Solar Thermal Energy System for Renewable Desalination

by Ryan McLaughlin

A THESIS

submitted to

Oregon State University

Honors College

in partial fulfillment of the requirements for the

degree of

Honors Baccalaureate of Science in Chemical Engineering (Honors Scholar)

Presented June 4, 2020 Commencement June 2020

AN ABSTRACT OF THE THESIS OF

Ryan McLaughlin for the degree of Honors Baccalaureate Science in Chemical Engineering presented on June 4, 2020. Title: Solar Thermal Energy System for Renewable Desalination.

Abstract approved:_____________________________________________________ Nicholas AuYeung

Natural freshwater availability is decreasing as the global population increases.

Advances in desalination technologies aim to produce fresh water in an economical way.

More sustainable energy sources are desired for these desalination plants to limit the carbon

footprint of water production since global carbon dioxide production is exacerbating

freshwater scarcity. Solar thermal energy is desirable because the technology is much more

efficient at producing the heat required for desalination than traditional photovoltaic solar

energy. Dr. Bahman Abbasi, Dr. Nicholas AuYeung, Dr. James Klausner, Dr. Jelena Srebric,

Dr. Chris Hagen, Dr. Behrooz Abbasi, Mohammed Elhashimi, and Deepak Sharma have

been designing a mobile desalination unit that utilizes humidification-dehumidification

cycles to desalinate water. The system is to be powered by a solar thermal system. The

primary objective of this report is to provide a comparison between available heat transfer

fluids and solar collectors used in this solar thermal system as well as provide a preliminary

economic analysis of the system.

Key Words: Desalination, Feasibility Analysis, Economic Analysis, Renewable Energy Corresponding e-mail address: ryanpmclaughlin34@gmail.com

©Copyright by Ryan McLaughlin June 4, 2020

Solar Thermal Energy System for Renewable Desalination

by Ryan McLaughlin

A THESIS

submitted to

Oregon State University

Honors College

in partial fulfillment of the requirements for the

degree of

Honors Baccalaureate of Science in Chemical Engineering (Honors Scholar)

Presented June 4, 2020 Commencement June 2020

Honors Baccalaureate of Science in Chemical Engineering project of Ryan McLaughlin presented on June 4, 2020. APPROVED: _____________________________________________________________________ Nicholas AuYeung, Mentor, representing Chemical Engineering _____________________________________________________________________ Bahman Abbasi, Committee Member, representing Mechanical Engineering _____________________________________________________________________ Mohammed Elhashimi, Committee Member, representing Mechanical Engineering _____________________________________________________________________ Deepak Sharma, Committee Member, representing Mechanical Engineering _____________________________________________________________________ Toni Doolen, Dean, Oregon State University Honors College I understand that my project will become part of the permanent collection of Oregon State University, Honors College. My signature below authorizes release of my project to any reader upon request. _____________________________________________________________________

Ryan McLaughlin, Author

Acknowledgements

I am so grateful for the endless support I have received from faculty, friends,

and family. I would first like to thank my thesis mentor Dr. Nicholas AuYeung. After

my previous mentor had left Oregon State, he had provided me with the opportunity

to join this project that he was working on. He pushed me and gave the support I

needed to navigate the a thesis defense during the Covid-19 pandemic. I am forever

thankful for the opportunity he provided me with to learn more about renewable

energy and desalination, two fields that I have always been passionate about.

I would also like to thank Dr. Bahman Abbasi, Mohammed Elhashimi, and

Deepak Sharma for letting me collaborate with them on this meaningful project as

well as supporting me as committee members. Your support, assistance, and

collaboration along the way was greatly appreciated.

This journey would also not have been possible without my best friend, Kelly

Landreth. She has been the best person to rough life’s rapids with and I could not

imagine life without you. Your love, support, and charisma mean the world to me.

You never fail to put a smile on my face.

I would like to thank my family for helping me get through college. I can’t

imagine how much work it was to raise me—so thank you JR, mom, and dad! I really

appreciate your endless love, support, and dad jokes. It has taken a village to get me

here and I can never begin to express my gratitude to the rest of my grandparents,

aunts, uncles, cousins, and other brother Jackson for helping me get where I am.

Table of Contents

1. Introduction ................................................................................................. 1

2. Background ................................................................................................. 3

Common Desalination Methods ............................................................................. 3

Current Desalination Project ................................................................................. 5

Solar Thermal Collection Technologies ................................................................. 6

Solar Thermal Heat Transfer Fluids ..................................................................... 9

Preliminary Solar Thermal Energy System Design ............................................... 11

3. Methodology ............................................................................................... 14

Solar Collector Comparison Criterion .................................................................. 14

Heat Transfer Fluid Comparison Criterion .......................................................... 16

Heat Exchanger Area ........................................................................................... 18

Economic Analysis ............................................................................................... 19

4. Results and Discussion ................................................................................ 23

Solar Thermal Collector Comparison ................................................................... 23

Heat Transfer Fluid Comparison ......................................................................... 27

Economic Analysis ............................................................................................... 32

References ............................................................................................................ 39

Appendix A – Desalination System ....................................................................... 44

Appendix B – Solar Thermal Collector Decision Matrix ....................................... 45

Appendix C – Heat Transfer Fluid Decision Matrix ............................................. 46

Equation 1: ................................................................................................................. 18

Equation 2: ................................................................................................................. 18

Equation 3: .................................................................................................................. 19

Table 1: Star Diagram Criterion ................................................................................. 17

Table 2: HX Fluid Volume Needed ............................................................................ 32

Table 3: System Cost .................................................................................................. 33

Table 4: Solar Thermal Collector Decision Matrix .................................................... 45

Table 5: Heat Transfer Fluid Decision Matrix ............................................................ 46

Figure 1: Energy Collection and Heat Transfer System ............................................. 12

Figure 2: Maximum Temperature Comparison .......................................................... 23

Figure 3: Power Generation per Radiated Area Comparison ..................................... 25

Figure 4: Collector Number Comparison ................................................................... 26

Figure 5: Heat Transfer Fluid Individual Star Diagrams ............................................ 28

Figure 6: Heat Transfer Fluid Combined Star Diagram ............................................. 29

Figure 7: Heat Transfer Fluid Cost Comparison ......................................................... 31

Figure 8: HT Fluid Temperature Change Sensitivity Analysis .................................. 34

Figure 9: Desalination System .................................................................................... 44

1

1. Introduction

844 million people—about 10% of the global population—do not have access

to clean water [1]. Global water insecurity issues are projected to be exacerbated as

carbon emissions increase, global climate changes and human population increases.

Drought is projected to affect 35% of global land mass this year [2]. More

specifically, Africa is extremely prone to water insecurity issues in the future because

of poor water infrastructure, political instability, and a high percentage of developing

nations prone to rapid population growth as well. Droughts exacerbate a paradox of

poverty in which poor, unstable countries are unable to develop water infrastructure

due to economic stress while water stress produces instability that prevents the

development and economic success. For instance, droughts between 1999 and 2000

dropped Kenya’s GDP a whole 16% [3]. According to the World Health Organization

and UNICEF, at least 44% of the population in Africa do not have access to safe and

reliable fresh water sources as of 2007 [2]. According to the UN, 80% of diseases and

30% of deaths are the result of unsafe drinking water [3]. Without the advanced water

purification technologies that developed countries have, water insecurity issues will

likely continue to plague these developing countries.

Desalination is a method of generating fresh water by removing the salts out

of saltwater. The salt and water are separated through several techniques including

but not limited to filtration, ion exchange, distillation, and humidification-

dehumidification cycles. Desalination helps increase the limited amount of fresh

water available on the planet.

2

Dr. Bahman Abbasi, Dr. Nicholas AuYeung, Dr. James Klausner, Dr. Jelena

Srebric, Dr. Chris Hagen, Dr. Behrooz Abbasi, Mohammed Elhashimi, and Deepak

Sharma have been designing a desalination unit to be deployed in Nigeria. The

desalination unit will purify 3500 liters per hour of fresh water. The system—outlined

in Appendix A—utilizes a humidification-dehumidification cycle to extract the solids

out of a saltwater stream to produce fresh water. Dry air is fed into a converging

nozzle which accelerates and heats it with low-grade solar thermal energy. The hot,

dry air is then used to atomize and vaporize the saltwater, which is fed into a cyclone

for separation. The warm humid air is then fed into a condenser that cools the stream

to condense the water out of the air stream. The air and condensed water stream is fed

into an air-water separator that separates the dry air and fresh, liquid water. The dry

air recycles back into the dry air converging nozzle section while the fresh, liquid

water is fed into a heat recuperator that preheats incoming cool saltwater before it is

fed into the atomizing area.

The purpose of this project is to generate a preliminary design of the solar

thermal collection system used to heat up the air in the compression tubes. The

system will consist of a solar thermal collector, heat transfer fluid, a storage tank,

piping, and a heat exchanger. The primary focus is to review what solar thermal

collectors and heat transfer fluids would be best for this system, but a secondary

analysis of system cost will be used to determine the feasibility of the system.

3

2. Background

Common Desalination Methods

Desalination methods are rapidly evolving as the need to generate more fresh

water increases with changing climate and increasing global population. This section

is meant to be a brief summary of the technologies currently available on the market,

but is not an exhaustive list of the desalination methods available. Between 2008 and

2012, the capacity of desalination plants increased globally from 52,333,950 cubic

meters per day to 79,000,000 cubic meters per day [4]. To be potable drinking water,

the total dissolved solids in the water—a measure of salinity—must be below 1000

milligrams per liter [5]. There are three general desalination methods: thermal

processes, membrane processes, and chemical processes [6].

Some of the most common thermal desalination processes include multi-stage

flash distillation, multi-effect distillation, thermal vapor compression, and mechanical

vapor compression [4]. The processes utilize thermal cycles to extract the solids out

of fresh water through phase separations [5]. Thermal desalination methods typically

require a significantly larger energy input than membrane methods, which causes

undesirably high energy costs of operation [4]. However, these processes are

extremely effective at treating large volumes of saltwater with high levels of salinity

and are desirable because they can utilize heat rather than electricity as an energy

source, which is much cheaper. The primary issue with the high-output thermal and

distillation plants is that most of them emit high levels of carbon dioxide [7]. Carbon

dioxide production is problematic because it further exacerbates climate change,

which is a driver for water insecurity because it increases drought frequency [3].

4

However, renewable sources of energy are used for some thermal and distillation

desalination processes to ensure more sustainable operation [7].

Reverse/forward osmosis, electro-dialysis, and nanofiltration are common

membrane desalination techniques [6]. Membrane desalination techniques work to

remove salt from water by creating a driving force for mass transfer across a

membrane to separate the solids and water [5]. These processes are associated with

large initial investments followed by low energy costs because most of them require

expensive, energy efficient technology [4]. However, many membrane processes

require pre-treatment to prevent the rapid fouling of the membrane used in the

process. Membrane processes usually rely on electrical energy rather than thermal

energy to generate a pressure drop in the system, which is more expensive [8].

Liquid-liquid extraction, adsorption, ion-exchange, and gas hydrate processes

are a few of the more common chemical desalination techniques [5]. As the name

suggests, chemical desalination techniques utilize chemical reactions to extract solids

out of the saltwater [6]. Currently, most of these processes are not considered

commercially feasible to treat highly saline water [5]. Solar thermal energy will be

used in the system as it is a more feasible and efficient off-grid energy source.

Humidification-dehumidification cycles are included in thermal desalination

methods. In these cycles, a hot air stream is humidified using a saltwater stream. The

solids in the saltwater are left behind during humidification or within in a cyclone

section that removes salt in some systems. The air then enters a dehumidifier—or

condenser—where the temperature of the air decreases until fresh water condenses

out of the air. The now dry air is recycled back into the humidification section and the

cycle repeats. This technology has great potential to be used in decentralized

5

applications with freshwater production rates less than several thousand cubic meters

per day [9].

Current Desalination Project

A multi-university team led by Oregon State University is working on

constructing the desalination unit that is projected to be capable of producing 3500

liters of fresh water per day from a water source that has 100,000 ppm of total

dissolved solids. The system—outlined in Appendix A—utilizes humidification-

dehumidification cycles to desalinate the source water. Dry air first enters a

converging nozzle with a heat exchanger powered by solar thermal energy. The dry

air then comes into contact with warm saltwater and atomizes it since the air is

traveling at high speeds. The now humid stream of air enters a cyclone system in

which any salt in the air stream is separated from the air. The humid air then enters a

condenser unit that cools the air stream down to get the water out of the air stream

through condensation. The air and condensed water are fed into an air-water separator

that separates the dry air and fresh, liquid water. The dry air recycles back into the

converging nozzle section while the fresh, liquid water is fed into a heat recuperator

that preheats cool saltwater before the saltwater is fed into the atomizing area.

The system is meant to provide fresh water for off-grid communities with scarce

fresh water sources that are near saltwater sources. The system will primarily be

powered by thermal energy so that the system can be sustainable and self-contained.

The purpose of this project is to review solar thermal collectors, heat transfer fluids,

as well as provide a preliminary design for the energy collection and heat transfer

system that will be used to power the system. The energy collection and heat transfer

system will be used to heat the dry air in the converging nozzles before the air stream

6

atomizes the saltwater. The heat collection and energy transfer system will be

designed to operate around 200 °C.

Solar Thermal Collection Technologies

Solar thermal collectors harvest the light energy from the sun and convert it into

thermal energy. Collector technologies may either be passive or tracking. Unlike

tracking collectors, passive collectors do not track solar movement across the sky

[10]. Most solar thermal technologies are broken into two general categories:

concentrating and non-concentrating. Concentrating technologies work by reflecting

incoming solar radiation by focusing solar onto a small absorption surface while non-

concentrating collectors have large absorption areas and do not concentrate incoming

solar radiation [11].

Concentrating solar thermal collectors are typically used for higher temperature

applications because they are generally able to achieve higher temperatures than non-

concentrating collectors [10]. Common concentrating collector technologies include

parabolic trough collectors, parabolic dish collectors, linear Fresnel collectors, and

heliostat field collector [11].

Parabolic trough collectors are systems which utilize U-shaped mirrors to reflect

solar radiation onto a typically rod-shaped receiver that is the length of the mirror

[12]. Parabolic dish collectors are comprised of a dish shaped mirror that focuses onto

a receiver at the focal point of the dish [10]. Since the parabolic dish collector is a

point focus rather than an axis focus, the parabolic dish collector can usually achieve

higher temperatures than parabolic trough collectors [12]. Linear Fresnel collectors

are comprised of structures of flat mirror strips that focus onto either one or multiple

rod-like receiver(s) that run along the length of the mirrors [10]. Linear Fresnel

7

mirrors mimic parabolic trough collector reflecting patterns, but are comprised of a

series of flat mirrors instead of one U-shaped mirror [13]. Most of these systems are

larger and less portable than parabolic dish and parabolic trough collectors; however,

there have been more recent research in more compact Linear Fresnel collector

systems. Heliostat collector systems are similar to Linear Fresnel collectors in the

sense that both are comprised of multiple reflectors focusing on one or multiple

common absorbers. Heliostat systems are comprised of a system of large mirrors that

direct solar radiation towards a central tower that acts a receiver [12]. Heliostat

collector systems are typically large and capable of producing massive power outputs

but require the largest land area and a substantial water supply [14]. The Ivanpah

Solar Electric Generating System in the United States is capable of producing 392

MW, but utilizes about 3500 acres of land [10].

Non-concentrating solar thermal collection systems are typically used for lower

temperature applications because the systems are unable to reach higher temperatures

without concentrating components. Non-concentrating collectors are desirable

because many have a higher efficiency than concentrating collectors [15]. However,

most non-concentrating collectors can only achieve a maximum temperature of

200 °C and most have poor efficiency at that temperature [11]. Non-concentrating

collectors include flat plate collectors, hybrid PVT collectors, and evacuated tube

collectors [11, 16].

Flat plate collectors generally consist of glazing covers containing tubes filled

with heat transfer fluid fixed on an absorber plate [11]. The collectors need to be

oriented to maximize incoming solar radiation since they are typically permanently

fixed in position [16]. Flat plate collectors are typically used at operating

8

temperatures below 100 °C and one of the cheapest solar thermal collectors [11].

Hybrid PVT (Photovolatic-Thermal) systems are capable of producing both heat and

electrical energy. The hybrid PVT collectors consist of a traditional photovoltaic cell

found in most traditional solar panels as well as an heat absorption plate attached to

the photovoltaic cell to capture heat that would otherwise be lost from the cell.

Hybrid PVT cells are regarded as a more efficient alternative to traditional solar

panels available on the market because Hybrid PVT collectors capture this heat that

would otherwise be lost; however, they are only efficient in low temperature

applications [17]. Evacuated tube collectors (ETCs) are the only non-concentrating

collectors capable of operating efficiently around 200 °C. There are two types of

ETCs: heat pipe and direct flow. Heat pipe ETCs are comprised of a series of hollow

glass cylinders under vacuum with a thermally conductive metal pipe running along

the length of the cylinder. The thermally conductive metal is fixed to an absorber

sheet that transfers incoming solar radiation to the pipe. The pipe is filled with a heat

transfer fluid that undergoes a phase change to store the captured solar energy [11].

Direct flow ETCs are comprised of hollow glass cylinders under vacuum with a flat

or curved absorber plate running along the diameter of the tube. A metal, thermally

conductive pipe containing heat transfer fluid is welded to the absorber plate. The

absorber plate captures incoming solar radiation and transfers to the heat transfer fluid

contained in the metal pipe [11, 18]. Direct flow ETCs are usually more suitable for

high temperature applications because they have lower thermal loss coefficients [18].

The solar collectors primarily researched in this report were parabolic trough

collectors, parabolic dish collectors, and direct flow evacuated tube collectors. The

reason of this selection was that these types of collectors were the most transportable

9

and efficient collectors for operation around 200 °C. The only other collectors

capable of efficient operation around 200 °C were heliostat and Linear Fresnel

collection systems, which were considered too difficult to transport between sites.

Solar Thermal Heat Transfer Fluids

The heat transfer fluids in the system need to be capable of operation between

ambient temperatures and 200 °C, since the solar thermal system will only be capable

of operation during daylight. Common heat transfer fluids for solar thermal collectors

include oils, molten salts, water/glycol mixtures, and pressurized gasses [19].

There are four categories of heat transfer oils used in solar thermal applications:

synthetic hydrocarbon oils, paraffin oils, silicone oils, and aromatic refined mineral

oils [19, 20]. Synthetic hydrocarbon oils are typically associated with higher thermal

conductivity and lower viscosity than mineral oils, which makes them more desirable

heat transfer fluids [19]. Synthetic oils also have a relatively long lifespan of 5 to 10

years and are generally non-toxic [21]. However, mineral oils are a byproduct of the

petroleum industry, which drives down the cost of the heat transfer fluid. Mineral oils

also have low viscosities, low toxicities, high stabilities, and high thermal

conductivities, which makes them a viable and cheaper alternative than synthetic oils

for this solar thermal system [22]. Paraffin oils are petroleum-based heat transfer fluid

with larger temperature ranges and lower viscosities than synthetic and mineral oils,

but are typically toxic and require double-walled heat exchangers [21]. Silicone oils

are an excellent option for solar thermal applications because they freeze at low

temperatures, boil at high temperatures, are non-corrosive, and very resistant to

oxidation. However, silicone oils usually have higher viscosities and low heat

capacities, which greatly increases the pumping power needed [23].

10

Molten salts are salt mixtures primarily composed of nitrates that are used for heat

transfer fluids. These salts are commonly used in high temperature applications

because they have high densities, high heat capacities, high thermal stabilities, and

low vapor pressures [24]. However, most of these molten salts have high melting

points that are problematic for operation because the salts can freeze in heat transfer

piping [25]. The melting points for molten salts typically range between 150 °C and

600 °C [26]. Most solar thermal plants have alternative energy sources during dark

hours to heat the system and prevent freezing salts from destroying heat transfer

piping [27]. Molten salts are not suitable for this solar thermal system because the

system is only operational during the day and the temperature of the system is too

low, which would cause molten salts to freeze.

Water and glycol mixtures are common heat transfer fluids for low to medium

temperature systems. The water and glycol mixtures are extremely desirable heat

transfer fluids because of their large range of operating temperatures, high heat

capacity, low viscosity, and price. However, water and glycol mixtures are only

thermally stable to temperatures up to 175 °C [28]. For this reason, water and glycol

mixtures are not suitable for this solar thermal system since it operates around

200 °C.

Pressurized gases are also commonly used for solar thermal applications.

Commonly used pressurized gases in solar thermal systems include air, carbon

dioxide, helium, hydrogen, and water [16]. Pressurized gasses are desirable heat

transfer fluids because they have much wider operating ranges and they are very

cheap [16, 29]. However, the fluid (e.g. density) and heat transfer properties of gases

limit the efficiency of pressurized gasses in solar thermal applications [16, 25].

11

The primary heat transfer fluids reviewed in this analysis are silicone, aromatic

mineral, paraffin, and synthetic oils. These heat transfer fluids were most suitable for

this solar thermal system because of their fluid, heat transfer, material compatibility,

and toxicology properties. Molten salts were excluded from this analysis because of

their high melting points and low material compatibility. Pressurized gases were

excluded from this analysis because of their unfavorable heat transfer properties.

Water and glycol mixtures were excluded from this analysis because of stability

concerns at the operating temperature of the system. Only non-toxic paraffin oils

were included in this analysis to ensure safe operation of the desalination system.

Preliminary Solar Thermal Energy System Design

The heat transfer and energy collection system will be comprised of four simple

components: the solar thermal collection system, heat transfer fluid storage, heat

transfer fluid pump, and heat exchanger. The system is outlined in Figure 1: Energy

Collection and Heat Transfer System. The solar thermal collection system collects

solar radiation and transfers the energy to a heat transfer fluid. The heat transfer fluid

storage is a tank in which thermal energy will be stored in the heat transfer fluid to

allow for 30 minutes of operation without light. This will help ensure that the process

is capable of running with scattered clouds that would decrease the amount of

incoming solar radiation that the solar thermal collection system can collect. The fluid

is pumped out of the heat transfer fluid storage tank by a pump. The pump will be a

self-priming oil pump capable of withstanding process temperatures exceeding

200 °C. The pump outlet flows into the heat exchanger, which will be used to heat the

air stream that atomizes the saltwater in the desalination unit. The objective of this

report is to provide a preliminary design of the energy collection and heat transfer

12

Heat Transfer Fluid to Air Heat Exchanger

Heat TransferFluid Storage

Heat TransferFluid Pump

Solar Thermal Collection System

Figure 1: Energy Collection and Heat Transfer System

Solar energy is collected and transferred to a heat transfer fluid in the solar thermal collection system. The fluid is then transported to a heat transfer fluid storage tank, which is sized to hold about 30 minutes of heat transfer fluid. The heat transfer pump provides the pressure throughout the system to move the fluid through the piping between components. The outlet of the heat transfer pump goes into the heat exchanger, which heats the process that is used to atomize the saltwater.

system that focuses on the solar thermal collection system and heat transfer fluid used

in the process as well as provide an estimate of the energy cost generated in the

system.

The solar thermal collection system will be designed to provide enough

energy to operate at 200 °C. The system will be designed to produce 400 liters per

hour of fresh water with an energy requirement of 54 kWh/m3 of water. A secondary

goal of the solar thermal collection system is to have it be compact or mountable on a

large bus so that the system may be easily transported to different locations

experiencing water insecurity. Another objective of the solar thermal collection

system is that the system is simple enough that it can be operated easily without

rigorous training. This would allow for a system to be easily deployed, operated, and

managed by local governments in underdeveloped countries.

13

The heat transfer fluid in the system will need to be stable beyond the high

operating temperature of 200 °C. Ideally, the stability and boiling point of the fluid

will be well above the operating temperature (more than 50 °C) so that the heat

transfer fluid would not degrade or cause hazards in the event that the system is out of

specification. Ideally, the heat transfer fluid will be non-toxic so that if the heat

transfer fluid were to enter the air stream or an operator was exposed to it, no adverse

health effects would develop. In addition to that, a non-toxic heat transfer fluid would

decrease the need for complex heat transfer fluid disposal and replacement. Heat

transfer fluid disposal is a very critical factor because most of the areas where the

desalination systems would be deployed would not have a means of treating

hazardous chemicals. In addition to that, the reactivity of the heat transfer fluid

should be low so that there is not a large flammability or corrosion risk. The heat

transfer properties, fluid properties, and cost are also important because they govern

the economic feasibility of the entire energy collection and heat transfer system.

The total cost of the system outlined in Figure 1: Energy Collection and Heat

Transfer System was used to determine a rough estimate for the cost of water

production in the desalination system. The cost of energy for the solar thermal energy

subsystem of the desalination unit was estimated by dividing the installed cost of the

system by the total energy would theoretically produce over the system lifetime. The

levelized cost of water for the energy system was estimated based upon the lifetime

and initial cost of the system and heat transfer fluid.

14

3. Methodology

Solar Collector Comparison Criterion

The solar collectors will be compared based upon the maximum temperature,

energy cost, and power per radiated area. Various solar thermal collectors were

selected based upon performance, ease of transportation, and operating temperature to

compare. The solar collectors compared include the Apricus ETC-20, SunMaxx Solar

VHP 10, SunMaxx Solar VDF 20, Solar Panels Plus SPP-25, Absolicon T160, Artic

Solar XCPC, Norwich Technologies SunTrap, and a parabolic dish collector from

researchers at Oxford University (UK). The Apricus ETC-20 is an evacuated tube,

heat pipe collector with 20 tubes [30]. The SunMaxx Solar VHP 10 and VDF 20 are

heat pipe and direct flow evacuated tube collectors, respectively. The VHP 10 and

VDF 20 have 10 and 20 heat collecting tubes, respectively [31, 32]. The Solar Panels

Plus SPP-25 is a 25 tube evacuated tube collector [33]. The Absolicon T160 is a

parabolic trough solar concentrator [34]. The Artic Solar XCPC is a panel comprised

of a series of small parabolic trough collectors [35]. Norwich Technologies SunTrap

is a parabolic trough receiver optimized for high temperature applications [36]. The

parabolic dish collector from Oxford is a parabolic dish collector still in the

experimental stage that is capable of high temperature operation [37].

The maximum temperature statistic is the stagnation temperature of the

collector. The stagnation temperature is the temperature achieved when the heat

transfer fluid in the collector is not flowing through the system [38]. While this

statistic does not provide specifics about the efficiency at different temperatures for

each collector, it does provide insight as to the temperatures at which the system is

capable of being efficient. Greater stagnation temperatures suggest higher operating

15

temperature ranges and greater efficiencies at higher temperatures since efficiency

generally decreases with temperature under ambient conditions.

The energy cost between solar collectors was compared by dividing the cost of

the unit by the peak output. The peak output of each collector was found from each

collector’s technical specification sheets provided by the company. The peak output is

the maximum power output for the system. The SRCC clear day standard of 2000

BTU/ft2-day or 1000 W/m2 of incoming solar radiation was used, depending on what

information was available for each collector. The cost for each unit was based upon

quotes or information from the company which develops the solar panels. A lower

energy cost indicates that a solar collector is more economically efficient, which

drives the cost of the energy collection and heat transfer system down.

The power per radiated area is calculated by dividing the peak output by the

cross-sectional area at the largest cross-section of the collector. This statistic is meant

to illustrate how much energy each collector can generate per space it takes up. The

power per radiated area is a useful statistic for the solar thermal system because it

provides insight about how portable the unit can be. If the power per radiated area is

greater, the solar thermal collection system will take up less space and be more

transportable. This is important when designing a desalination unit that is meant to be

portable.

A summary of the collectors explored along with the statistics of each collector

may be found in Table 4: Solar Thermal Collector Decision Matrix. The summary

also includes two collectors—a parabolic dish collector from a research team at

Oxford and the SunTrap absorption tube—that have the potential to be a great fit for

the system, but not enough information was available to include them in the report.

16

Heat Transfer Fluid Comparison Criterion

The heat transfer fluids that will be compared were selected based upon their

performance and ability to withstand process temperatures. Only mineral, synthetic,

paraffin, and silicone oils were compared because of the reasons outlined in the Solar

Thermal Heat Transfer Fluids section. The heat transfer fluids compared are

Duratherm S, Duratherm 630, Therminol XP, and Therminol 68. Each heat transfer

fluids compared will be based upon safety, material compatibility, thermodynamic

properties, heat transfer properties, fluid properties, and cost. A star diagram will be

used to compare the safety, material compatibility, thermodynamic properties, heat

transfer properties, and fluid properties while a bar graph will be used to compare the

cost of fluids.

The star diagram criterion is summarized in Table 1: Star Diagram Criterion. The

safety section describes how toxic, flammable and reactive the substance is. This is

quantified by summing the health, flammability, and reactivity section of the NFPA

rating. A lower NFPA score indicates a less hazardous material. The compatibility

section quantifies the corrosivity and fouling/oxidizing tendency of the heat transfer

fluid. The thermodynamic properties illustrate the operating range of the fluid which

includes the freezing point, boiling point, and point at which the substance is

unstable. The heat transfer properties section is a way of illustrating the heat transfer

performance of the fluid. This includes the heat capacity and thermal conductivity.

The fluid properties section includes the density, kinematic viscosity, and thermal

expansion coefficient of the fluid. Greater scores in each category indicate higher

performance in that area. The heat transfer fluid with the greatest area on the star

diagram has the greatest overall performance.

17

1 Combined NFPA score greater than 9 1 Corrosive to most heat

transfer metals

2 Combined NFPA score between 7-9. 2 Meets criteria of above and

below options.

3 Combined NFPA score between 5-7 3

Relatively non-corrosive to most heat transfer metals but prone to fouling and

oxidation.

4 Combined NFPA score between 3-5. 4 Meets criteria of above and

below options.

5 Combined NFPA score less than 3. 5 Non-corrosive and non-

fouling

1Boiling Point < 200 - 250°C

Unstable < 280 - 300°C Freezing Point 5 - > 10°C

1Cp less than 2.2 kJ/kg-K

Thermal Conductivity less than 0.09 W/m-K

1

Kinematic Viscosity > 0.8 cP Density < 700 kg/m3

Thermal Expansion Coefficient >12 %/°C

2 Meets criteria of above and below options. 2 Meets criteria of above and

below options. 2 Meets criteria of above and below options.

3Boiling Pt.: 250 - 310°C

Unstable: 300- 340°C Freezing Pt: -5 - 5°C

3Cp between 2.2 and 2.6

kJ/kg-K Thermal Conductivity between 0.09

and 0.11 W/m-K

3

Kin. Viscosity: 0.8 - 0.7 cP Density: 700 - 800 kg/m3

Thermal Expansion Coefficient: 10-12 %/°C

4 Meets criteria of above and below options. 4 Meets criteria of above and

below options. 4 Meets criteria of above and below options.

5Boiling Point > 310°C

Unstable > 340°C Freezing Point < -5 °C

5Cp greater than 2.6 kJ/kg-K

Thermal Conductivity greater than 0.11 W/m-K

5

Kinematic Viscosity < 0.7 cP Density > 800 kg/m3

Thermal Expansion Coefficient < 10 %/°C

Thermodynamic Properties Heat Transfer Properties Fluid Properties

Star Diagram CriterionSafety Compatibility

Table 1: Star Diagram Criterion

Below is the star diagram criterion for comparing the safety, compatibility, thermodynamic properties, heat transfer properties, and fluid properties. A greater number corresponds to greater performance. The safety section is rated based upon the NFPA safety rating of the chemical. The NFPA score is calculated by adding up all of the numbers in the NFPA safety rating. The compatibility section helps describe the corrosivity and resistance to fouling/oxidation of the heat transfer fluid.

18

The cost of each heat transfer fluid will be calculated based upon an energy

basis. The purpose of this is to take out factors in density when calculating the cost of

each fluid because the cost per unit energy is a more telling parameter of economic

performance. The cost of each fluid is calculated by dividing the cost per unit volume

by the density, heat capacity, and operating temperature change. The operating

temperature change—5 °C—is the temperature change that the heat transfer fluid will

experience in the heat exchanger that heats the air used in the desalination process.

This temperature change will be modified accordingly when optimizing the heat

exchanger in the desalination system. A lower energy cost is desirable because that

indicates less money needs to be spent on heat transfer fluid in the process.

A summary of the heat transfer fluid properties and their performance may be

found in Table 5: Heat Transfer Fluid Decision Matrix.

Heat Exchanger Area

The total heat exchanger area for the process was calculated using the

following equation:

Equation 1:# = %&∆()*

where # is the energy demand, % is the overall heat transfer coefficient, & is the heat

exchanger area, and ∆()* is the log mean temperature difference. # was calculated

using the specified basis of 400 liters of fresh water produced per hour and ceiling

energy use of 54 kW-hr of energy needed per cubic meter of water purified.

% was calculated with the following equation:

Equation 2:+

,=

+

-.+

0

1+

+

-2

where ℎ4 is the convective heat transfer coefficient of the inside fluid (air – assumed

to be 100 W/m2-°C), ℎ5 is the convective heat transfer coefficient of the outside fluid

19

(heat transfer fluid – assumed to be 5000 W//m2-°C), 6 is the thickness of the heat

exchanger wall (assumed to be 1.5 mm), and 7 is the thermal conductivity of the heat

exchanger material (assumed to be 20 W/m-°C, that of stainless steel). This led to an

overall heat transfer coefficient of 97 W/m2-°C.

∆()* was calculated with the following equation:

Equation 3: ∆()* =89:,<=9>,?@=89:,?=9>,<@

ABC8D:,<ED>,?@

8D:,?ED>,<@F

where (G,+ is the temperature of the heat transfer fluid entering the heat exchanger

(200 °C), (G,H is the temperature of the heat transfer fluid leaving the heat exchanger

(195 °C), (I,+ is the temperature of the air entering the heat exchange (70 °C), and

(I,H is the temperature of the air leaving the heat exchanger (125 °C). The log mean

temperature difference is 98 K. The total heat exchanger area needed with these

specifications is 2.3 m2.

Economic Analysis

For the economic analysis, a few assumptions need to be made since this

report focuses primarily on finding the ideal heat transfer fluid and solar collector

rather than completely designing the system. The assumptions for the basis of the 10

year economic analysis are:

• The total heat transfer fluid needed for the system will be that of 30 minutes

of heat transfer fluid moving through the heat exchanger

• The piping, valves, and fittings is 3% of the tank cost [39]

• The electrical work and instrumentation is 7% of the tank cost [39]

• The storage tank will be sized to house 1.5x the heat transfer fluid in the

system

20

• The heat transfer fluid will need to be replaced once every five years

• The system will produce 400 liters of fresh water per hour for 8 hours

• The system will be active 355 days out of the year, with 10 days of downtime

and clouds assumed

• The incoming solar energy during the 8 hours of operation will be 80% of the

SRCC clear sky standard

• Installed costs of equipment are 1.3x higher than the capital cost [39]

• The 2010, 2011 and 2020 CEPCIs are 532.9, 585.7 and 668, respectively [40,

41, 42]

• There is a 10 year life time for the solar collectors

• There is a salvage value of 0 after the 10 year period

The cost per cubic meter of fresh water generated will be calculated over a 10

year period based upon the energy demand of the system. The total cost per cubic

meter of water will be calculated with the following equation:

JKL0MN =JI5AAMO05N ∗ QI5AAMO05N + ∑JI5ST5BMB0U + JVWST +

6XAW4Y6UZU0MS

∗ J[AW4Y

\]L0MN

where JKL0MN is the cost of water per cubic meter, JI5AAMO05N is the cost one collector,

QI5AAMO05N is the number of collectors needed, ^_TMNL045B is the energy needed for

operation, ∑JI5ST5BMB0U is the installed cost of piping, valves, fittings,

instrumentation, storage tanks, storage tank supports, site work, foundations,

electrical work, and instrumentation, JVWST is the installed cost of the pump in the

system, 6XAW4Y is the lifetime of the heat transfer fluid, 6UZU0MS is the lifetime of the

21

system, J[AW4Y is the total cost of heat transfer fluid needed, and \]L0MN is the volume

of water produced over the lifetime of the plant.

JI5AAMO05N was calculated by multiplying collector cost estimates or quotes by

the installation factor (1.3) to account for installation costs. QI5AAMO05N was calculated

dividing the power requirements of the system (54 kWh/m3 water, 400 L/hr of water)

by 80% of the peak output under SRCC clear day conditions. ∑JI5ST5BMB0U was

calculated by summing the installed energy costs for piping, valves, fittings,

instrumentation, storage tanks, storage tank supports, site work, foundations,

electrical work, and instrumentation. The cost of the storage tank and pump were

found based upon the following cost correlation:

J4 = JNMX NMX a4

NMXbc

where J4 is the cost of the storage tank or pump, JNMXis the reference cost per physical

parameter, NMX is the reference physical parameter that the cost is based upon, 4 is

the actual physical parameter that the cost is based upon, and b is an empirically

determined sizing exponent [43]. For the pump, JNMX, NMX, and d were

$357.13/(m3/hr), 0.98 m3/hr, and 0.6, respectively. 4 was the volumetric flow rate of

heat transfer fluid for the pump. For the tank,JNMX, NMX, and d were $3883/m3, 0.32

m3, and 0.5, respectively. 4 was the total volume of heat transfer fluid multiplied by

a factor of 1.5 to allow safe operation with thermal expansion. 6XAW4Y (5 years) and

6UZU0MS (10 years) were assumed values based upon reasonable heat transfer fluid and

system lifetimes [19,39]. J[AW4Y was based upon cost estimates and quotes provided in

Table 5: Heat Transfer Fluid Decision Matrix. \]L0MN was calculated by multiplying

22

the volumetric production of water (400 L/hr) by the daily operational time (8 hours

per day) and by the yearly operation time (355 days).

The levelized cost of energy for the CAPEX of the energy system will be

calculated by dividing the CAPEX of the installed equipment of the energy system by

the energy produced over a 10 year. The levelized cost of water will be calculated for

the two highest performing collectors using the highest performing heat transfer fluid.

This will be calculated by dividing the CAPEX of the installed equipment of the

energy system by the total water produced over 10 years of operation.

23

4. Results and Discussion Solar Thermal Collector Comparison

The solar thermal collectors most viable for the operating temperature of 200 °C

were the parabolic trough (Absolicon T160) and XCPC (Artic Solar XCPC)

technologies. The maximum temperature data along with the energy cost for each

collector may be found in Table 4: Solar Thermal Collector Decision Matrix. The

maximum temperature comparison between the collection technologies is illustrated

in below:

Figure 2: Maximum Temperature Comparison

The maximum temperature of each solar collector is the maximum stagnation temperature of the solar collector. While this does not provide specific performance data at the operating temperature of 200°C, it provides a proxy for the highest temperatures at which the collectors can operate efficiently. The x-axis of this figure is calculated by dividing the unit cost of the system by the peak output under SRCC clear sky conditions. The statistics and full name of each collector may be found in Table 4: Solar Thermal Collector Decision Matrix.

ApricusVHP 10 VDF 20

Absolicon

Artic

SPP

0

100

200

300

400

500

0 500 1000 1500 2000 2500 3000

Max

Tem

pera

ture

(°C)

USD/kW

24

The Absolicon T160 and Artic Solar XCPC can achieve significantly higher

temperatures than the other solar collectors. This was expected as the concentrating

technologies of the two collectors allows for much higher operating temperatures.

The maximum recommended operating temperature for the Absolicon T160 is 175 °C

[34]. However, this maximum operating temperature is based upon pressurized steam

generation rather than a thermal oil as the heat transfer fluid. Other lab studies have

shown that the Absolicon T160 is capable of higher temperature operation (greater

than 200 °C) can achieve stagnation temperatures up to 460 °C [44]. The Artic Solar

XCPC on the other hand, is advertised to have capture efficiencies greater than 50%

when operating around 200 °C and has an estimated stagnation temperature of about

320 °C [35, 45]. While the Artic Solar XCPC maximum operating temperature may

be lower than the Absolicon T160, the Artic Solar XCPC may have greater operating

efficiencies. Power generation statistics need to be requested from both companies to

make a more informed decision about which collector has a greater energy output at

the process conditions since collector efficiency varies so much with temperature.

The collector power generation per amount of space that the collector takes up

is a critical parameter because it describes the transportability of the system. The

transportability of the system is very important because the desalination system will

be based upon a design that is easily moveable between areas with people

experiencing water insecurity. The comparison between power per radiated area—

which compares energy generation—is illustrated in Figure 3: Power Generation per

Radiated Area Comparison.

25

Figure 3: Power Generation per Radiated Area Comparison

The figure above illustrates the comparison between power generated and the space taken up by the solar thermal collector. The power used is the power generated under SRCC clear sky conditions during ideal operation (i.e. 1000 W/m2). The radiated area is the cross-sectional area at the largest cross-section of the collector. The dimensions and power data for each collector can be found in Table 4: Solar Thermal Collector Decision Matrix. The power generation per radiated area comparison further supports that the

Absolicon T160 and Artic Solar XCPC are the most viable solar thermal collector

candidates. The Absolicon T160 and Artic Solar XCPC had a power per radiated area

54% and 24% greater than the next best system (Apricus ETC-20), respectively. The

greater power per radiated area means that the Absolicon T160 and Artic Solar XCPC

would take up the least amount of space per energy generated. This is further shown

when comparing how many units would be needed to meet the energy demand of the

desalination unit (see Economic Analysis section calculations about number of units

needed) as explained in Figure 4: Collector Number Comparison.

Absolicon Artic Apricus SPP VDF 20 VHP 10

Pow

er p

er R

adia

ted

Area

(kW

/m2 )

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

26

Figure 4: Collector Number Comparison

The number of collectors was calculated based upon the energy output of each collector under conditions of 80% SRCC clear day rating for 8 hours of operation. This is plotted against the cost per unit, which is the collector cost. The collector cost along with the power generation of each collector may be found in Table 4: Solar Thermal Collector Decision Matrix. The solar thermal collectors with the smallest rectangular area between the axis and

point are the highest performing collectors in this comparison because the total cost

of the collection system may be calculated by multiplying the axis together. The

lowest total collection system cost of the collectors capable of efficient operation at

process temperatures are the Apricus ETC-20, Artic Solar XCPC, and Absolicon

T160. The total collection system cost of the Apricus ETC-20, Artic Solar XCPC, and

Absolicon T160 are $20,000, $20,700 and $14,000, respectively. The Absolicon T160

is the cheapest option by about $6,000 for the system.

While these statistics appear to point to a clear option, one important detail is

not captured by the statistics presented. Artic Solar XCPC is a passive collection

system while the Absolicon T160 requires a tracking system [34, 35]. The tracking

Apricus

VHP 10

VDF 20

Absolicon

Artic

SPP

0

10

20

30

40

50

60

70

80

$0 $500 $1,000 $1,500 $2,000 $2,500

Colle

ctor

s Nee

ded

at 8

0% P

eak O

utpu

t

Cost per Unit

27

system could pose huge maintenance and installation costs that are not included in

solar collector quotes. The primary advantage of the Artic Solar XCPC technology is

that the system can achieve efficient operation at high temperatures without a tracking

system. More technical information from each company about the cost of tracking

systems along with the power output at the operating temperature is needed before

deciding between the Artic Solar XCPC and Absolicon T160 technologies. In

addition to these solar collecting technologies, further exploration of the parabolic

dish collector from Oxford and parabolic trough SunTrap Receiver from Norwich

technologies is necessary before purchasing a solar thermal collector. Not enough

information was available publicly about these projects to compare them, but each

shows promise for efficient high temperature thermal collector operation. The

Harvard parabolic dish collector and Norwich SunTrap receiver are capable of

achieving temperatures greater than 500 °C and 750 °C, respectively. All information

found regarding these projects is included in Table 4: Solar Thermal Collector

Decision Matrix.

Heat Transfer Fluid Comparison

The heat transfer fluids included in the comparison are Duratherm S (silicone

oil), Duratherm 630 (paraffin oil), Therminol XP (mineral oil), and Therminol 68

(synthetic oil). The first comparison was based upon fluid performance in the

categories of safety, material compatibility, thermodynamic properties, heat transfer

properties, and fluid properties. The star diagram criterion can be found in Table 1:

Star Diagram Criterion. The comparison was made in the form of a star diagram to

compare some of the more qualitative categories in a more quantitative manner.

28

1

2

34

5Safety Rating

CompatibilityRating

ThermodynamicProperties

Rating

HT PropertiesRating

Fluid PropertyRating

Duratherm 630

HT PropertiesRating

Fluid PropertyRating

1

23

45

Safety Rating

CompatibilityRating

ThermodynamicProperties

Rating

HT PropertiesRating

Fluid PropertyRating

Therminol 68

HT PropertiesRating

Fluid PropertyRating

1

2

3

4

5Safety Rating

CompatibilityRating

ThermodynamicProperties

Rating

HT PropertiesRating

Fluid PropertyRating

Therminol XP

Figure 5: Heat Transfer Fluid Individual Star Diagrams

The heat transfer fluids Duratherm S (silicone oil), Duratherm 630 (paraffin oil), Therminol XP (mineral oil), and Therminol 68 (synthetic oil) were compared on the basis of safety, material compatibility, thermodynamic properties, heat transfer properties, and fluid properties. The star diagram criterion for each category may be found in Table 1: Star Diagram Criterion.

All four heat transfer fluids have good safety ratings because that was the

primary concern when designing the system. However, Therminol 68 has a lower

rating than the other three heat transfer fluids because it has an NFPA health rating of

2 rather than 0. For this reason, the other heat transfer fluids were more desirable

because there likely will not be chemical treatment facilities near where the

desalination unit will be deployed. This is a huge concern because improper disposal

of the heat transfer fluid could cause adverse health effects or environmental damage

if the substance is toxic (e.g. Therminol 68). The other somewhat esoteric but very

important category is the compatibility rating. The compatibility rating combines the

material compatibility and resistance to fouling or oxidation of each heat transfer

29

fluid. Duratherm S and Duratherm 630 especially excelled in that category because

both are compatible with standard heat exchanger materials and are very resistant to

fouling. Duratherm S is especially resistant to fouling and may be used in open tank

applications. Therminol XP also has low fouling, but is potentially corrosive to

copper piping since it is a mineral oil. No corrosivity data was available and

Therminol did not respond to an inquiry about the material compatibility of the heat

transfer fluid. Therminol 68 was much more prone to oxidation and required a

carefully monitored inert environment, which caused it to have a lower compatibility

rating. The thermodynamic properties, fluid properties, and heat transfer properties

help indicate which heat transfer fluid has the best overall performance.

Figure 6: Heat Transfer Fluid Combined Star Diagram

The heat transfer fluids Duratherm S (silicone oil), Duratherm 630 (paraffin oil), Therminol XP (mineral oil), and Therminol 68 (synthetic oil) were compared on the basis of safety, material compatibility, thermodynamic properties, heat transfer properties, and fluid properties. The star diagram criterion for each category may be found in Table 1: Star Diagram Criterion. The combined star diagram better illustrates the differences in performance

between Therminol XP, Therminol 68, Duratherm S, and Duratherm 630. Duratherm

630 clearly covers the most area on the star diagram, which indicates that it is a much

better candidate than the other heat transfer fluids as far as performance is concerned.

1

2

3

4

5Safety Rating

Compatibility Rating

ThermodynamicProperties RatingHT Properties Rating

Fluid Property Rating

Therminol 68 Therminol XP

Duratherm S Duratherm 630

30

Duratherm S fell short in the fluid, heat transfer, and thermodynamic properties

because of the general properties of silicone oil. The silicone oil forms vapor near

operating temperatures, has a high viscosity that decreases pumpability, and has a low

heat capacity. Conversely, Duratherm 630 has a high boiling point, very low

viscosity, and high heat capacity which make it the best candidate for a heat transfer

fluid. The specification sheet and NFPA rating state that Duratherm 630 is non-toxic

and non-reportable. However, the manufacturer classified Duratherm 630 as a

paraffin oil which seems unlikely given that most paraffin oils are more toxic. The

toxicology information should be verified with the company before installation. In

addition to the discussed heat transfer fluids, additional research should be conducted

to find out more information about Los Alamos National Labs CX-500 heat transfer

fluid. The fluid is stable up to 570 °C, has a low viscosity, and has a -40 °C gel point

[36]. CX-500 would be a great heat transfer fluid candidate because the fluid would

be capable of withstanding the higher temperatures associated with the Oxford and

Norwich solar thermal collectors. However, minimal information about this fluid was

available online during this review.

The economy of each heat transfer fluid was calculated on a per energy basis

(kJ). The comparison by a per energy basis eliminates the bias of fluid properties

when comparing the heat transfer fluids. A per energy basis provides a proxy for

comparing all fluids with the same basis of energy needed for the desalination

process. The energy cost of each heat transfer fluid was calculated with the same

initial (200 °C) and final operating temperature (195 °C) in the heat exchanger that

transfers energy to the air stream in the desalination process.

31

Figure 7: Heat Transfer Fluid Cost Comparison

The heat transfer fluids Duratherm S (silicone oil), Duratherm 630 (paraffin oil), Therminol XP (mineral oil), and Therminol 68 (synthetic oil) were compared on the basis of the cost per unit energy. The energy basis outlined in the Heat Transfer Fluid Comparison Criterion section was used. Thermal properties are based upon specifications provided by the manufacturers at 200 °C. The performance information for each heat transfer fluid is available in Table 5: Heat Transfer Fluid Decision Matrix. Pricing was not available for Therminol 68, so it was assumed to be that of Therminol XP. Duratherm 630 also outperforms the other heat transfer fluids in energy economy.

Duratherm 630 has an energy cost 25% lower than the next cheapest heat transfer

fluid (Therminol XP). Further comparison was performed to analyze which heat

transfer fluid was the most spatially efficient. The amount of heat transfer fluid

needed for energy demands is outlined in the in Heat Transfer Fluid Comparison

Criterion section. A lower volume of heat transfer fluid is desirable because it will

drive down shipping costs and the tank volume needed for the system. This is

especially important in a mobile desalination unit because space is very limited.

$0.00

$0.50

$1.00

$1.50

$2.00

$2.50

$3.00

Therminol 68 Therminol XP Duratherm S Duratherm 630

Cost

per

kJ o

f Ene

rgy

32

HT Fluid Cp 2.2 J/g°C 2.6 J/g°C 2.034 J/g°C 2.539 J/g°C

HT Fluid Flow Rate 1.964 kg/s 1.662 kg/s 2.124 kg/s 1.701 kg/s

HT Fluid Density 898 kg/m3

761 kg/m3

811 kg/m3

741 kg/m3

Time of Dark Operation 0.5 hr 0.5 hr 0.5 hr 0.5 hr

Mass of Fluid Needed for Storage 3535 kg 2991 kg 3823 kg 3063 kg

Volume of Fluid Needed for Storage 3.94 m3

3.93 m3

4.71 m3

4.13 m3

Total Cost of HT Fluid $30,437 $30,391 $99,622 $22,656

Duratherm S Duratherm 630HX Fluid Volume Needed

Therminol 68 Therminol XP

Table 2: HX Fluid Volume Needed

The table above is a comparison between the heat transfer fluids in regards to mass, volume, and cost. The mass of fluid needed for storage was based upon the heat transfer fluid flow rate, which was based on the energy demand outlined in the Heat Transfer Fluid Comparison Criterion section. The energy demand was calculated based upon water generation. The energy demand was then used to calculate the heat transfer fluid mass flow rate needed by dividing the energy demand by the heat capacity and operating temperature change of the fluid in the heat exchanger (5 °C). All material properties are that at 200 °C and were provided by manufacturers in technical data sheets. The volume of fluid needed was calculated on the basis of running for 30 minutes without light.

Therminol XP is more spatially efficient than Duratherm 630 because it requires 0.2

m3 less of fluid. While this difference may seem trivial, this is the equivalent to about

one 55 gallon drum, which takes up a lot of space. However, Duratherm 630 is

significantly cheaper than Therminol XP and the other heat transfer fluids, which

makes it a more favorable choice for a heat transfer fluid.

Economic Analysis

The solar thermal energy system must be capable of generating fresh water at

a competitive price based upon the energy demand and other assumptions outlined in

the Economic Analysis section. The price of water was calculated for the two most

economical collectors: the Absolicon T160 and Artic Solar XCPC. The heat transfer

fluid used was also the highest performing and most economical option: Duratherm

630. The cost of water for each collector based upon the entirety of the system is in

Table 3: System Cost.

33

Table 3: System Cost

This table is an economic summary of the system over a 10-year period. The pump was sized using methods for a single stage centrifugal pump outlined in Towler et al. [42]. The solar thermal collectors and heat transfer fluids were assumed to have 10 and 5 year lifetimes, respectively. The method for costing the fluid, the method for costing other components, and other assumptions may be found in the Economic Analysis section.

The Absolicon T160 is a more economical option and also occupies less space than

the Artic Solar XCPC system from this analysis. However, the Artic Solar XCPC has

a higher recommended operating temperature, which may indicate that the Artic Solar

XCPC system. More research also needs to be conducted about the supports of each

system, since the Absolicon T160 requires a tracking system and the Artic Solar

XCPC does not.

The energy cost was comparable to that of other solar thermal energy systems.

The levelized CAPEX per unit of energy for the Absolicon T160 and Artic Solar

XCPC systems were $0.049 and $0.055, respectively. The cost for the Absolicon

T160 collector might be higher since the tracking system is not included in the

collector cost. This cost is extremely competitive; however, transportation and

operational costs may lead to an increase in the energy cost. The levelized cost of

water for the energy collection sub-system is $6.34 per cubic meter of water for the

Absolicon T160 and $7.11 per cubic meter of water for the Artic Solar XCPC. This is

Number of Units NeededCost of Collector

Total Cost of Solar CollectorsPump Cost

Total Cost of FluidEnergy Over 10 years (kWh)

Cost of TankCost of piping, valves, and fittings

Cost of electrical and instrumentationTotal Installed Cost

Levelized Energy Cost of Energy System CAPEX per kWh

Levelized Cost of Energy System CAPEX per Cubic Meter of Water

$6.34 $7.11

7 18

$346

$4,938$148

$0.049 $0.055

$72,049 $80,759

System Cost with Duratherm 630

$14,000 $20,700

Absolicon T160 Artic Solar XCPC

$2,000 $1,150

$1,136$45,3111472256

34

$0.00

$1.00

$2.00

$3.00

$4.00

$5.00

$6.00

$7.00

$0

$10,000

$20,000

$30,000

$40,000

$50,000

0 20 40 60

Leve

lized

Cos

t of W

ater

per m

3

HT Fl

uid

Cost

Ove

r Life

time

HT Fluid Temperature Change (°C)

not a very competitive price for water at a mass production scale, but is reasonable for

the smaller desalination system at hand. The primary cost driver of the system was

the heat transfer fluid cost, which accounted for more than 60% of the CAPEX over a

10 year period. The cost of heat transfer fluids may be lower since the cost of heat

transfer fluid is a based upon a 55 gallon basis and the total volume of heat transfer

fluid purchased would be about 1100 gallons.

A sensitivity analysis of the operating temperature difference of the heat

transfer fluid in the heat exchanger was performed to see how the parameter affected

the total cost of heat transfer fluid as well as the levelized cost of water production.

The temperature change was varied between 5 °C and 65 °C for a system using the

Absolicon T160 collectors and Duratherm 630 heat transfer fluid (Figure 8).

Figure 8: HT Fluid Temperature Change Sensitivity Analysis

The temperature change in the heat exchanger of the heat transfer fluid was varied to see the effect on the total cost of heat transfer fluid over the lifetime of the system as well as the levelized cost of water. The system analyzed utilized the Duratherm 630 heat transfer fluid and Absolicon T160 collector. The triangles represent the levelized cost of water and the circles represent the total cost of heat transfer fluid over the lifetime of the system. The costing methodologies may be found in the Economic Analysis section.

35

The operating temperature change of the heat transfer fluid had a profound effect on

the levelized cost of water and heat transfer fluid total cost. Changing the temperature

change from 5 °C to 15 °C dropped the levelized cost of water and heat transfer fluid

cost over the system lifetime by 47% and 67%, respectively. The levelized cost of

water and heat transfer fluid cost at 15 °C is $3.36 and $15,104, respectively.

However, operating temperature changes beyond 15 °C may not be feasible since the

solar thermal collectors may not be able to heat the fluid fast enough during collection

[43]. A more in depth analysis involving the solar thermal collector companies needs

to be performed to assess the feasibility of different operating temperature changes.

36

Conclusions and Recommendations

This analysis was meant to serve as a comparison between available solar

thermal and heat transfer fluid technologies on the market as well as provide a rough

economic analysis of the most optimal combination of technologies. The designed

system will use a paraffin heat transfer oil in combination with parabolic trough

collecting technology.

Out of the solar collectors examined, either the Artic Solar XCPC or

Absolicon T160 should be implemented in the system. The benefits of the Artic Solar

XCPC unit is that it is a passive collector and proven to be efficient at high

temperatures operation. The strengths of the Absolicon T160 is that the system is

more cost and space efficient, which helps drive down the cost and size of the

collection system. Further research needs to be conducted about the cost of the

trackers that the Absolicon T160 would require. In addition to that, more information

about the Oxford parabolic dish collector and SunTrap absorber should be obtained

before making a final decision about the solar collector used in the system.

Two other heat transfer fluids are recommended to further pursue as viable

candidates. These heat transfer fluids are Duratherm G and Los Alamos National

Labs CX-500. Duratherm G is a high temperature glycol mixture capable of operation

up to 260 °C while CX-500 is a nanofluid capable of operation above 500 °C.

Duratherm G could be a viable candidate because glycol mixtures have large fluid

densities, high heat capacities, high resistance to oxidation, high thermal

conductivities, low corrosivity, and are relatively non-toxic. Research about the CX-

500 heat transfer fluid should be pursued if the SunTrap or parabolic trough

concentrator from Oxford are used for solar collection. CX-500 would allow for

37

greater operating temperatures, which has the potential to increase the operating

temperature of the air stream of the desalination system. A greater temperature in the

air stream would increase the amount of water that can be atomized, which would

increase water production. However, Duratherm 630 is currently recommended for

because it was the most economical fluid found in this preliminary analysis.

The optimized energy cost and levelized cost of water were $0.026 per kWh

and $3.36 per cubic meter of fresh water generated, respectively. The cost of water is

likely an overestimate and more research needs to be conducted to determine the

required volume and lifetime of the heat transfer fluid. Once the tank has been sized

for the system and an operating temperature change is determined, a better estimate

for the heat transfer fluid required and the cost of collectors can be obtained to further

drive down the estimated cost of water production. An analysis of the water

generation rate versus the operating temperature should also be conducted. A lower

operating temperature would increase the energy efficiency of the system because

collector efficiency generally decreased as temperatures increased. In addition to that,

a feasibility analysis of the operating temperature difference should be performed to

see what the optimal temperature change of heat transfer fluid is. The amount of heat

transfer fluid had a profound effect on the CAPEX of the project as it accounted for

more than 60% of the installed costs. Finding an optimal operating temperature

change in the desalination heat exchanger can reduce the amount of heat transfer fluid

needed. A typical temperature difference between the inlet and outlet of a collector is

about 15 °C, so heat exchanger operating temperature changes for the heat transfer

fluid between the inlet and outlet should not be significantly greater than that [46]. A

more in depth analysis involving the solar thermal collector distributors will need to

38

be performed to assess system performance at different operating temperature

changes.

A FMEA and operational feasibility analysis should be conducted before the

system design is finalized. One primary objective of the desalination project is to

have local governments in underdeveloped countries to be operating this technology.

Safe and easy operation of the systems is paramount to ensure the safety of operators,

reduce the risk of water contamination with process chemicals, and reduce reliance on

technical aid.

39

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44

Appendix A – Desalination System Figure 9: Desalination System The system utilizes a humidification-dehumidification cycle to extract the solids out of a saltwater stream to produce fresh water. Dry air is fed into a converging nozzle which compresses while solar thermal energy is used to increase the temperature of the air. The hot, dry air is then used to atom saltwater, which is fed into a cyclone that separates out the salt particles. The warm humid air is then fed into a condenser which is cools the stream to condense water out of the air. The air and condensed water stream is fed into an air-water separator that separates the dry air and fresh, liquid water. The dry air recycles back into the dry air converging nozzle section while the fresh, liquid water is fed into a heat recuperator that preheats cool saltwater before it is fed into the atomizing area.

45

General Specific Model Identifier on Graph Positives Negatives Dimensions/

MaterialRadiated

Area (m2)Performance Cost Peak Output

(kW)

Optimum Operating

Range (°C)

Max Temperatur

e (°C)

Energy Cost $/kW

Power per Radiated Area

(kW/m2)

Evacuated Tube Collector

Company that has many mounting options as well as

solar collection systems. Could purchase manifold then purchase

higher temperature evacuated tubes for the system.

Apricus ETC-20 Apricus Many different options for mounting and systems.

Seems to have a low operating temperature. No specification of

max operating temperature. Could purchase manifold then

buy evacuated tubes separately.

2.005m X 1.496m X 0.136m. High

purity copper pipes with aluminum heat

transfer fins.

3.00

1342 W for the ETC-20 which is 1.89 m2. Max stagnation

temperature of 228°C. Recommended flow rate of 1.4

L/min.

$1,000 1.342 0-120 *** 228 745 0.447

Evacuated Tube Collector

Seems like a cheaper version than Apricus. Seems to be about the same quality of product with

worse customer service.

SunMaxx Solar VHP10 VHP 10Very cheap. Compact.

Different sizes and mounting options.

Max stagnation temperature lower than 250°C.

37.5" X 79". 88 lbs. Water or glycol operating fluid.

1.91

14 kBTU/panel/day SRCC Clear C Rating. Max stagnation

temperature of 220°C. 0.2-3 GPM. Storage tank: 30-40 gal.

$93 0.342 0-100 *** 220 272 0.179

Evacuated Tube Collector

Seems like a cheaper version than Apricus. Seems to be about the same quality of product with worse customer service. Direct

flow pipe.

SunMaxx Solar VDF20 VDF 20Very cheap. Different

sizes and mounting options.

Max stagnation temperature lower than 250°C.

60" X 64.7". 115 lbs. Water operating fluid. Copper pipe and aluminum fins.

2.50

22.8 kBTU/panel/day SRCC Clear C Rating. Max stagnation

temperature of 220°C. 0.2-3 GPM. Storage tank: 30-40 gal.

$1,600 0.567 0-120 *** 220 2822 0.226

Evacuated Tube Collector

10 year warranty. Variety of solar thermal collectors and

storage units. SRCC certified.Solar Panels Plus SPP-25 SPP

Seems like a very reliable company with a long

warranty. Higher stagnation temp.

Not very efficient at 250°C operation. May work better if

different HX fluid used.

81" x 79". 183 lbs. Copper tubing. Stainless steel

frame. Rock wool insulation.

4.13

6000-39000 BTU/ft2/day. Max stagnation temperature of 250

°C. 0.8-1.2 GPM recommended.

$1500 *** 0.95 0-150 *** 250 1579 0.230

Parabolic Trough

Collector

Used in a variety of industries and research

papers. Seems very promising for high

temperature applications. Used in many large scale

thermal plants.

Absolicon T160 Absolicon

Capable of producing steam at 160°C. Likely

can produce 250°C fluid. One of highest efficiency

concentrators.

Not many technical specs. Need more information.

5.49m X 1.056m. 148 lbs. Tyfocor

heat transfer fluid.5.80 430-4200 W per collector from

third party tests. $2000 *** 4 0-175 460 500 0.690

Parabolic Trough

Collector

Seems like a very promising technology. Very organized

company transparent with costs. Excel sheets available with cost

breakdown for installation. Project sizing calculators and

tools available.

Artic Solar XCPC ArticGreater than 50%

efficiency in excess of 200°C.

Not too much specific information about energy

generation I found.

2.208 m x 1.221 m x 0.295 m 2.70

Depends on project set up. Greater than 50% efficiency at

temps upward of 200°C.$1,150 1.5 150-200 320 767 0.556

Parabolic Trough Panel

Collector

Norwich Technologies President, Joes Stettenheim:

stettenheim@norwitech.com 802-384-1333

Los Alamos NL x Norwich Technologies x

US Department of Energy

Extremely high temperature application (750°C). Funded project

with department of energy. Collaboration?

Not much information available No information available

No information available No information available No information

availableNo information

availableNo information

available 750 No information available

No information available

Parabolic Dish Collector Harvard project Parabolic Dish Collector

Extremely high temperature application

(greater than 500°C)Not much information available No information

availableNo information

available No information available No information available

No information available

No information available 500 No information

availableNo information

available

Appendix B – Solar Thermal Collector Decision Matrix Table 4: Solar Thermal Collector Decision Matrix

The table above is a summary of the information collected when researching each solar collector. IT also includes the comparison statistics used in the analysis. Any number with “***” indicates that the cost is an estimate based upon similar technologies. All underlined text represents a hyperlink. The technical data may be found from the following sources: [30–37, 44, 45, 47].

46

Appendix C – Heat Transfer Fluid Decision Matrix Table 5: Heat Transfer Fluid Decision Matrix

The star diagram criterion is summarized in Table 1: Star Diagram Criterion. The safety section describes how toxic, flammable and reactive the substance is. This is quantified by summing the health, flammability, and reactivity section of the NFPA rating. A lower NFPA score indicates a less hazardous material. The compatibility section quantifies the corrosivity and fouling/oxidizing tendency of the heat transfer fluid. The thermodynamic properties illustrate the operating range of the fluid which includes the freezing point, boiling point, and point at which the substance is unstable. The heat transfer properties section is a way of illustrating the heat transfer performance of the fluid. This includes the heat capacity and thermal conductivity. The fluid properties section includes the density, kinematic viscosity, and thermal expansion coefficient of the fluid. Greater scores in each category indicate higher performance in that area. The cost of each heat transfer fluid will be calculated based upon an energy basis. The purpose of this is to take out factors in density when calculating the cost of each fluid because the cost per unit energy is a more telling parameter of economic performance. The cost of each fluid is calculated by dividing the cost per unit volume by the density, heat capacity, and operating temperature change. The operating temperature change—5 °C—is the temperature change that the heat transfer fluid will experience in the heat exchanger that heats the air used in the desalination process. A lower energy cost is desirable because that indicates less money needs to be spent on heat transfer fluid in the process. The technical data may be found from the following sources: [48–52].