Solar Thermal Power: Opportunities and Obstacles

EE351

Spring 2014

PREPARED BY:

Trigg Ruehle

PAPER DUE DATE: April 24, 2014

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ABSTRACT/EXECUTIVE SUMMARY

Solar thermal energy is a way to harness the thermal energy from the radiant light from the sun

to be used for many different applications: Heating air, heating water, cooking, distillation,

ventilation, drying, and power generation. Solar thermal power works much like a steam cycle in

a plant. It magnifies light using lenses or mirrors to heat water to steam to either run a turbine or

generator. Higher temperatures come with more problems as normal materials can’t be used.

New technologies using salt solutions at these high temperatures can make these plants

economically feasible to produce power for the future.

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INTRODUCTION/BACKGROUND

Solar energy is one of the most widely available energies in the world. Radiant light from

the sun can be used in many different applications. Solar energy can be captured in many

different ways from collectors to panels. There are two types of ways that radiant energy can be

captured either passive or active solar capture. Active capture is the use of solar panels to capture

and convert this radiant light into a usable energy by heating a working fluid to power a turbine

or by the photovoltaic effect. Passive solar capture is when spaces are designed or used to

naturally circulate the radiant energy to be used in circulating air in buildings and other designs.

Photovoltaic cells work by irradiating a semiconductor and creating electricity from the

light irradiated upon it. These cells generate electrical power by directly converting the radiation

from the sun into a direct current. The photovoltaic effect is defined as the process of converting

solar energy into a usable from of electricity through exciting electrons in a semiconductor.

When radiation in the form of light reacts with the material electrons in the semiconductor

become excited from the thermal energy and are released from their valence band and are thrown

into the conduction band where they become free electrons. These very excited electrons become

accelerated which creates an electromagnetic force that is created as the light irradiates the

surface. These highly excited electrons are discharged from the surface of the material into the

other half of the semiconductor which creates an electric charge and current. As seen below as

the light energy is harnessed the electrons flee from the surface as they are excited. Exciting

these electrons creates electrical energy that can be harnessed and used for powering many

different devices and other applications. This excited energy is then coupled to an external load

that requires a current and voltage. This current and voltage is mostly due to the electrons that

become excited and move through the band. [1]

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 Figure  1:  Photovoltaic  Cell  Diagram  [2]

Photovoltaic cells can be used in many different environments and many different applications.

Solar thermal energy works much like photovoltaic cells in that they convert radiant light into

usable energy in the form of heat. Photovoltaic cells are completely different than solar thermal

energy in that they don’t take advantage of the photovoltaic effect but harness that actual heat in

the light. In the following sections solar thermal energy will be discussed.

There are three types of solar thermal collectors that work at much different temperatures: Low-

temperature solar thermal systems, Medium-temperature solar thermal systems, and High-

temperature solar thermal systems. Low-temperature collectors are usually flat plates that do not

concentrate the light but rather take on the original form of the light. These collectors are used in

applications where high temperature heat is unnecessary and range from 50 to around 100

degrees Celsius. [3] Some applications for them can be heating spas and pools, heating air for

commercial or residential use, and heating water for residential and commercial use. As seen

below is a low-temperature collector that uses the suns radiant light to be used to heat a working

fluid in this case water. [3]

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 Figure  2:  Low-­‐temperature  collector  on  a  house  [4]

First the radiant energy comes from the sun and is absorbed on the flat plate solar collector. A

glycol solution with a high heat transfer rate is pumped underneath the panels absorbing the

radiation given off by the radiant light. Since this glycol has such a high heat transfer rate it

absorbs almost all of the heat from the radiant light. This glycol is then pumped using a pump to

become in contact with water to be used in heating the house. A heat exchangers exchanges the

heat from the glycol solution into the potable water. This water is then put into a conventional

water heater where it is mixed with cold water to achieve the desired temperature. The

differential controller and the pump can all be run off about 9KW of electricity a month or about

a dollar. [5] If they are coupled with a 12V battery much of the power from the pump and

differential controller can be offset and used off the grid. This heated water can be used in

heating the house as well as for hot water for things like bathing, cooking, and cleaning. Glycol

has a very long storage life and since it is never in direct contact with the water it can be used for

up to 30 years without needing to be replaced. [6] These low-temperature collectors can play a

huge role in third world countries where electricity is not easily available. In developing

countries like Africa with intense sunlight there is a huge opportunity to use these systems for

bathing, cleaning, and for other hot water applications. This would allow these people in these

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poor countries to be provided with the hot water necessary for their day-to-day activities. Other

examples of low-temperature collectors have been used all the way back to the Romans. The

Romans would use huge chimneys on their buildings which when heated by the sun would cause

air circulation throughout the building cause by the different air densities. These principles are

still being used as attic solar fans are being implemented to keep air circulating throughout your

house while not over burdening the HVAC units. The oldest example of low-temperature

collectors can be found in the Dead Sea where basins were used to dry seawater to harness sea

salt to be used in preserving meat. [7] Solar drying can play a huge role in the drying of wood

products, meat products and other agriculturally products. Drying these products by the use of a

solar collector can effectively drop the prices of these products. Since solar energy is readily

available it can play a huge role in the prices of dried meats and other agricultural products. Solar

drying is also very environmentally friendly and can play a big role in eliminating green house

gases as industrial strength dryers are not used which generate greenhouse gases. In many third

world countries solar drying is used in lieu of industrial dryers.

Medium-temperature collectors can be used in applications where temperatures from 100

degrees Celsius to 200 degrees Celsius are needed such as drying, cooking, and other industrial

applications. Solar cookers need high heat in order to sterilize and cook the products being

consumed. Solar cooking plays a huge role in environmental quality as it uses less firewood that

in turn will help reduce green house gases by capturing carbon in the atmosphere. Solar cooking

also eliminates harmful smoke and other inhalants that would be created from the combustion of

firewood. [8]

High-temperature collectors are a little more complicated and can be used in applications where

much higher temperatures are needed such as powering a turbine, drying processes, sterilization,

and power generation. These high-temperature collectors need a lot of bright sunlight to

effectively be able to produce the intensity so in places with low levels of sunlight this

technology is virtually unfeasible. Mirrors and lenses are used to intensify the light to achieve

high temperatures needed for power generation applications. This intensifying of the radiant light

is known as CSP or Concentrated Solar Power. Concentrating this light helps reduce the land

area used for collectors which helps reduce the environmental impact. Shown below is the layout

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of a typical Concentrated Solar Power Plant. The plant below incorporates a tower to intensify

the light which will be explained in detail in the following section.

 

Figure  3:  Typical  CSP  Layout  [9]  

 

There are many different ways that power can be produced using light radiation. In the following

sections photovoltaic cells were introduced to create a charge from the excitement of electrons

through a semiconducting material. The energy produced in this process is not nearly enough to

power huge machinery or be feasible in industrial applications. Using High-temperature

collectors much more energy can be harnessed thorough this concentration of heat. These

Concentrated Solar Power plants work by concentrated the radiant light to extremely high

temperature using many different techniques that will be discussed further. As the temperature of

the working fluid increases more power generating applications may be used. Temperatures

generated up to 600 degrees Celsius can be used to power a steam turbine which can then power

a generator to create usable electricity. The efficiency of the steam turbine is related directly to

the operating temperatures and is usually around 41%. [10] There are different ways that this

light can be concentrated to achieve the necessary temperatures needed to power a steam turbine.

As shown below curved parabolic troughs can be used to concentrate this light.

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 Figure  4:  Parabolic  trough  plant  layout  [11]

These parabolic troughs are curved to concentrate this light onto a glass tube that contains some

sort of liquid usually glycol because of its high heat transfer properties. This glass tube is

positioned at the focal point to allow maximum concentration of light. These troughs are

positioned east to west and can tilt to allow maximum insolation from the sun. This heat transfer

fluid can be made up of many different solutions: glycol, salt solutions, water, and even oil.

Using a fluid with the highest heat transfer rate is effective to be able to store and transfer the

heat produced from magnifying the sun’s rays. This heated fluid is then sent to a heat exchanger

which transfers the stored heat to either a gas or liquid water turned to steam. This steam or gas

is then sent to a turbine that runs a generator that creates electricity from the work of the turbine.

[12] These plants consist of man different troughs connected to maximize heat transfer. Shown

below is a typical setup of the trough systems.

 Figure  5:  Trough  system  layout  [13]

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Another way to superheat a heat transfer fluid is with the use of solar power towers also known

as heliostat power plants. This is a type of solar furnace that uses movable mirrors to focus the

sun’s rays on the tower itself in a collector. This resulting intense ray of light is used to heat

usually water to a steam to then power a turbine. Shown below is a typical layout of a heliostat

power plant. The image below uses a molten salt solution that can effectively be used with

temperatures up to 1050 degrees Celsius. [14] A heat engine has a higher efficiency the hotter

the working fluid gets so using a fluid with the highest heat transfer rate will increase the

efficiency of the cycle resulting in more power produced.

 Figure  6:  Heliostat  Power  Plant  [15]  

As shown the collector field focuses the rays to a tower which superheats the salt solution. This

salt solution can either be stored in a thermal storage system to be used at night or at a different

time or can be sent to a heat exchanger. At the heat exchanger this molten salt transfers heat to

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water to be used to power a turbine. A reheater can be used to reheat the steam which is still hot

after exiting the turbine to be used to power another turbine resulting in a higher efficiency. This

One major advantage of using solar towers versus parabolic troughs is the increase in

temperature. Thermal energy can be converted to electricity much easier at higher temperatures

with a much greater efficiency. These higher temperature fluids can also be stored longer and

cheaper than storing a fluid at a lower temperature. NREL did a study that showed the levelized

cost of these two systems in 2020 if the prices of their materials are decreasing at the present

rate. It was found that the cost drastically increased from 5.47 cents per kilowatt-hour for

heliostat power plants to 6.21 cents per kilowatt-hour for parabolic trough power plants. It was

also found that the capacity factors were much different between these two systems due to the

difference in operating temperatures. Since the operating temperature of the heliostat power plant

is much hotter it was found to have a capacity factor of around 72.9% compared to a capacity

factor of 56.2% for the parabolic trough power plants. [16]

Another way that power can be produced from radiant light is through the use of a CSP-Stirling

system. A dish is used to concentrate light much like a solar tower. This intense beam of light is

used to heat a working fluid which then powers a Stirling engine. Shown below is a good

example of what a Stirling dish looks like.

 

Figure  7:  Stirling  Dish  [17]

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Concentrated Solar Power can play a huge role in power generation in places where there is a lot

of sunlight like the Mojave Desert and other deserts on earth. These plants were first introduced

in the 1980’s when energy was a major concern and gas prices were on the rise. The largest

Concentrated Solar Power plant is the SEGS power plant in the Mojave Desert in California.

This plant is rated at 377 MW. [18]

CONCLUSIONS

Solar thermal energy not only can help power the future but also help the environment. Burning

coal and other fossil fuels emit not only green house gases but create many other emissions that

can create health hazards. Nuclear power is clean and efficient but creates harmful radioactive

byproducts that can potentially be used for nuclear weapons. There is also numerous health

hazards that come nuclear power. Solar thermal power is very clean and readily available in

many parts of the country. Much of the southern portion of the country is covered in heavily

solar irradiated areas that can benefit greatly from solar thermal energy.

When looking at the levelized cost of solar thermal energy it is much higher than other forms of

energy. Solar power plants are very costly compared to other sources of energy due to the high

prices of their materials. Nuclear and coal fired plants can be used around the clock and are

independent of the sun but still have a high initial investment. These solar thermal plants heavily

rely on the sun. If the sun isn’t out and there is no thermal storage then these plants cannot

produce energy. One main difference between these technologies is that there are no

transmission lines between the source and the plant. The sun irradiates all the energy needed for

the power production. In nuclear plants and fossil fuel burning plants containment is necessary to

harness the energy and transport the energy. In the developing world this can be very useful in

creating usable energy, as there is not a huge energy infrastructure. As shown below the cost of

solar thermal power is pretty high even with predictions in price drops of materials. These

predictions are usually very conservative.

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                                             Figure  8:  Various  LCOE  of  Energy  Sources  [19]

The future of solar thermal plants lies heavily on discovering cheaper materials to be used in

heliostat power plants as well as parabolic trough plants. With cheaper and more durable

materials the levelized cost of these systems can decrease dramatically and become economically

feasible to produce energy. These materials will allow the efficiencies of these cycles to

drastically increase. Shown below is how much the efficiencies of these plants can increase with

respect to the temperature of the working fluid. This fluid temperature relies heavily on keeping

the fluids insulated and using isothermal materials that as of right now are very expensive. These

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graphs were calculated using the Carnot efficiency equation 1- Tc/Th. Tc being the temperature

of the cold reservoir and Th being the temperature of the working fluid.

 Figure  9:  Plant  Efficiency  versus  Working  Fluid  Temperature  [20]  

 

Solar  power  plants  are  very  inefficient  compared  to  other  sources  of  energy  due  to  the  high  prices  of  

their  materials.  Nuclear  and  coal  fired  plants  can  be  used  around  the  clock  and  are  independent  of  the  

sun.  These  solar  thermal  plants  heavily  rely  on  the  sun.  If  the  sun  isn’t  out  and  there  is  no  thermal  

storage  then  these  plants  cannot  produce  energy.  One  main  difference  between  these  technologies  is  

that  there  are  no  transmission  lines  between  the  source  and  the  plant.  The  sun  irradiates  all  the  energy  

needed  for  the  power  production.  In  nuclear  plants  and  fossil  fuel  burning  plants  containment  is  

necessary  to  harness  the  energy  and  transport  the  energy.  In  the  developing  world  this  can  be  very  

useful  in  creating  usable  energy,  as  there  is  not  a  huge  energy  infrastructure.    

 

Another  huge  effect  on  the  efficiency  of  these  solar  thermal  plants  is  new  working  fluids.  With  the  use  of  

super  high  heat  transfer  fluids  the  efficiencies  of  these  plants  can  dramatically  increase.  One  new  

technology  that  is  aiming  at  increasing  solar  power  efficiency  is  suspending  nano-­‐particles  within  the  

working  fluid.  These  nano-­‐particles  can  absorb  light  much  more  efficiently  than  the  working  fluid  alone.  

Another  way  that  efficiency  of  these  plants  can  increase  is  coupling  photovoltaic  cells  into  the  mirrors  

and  heliostats.  Instead  of  just  reflecting  and  focusing  the  light,  these  mirrors  can  also  use  the  

photovoltaic  effect  to  create  some  energy  from  this  light  that  can  be  used  in  powering  the  pumps  and  

other  devices  used  in  transferring  the  working  fluid.    New  salts  can  be  used  to  create  a  hotter  working  

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fluid  that  can  be  used  to  generate  much  higher  temperatures  for  the  turbines.    Using  new  salts  can  

effectively  lead  to  longer  storage  times.  With  longer  storage  times  and  less  heat  escaping,  Solar  thermal  

power  can  be  used  during  times  of  bad  weather  and  in  times  where  other  energy  sources  are  not  

economically  feasible.  New  storage  tanks  to  store  the  molten  salt  can  effectively  lead  to  a  more  efficient  

plant.  The  use  of  more  isothermal  materials  can  lead  to  much  higher  efficiencies.    Overall  solar  thermal  

energy  can  play  a  viable  role  in  energy  production  in  the  future  as  prices  for  the  materials  become  more  

economical.  As  new  technologies  create  hotter  working  fluids  that  can  be  stored  more  effectively  solar  

thermal  power  can  power  many  cities  in  the  future.    

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BIBLIOGRAPHY/REFERENCES

[1] Luque, Antonio and Hegedus, Steven (2003). Handbook of Photovoltaic Science and

Engineering. John Wiley and Sons. ISBN 0-471-49196-9.

[2] Principles of Electricity Generation by Photovoltaic Cells. Nisshin Electric Co., Ltd.

[3] Norton, Brian (2013). Harnessing Solar Heat. Springer. ISBN 978-94-007-7275-5.

[4] Solar Fuels and Artificial Photosynthesis. Royal Society of Chemistry 2012

http://www.rsc.org/ScienceAndTechnology/Policy/Documents/solar-fuels.asp Web. 22 Apr.

2014

[5] Bradford, Travis (2006). Solar Revolution: The Economic Transformation of the Global Energy Industry. MIT Press. Web. 22 Apr. 2014.

[6] Mills, David (2004). "Advances in solar thermal electricity technology". Solar Energy

[7] "Design of Solar Cookers". Arizona Solar Center. Web. 22 Apr. 2014

[8] Smil, Vaclav (2003). Energy at the Crossroads: Global Perspectives and Uncertainties. MIT

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[9] "Solar Energy Technologies and Applications". Canadian Renewable Energy Network. Web.

22 Apr. 2014.

[10] "Advantages of Using Molten Salt". Sandia National Laboratory. Web. 22 Apr. 2014.

[11] Wong B., Thornton J. (2013). Integrating Solar & Heat Pumps. Presentation. Renewable Heat Workshop. Web. 22 Apr. 2014  

[12] Energy and Environmental Analysis (2008). "Technology Characterization: Steam

Turbines" (PDF). Report prepared for U.S. Environmental Protection Agency. Web. 22 Apr.

2014.

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[13] United States Department of Energy: http://www.nrel.gov/solar/parabolic_trough.html Web.

22 Apr. 2014.

[14] "Solar Process Heat". Nrel.gov. 2013-04-08. Web. 22 Apr. 2014.

[15] "California's First Molten Salt Solar Energy Project Gets Green Light." Inhabitat

Sustainable Design Innovation. 16 Dec. 2010. Web. 22 Apr. 2014.

[16] "Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts". Nrel.gov. 2010-09-23. Web. 22 Apr. 2014.  

[17] Solar Stirling Dish. http://www.wapa.gov/es/pubs/esb/1998/98Aug/at_solargen.htm Web.

22 Apr. 2014.

[18] "Google's Goal: Renewable Energy Cheaper than Coal.” November 27, 2007. Google.com.

Web. 22 Apr. 2014.

[19] "LCOE Of New Electricity Generating Technologies." IER. Institute for Energy Research, n.d. Web. 22 Apr. 2014.

[20] Çengel, Yunus A., and Michael A. Boles. "6-7." Thermodynamics: An Engineering Approach. 7th ed. New York: McGraw-Hill, 2011. Web. 22 Apr. 2014.

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