Description of Solar Thermal Power and Its applications for the future.

1. Solar Thermal Power: Opportunities and
Obstacles
EE351
Spring 2014
PREPARED BY:
Trigg Ruehle
PAPER DUE DATE: April 24, 2014

2. 2
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.

3. 3
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]

4. 4
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

6. 6
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

7. 7
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.

8. 8
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]

9. 9
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

10. 10
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]

11. 11
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

13. 13
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

14. 14
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.

15. 15
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
Press. Web. 22 Apr. 2014
[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.

16. 16
[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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