Solar Concentrator

1. Concentrating Collectors

2. Table of Contents
1.Introduction.......................................................4
2.Concentrating collectors...................................5
3.Types of concentrating collectors.....................6
3.1. Parabolic trough system.............................7
3.2. Parabolic dish system.... ...........................11
3.3. Power tower system...................................14
3.4. Stationary concentrating solar collectors....16

3. 4.Working principles of concentrating collectors..17
4.1. Trough Systems..........................................18
4.2. Dish Systems...............................................21
4.3. Central Receiver Systems...........................23
5. Technology Comparison...................................25
6. Calculations.......................................................28
7. Economic and Environmental Considerations..37
8. Conclusions.......................................................39
References........................................................41

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

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

6. 3. Types of concentrating
collectors
Parabolic trough system
Parabolic dish
Power tower
Stationary concentrating collectors
There are four basic types of concentrating collectors:

7. 3.1. Parabolic trough
system
Parabolic troughs are devices that are shaped like the
letter “u”. The troughs concentrate sunlight onto a
receiver tube that is positioned along the focal line of the
trough. Sometimes a transparent glass tube envelops the
receiver tube to reduce heat loss [3].
Figure 3.1.2 Parabolic trough system [3].
Figure 3.1.1 Crossection of parabolic trough [4].
The parabolic trough sytem is
shown in the figure 3.1.2 below.
Their shapes are like letter “u”
as shown figure 3.1.1 below.

8. Parabolic troughs often use single-axis or dual-axis
tracking.
Figure 3.1.3 One Axis Tracking Parabolic Trough
with Axis Oriented E-W [8].
Figure 3.1.4 Two Axis Tracking Concentrator [8].
The below figure 3.1.3 shows one axis
tracking parabolic trough with axis
oriented E-W.
The below figure 3.1.4 shows two
axis tracking concentrator.

9. Temperatures at the receiver can reach 400 °C and
produce steam for generating electricity. In California,
multi-megawatt power plants were built using parabolic
troughs combined with gas turbines [3].
Parabolic trough combined with gas turbines is shown
figure 3.1.5 below.
Figure 3.1.5 Parabolic trough combined with gas turbines [4].

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

11. 3.2. Parabolic dish
systems
A parabolic dish collector is similar in appearance to
a large satellite dish, but has mirror-like reflectors
and an absorber at the focal point. It uses a dual
axis sun tracker [3].
Figure 3.2.2 Parabolic dish collector with a mirror-
like reflectors and an absorber at the focal point
[Courtesy of SunLabs - Department of Energy] [3].
Figure 3.2.1 Crossection of parabolic dish [4].
The below figure 3.2.1 shows
crossection of parabolic dish.
The Parabolic dish collector is
shown in the below figure 3.2.2.

12. A parabolic dish system uses a computer to track the sun
and concentrate the sun's rays onto a receiver located at the
focal point in front of the dish. In some systems, a heat
engine, such as a Stirling engine, is linked to the receiver to
generate electricity. Parabolic dish systems can reach 1000
°C at the receiver, and achieve the highest efficiencies for
converting solar energy to electricity in the small-power
capacity range [3].
Figure 3.2.3 Solar dish stirling engine [9].
The right figure 3.2.3
shows the solar dish
stirling engine.

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

14. 3.3. Power tower system
A heliostat uses a field of dual axis sun trackers that
direct solar energy to a large absorber located on a
tower. To date the only application for the heliostat
collector is power generation in a system called the
power tower [3].
Figure 3.3.2 Heliostats [4].
Figure 3.3.1 Power tower system [4].
Heliostats are shown in
the figure 3.3.2 below.
The Power tower system is
shown in the figure 3.3.1 below.

15. A power tower has a field of large mirrors that follow
the sun's path across the sky. The mirrors concentrate
sunlight onto a receiver on top of a high tower. A
computer keeps the mirrors aligned so the reflected
rays of the sun are always aimed at the receiver, where
temperatures well above 1000°C can be reached. High-
pressure steam is generated to produce electricity [3].
The power tower system with heliostats is shown in the
figure 3.3.3 below.
Figure 3.3.3 Power tower system with heliostats [4].

16. 3.4. Stationary concentrating
solar collectors
Stationary concentrating collectors use compound
parabolic reflectors and flat reflectors for directing solar
energy to an accompanying absorber or aperture through
a wide acceptance angle. The wide acceptance angle for
these reflectors eliminates the need for a sun tracker.
This class of collector includes parabolic trough flat plate
collectors, flat plate collectors with parabolic boosting
reflectors, and solar cooker. Development of the first two
collectors has been done in Sweden. Solar cookers are
used throughout the world, especially in the developing
countries [3].

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

18. 4.1. Trough Systems
These solar collectors use mirrored parabolic troughs to
focus the sun's energy to a fluid-carrying receiver tube
located at the focal point of a parabolically curved trough
reflector [5].It is shown in the figure 4.1.1 below.
Figure 4.1.1 Parabolic trough with mirrored parabolic troughs [10].

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

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

21. 4.2. Dish Systems
Dish systems use dish-shaped parabolic mirrors as
reflectors to concentrate and focus the sun's rays onto a
receiver, which is mounted above the dish at the dish
center. A dish/engine system is a stand alone unit
composed primarily of a collector, a receiver, and an
engine. It works by collecting and concentrating the sun's
energy with a dishshaped surface onto a receiver that
absorbs the energy and transfers it to the engine. The
engine then converts that energy to heat. The heat is
then converted to mechanical power, in a manner similar
to conventional engines, by compressing the working
fluid when it is cold, heating the compressed working
fluid, and then expanding it through a turbine or with a
piston to produce mechanical power. An electric
generator or alternator converts the mechanical power
into electrical power.

22. Each dish produces 5 to 50 kW of electricity and can
be used independently or linked together to increase
generating capacity. A 250-kW plant composed of
ten 25-kW dish/engine systems requires less than an
acre of land. Dish/engine systems are not
commercially available yet, although ongoing
demonstrations indicate good potential. Individual
dish/engine systems currently can generate about 25
kW of electricity. More capacity is possible by
connecting dishes together. These systems can be
combined with natural gas, and the resulting hybrid
provides continuous power generation [5].
Figure 4.2.1 Combination of parabolic dish system [4].
The right figure 4.2.1
shows the combination of
parabolic dish system.

23. 4.3. Central Receiver Systems
Central receivers (or power towers) use thousands of
individual sun-tracking mirrors called "heliostats" to
reflect solar energy onto a receiver located on top of tall
tower. The receiver collects the sun's heat in a heat-
transfer fluid (molten salt) that flows through the
receiver. The salt's heat energy is then used to make
steam to generate electricity in a conventional steam
generator, located at the foot of the tower. The molten
salt storage system retains heat efficiently, so it can be
stored for hours or even days before being used to
generate electricity [5]. In this system, molten-salt is
pumped from a “cold” tank at 288 deg.C and cycled
through the receiver where it is heated to 565 deg.C
and returned to a “hot” tank. The hot salt can then be
used to generate electricity when needed. Current
designs allow storage ranging from 3 to 13 hours [4].

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

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

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

27. Parabolic Trough Dish/Engine Power Tower
Size 30-320 MW 5-25 kW 10-200 MW
Operating Temperature
(ºC/ºF)
390/734 750/1382 565/1049
Annual Capacity Factor 23-50 % 25 % 20-77 %
Peak Efficiency 20%(d) 29.4%(d) 23%(p)
Net Annual Efficiency 11(d)-16% 12-25%(p) 7(d)-20%
Commercial Status
Commercially Scale-up
Prototype
Demonstration AvailableDemonstration
Technology
Development Risk
Low High Medium
Storage Available Limited Battery Yes
Hybrid Designs Yes Yes Yes
Cost USD/W 2,7-4,0 1,3-12,6 2,5-4,4
(p) = predicted; (d) = demonstrated;
Table 5.1 Key features of the three solar technologies [4].
Table 5.1 highlights the key features of the three solar technologies.

28. 6. Calculations
Heat from a solar collector may be used to drive a heat
engine operating in a cycle to produce work. A heat
engine may be used for such applications as water
pumping and generating electricity.
The thermal output Qout of a concentrating collector
operating at temperature T is given by
Qout = F'[gamma.Ainqin - U.Arec(T - Ta)],
Ain : the area of the incident solar radiation (m2).

29. Arec : the area of the receiver (m2)
gamma:optical efficiency
qin : the incident solar irradiation (W/m2)
Ta :the ambient temperature (°C)
U :the heat loss coefficient (W/m2K)
F’ :collector efficiency factor
The quantity Ain/Arec is called the concentration ratio.

30. High concentration ratios are obtained by making Ain the
area of a system of mirrors designed to concentrate the
solar radiation received onto a small receiver of area Arec.
Heat losses from the receiver are reduced by the smaller
size of the receiver. Consequently, high concentration
ratios give high collector temperatures. The stagnation
temperature Tmax is given by:
gamma.Ainqin = U.Arec(Tmax - Ta).

31. For example, if the optical efficiency is gamma = 0.8,
the incident solar irradiation is qin = 800W/m2, the
ambient temperature is Ta = 30°C, and the heat loss
coefficient is U = 10W/m2K, then a concentration ratio
Ain/Arec = 1 (no concentration) gives Tmax = 94°C, and a
concentration ratio Ain/Arec = 10 gives Tmax = 670°C.

32. The collector efficiency etac at operating temperature T is
etac=Qout/Ainqin = F'[gamma-U.Arec(T -Ta)/Ainqin]
= F'gamma(Tmax - T)/(Tmax - Ta).
The available mechanical power from the thermal power
output of the collector that would be obtained using a Carnot
cycle is Qout(1 - Ta/T), where the temperatures are absolute
temperatures.

33. The second law efficiency eta2 of a heat engine is
defined by
eta2=(mechanical power delivered)
/(available mechanical power).
Suppose a heat engine with second law efficiency eta2
uses as input the thermal power Qout from the solar
collector. The first law efficiency of the engine is
eta1 = (mechanical power delivered)/Qout = eta2(1 - Ta/T),

34. where Tmax depends on the design of the collector and
on the solar radiation input qin. Now, given F', gamma,
eta2, Ta, and Tmax, we can find the maximum efficiency
obtainable, and the optimum operating temperature Topt
from the condition d(eta)/dT = 0. This occurs at the
optimum temperature
Topt = [TmaxTa],
and the maximum efficiency is obtained by putting
T = Topt in the equation
eta = etac.eta1.
½

35. For example, putting F' = 0.9, gamma = 0.8, eta2 = 0.6,
Ta = 30°C = 303K, we get the efficiencies etamax for
different degrees of concentration shown in Table 6.1.
Very low overall efficiencies are obtained unless
operating temperatures greater than 500°C are used.
Expensive concentrating systems are needed to reach
these high temperatures, so commercial viability is
difficult [12].

36. Efficiencies for Converting Solar Radiation to Work
Tmax Topt etamax
100°C 63°C 2.2%
200°C 106°C 4.8%
400°C 179°C 8.5%
800°C 297°C 13.2%
1600°C 480°C 18.4%
Table 6.1. Different degrees of concentration [12].

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

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

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

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

41. References
[1]http://aloisiuskolleg.www.de/schule/fach
bereiche/comenius/charles/solar.html
[2]http://www.tpub.com/utilities/index.html
[3]http://www.canren.gc.ca/tech.appl/index
.asp

42. [4]http://www.geocities.com/dieret/re/Solar
/solar.html
[5]http://www.eren.doe.gov/menus/energy
ex.html
[6]http://www.powerfromthesun.net/chapte
r1/Chapter1.html
[7]http://www.eere.energy.gov/
[8]http://rredc.nrel.gov/solar/pubs/redbook/
interp.html

43. [9]http://www.sunwindsolar.com/a solar/
optics html
[10]http://www.eere.energy/gov/solar/solar.
heating html
[11]http://www.energylan.sandia.gov/sunlab/
stfuture.html
[12]http://www.jgsee.kmutt.ac.th/exell/Solar/
Conversion.html