PHYS 471PHYS 471Solar Energy Solar Energy

2004-12004-1

Concentrating CollectorsConcentrating Collectors

Instructor : Prof.Dr Ahmet EcevitInstructor : Prof.Dr Ahmet Ecevit

Prepared by:Prepared by:Serkan KapucuSerkan Kapucu

Table of ContentsTable of Contents1.Introduction.......................................................41.Introduction.......................................................4

2.Concentrating collectors...................................52.Concentrating collectors...................................5

3.Types of concentrating collectors.....................63.Types of concentrating collectors.....................6

3.1. Parabolic trough system.............................73.1. Parabolic trough system.............................7

3.2. Parabolic dish system.... ...........................113.2. Parabolic dish system.... ...........................11

3.3. Power tower system...................................143.3. Power tower system...................................14

3.4. Stationary concentrating solar collectors....163.4. Stationary concentrating solar collectors....16

4.Working principles of concentrating collectors..174.Working principles of concentrating collectors..17

4.1. Trough Systems..........................................184.1. Trough Systems..........................................18

4.2. Dish Systems...............................................214.2. Dish Systems...............................................21

4.3. Central Receiver Systems...........................234.3. Central Receiver Systems...........................23

5. Technology 5. Technology ComparisonComparison...................................25...................................25

6. 6. Calculations.......................................................28Calculations.......................................................28

7. Economic and Environmental Considerations..377. Economic and Environmental Considerations..37

8. 8. Conclusions.......................................................39Conclusions.......................................................39

References........................................................41References........................................................41

1. Introduction1. IntroductionFor applications such as air conditioning, central power For applications such as air conditioning, central power generation, and numerous industrial heat requirements, flat generation, and numerous industrial heat requirements, flat plate collectors generally cannot provide carrier fluids at plate collectors generally cannot provide carrier fluids at temperatures sufficiently elevated to be effective. They may temperatures sufficiently elevated to be effective. They may be used as first-stage heat input devices; the temperature be used as first-stage heat input devices; the temperature of the carrier fluid is then boosted by other conventional of the carrier fluid is then boosted by other conventional heating means. Alternatively, more complex and expensive heating means. Alternatively, more complex and expensive concentrating collectors can be used. These are devices concentrating collectors can be used. These are devices that optically reflect and focus incident solar energy onto a that optically reflect and focus incident solar energy onto a small receiving area. As a result of this concentration, the small receiving area. As a result of this concentration, the intensity of the solar energy is magnified, and the intensity of the solar energy is magnified, and the temperatures that can be achieved at the receiver (called temperatures that can be achieved at the receiver (called the "target") can approach several hundred or even several the "target") can approach several hundred or even several thousand degrees Celsius. The concentrators must move thousand degrees Celsius. The concentrators must move to track the sun if they are to perform effectively [1].to track the sun if they are to perform effectively [1].

2. Concentrating 2. Concentrating collectorscollectors

Concentrating, or focusing, collectors intercept direct Concentrating, or focusing, collectors intercept direct radiation over a large area and focus it onto a small radiation over a large area and focus it onto a small absorber area. These collectors can provide high absorber area. These collectors can provide high temperatures more efficiently than flat-plate collectors, since temperatures more efficiently than flat-plate collectors, since the absorption surface area is much smaller. However, the absorption surface area is much smaller. However, diffused sky radiation cannot be focused onto the absorber. diffused sky radiation cannot be focused onto the absorber. Most concentrating collectors require mechanical equipment Most concentrating collectors require mechanical equipment that constantly orients the collectors toward the sun and that constantly orients the collectors toward the sun and keeps the absorber at the point of focus. Thereforekeeps the absorber at the point of focus. Therefore;; there there are many types of concentrating collectors [2].are many types of concentrating collectors [2].

3. Types of concentrating 3. Types of concentrating collectorscollectors

Parabolic trough systemParabolic trough system

Parabolic dish Parabolic dish 

Power towerPower tower

Stationary concentrating collectors Stationary concentrating collectors 

There are four basic types of concentrating collectors:

3.1. Parabolic trough 3.1. Parabolic trough systemsystem  

Parabolic troughs are devices that are shaped like the Parabolic troughs are devices that are shaped like the letter “u”letter “u”.. The troughs concentrate sunlight onto a The troughs concentrate sunlight onto a receiver tube that is positioned along the focal line of the receiver tube that is positioned along the focal line of the trough. Sometimes a transparent glass tube envelops the trough. Sometimes a transparent glass tube envelops the receiver tube to reduce heat loss [3].receiver tube to reduce heat loss [3].

Figure 3.1.2 Parabolic trough system [3]. Figure 3.1.1 Crossection of parabolic trough [4].

The parabolic trough sytem is The parabolic trough sytem is shown in the figure 3.1.2 below.shown in the figure 3.1.2 below.

TTheir shapes are like letter heir shapes are like letter “u”“u” as shown figure 3.1.1 below.as shown figure 3.1.1 below.

Parabolic troughs often use single-axis or dual-axis Parabolic troughs often use single-axis or dual-axis trackingtracking..

Figure 3.1.3 One Axis Tracking Parabolic Trough with Axis Oriented E-W [8].

Figure 3.1.4 Two Axis Tracking Concentrator [8].Axis Tracking Concentrator [8].

The below figure 3.1.3 shows one axis The below figure 3.1.3 shows one axis tracking parabolic trough with axis tracking parabolic trough with axis oriented E-W.oriented E-W.

The below figure 3.1.4 shows two The below figure 3.1.4 shows two axis tracking concentrator.axis tracking concentrator.

Temperatures at the receiver can reach 400 °C and Temperatures at the receiver can reach 400 °C and produce steam for generating electricity. In California, produce steam for generating electricity. In California, multi-megawatt power plants were built using parabolic multi-megawatt power plants were built using parabolic troughs combined with gas turbines [3].troughs combined with gas turbines [3].Parabolic trough combined with gas turbines is shown Parabolic trough combined with gas turbines is shown figure 3.1.5 below.figure 3.1.5 below.

Figure 3.1.5 Parabolic trough combined with gas turbines [4].

Cost projections for trough technology are higher than Cost projections for trough technology are higher than those for power towers and dish/engine systems due in those for power towers and dish/engine systems due in large part to the lower solar concentration and hence large part to the lower solar concentration and hence lower temperatures and efficiency.However with long lower temperatures and efficiency.However with long operating experience, continued technology operating experience, continued technology improvements, and operating and maintenance cost improvements, and operating and maintenance cost reductions, troughs are the least expensive, most reductions, troughs are the least expensive, most reliable solar thermal power production technology for reliable solar thermal power production technology for near-term [4].near-term [4].

3.2. Parabolic dish 3.2. Parabolic dish systems systems 

A parabolic dish collector is similar in appearance to A parabolic dish collector is similar in appearance to a large satellite dish, but has mirror-like reflectors a large satellite dish, but has mirror-like reflectors and an absorber at the focal point. It uses a dual and an absorber at the focal point. It uses a dual axis sun tracker [3]. axis sun tracker [3].

Figure 3.2.2 Parabolic dish collector with a mirror-like reflectors and an absorber at the focal point [Courtesy of SunLabs - Department of Energy] [3].

Figure 3.2.1 Crossection of parabolic dish [4].

The below figure 3.2.1 shows The below figure 3.2.1 shows crossection of parabolic dish.crossection of parabolic dish.

The Parabolic dish collector is The Parabolic dish collector is shown in the below figure 3.2.2.shown in the below figure 3.2.2.

A parabolic dish system uses a computer to track the sun A parabolic dish system uses a computer to track the sun and concentrate the sun's rays onto a receiver located at the and concentrate the sun's rays onto a receiver located at the focal point in front of the dish.  In some systems, a heat focal point in front of the dish.  In some systems, a heat engine, such as a Stirling engine, is linked to the receiver to engine, such as a Stirling engine, is linked to the receiver to generate electricity. Parabolic dish systems can reach 1000 generate electricity. Parabolic dish systems can reach 1000 °C at the receiver, and achieve the highest efficiencies for °C at the receiver, and achieve the highest efficiencies for converting solar energy to electricity in the small-power converting solar energy to electricity in the small-power capacity range [3].capacity range [3].

Figure 3.2.3 Solar dish stirling engine [9].

The right figure 3.2.3 The right figure 3.2.3 shows the solar dish shows the solar dish stirling engine.stirling engine.

Engines currently under consideration include Stirling Engines currently under consideration include Stirling and Brayton cycle engines. Several prototype and Brayton cycle engines. Several prototype dish/engine systems, ranging in size from 7 to 25 kW dish/engine systems, ranging in size from 7 to 25 kW have been deployed in various locations in the USA. have been deployed in various locations in the USA. High optical efficiency and low start up losses make High optical efficiency and low start up losses make dish/engine systems the most efficient of all solar dish/engine systems the most efficient of all solar technologies. A Stirling engine/parabolic dish system technologies. A Stirling engine/parabolic dish system holds the world’s record for converting sunlight into holds the world’s record for converting sunlight into electricity. In 1984, a 29% net efficiency was electricity. In 1984, a 29% net efficiency was measured at Rancho Mirage, California [4].measured at Rancho Mirage, California [4].

3.3. Power tower system 3.3. Power tower system A heliostat uses a field of dual axis sun trackers that A heliostat uses a field of dual axis sun trackers that direct solar energy to a large absorber located on a direct solar energy to a large absorber located on a tower. To date the only application for the heliostat tower. To date the only application for the heliostat collector is power generation in a system called the collector is power generation in a system called the power towerpower tower [3]. [3].

Figure 3.3.2 Heliostats [4].Figure 3.3.2 Heliostats [4].Figure 3.3.1 Power tower system [4].

Heliostats are shown in Heliostats are shown in the figure 3.3.2 below.the figure 3.3.2 below.

The Power tower system is The Power tower system is shown in the figure 3.3.1 below.shown in the figure 3.3.1 below.

A power tower has a field of large mirrors that follow the A power tower has a field of large mirrors that follow the sun's path across the sky. The mirrors concentrate sun's path across the sky. The mirrors concentrate sunlight onto a receiver on top of a high tower. A sunlight onto a receiver on top of a high tower. A computer keeps the mirrors aligned so the reflected computer keeps the mirrors aligned so the reflected rays of the sun are always aimed at the receiver, where rays of the sun are always aimed at the receiver, where temperatures well above 1000°C can betemperatures well above 1000°C can be reached.reached. High-High-pressure steam is generated to produce electricity [3].pressure steam is generated to produce electricity [3].The power tower system with heliostats is shown in the The power tower system with heliostats is shown in the figure 3.3.3 below.figure 3.3.3 below.

Figure 3.3.3 Power tower system with heliostats [4]. 

3.4. Stationary concentrating 3.4. Stationary concentrating solar collectorssolar collectors

Stationary concentrating collectors use compound Stationary concentrating collectors use compound parabolic reflectors and flat reflectors for directing solar parabolic reflectors and flat reflectors for directing solar energy to an accompanying absorber or aperture through energy to an accompanying absorber or aperture through a wide acceptance angle. The wide acceptance angle for a wide acceptance angle. The wide acceptance angle for these reflectors eliminates the need for a sun tracker. these reflectors eliminates the need for a sun tracker. This class of collector includes parabolic trough flat plate This class of collector includes parabolic trough flat plate collectors, flat plate collectors with parabolic boosting collectors, flat plate collectors with parabolic boosting reflectors, and solar cooker. Development of the first two reflectors, and solar cooker. Development of the first two collectors has been done in Sweden. Solar cookers are collectors has been done in Sweden. Solar cookers are used throughout the world, especially in the developing used throughout the world, especially in the developing countries [3].countries [3].

4. Working principles of 4. Working principles of concentrating collectorsconcentrating collectors

Unlike solar (photovoltaic) cells, which use light to produce Unlike solar (photovoltaic) cells, which use light to produce electricity, concentrating solar power systems generate electricity, concentrating solar power systems generate electricity with heat. Concentrating solar collectors use electricity with heat. Concentrating solar collectors use mirrors and lenses to concentrate and focus sunlight onto mirrors and lenses to concentrate and focus sunlight onto a thermal receiver, similar to a boiler tube. The receiver a thermal receiver, similar to a boiler tube. The receiver absorbs and converts sunlight into heat. The heat is then absorbs and converts sunlight into heat. The heat is then transported to a steam generator or engine where it is transported to a steam generator or engine where it is converted into electricity. There are three main types of converted into electricity. There are three main types of concentrating solar power systems: parabolic troughs, concentrating solar power systems: parabolic troughs, dish/engine systems, and central receiver systems.dish/engine systems, and central receiver systems.These technologies can be used to generate electricity for These technologies can be used to generate electricity for a variety of applications, ranging from remote power a variety of applications, ranging from remote power systems as small as a few kilowatts (kW) up to grid systems as small as a few kilowatts (kW) up to grid connected applications of 200-350 megawatts (MW) or connected applications of 200-350 megawatts (MW) or more. A concentrating solar power system that produces more. A concentrating solar power system that produces 350 MW of electricity displaces the energy equivalent of 350 MW of electricity displaces the energy equivalent of 2.3 million barrels of oil [5].2.3 million barrels of oil [5].

4.1. Trough Systems4.1. Trough SystemsThese solar collectors use mirrored parabolic troughs to These solar collectors use mirrored parabolic troughs to focus the sun's energy to a fluid-carrying receiver tube focus the sun's energy to a fluid-carrying receiver tube located at the focal point of a parabolically curved trough located at the focal point of a parabolically curved trough reflectorreflector [5].It is shown in the figure 4.1.1 below. [5].It is shown in the figure 4.1.1 below.

Figure 4.1.1 Parabolic trough with mirrored parabolic troughs [10].

The energy from the sun sent to the tube heats oil The energy from the sun sent to the tube heats oil flowing through the tube, and the heat energy is then flowing through the tube, and the heat energy is then used to generate electricity in a conventional steam used to generate electricity in a conventional steam generator. Many troughs placed in parallel rows are generator. Many troughs placed in parallel rows are called a "collector field." The troughs in the field are all called a "collector field." The troughs in the field are all aligned along a northsouth axis so they can track the aligned along a northsouth axis so they can track the sun from east to west during the day, ensuring that sun from east to west during the day, ensuring that the sun is continuously focused on the receiver pipes. the sun is continuously focused on the receiver pipes. Individual trough systems currently can generate Individual trough systems currently can generate about 80 MW of electricity.about 80 MW of electricity.

Trough designs can incorporate thermal storage-Trough designs can incorporate thermal storage-setting aside the heat transfer fluid in its hot phase setting aside the heat transfer fluid in its hot phase allowing for electricity generation several hours into allowing for electricity generation several hours into the evening. Currently, all parabolic trough plants are the evening. Currently, all parabolic trough plants are "hybrids," meaning they use fossil fuels to "hybrids," meaning they use fossil fuels to supplement the solar output during periods of low supplement the solar output during periods of low solar radiation. Typically, a natural gas-fired heat or a solar radiation. Typically, a natural gas-fired heat or a gas steam boiler/gas steam boiler/reheaterreheater is used. Troughs also can is used. Troughs also can be integrated with existing coal-fired plants [5].be integrated with existing coal-fired plants [5].

4.2. Dish Systems4.2. Dish SystemsDish systems use dish-shaped parabolic mirrors as Dish systems use dish-shaped parabolic mirrors as reflectors to concentrate and focus the sun's rays onto a reflectors to concentrate and focus the sun's rays onto a receiver, which is mounted above the dish at the dish receiver, which is mounted above the dish at the dish centcenterer. A dish/engine system is a stand alone unit . A dish/engine system is a stand alone unit composed primarily of a collector, a receiver, and an composed primarily of a collector, a receiver, and an engine. It works by collecting and concentrating the sun's engine. It works by collecting and concentrating the sun's energy with a dishshaped surface onto a receiver that energy with a dishshaped surface onto a receiver that absorbs the energy and transfers it to the engine. The absorbs the energy and transfers it to the engine. The engine then converts that energy to heat. The heat is engine then converts that energy to heat. The heat is then converted to mechanical power, in a manner similar then converted to mechanical power, in a manner similar to conventional engines, by compressing the working to conventional engines, by compressing the working fluid when it is cold, heating the compressed working fluid when it is cold, heating the compressed working fluid, and then expanding it through a turbine or with a fluid, and then expanding it through a turbine or with a piston to produce mechanical power. An electric piston to produce mechanical power. An electric generator or alternator converts the mechanical power generator or alternator converts the mechanical power into electrical power.into electrical power.

Each dish produces 5 to 50 kW of electricity and can Each dish produces 5 to 50 kW of electricity and can be used independently or linked together to increase be used independently or linked together to increase generating capacity. A 250-kW plant composed of ten generating capacity. A 250-kW plant composed of ten 25-kW dish/engine systems requires less than an 25-kW dish/engine systems requires less than an acre of land. Dish/engine systems are not acre of land. Dish/engine systems are not commercially available yet, although ongoing commercially available yet, although ongoing demonstrations indicate good potential. Individual demonstrations indicate good potential. Individual dish/engine systems currently can generate about 25 dish/engine systems currently can generate about 25 kW of electricity. More capacity is possible by kW of electricity. More capacity is possible by connecting dishes togetherconnecting dishes together. . These systems can be These systems can be combined with natural gas, and the resulting hybrid combined with natural gas, and the resulting hybrid provides continuous power generation [5].provides continuous power generation [5].

Figure 4.2.1 Combination of parabolic dish system [4].

The right figure 4.2.1 The right figure 4.2.1 shows the combination of shows the combination of parabolic dish system.parabolic dish system.

4.3. Central Receiver Systems4.3. Central Receiver SystemsCentral receivers (or power towers) use thousands of Central receivers (or power towers) use thousands of individual sun-tracking mirrors called "heliostats" to individual sun-tracking mirrors called "heliostats" to reflect solar energy onto a receiver located on top of tall reflect solar energy onto a receiver located on top of tall tower. The receiver collects the sun's heat in a heat-tower. The receiver collects the sun's heat in a heat-transfer fluid (molten salt) that flows through the transfer fluid (molten salt) that flows through the receiver. The salt's heat energy is then used to make receiver. The salt's heat energy is then used to make steam to generate electricity in a conventional steam steam to generate electricity in a conventional steam generator, located at the foot of the tower. The molten generator, located at the foot of the tower. The molten salt storage system retains heat efficiently, so it can be salt storage system retains heat efficiently, so it can be stored for hours or even days before being used to stored for hours or even days before being used to generate electricity [5]. In this system, molten-salt is generate electricity [5]. In this system, molten-salt is pumped from a “cold” tank at 288 deg.C and cycled pumped from a “cold” tank at 288 deg.C and cycled through the receiver where it is heated to 565 deg.C through the receiver where it is heated to 565 deg.C and returned to a “hot” tank. The hot salt can then be and returned to a “hot” tank. The hot salt can then be used to generate electricity when needed. Current used to generate electricity when needed. Current designs allow storage ranging from 3 to 13 hours [4]. designs allow storage ranging from 3 to 13 hours [4].

Figure 4.3.1 The process of molten salt storage [11].

Figure 4.3.1 shows the process of molten salt storage.Figure 4.3.1 shows the process of molten salt storage.

5. Technology Comparison5. Technology Comparison Towers and troughs are best suited for large, grid-Towers and troughs are best suited for large, grid-connected power projects in the 30-200 MW size, connected power projects in the 30-200 MW size, whereas, dish/engine systems are modular and can be whereas, dish/engine systems are modular and can be used in single dish applications or grouped in dish farms used in single dish applications or grouped in dish farms to create larger multi-megawatt projects. Parabolic to create larger multi-megawatt projects. Parabolic trough plants are the most mature solar power trough plants are the most mature solar power technology available today and the technology most technology available today and the technology most likely to be used for near-term deployments. Power likely to be used for near-term deployments. Power towers, with low cost and efficient thermal storage, towers, with low cost and efficient thermal storage, promise to offer dispatchable, high capacity factor, solar-promise to offer dispatchable, high capacity factor, solar-only power plants in the near future. only power plants in the near future.

The modular nature of dishes will allow them to be used in The modular nature of dishes will allow them to be used in smaller, high-value applications. Towers and dishes offer smaller, high-value applications. Towers and dishes offer the opportunity to achieve higher solar-to-electric the opportunity to achieve higher solar-to-electric efficiencies and lower cost than parabolic trough plants, efficiencies and lower cost than parabolic trough plants, but uncertainty remains as to whether these technologies but uncertainty remains as to whether these technologies can achieve the necessary capital cost reductions and can achieve the necessary capital cost reductions and availability improvements. Parabolic troughs are currently availability improvements. Parabolic troughs are currently a proven technology primarily waiting for an opportunity to a proven technology primarily waiting for an opportunity to be developed. Power towers require the operability and be developed. Power towers require the operability and maintainability of the molten-salt technology to be maintainability of the molten-salt technology to be demonstrated and the development of low cost heliostats. demonstrated and the development of low cost heliostats. Dish/engine systems require the development of at least Dish/engine systems require the development of at least one commercial engine and the development of a low cost one commercial engine and the development of a low cost concentratorconcentrator [4].[4].

   Parabolic TroughParabolic Trough Dish/EngineDish/Engine Power TowerPower Tower

SizeSize 30-320 MW30-320 MW 5-25 kW5-25 kW 10-200 MW10-200 MW

Operating Temperature Operating Temperature (ºC/ºF)(ºC/ºF) 390/734 390/734  750/1382750/1382 565/1049565/1049

Annual Capacity Factor Annual Capacity Factor  23-50 %23-50 % 25 %25 % 20-77 %20-77 %

Peak EfficiencyPeak Efficiency   20%(d)20%(d)   29.4%(d)29.4%(d) 23%(p)23%(p)

Net Annual Efficiency Net Annual Efficiency  11(d)-16%11(d)-16% 12-25%(p)12-25%(p) 7(d)-20%7(d)-20%

Commercial StatusCommercial Status Commercially Scale-up Commercially Scale-up Prototype Prototype  Demonstration Demonstration  AvailableDemonstrationAvailableDemonstration

Technology Technology Development RiskDevelopment Risk LowLow HighHigh MediumMedium

Storage AvailableStorage Available LimitedLimited BatteryBattery YesYes

Hybrid DesignsHybrid Designs YesYes YesYes YesYes

Cost USD/WCost USD/W 2,7-4,02,7-4,0 1,3-12,61,3-12,6 2,5-4,42,5-4,4

(p) = predicted; (d) = demonstrated;

Table 5.1 Key features of the three solar technologies [4].

Table 5.1 highlights the key features of the three solar technologies.Table 5.1 highlights the key features of the three solar technologies.

6. Calculations6. Calculations

Heat from a solar collector may be used to drive a heat Heat from a solar collector may be used to drive a heat

engine operating in a cycle to produce work. A heat engine operating in a cycle to produce work. A heat

engine may be used for such applications as water engine may be used for such applications as water

pumping and generating electricity.pumping and generating electricity.

The thermal output QThe thermal output Qoutout of a concentrating collector of a concentrating collector

operating at temperature T is given byoperating at temperature T is given by

QQoutout = F'[ = F'[gammagamma.A.Aininqqinin - U.A - U.Arecrec(T - T(T - T

aa)], )],

AAinin :: the area of the incident solar radiation ( the area of the incident solar radiation (mm22).).

AArecrec :: the area of the receiver ( the area of the receiver (mm22))

gamma:optical efficiencygamma:optical efficiency

qqinin : the incident solar irradiation ( : the incident solar irradiation (W/mW/m22))

TTaa :the ambient temperature ( :the ambient temperature (°C°C))

U :the heat loss coefficient (U :the heat loss coefficient (W/mW/m22KK))

F’ :collector efficiency factorF’ :collector efficiency factor

The quantity AThe quantity Ainin/A/Arecrec is called the is called the concentration ratioconcentration ratio..

High concentration ratios are obtained by making AHigh concentration ratios are obtained by making Ainin the the

area of a system of mirrors designed to concentrate the area of a system of mirrors designed to concentrate the

solar radiation received onto a small receiver of area Asolar radiation received onto a small receiver of area Arecrec. .

Heat losses from the receiver are reduced by the smaller Heat losses from the receiver are reduced by the smaller

size of the receiver. Consequently, high concentration size of the receiver. Consequently, high concentration

ratios give high collector temperatures. The stagnation ratios give high collector temperatures. The stagnation

temperature Ttemperature Tmaxmax is given by: is given by:

gammagamma.A.Aininqqinin = U.A = U.Arecrec(T(Tmaxmax - T - T

aa).).

For example, if the optical efficiency is For example, if the optical efficiency is gammagamma = 0.8, = 0.8,

the incident solar irradiation is qthe incident solar irradiation is qinin = 800W/m = 800W/m22, the , the

ambient temperature is Tambient temperature is Taa = 30°C, and the heat loss = 30°C, and the heat loss

coefficient is U = 10W/mcoefficient is U = 10W/m22K, then a concentration ratio K, then a concentration ratio

AAinin/A/Arecrec = 1 (no concentration) gives T = 1 (no concentration) gives Tmaxmax = 94°C, and a = 94°C, and a

concentration ratio Aconcentration ratio Ainin/A/Arecrec = 10 gives T = 10 gives T

maxmax = 670°C. = 670°C.

The collector efficiency The collector efficiency etaetacc at operating temperature T is at operating temperature T is

etaetacc=Q=Qoutout/A/Aininqqinin = F'[ = F'[gammagamma-U.A-U.A

recrec(T -T(T -Taa)/A)/A

ininqqinin] ]

= F'= F'gammagamma(T(Tmaxmax - T)/(T - T)/(Tmaxmax - T - T

aa). ).

The available mechanical power from the thermal power The available mechanical power from the thermal power

output of the collector that would be obtained using a Carnot output of the collector that would be obtained using a Carnot

cycle is Qcycle is Qoutout(1 - T(1 - T

aa/T), where the temperatures are absolute /T), where the temperatures are absolute

temperatures.temperatures.

The The second law efficiencysecond law efficiency etaeta22 of a heat engine is of a heat engine is

defined bydefined by

etaeta22=(mechanical power delivered)=(mechanical power delivered)

/(available mechanical power). /(available mechanical power).

Suppose a heat engine with second law efficiency Suppose a heat engine with second law efficiency etaeta22

uses as input the thermal power Quses as input the thermal power Qoutout from the solar from the solar

collector. The first law efficiency of the engine iscollector. The first law efficiency of the engine is

etaeta11 = (mechanical power delivered)/Q = (mechanical power delivered)/Qoutout = = etaeta22(1 - T(1 - T

aa/T),/T),

where Twhere Tmaxmax depends on the design of the collector and depends on the design of the collector and

on the solar radiation input qon the solar radiation input qinin. Now, given F', . Now, given F', gammagamma, ,

etaeta22, T, Taa, and T, and T

maxmax, we can find the maximum efficiency , we can find the maximum efficiency obtainable, and the optimum operating temperature Tobtainable, and the optimum operating temperature T

optopt from the condition d(from the condition d(etaeta)/dT = 0. This occurs at the )/dT = 0. This occurs at the optimum temperatureoptimum temperature

TToptopt = [T = [TmaxmaxTTaa], ],

and the maximum efficiency is obtained by putting and the maximum efficiency is obtained by putting

T = TT = Toptopt in the equation in the equation

etaeta = = etaetacc..etaeta11..

½½

For example, putting F' = 0.9, For example, putting F' = 0.9, gammagamma = 0.8, = 0.8, etaeta22 = 0.6, = 0.6,

TTaa = 30°C = 303K, we get the efficiencies = 30°C = 303K, we get the efficiencies etaetamaxmax for for

different degrees of concentration shown in Table 6.1. different degrees of concentration shown in Table 6.1. Very low overall efficiencies are obtained unless Very low overall efficiencies are obtained unless operating temperatures greater than 500°C are used. operating temperatures greater than 500°C are used. Expensive concentrating systems are needed to reach Expensive concentrating systems are needed to reach these high temperatures, so commercial viability is these high temperatures, so commercial viability is difficult [12].difficult [12].

Efficiencies for Converting Solar Radiation to WorkEfficiencies for Converting Solar Radiation to Work

TTmaxmax TToptopt etaetamaxmax

100°C100°C 63°C63°C 2.2%2.2%

200°C200°C 106°C106°C 4.8%4.8%

400°C400°C 179°C179°C 8.5%8.5%

800°C800°C 297°C297°C 13.2%13.2%

1600°C1600°C 480°C480°C 18.4%18.4%

Table 6.1. Different degrees of concentration [12].

7. Economic and Environmental 7. Economic and Environmental ConsiderationsConsiderations

The most important factor driving the solar energy The most important factor driving the solar energy system design process is whether the energy it system design process is whether the energy it produces is economical. Although there are factors other produces is economical. Although there are factors other than economics that enter into a decision of when to use than economics that enter into a decision of when to use solar energy; i.e. no pollution, no greenhouse gas solar energy; i.e. no pollution, no greenhouse gas generation, security of the energy resource etc., design generation, security of the energy resource etc., design decisions are almost exclusively dominated by the decisions are almost exclusively dominated by the ‘levelized energy cost’. This or some similar economic ‘levelized energy cost’. This or some similar economic parameter, gives the expected cost of the energy parameter, gives the expected cost of the energy produced by the solar energy system, averaged over the produced by the solar energy system, averaged over the lifetime of the systemlifetime of the system. . 

Commercial applications from a few kilowatts to Commercial applications from a few kilowatts to hundreds of megawatts are now feasible, and plants hundreds of megawatts are now feasible, and plants totaling 354 MW have been in operation in California totaling 354 MW have been in operation in California since the 1980s. Plants can function in dispatchable, since the 1980s. Plants can function in dispatchable, grid-connected markets or in distributed, stand-alone grid-connected markets or in distributed, stand-alone applications. They are suitable for fossil-hybrid operation applications. They are suitable for fossil-hybrid operation or can include cost-effective storage to meet or can include cost-effective storage to meet dispatchability requirements. They can operate dispatchability requirements. They can operate worldwide in regions having high beam-normal insolation, worldwide in regions having high beam-normal insolation, including large areas of the southwestern United States, including large areas of the southwestern United States, and Central and South America, Africa, Australia, China, and Central and South America, Africa, Australia, China, India, the Mediterranean region, and the Middle East, . India, the Mediterranean region, and the Middle East, . Commercial solar plants have achieved levelized energy Commercial solar plants have achieved levelized energy costs of about 12-15¢/kWh, and the potential for cost costs of about 12-15¢/kWh, and the potential for cost reduction are expected to ultimately lead to costs as low reduction are expected to ultimately lead to costs as low as 5¢/kWh [6].as 5¢/kWh [6].

8. Conclusions8. ConclusionsConcentrating solar power technology for electricity Concentrating solar power technology for electricity generation is ready for the market. Various types of generation is ready for the market. Various types of single and dual-purpose plants have been analysed single and dual-purpose plants have been analysed and tested in the field. In addition, experience has been and tested in the field. In addition, experience has been gained from the first commercial installations in use gained from the first commercial installations in use worldwide since the beginning of the 1980s. Solar worldwide since the beginning of the 1980s. Solar thermal power plants will, within the next decade, thermal power plants will, within the next decade, provide a significant contribution to an efficient, provide a significant contribution to an efficient, economical and environmentally benign energy supply economical and environmentally benign energy supply both in large-scale gridconnected dispatchable markets both in large-scale gridconnected dispatchable markets and remote or modular distributed markets. Parabolic and remote or modular distributed markets. Parabolic and Fresnel troughs, central receivers and parabolic and Fresnel troughs, central receivers and parabolic dishes will be installed for solar/fossil hybrid and solar-dishes will be installed for solar/fossil hybrid and solar-only power plant operation. In parallel, decentralised only power plant operation. In parallel, decentralised process heat for industrial applications will be provided process heat for industrial applications will be provided by low-cost concentrated collectors.by low-cost concentrated collectors.

Following a subsidised introduction phase in green Following a subsidised introduction phase in green markets, electricity costs will decrease from 14 to 18 markets, electricity costs will decrease from 14 to 18 Euro cents per kilowatt hour presently in Southern Euro cents per kilowatt hour presently in Southern Europe towards 5 to 6 Euro cents per kilowatt hour in Europe towards 5 to 6 Euro cents per kilowatt hour in the near future at good sites in the countries of the the near future at good sites in the countries of the Earth’s sunbelt. After that, there will be no further Earth’s sunbelt. After that, there will be no further additional cost in the emission reduction by CSP. This, additional cost in the emission reduction by CSP. This, and the vast potential for bulk electricity generation, and the vast potential for bulk electricity generation, moves the goal of longterm stabilisation of the global moves the goal of longterm stabilisation of the global climate into a realistic range. Moreover, the problem climate into a realistic range. Moreover, the problem of sustainable water resources and development in of sustainable water resources and development in arid regions is addressed in an excellent way, making arid regions is addressed in an excellent way, making use of highly efficient, solar powered co-generation use of highly efficient, solar powered co-generation systems. However, during the introduction phase, systems. However, during the introduction phase, strong political and financial support from the strong political and financial support from the responsible authorities is still required, and many responsible authorities is still required, and many barriers must be overcome [7].barriers must be overcome [7].

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