.1 Silver Polymer Sheets

Reflec tech surface is very smooth. Its uses pure silver to provide high specular reflectance and multiple layers of polymer films to protect against Ultraviolet radiations and moisture.

PROPERTIES:

1. Specular Reflectance is 94%

2. Thickness is 0.1mm and also available in different thickness

3. Maximum Operating Temperature is 60 °C

ADVANTAGES:

1. High Reflectance than others

2. Low Cost About 30% than other reflectors

3. Out Door Wheatherable

4. Lighter in weight than glass, less costly, more durable

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5. Good Replacement to Ordinary Mirrors

3.8.2 Different Reflective Sheets

There are some other different reflective sheets or films shown in table.

Reflective Surface Advantages Disadvantages

Black silvered glass

High reflectance Load carrying capacity Surface accuracy Good weathering

protection

Very expensive Specialized

equipment needed for manufacture

Bare aluminum

Reflectance up to 95% possible

Inexpensive Various gauges

available Light weight

No weathering protection

Highly susceptible to abrasion

Requires stable, accurate parabolic structure

Polished stainless steel

Inexpensive Various gauges

available

Low reflectance(less than 70%)

Susceptible to abrasion

Requires stable, accurate parabolic str

Aluminized acrylic film

Reflectance up to 85% possible

Relativity inexpensive Some weathering

protection



Out of production

Advanced composite film

Very high reflectance (> 97%)

Non-metallic composite



No weathering protection

Very expensiveTable 3.1: Comparison between Different Reflective Sheets

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3.9 Absorber

For the Parabolic trough collectors the absorber pipe consists of a stainless steel tube with a length of 4 meters and a thickness of 70mm. A glass pipe surrounds the tube (see figure 3.16). The glass tube allows evacuating of the area between the absorber tube and the glass pipe in order to minimize convection and conduction heat losses.

Figure 3.24: Absorber tube of a parabolic trough collector [REB]

The vacuum also serves to protect the highly sensitive coating. Nowadays, such selective coatings remain stable in temperatures of 450°C upwards to 500°C. On average the solar absorption is currently above 95% and at an operational temperature of around 400°C the emissivity is below 14%. This leads to an optical efficiency of around 80% for upcoming perpendicular radiation. Furthermore the hydrogen getter (see figure 3.16) absorbs the hydrogen, which is getting through the glass pipe and the stainless steel pipe by diffusion. A membrane finally “pumps” the hydrogen out of the vacuum. As a final point, glass/metal joints realize extension bellows compensating the thermal expansion of the pipe, and the connection between the glass pipe and the metal structure.All rays entering the parabolic reflector are concentrated at single point (the focal point), located 1/2 the distance of the arc's radius. A Parabolic Trough Mirror type solar array is engineered so as to place the Heat Collection Element (HCE) precisely at the Fp. The solar array will track the East to West movement of the sun. The HCE is designed to absorb and collect incident rays reflecting off the parabolic mirror but, of course, some incident rays will strike the HCE directly as it is located in front of the mirror. As a result, there will be some reflections from the glass coating the HCE; however, these reflections will be minor as the HCEs are designed to absorb sunlight, not reflect it.

The receiver has glass-to-metal seals and metal bellows to accommodate for differing thermal expansions between the steel tubing and the glass envelop. They also help achieve the necessary vacuum-tight enclosure.

52

We use steel pipe coated with cermets materials. Steel is used due to its high thermal conductivity, due to greater resistance of corrosion and high melting point. Different materials with different thermal conductivity are shown in table.

Materials Melting point (ºC) Temperature (ºF)

Thermal Conductivity Btu/(hr o F ft)

Silver 980 300 235

Tin 240 300 39

Zink 419 300 67

Lead 328 300 18

Copper 1080 300 225

aluminum 665 300 144

Table 3.2: Different Materials with Melting point and Thermal Conductivity

The vacuum-tight enclosure primarily serves to significantly reduce heat losses at high-operating temperatures. It also protects the solar-selective absorber surface from oxidation.

The selective coating on the steel tube has good solar absorptance and a low thermal emittance for reducing thermal radiation losses. The glass cylinder features an anti-reflective coating to maximize the solar transmittance. Getters metallic compounds designed to absorb gas molecules are installed in the vacuum space to absorb hydrogen and other gases that permeate into the vacuum annulus over time.

Equation 3.9.1 shows an energy balance for a receiver.

Qout=Q|¿|−Qloss (3.9 .1)¿

where:

Qout = Useful energy transferred to working fluidQ|¿|¿ = Energy collected by the absorberQloss = Receiver energy losses

and total receiver efficiency ηrec, is given by Equation 3.9.2.

ηrec=Qout

Q|¿|(3.9 .2)¿

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HCEs are designed to minimize heat loss to the environment while letting in and absorbing as much sunlight as possible. Several design features that make this possible.

1. The annulus between the absorber tube and the transparent glass cylindrical envelope is evacuated to prevent heat conduction/convection from the hot absorber tube to the cooler glass envelope.

2. Irradiative heat loss from the absorber is minimized by coating the absorber tube with a selective surface that has high solar absorption but low thermal emittance. The effect of heat conduction at the ends is reduced by making long HCEs.

3. Finally, the diameter of the absorber is small relative to the collecting aperture of the reflector, thereby decreasing the surface area associated with heat loss.

3.10 Solar Field

Inside of the absorber tube a heat transport medium, mainly synthetic oil, is used to collect the thermal energy and transport it to heat exchangers for producing steam in order to operate a steam turbine. During the transportation of the oil thermal losses can be recognized, and also further losses during the concentration process have to be taken into account.

3.10.1 Optical Losses

Energy losses not only occur because of geometrical reasons like shading or un- irradiated absorber parts, but also from the material properties of the mirror, the hull pipe, and the absorber. Therefore, some operating figures will be defined. They are all dependent on the design quality of the collector elements being used.

• Reflectivity of the mirror:A small part of the incidence radiation is not reflected to the absorber, because the reflectivity of the mirror being finite. The mirror absorbs a part of the incoming radiation. As an average the reflection coefficient of a mirror used in solar thermal abdications can be set to ρ=0.93

• Contamination of the mirror:The part of irradiation, which is absorbed by the mirror, is increasing with ongoing contamination of the mirror surface. Taking into consideration frequently washing procedure the contamination factor can be estimated with δ=0.98

• Transmission factor of the mirror:

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The glass on the top of the mirror also partly absorbs the irradiation. The irradiation has to pass the glass cover two times, which leads to a transmission coefficient of around τ s=0.99

• Quality factor of the mirror:The quality factors depending on the production processes as well as the erection on site. For example the absorber tube is not exactly mounted in the focal point of the mirror additional losses will occur. Also different focal length of the mirror plats will lead to additional losses. Nowadays a quality factor of γ=0.90 is assumed.

• Transmission factor of the hull pipe:A small part of the reflected irradiation is again reflected by the glass pipe, which surrounds the absorber tube. This transmission coefficient can be set here to τ H=0.95

• Absorption factor of the absorber pipe:At the absorber tube not all of the reflected radiation will be absorbed. Due to physical conditions a part of the radiation will always be reflected. The absorption factor can be estimated with α=0.95.

Taking into consideration all the additional factors above it is finally possible to calculate the amount of energy received per square meter absorber pipe:

GA=IDRδρα τ H τ S2=IDR ηopt (3.10.1)

As displayed in equation (3.10.1) the combination of all quality factors can be summarized as the optical efficiency of the mirrorηopt. In the formula above GA represents the irradiation reaching the absorber pipe per square meter. Now the part of how the transformation into thermal energy and consequently, the losses occurring during transportation in the absorber pipe is described.

3.10.2 Heat Losses

Until now it has been described how the energy of the direct solar radiation reaches the surface of the absorber. Here the form of energy is now changed into thermal energy. This can be described by constructing an energy balance equation like:

GA Aσ=Q n+ ˙Q losses(3.10 .2)

Here A is the absorber area and σ is the absorbance of the absorber. Qn identifies the useful thermal energy collected in the thermal oil and the thermal losses are combined inQlosses.Physically there are three different types of heat transportation, occurring naturally. These are heat transport according to convention, conduction and radiation. However

55

these phenomena are the reason for the thermal losses in the solar field. The losses depending on conduction and convection can be set in a first approach in a linear relation with the difference between the average absorber temperature and the ambient temperature:

Qcc=U CC A (T a−TU ) (3.10 .3)

UCC is the heat transfer coefficient, which is adjusted by measurement results. The

average absorber temperature is labeled T a , and T U is the ambient temperature. In addition to the convection and conduction losses, the thermal radiation losses must be taken into consideration. Therefore the heat flux between two surfaces by thermal radiation is described as:

˙Qrad=σ (T 2

4−T14)

1−ε1

ε1 A1

+1

A1 F1,2

+1−ε2

ε2 A2

(3.10 .4)

T represents the temperature for each of the two surfaces, F is the area angle between the surfaces, A is the area of the surfaces, σ is the Stefan Boltzmann constant, and, ε is the emission coefficient for each surfaces. In case of a solar field it can be assumed that the absorber area is relatively small compared to the ambient area. This allows simplifying equation (3.10.4) to:

˙Qrad=ε1 A1σ (T¿¿24−T 14)(3.10 .5)¿

In the formula above the emissivity of the environment is considered with one. A1 displays the area of the absorber. Now the energy used for heating up of the collector, for example in the morning hours can be calculated by:

QC=A U C (T c−T a )(3.10 .6)

UC shows the heat transfer coefficient of the collector and Tc is the collector

temperature. Putting all these thermal losses together and combining it with formula(3.10.2) the amount of thermal energy “produced” by the solar field can be calculated out of :

Qn=GαA−A U CC ( Tc−T a )−σεc A (T¿¿C4−T a4)−A UC (T c−T a ) (3.10.7)¿

Consequently, the amount of energy collected in the absorber must be transported either into storage or to the power block. Here additional losses in the absorber pipe will be occurring. Finally these piping losses can be calculated according to:

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QP=U P ( T F−Ta ) (3.10.8)

Now all necessary relations for the description of how the energy provided by the sun is harvested and distributed either into storage or to the power block are described.

3.11 Heat Exchanger

Heat exchangers are devices used to transfer heat between two or more fluid streams at different temperatures. Heat exchangers find widespread use in power generation, chemical processing, electronics cooling, air-conditioning, refrigeration, and automotive applications. In this topic we will examine the basic theory of heat exchangers and consider many applications. In addition, we will examine various aspects of heat exchanger design and analysis.

3.11.1 Heat Exchanger Classification

Due to the large number of heat exchanger configurations, a classification system was devised based upon the basic operation, construction, heat transfer, and flow arrangements. The following classification as outlined by Kakac and Liu (1998) will be discussed:o Recuperators and regenerators

o Transfer processes: direct contact or indirect contact

o Geometry of construction: tubes, plates, and extended surfaces

o Heat transfer mechanisms: single phase or two phase flow

o Flow Arrangement: parallel flow, counter flow, or cross flow

3.11.2 Flow Arrangement

There are three primary classifications of heat exchangers according to their flow arrangement.

In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side.

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Figure 3.25: Parallel Flow Arrangement

In counter-flow heat exchangers the fluids enter the exchanger from opposite ends. The counter current design is the most efficient, in that it can transfer the most heat from the heat (transfer) medium due to the fact that the average temperature difference along any unit length is greater. See countercurrent exchange.

Figure 3.26: Counter Current Flow Arrangement

In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger.

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Figure 3.27: Cross Flow Arrangement

For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance to fluid flow through the exchanger. The exchanger's performance can also be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence.

The driving temperature across the heat transfer surface varies with position, but an appropriate mean temperature can be defined. In most simple systems this is the "log mean temperature difference" (LMTD). Sometimes direct knowledge of the LMTD is not available and the NTU method is used.

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3.11.3 Types of Heat Exchanger

3.11.3.1 Shell and Tube Heat Exchanger

Figure 3.28: Shell and Tube Exchanger

Shell and tube heat exchangers consist of a series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. Shell and tube heat exchangers are typically used for high-pressure applications (with pressures greater than 30 bar and temperatures greater than 260 °C).

This is because the shell and tube heat exchangers are robust due to their shape.Several thermal design features must be considered when designing the tubes in the shell and tube heat exchangers:

Tube diameter: Using a small tube diameter makes the heat exchanger both economical and compact. However, it is more likely for the heat exchanger to foul up faster and the small size makes mechanical cleaning of the fouling difficult. To prevail over the fouling and cleaning problems, larger tube diameters can be used. Thus to determine the tube diameter, the available space, cost and the fouling nature of the fluids must be considered.

Tube thickness: The thickness of the wall of the tubes is usually determined to ensure:

o There is enough room for corrosiono That flow-induced vibration has resistance

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o Axial strengtho Availability of spare partso Hoop strength (to withstand internal tube pressure)o Buckling strength (to withstand overpressure in the shell)

Tube length: Heat exchangers are usually cheaper when they have a smaller shell diameter and a long tube length. Thus, typically there is an aim to make the heat exchanger as long as physically possible whilst not exceeding production capabilities. However, there are many limitations for this, including space available at the installation site and the need to ensure tubes are available in lengths that are twice the required length (so they can be withdrawn and replaced). Also, long, thin tubes are difficult to take out and replace.

Tube pitch: When designing the tubes, it is practical to ensure that the tube pitch (i.e., the centre-centre distance of adjoining tubes) is not less than 1.25 times the tubes' outside diameter. A larger tube pitch leads to a larger overall shell diameter, which leads to a more expensive heat exchanger.

Tube corrugation: This type of tubes, mainly used for the inner tubes, increases the turbulence of the fluids and the effect is very important in the heat transfer giving a better performance.

Tube Layout: Refers to how tubes are positioned within the shell. There are four main types of tube layout, which are, triangular (30°), rotated triangular (60°), square (90°) and rotated square (45°). The triangular patterns are employed to give greater heat transfer as they force the fluid to flow in a more turbulent fashion around the piping. Square patterns are employed where high fouling is experienced and cleaning is more regular.

Baffle Design: Baffles are used in shell and tube heat exchangers to direct fluid across the tube bundle. They run perpendicularly to the shell and hold the bundle, preventing the tubes from sagging over a long length. They can also prevent the tubes from vibrating. The most common type of baffle is the segmental baffle. The semicircular segmental baffles are oriented at 180 degrees to the adjacent baffles forcing the fluid to flow upward and downwards between the tube bundles. Baffle spacing is of large thermodynamic concern when designing shell and tube heat exchangers. Baffles must be spaced with consideration for the conversion of pressure drop and heat transfer. For thermo economic optimization it is suggested that the baffles be spaced no closer than 20% of the shell’s inner diameter. Having baffles spaced too closely causes a greater pressure drop because of flow redirection. Consequently having the baffles spaced too far apart means that there may be cooler spots in the corners between baffles. It is also important to ensure the baffles are spaced close enough that the tubes do not sag. The other main type of baffle is the disc and donut baffle, which consists of two concentric baffles. An outer, wider baffle looks like a donut, whilst the inner baffle is shaped like a disk. This type of baffle forces the fluid to pass around

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each side of the disk then through the donut baffle generating a different type of fluid flow.

3.11.3.2 Plate Heat Exchanger

Another type of heat exchanger is the plate heat exchanger. One is composed of multiple, thin, slightly separated plates that have very large surface areas and fluid flow passages for heat transfer. This stacked-plate arrangement can be more effective, in a given space, than the shell and tube heat exchanger. Advances in gasket and brazing technology have made the plate-type heat exchanger increasingly practical. In HVAC applications, large heat exchangers of this type are called plate-and-frame; when used in open loops, these heat exchangers are normally of the gasket type to allow periodic disassembly, cleaning, and inspection. There are many types of permanently bonded plate heat exchangers, such as dip-brazed, vacuum-brazed, and welded plate varieties, and they are often specified for closed-loop applications such as refrigeration. Plate heat exchangers also differ in the types of plates that are used, and in the configurations of those plates. Some plates may be stamped with "chevron", dimpled, or other patterns, where others may have machined fins and/or grooves.

Figure 3.29: Conceptual diagram of a plate and frame heat exchanger

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3.11.3.3 Plate and shell heat exchanger

A third type of heat exchanger is a plate and shell heat exchanger, which combines plate heat exchanger with shell and tube heat exchanger technologies. The heart of the heat exchanger contains a fully welded circular plate pack made by pressing and cutting round plates and welding them together. Nozzles carry flow in and out of the plate pack (the 'Plate side' flow path).The fully welded plate pack is assembled into an outer shell that creates a second flow path ( the 'Shell side'). Plate and shell technology offers high heat transfer, high pressure, high operating temperature, compact size, low fouling and close approach temperature. In particular, it does completely without gaskets, which provides security against leakage at high pressures and temperatures.

Figure 3.30: Plate and shell heat exchanger

3.11.4 Heat Transfer Rate

It is given by [12]

q=U . A . ΔTm (3.11.4 .1)

U = overall heat transfer coefficient

A = surface area of exchanger

ΔT m = mean temperature difference between two fluid

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Figure 3.31: Graph of Heat Rate Transfer

The heat transferred through an element of area A can be written as

q=[mc . cc . (t2−t1 )c ](3.11.4 .2)

¿−[mh .ch . (t2−t1 )h ](3.11.4 .3)

through an area dA may b written as:

dq=mc∗cc∗dT c(3.11.4 .4)

dq=−mh∗ch∗dT h

So overall heat can be written as:

dq=U (T h−T c ) dA(3.11.4 .5)

From equation (3.11.4.4)

dT h=−dq

mh∗ch

And

dT c=dq

mc∗cc

So

d (T h−T c)=−dq ( 1mhch

+1

mc cc)(3.11.4 .6)

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So put values of (dq) from equation (3.11.4.5)

d( Th−Tc )( Th−Tc )

=−U ( 1mh ch

+ 1mc cc

)dA (3.11.4 .7)

Now integrate it between (3.11.4 .1) & (3.11.4.4) condition of graph

ln(T h2−Tc 2 )(T h1−Tc 1 )

=−UA ( 1mh ch

+ 1mc cc

)(3.11.4 .8)

From equation (3.11.4.4) put values of mh ch & mc cc in equation (3.11.4.8)

ln (T 2−T 2)( T1−T 1 )

=−UA [ (T h 1−T h2 )q

+(T c2−Tc 1 )

q ]q=UA

( Th 2−T c 2 )−(T h 1−T c1 )

ln [T h 2−T c2

T h 1−T c1 ]S0

∆ T m=(T h2−T c2 )−(T h1−Tc 1 )

ln [ Th2−T c 2

Th1−T c 1 ]

3.12 Thermal Storage

Regardless of the receiver design, CSP system benefit from thermal for dispatchable energy production.CSP plants without storage can only produce power while the sun is shining although the thermal inertia of the receiver allows for some flexibility due to cloud transient, on the order of a minute or so, as opposed to the truly instantaneous output of photovoltaic system. A well-designed thermal storage system allows for heat extraction irrespective of instantaneous solar conditions so power production can occur shifted relative to the maximum solar resources. However, some demonstration CSP plants have been designed with a co-firing gas turbine scheme as an alternative to thermal storage system. This provides the turbine with constant input power, regardless of solar fluctuations. Unfortunately, the gas turbine is driven at nights and during period of low insulation, offsetting the truly “clean” energy benefits the CSP field may provide.

Thermal energy storage can be divided into three categories: sensible, latent or chemical. Sensible heat systems rely on a temperature increase within media to store

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energy. Latent heat storage utilizes phase change materials, usually at constant temperature, releasing their enthalpies of fusion or vaporization. Alternatively, reversible endothermic reactions can be used to provide chemical heat storage; functional requirements include high energy density, operating temperature compatibility, excellent heat transfer characteristics, low losses, ease of control, safety, durability, mechanical and chemical stability and low storage system costs.

Current CSP system which address thermal storage utilize remote sensible heat storage of various design, dependent on the receiver HTF: tanks of pumped molten nitrate salts, banks of thermal oil-filled steel pipe bundles encased in concrete, or hot-air heated hollow refractory brick chamber. These designs required an active heat-transfer fluid flow, with associated high-temperature pumping issue and costs.

Many applications for working fluids require a steady thermal output. Heat may be stored in the receiver to act as a damper for heat transfer. When momentary cloud cover blocks energy input to the system the working fluid can obtain energy from the stored heat. If a receiver is producing steam to run a turbine and the thermal input decreased, the turbine blades could be damaged if liquid water is injected. A system equipped with heat storage could generate the steam after decreased solar input, decreasing the risk of outputting liquid water.Energy can be stored in any material by increasing its temperature. The amount of energy stored is proportional to the mass, heat capacity and temperature increase. The storage capacity of a mass can be made more efficient if it undergoes a phase change. This effect increases thermal storage without increasing receiver temperature. The total energy stored in a material that undergoes a phase change is given in Equation (3.12.1).

Qs=m [C solid (T ¿−T1 )+ λ+C liquid (T 2−T ¿) ](3.12.1)

Where:

Qs = Energy storage

m= Mass of materialC solid= Heat capacity of solid phaseC liquid= Heat capacity of liquid phaseT 1= Initial material temperature

T ¿= Phase change temperatureT 2=Final material temperature

3.12.1 Heat Transfer

Most heat transfer in a concentrating collector system occurs at the receiver. Energy from insulation is reflected onto the absorber and leaves the system via the working fluid and thermal losses. Heat is lost through all three modes of heat transfer;

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radiation, conduction and convection. The thermal efficiency of a concentrator system (ηtherm) is given by Equation (3.12.1.1).

ηtherm=Qout /Q¿=1−QL

Q¿(3.12.1 .1)

in this equation:

Q¿=Adish . I (3.12.1 .2)

QL=Q rad+Q conv+Qcond(3.12.1 .3)

And

Q¿ = Energy incident on dish

I = Direct normal insulation

Qout= Energy absorbed by working fluid

3.12.2 Molten Salt Thermal Storage

Molten salt is a preferred media for high temperature direct thermal storage, whereby a single fluid function as both the receiver HTF and the sensible heat storage medium. Direct systems eliminate the need for a heat exchanger between the receiver HTF and the storage media. Molten salts have high densities and specific heats, which increases volumetric storage efficiency. Additionally, they can be formulated to operate across various temperature ranges and have very low vapor pressure, enabling them to be used in unpressurized system. Molten salts are cheaper and more environmentally friendly than organic heat transfer oil used in parabolic trough system. However, as mentioned previously, molten salts have relatively high melting temperatures.

Tow designs are used for molten salt direct thermal storage: toe-tank system and single tank thermo cline systems. In a tow tank system, salt is heated by the receiver and directly stored in a hot tank. From the hot tank, salt is pumped to a heat exchanger, or steam generator, for the power cycle, where heat is extracted and its temperature is reduced. From here, it is pumped to a cold storage tank; the cycle repeats when the salt is then pumped to the receiver to be reheated. The advantage to

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the two-tank system is that the cold and hot salts are stored separately; however, two tanks must be constructed with each capable of storing the entire system volume. Daily temperature and pressure cycling of the tank walls are severe as the salt volume is transferred nearly completely from one tank to the other.

In contrast, a thermo cline system uses one tank, whereby the hot and cold salts are stored in the same tank. In traditional CSP system, cost-savings have been obtained with single tank system relying on temperature stratification via natural thermo cline formation. The hot salt, with reduced density, floats above the cold salt. Hot salt is extracted from the top of the tank and cold salt is returned to the bottom. Care must be taken to design the tank proportions and locate the extraction and return ports so fluid motion does not disturb the thermo cline. The stratification which forms can be enhanced with the use of solid filler materials within the tank, reducing mixing and stabilizing the thermo cline zone that forms between the hot and cold fluids. Tests using dual media thermo cline zone that forms between the hot and cold fluids. Tests using dual media thermo cline tanks with silica particles (sand) in molten nitrate salts, while confirming chemical stability, have shown the filler material tends to settle and pack over time due to the vertical cycling of the thermo cline’s position, as the system is charged and depleted respectively.

3.13 Steam Turbine

A steam turbine is an axial turbo engine, in which the thermal energy stored in the steam is converted into mechanical, mainly rotational energy. For the transition of the enthalpy into kinetic energy the working medium is seeded up in the nozzles. These nozzles are composed by the outlines of the guidance wheels. After this a switch of flow direction of the working medium takes place by using the rotating wheels. As a reaction of the impulse forces occurring now on the wheels, a torque is created and transferred to the turbine shaft. The set of a guidance wheel and a rotational wheel is called turbine stage. Figure 3.24 gives an overview of the construction concept of a modern steam turbine.

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Figure 3.32: Schematic drawing of a high-pressure turbine [KWT]

The construction schema above shows the inner cover a, the outer cover b, the labyrinth sealing c and the turbine shaft d.Furthermore, figure 3.24 is giving an overview of how the thermal energy stored in the steam is transferred into the kinetic energy of the shaft. Simplified the power produced by a steam turbine can be calculated out of the enthalpy drop over the turbine, the steam mass flow, and the turbine efficiency:

PT=ηT m Δh (3.13.1)

The turbine efficiency ηT is depending on several losses occurring during the energy conversion in the turbine. For an easier understanding of the turbine efficiency we can calculate the efficiency like:

ηT=ηi ηmech (3.13 .2)

In formula (3.13.2) ηi is considered as the inner efficiency. It is depending on losses occurring on the wheels, which are manly friction losses. Gap losses depending on the design of the turbine and ventilation losses are also recognized in this factor. Furthermore, losses depending on wet steam and losses occurring during the steam are leaving the turbine because of rearrangement in the flow direction. As a benchmark the inner efficiency for modern turbine can be assumed to be between 93% and 95%.The mechanical efficiency ηmech includes steam losses in the labyrinth sealing as well as friction losses between shaft and bearings. Because of using modern hydraulic bearings for carrying the turbine shaft the efficiency here can be assumed whit 98% or

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99%. All the assumptions above are related to the design point of the steam turbine, which is normally at the maximum rated power of the turbine.The turbine power however is controlled by the steam mass flow. This is affecting the efficiency as well as the net power output of the generator. The relationship between the steam mass flows for the different operation modes (full load or part load) are described by the cone law of Stodola:

mT

m0

=√ PαT2 −PωT

2 T α 0

Pα02 −Pω 0

2 T αT

(3.13 .3)

Here α stand for the entrance of the turbine and ω stands for the turbine outlet, 0 characterizes the full load operation, where T represents part load behavior. Over the years, several possibilities like fixed pressure operation, sliding control or equivalent sliding pressure have been established for the control of the steam turbine. Based on equations (3.13.3), the operation mode of the steam turbine, and also the part load behavior of a CSPP can be assumed.

3.14 DC Generator

An electric generator is a device used to convert mechanical energy into electrical energy.

The generator is based on the principle of electromagnetic induction discovered in 1831 by Michael Faraday. Faraday discovered that if an electric conductor, like a copper wire, is moved through a magnetic field, electric current will flow in the conductor. So the mechanical energy of the moving wire is converted into the electric energy of the current that flows in the wire.

3.14.1 How DC Generators Work

The commutator rotates with the loop of wire just as the slip rings do with the rotor of an AC generator. Each half of the commutator ring is called a commutator segment and is insulated from the other half. Each end of the rotating loop of wire is connected to a commutator segment. Two carbon brushes connected to the outside circuit rest against the rotating commutator. One brush conducts the current out of the generator, and the other brush feeds it in. The commutator is designed so that, no matter how the current in the loop alternates, the commutator segment containing the outward-going current is always against the "out" brush at the proper time. The armature in a large DC generator has many coils of wire and commutator segments. Because of the commutator, engineers have found it necessary to have the armature serve as the rotor(the rotating part of an apparatus) and the field structure as the stator (a stationary portion enclosing rotating parts). Which is the inverse of an AC Generator.

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Figure 3.33: Working Principle of DC Generator

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Chapter # 4

Comparison between Active and Passive Solar System

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4.1 Solar Thermal vs. Photovoltaic

It is important to understand that solar thermal technology is not the same as solar panel, or photovoltaic, technology. Solar thermal electric energy generation concentrates the light from the sun to create heat, and that heat is used to run a heat engine, which turns a generator to make electricity. The working fluid that is heated by the concentrated sunlight can be a liquid or a gas. Different working fluids include water, oil, salts, air, nitrogen, helium, etc. Different engine types include steam engines, gas turbines, Stirling engines, etc. All of these engines can be quite efficient, often between 30% and 40%, and are capable of producing 10’s to 100’s of megawatts of power.

Photovoltaic, or PV energy conversion, on the other hand, directly converts the sun’s light into electricity. This means that solar panels are only effective during daylight hours because storing electricity is not a particularly efficient process. Heat storage is a far easier and efficient method, which is what makes solar thermal so attractive for large-scale energy production. Heat can be stored during the day and then converted into electricity at night. Solar thermal plants that have storage capacities can drastically improve both the economics and the dispatch ability of solar electricity.

4.2 Photovoltaic electricity generation

Photovoltaic (PV) solar technologies generate electricity by exploiting the photovoltaic effect. Light shining on a semiconductor such as silicon (Si) generates electron-hole pairs that are separated spatially by an internal electric field created by introducing special impurities into the semiconductor on either side of an interface known as a p-n junction. This creates negative charges on one side of the interface and positive charges are on the other side (Figure 3.5). This resulting charge separation creates a voltage. When the two sides of the illuminated cell are connected to a load, current flows from one side of the device via the load to the other side of the cell. The conversion efficiency of a solar cell is defined as a ratio of output power from the solar cell with unit area (W/cm²) to the incident solar irradiance. The maximum potential efficiency of a solar cell depends on the absorber material properties and device design. One technique for increasing solar cell efficiency is with a multijunction approach that stacks specially selected absorber materials that can collect more of the solar spectrum since each different material can collect solar photons of different wavelengths.

PV cells consist of organic or inorganic matter. Inorganic cells are based on silicon or non-silicon materials; they are classified as wafer-based cells or thin-film cells. Wafer-based silicon is divided into two different types: monocrystalline and multicrystalline (sometimes called ‘polycrystalline’).

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Figure 4.34: Generic schematic cross-section illustrating the operation of an illuminated solar cell

4.3 Photovoltaic Application

Photovoltaic applications include PV power systems classified into two major types: those not connected to the traditional power grid (i.e., off-grid applications) and those that are connected (i.e., grid-connected applications). In addition, there is a much smaller, but stable, market segment for consumer applications.

Off-grid PV systems have a significant opportunity for economic application in the un-electrified areas of developing countries.

Off-grid centralized PV mini-grid systems have become a reliable alternative for village electrification over the last few years. In a PV mini-grid system, energy allocation is possible. For a village located in an isolated area and with houses not separated by too great a distance, the power may flow in the mini-grid without considerable losses. Centralized systems for local power supply have different technical advantages concerning electrical performance, reduction of storage needs, availability of energy, and dynamic behavior. Centralized PV mini-grid systems could be the least-cost options for a given level of service, and they may have a diesel generator set as an optional balancing system or operate as a hybrid PV-wind-diesel

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system. These kinds of systems are relevant for reducing and avoiding diesel generator use in remote areas

Grid-connected PV systems use an inverter to convert electricity from direct current (DC) as produced by the PV array to alternating current (AC), and then supply the generated electricity to the electricity network. Compared to an off-grid installation, system costs are lower because energy storage is not generally required, since the grid is used as a buffer.

These systems have a number of advantages: distribution losses in the electricity network are reduced because the system is installed at the point of use; extra land is not required for the PV system, and costs for mounting the systems can be reduced if the system is mounted on an existing structure; and the PV array itself can be used as a cladding or roofing material, as in building-integrated PV.

4.4 Solar Thermal System Characteristics

4.4.1 Application

Large-scale Grid Connected Power:

The primary application for parabolic trough power plants is large-scale grid connected power applications in the 30 to 300 MW range. Because the technology can be easily hybridized with fossil fuels, the plants can be designed to provide firm peaking to intermediate load power. The plants are typically a good match for applications in the U.S. southwest where the solar radiation resource correlates closely with peak electric power demands in the region. The existing SEGS plants have been operated very successfully in this fashion to meet SCE’s summer on-peak time-of-use rate period. Figure 4 shows the on-peak performance of the SEGS III through SEGS VII plants that are operated by KJC Operating Company. The chart shows that all 5 plants have produced greater than 100% of their rated capacity during the critical on-peak period between 1200 and 1800 PDT on weekdays during June through September. This demonstrates the continuous high availability these plants have been able to achieve. Note that 1989 was the first year of operation for SEGS VI and SEGS VII.

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Figure 4.35: On-peak capacity factors for five 30 MW SEGS plants during

1988 to 1996

Domestic Market:

The primary domestic market opportunity for parabolic trough plants is in the Southwestern deserts where the best direct normal solar resources exist. These regions also have peak power demands that could benefit from parabolic trough technologies. In particular, California, Arizona, and Nevada appear to offer some of the best opportunities for new parabolic trough plant development. However, other nearby states may provide excellent opportunities as well. The current excess of electric generating capacity in this region and the availability of low cost natural gas make future sustained deployment of parabolic trough technology in this region unlikely unless other factors come into play. However, with utility restructuring, and an increased focus on global warming and other environmental issues, many new opportunities such as renewable portfolio standards and the development of solar enterprise zones may encourage the development of new trough plants. All of the existing Luz-developed SEGS projects were developed as independent power projects and were enabled through special tax incentives and power purchase agreements such as the California SO-2 and SO-4 contracts.

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International Markets:

With the high demand for new power generation in many developing countries, the next deployment of parabolic troughs could be abroad. Many arid regions in developing countries are ideally suited for parabolic trough technologies. India, Egypt, Morocco, Mexico, Brazil, Crete (Greece), and Tibet (China) have expressed interest in trough technology power plants. Many of these countries are already planning installations of combined cycle projects. For these countries, the trough ISCCS design may provide a cheap and low risk opportunity to begin developing parabolic trough power plants. In regions such as Brazil and Tibet that have good direct normal solar resources and existing large hydroelectric and/or pumped storage generation resources, parabolic trough technologies can round out their renewable power portfolio by providing additional generation during the dry season.

4.4.2 Benefits

Least Cost Solar Generated Electricity:

Trough plants currently provide the lowest cost source of solar generated electricity available. They are backed by considerable valuable operating experience. Troughs will likely continue to be the least-cost solar option for another 5-10 years depending on the rate of development and acceptance of other solar technologies.

Daytime Peaking Power:

Parabolic trough power plants have a proven track record for providing firm renewable daytime peaking generation. Trough plants generate their peak output during sunny periods when air conditioning loads are at their peak. Integrated natural gas hybridization and thermal storage have allowed the plants to provide firm power even during non-solar and cloudy periods.

Environmental:

Trough plants reduce operation of higher-cost, cycling fossil generation that would be needed to meet peak power demands during sunny afternoons at times when the most photochemical smog, which is aggravated by NO emissions from power plants, is produced.

Economic:

The construction and operation of trough plants typically have a positive impact on the local economy. A large portion of material during construction can generally be supplied locally. Also trough plants tend to be fairly labor-intensive during both construction and operation, and much of this labor can generally be drawn from local labor markets.

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4.4.3 Impacts

HTF Spills/Leaks:

The current heat transfer fluid (Monsanto Therminol VP-1) is an aromatic hydrocarbon, biphenyl-diphenyl oxide. The oil is classified as non-hazardous by U.S. standards but is a hazardous material in the state of California. When spills occur, contaminated soil is removed to an on-site bio-remediation facility that utilizes indigenous bacteria in the soil to decompose the oil until the HTF concentrations have been reduced to acceptable levels. In addition to liquid spills, there is some level of HTF vapor emissions from valve packing and pump seals during normal operation. Although the scent of these vapor emissions is often evident, the emissions are well within permissible levels.

Water:

Water availability can be a significant issue in the arid regions best suited for trough plants. The majority of water consumption at the SEGS plants (approximately 90%) is used by the cooling towers. Water consumption is nominally the same as it would be for any Rankine cycle power plant with wet cooling towers that produced the same level of electric generation. Dry cooling towers can be used to significantly reduce plant water consumption; however, this can result in up to a 10% reduction in power plant efficiency. Waste water discharge from the plant is also an issue. Blowdown from the steam cycle, demineralizer, and cooling towers must typically be sent to an evaporation pond due to the high mineral content or due to chemicals that have been added to the water.

Land:

Parabolic trough plants require a significant amount of land that typically cannot be used concurrently for other uses. Parabolic troughs require the land to be graded level. One opportunity to minimize the development of undisturbed lands is to use parcels of marginal and fallow agricultural land instead. A study sponsored by the California Energy Commission determined that 27,000 MW of STE plants could be built on marginal and fallow e agricultural land in Southern California. A study for the state of Texas showed that land use requirements for parabolic trough plants are less than those of most other renewable technologies (wind, biomass, hydro) and also less than those of fossil when mining and drilling requirements are included. Current trough technology produces about 100 kWh/yr/m² of land.

Hybrid Operation:

Solar/fossil hybrid plant designs will operate with fossil fuels during some periods. During these times, the plant will generate emissions consistent with the fuel.

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Chapter # 5

Experimental Tests, Results, Conclusions and Future Work

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5.1 Data

This chapter contains dimensions of parabolic trough prototype, experimental results and analysis of thermal heat, and losses of heat at different time with different days. And also, how can we reduce thermal losses in our prototype. Tests were performed to determine the thermal convention of energy into electrical energy. Water was pumped through the central receiver where it was heated and flow rate and temperature rises of the water and production of steam were recorded to determine the collected energy and pressure were also recorded at different temperature.

5.1.1 Dimensions of Parabolic trough Prototype

Table shows the measurements and dimensions that were used in our prototype.

Area of parabola 25ft²

Depth of Parabola 13 inch

Diameter of parabola 58.5 inch

Focal point 16.5 inch

Rim angle θ 83.2º

Length of receiver(copper) 84 inch

Length of glass tube 84 inch

Diameter of copper tube 1 inch

Diameter of glass tube 1.5 inch

Gap between glass and copper tube 0.3 inch

Reflection coefficient of the Reflective surface

94%

5.1.2 Generator Specification

We used a small DC generator for power generation. The specifications of generator are given below in table.

Model Number 1.13.018.086

Rated Capacity 38.4Watt

Rated Voltage 24 Volts

Rated current 1.6 Amperes

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No. of Poles 2

Rated RPM 3000 rpm

5.2 Experiment Results

These experiments were performed in Comsats Abbottabad at different time. These result shows us we can get a large amount of thermal heat from sun at a small temperature.

Day Time Day Temp. Day Atmospheric Pressure

Temp. of Water in Receiver

Pressure of Steam

5-12-2012 10:20 19ºC 0.84 Bar 60ºC 0.5 Bar

5-12-2012 10:30 19ºC 0.84 Bar 80ºC 0.7 Bar

5-12-2012 10:40 19ºC 0.84 Bar 115ºC 1.5 Bar

6-12-2012 10:30 13ºC 0.86 Bar 80ºC 0.9 Bar

6-12-2012 10:45 13ºC 0.86 Bar 110ºC 1.4 Bar

8-12-2012 10:20 15ºC 0.87 Bar 95ºC 1.2 Bar

8-12-2012 10:40 15ºC 0.87 Bar 120ºC 2 bar

11-12-2012 11:00 17ºC 0.87 Bar 80ºC 0.9 Bar

11-12-2012 11:30 17ºC 0.87 Bar 130ºC 2.9 Bar

14-12-2012 10:30 16ºC 0.86 Bar 90ºC 1.2 Bar

14-12-2012 11:15 16ºC 0.86 Bar 150ºC 4 bar

5.2.1 Average Calculation Depending on Above Experimental Results

Max. O/p Temp. Gained

Max. o/p Steam Pressure

Turbine RPM

Generator RPM

Out-put Voltage

Out-put Current

Power

Watt

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Gained

150ºC 4 Bar 800 rpm 500rpm 4 volt 0.7 Ampere 2.8 W

5.2.2 Parabolic Trough Prototype

Figure 5.1 shows our project prototype which is known as Parabolic Trough Prototype.

Figure 5.36: Parabolic Trough Prototype

5.3 System Characterization

Energy losses for each subsystem are estimated below. Calculations are made to determine mirror, concentration, radiation and convective losses.

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5.3.1 Mirror Loss

The Refectech film has a solar reflectance of 94%. At an average insulation of 946 W/m², incident radiation was 13.2 kW. The loss due to the reflector was 6%, which corresponds to 792 W. As such, 12.4 kW was reflected by the mirror.

5.3.2 Concentrator Geometry Loss

Angular error was estimated for the fiberglass dish to determine the intercept factor. The error was estimated to the nearest integer multiple of the industrial standard error, which is 6.7 mrad. A series of boards with a center hole cut out were constructed. The center hole was cut with the diameter equal to the 99% cutoff diameter for a particular standard error. For the case of 5 standard errors (5*6.7 mrad angular error) the 99% cutoff diameter for a concentrator with a focal length of 2.13 meters is 38.5 cm. This means 99% of the reflected light should shine inside this circle. The fiberglass concentrator built corresponded closest to an angular error 6 times the industrial standard. The concentrator angular error was 40.2 +/- 6.7 mrad. It can be seen that 60%, or 7.44 kW, of the radiation strikes inside the cavity aperture and 25%, or 3.1 kW, of radiation strikes the absorber plate. The remaining 15% of reflected radiation was lost completely, which amounted to 1.86 kW.

5.3.3 Absorption Loss

When impinging radiation reflects off the absorber it is considered an absorption loss. The absorptivity coefficient of the cavity and plate surface was 90%. As such, 10%, or 310 W, of radiation that hit the absorber plate was lost. The cavity surface had the same absorptivity as the plate, but it recaptured some of the reflected radiation. When light was reflected off the surface it hit another surface, giving it a much higher chance of being absorbed. The effective absorptivity of the cavity was 98.7%. Only 164 W was lost by reflection out of the cavity. The total absorption losses were calculated to be 474 W.

5.3.4 Radiation Loss

Radiation losses were estimated for the cavity and plate absorber separately. Equation (5.3.1) gives energy lost due to radiation. For a cavity, the radiated energy is calculated using the effective emissivity given in Equation (5.3.2). The emissivity of the black chrome coating on the absorber surface is 0.15. For the absorber plate, the amount of radiated energy at 800_C is 0.6 kW. The cavity helped to reduce radiation losses by recapturing radiated energy. Even though the cavity surface area is nearly 4 times that of the absorber plate, the losses were only slightly higher at 0.85 kW. The total loss from the absorber due to radiation was calculated at 1.5 kW.

Qrad=εeff . σ . Aa .(T s4−T ∞

4 )(5.3 .1)

ε eff=εcav

εcav+(1−εcav ) .¿¿

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In this equation

ε eff= Effective emissivity of cavityε cav= Emissivity of cavity surfaceσ = Stefan-Boltzmann constantAa= Surface area of apertureAcav= Surface area inside cavityT s= Surface temperature of receiverT ∞= Temperature of surroundings

5.3.5 Convection Loss

Natural convection was the largest contributor to heat loss. At a cavity angle of 53_

buoyancy forces kept air continuously moving over the absorber, taking great amounts of heat. Using Equation (5.3.3) energy loss due to natural convection from a cavity can be found. The convective heat loss from the cavity was calculated at 1.3 kW. The absorber plate was modeled as an inclined at plate. Equation (5.3.5) gives the average Nusselt number that must be used with Equation (5.3.3) to find convection from an inclined plate. Natural convection from the absorber plate was calculated at 1.1 kW. The total heat loss due to convection was 2.4kW.

Qcond=h . Acav . (T s−Tamb )(5.3 .3)

h=N u f . kamb

L(5.3 .4)

Nu=0.088 .Gr13 .(T s/T amb)

.18 . cosθ2.47 .(L/ Dcav )8(5.3 .5)

In this equation

s=1.12−0.98 ( L/ Dcav )

Gr=g . β . (T s−T f ) . L3

v2

And

h= Average heat transfer coe_cientAcav= Cavity areaT s= Temperature of receiver surfaceT amb= Ambient fluid temperatureT f = Average fluid temperature

Nu= Nusselt number

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Gr= Grashoff numberk f= Thermal conductivity of ambient fluid

L= Characteristic length of aperture opening (diameter)Dcav= Diameter of cavity

v= Kinematic viscosityθ= Angle receiver makes with zenith (at θ = 0º the receiver is horizontal)g= Gravityβ= Volumetric thermal expansion coefficient

5.3.6 Remaining Thermal Loss

A simple First Law energy balance reveals that an additional 1.1 kW are lost. This is due to a combination of many small losses. The largest being reflector wear, conduction and imperfect insulation. The mirrored surface had a maximum reflective efficiency of 94%. In reality the efficiency of the mirrored surface on Solar 2 was not this high. Dust and dirt were blown onto the reflector and blocked sunlight. Discolorations formed on sections of the mirror after water pooled on them for months. If the wear was enough to reduce reflectivity to 90%, an additional 528 W were lost.Conduction occurred at the joining of the receiver and the connection arm. This was reduced as much as possible by placing insulation between the plates that are bolted together. The insulation will conduct a small amount, but the stainless steel bolts that directly connect the receiver to the arm are good conductors of heat.The final major contributor is the insulation surrounding the receiver. The insulation was not one continuous piece covering the receiver. Hot air could work its way out of the insulation carrying heat with it. The insulation itself heated a small amount and convicted heat to the ambient air.

5.3.7 Reducing Losses

Figure 5.2 shows a breakdown of the relative losses for Solar 2. Convection, radiation and concentrator geometry error make up 71% of all of the losses. Each of these three systems is primarily dependent upon the angular error of the concentrator, which determines the absorber aperture diameter. If the aperture radius was reduced, these losses would decrease exponentially.

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Figure 5.37: Distribution of Thermal Losses

By increasing the optical efficiency of the concentrator, cavity losses will be greatly reduced. Increasing the efficiency of the concentrator should be the primary focus to increase total system efficiency. Absorptive losses, which contributed 6% of all losses, can also be reduced. 64% of absorption losses were from the at plate absorber, even though only 25% of reflected radiation struck the at plate. By optimizing the cavity aperture to the concentrator the at plate absorber will be unnecessary. This will automatically eliminate a large portion of the absorption losses without increasing system cost. The remaining 23% of losses came from sources that will be more complicated to reduce.

Mirror losses cannot be decreased easily as it will be very difficult to find a material with such a high reflectivity and weathering capabilities for a reasonable price. Lastly, insulation losses can be minimized by building an airtight housing around the insulation. The insulation used was open to the elements. Wind and rain wear down the insulation and destroyed parts of it. Housing can be built to keep air from entering the insulation and weather out, but it will increase system cost and complexity.

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5.4 Conclusion

Concentrating solar power technology for electricity generation is ready for the market. Various types of single- and dual-purpose plants have been analyzed and tested in the field. In addition, experience has been gained from the first commercial installations in use worldwide since the beginning of the 1980s. Solar thermal power plants will, within the next decade, provide a significant contribution to an efficient, economical and environmentally benign energy supply both in large-scale grid connected dispatch able markets and remote or modular distributed markets.

Solar technologies have the potential to be major contributors to the global energy supply. The ability to dispatch power allows large-scale central solar technologies to provide 50% or more of the energy needs in sunny regions around the world. In addition, because parabolic trough technology is built from commodity materials such as glass, steel, and concrete, and standard utility power generation equipment, it is possible to scale-up and rapidly deploy new trough power plants. Large-scale solar technologies can provide energy price stability as well as quality jobs to the local community. Solar energy has the potential to become the major new domestic energy resource in the 21st century.

5.5 Future Work

Improve concentrator geometry. This will make the largest difference in system efficiency.

Decrease cavity aperture size. With an improved concentrator, the absorber radius should be decreased to take advantage of the improved optical efficiency. This will decrease radiation and convection.

Eliminate the at plate absorber. Use only the cavity absorber and insulate all other surfaces on the receiver.

Add a pump. A water pump will allow for steady steam production.

Correct tracking system. Program safety procedures to keep tracking system from moving when clouds block sunlight.

Increase boiler support. By stiffening the receiver arm, the tracking will err less in the morning and evening. Add steam turbine. The system is now ready to test with a micro steam turbine.

Using evacuated tube. By using evacuated receiver tube heat losses will minimum. And due to minimum heat loss thermal efficiency will increase.

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References

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http://www.physics.sjsu.edu/facstaff/becker/physics51/generator.htm

passive solar system

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