RE Feature
USING SOLAR THERMAL ENERGY
TO GENERATE ELECTRICITY
Concentrated Solar Power seems to be one of the most promising
technologies that can help meet the growing demand for energy
in the country. Nakul Varier and Rutuja Londhe look into the
various facets of this technology while focusing especially on
power generation though the Parabolic Trough.
CSP uses
various mirror
configurations
to focus the sun’s
radiation on to
a receiver that
absorbs thermal
energy to drive
steam turbines
connected to
generators,
thus producing
electricity.
32
| Akshay Urja | April 2014
E
nergy plays a vital role in the economic growth of a developing country like India.
Due to rapid depletion of fossil fuels, alternative renewable energy sources should
be developed and implemented in order to cope with such humungous energy
requirements. Solar energy seems to be one of the most promising alternative
energy resources. Solar radiation consists of both light and heat energy. This article
focuses on harnessing heat energy from solar radiation to generate electricity. The
technology used for this purpose is known as Concentrated Solar Power (CSP).
Figure 1: Block Diagram of CSP
Notes: A Solar energy source, B Collector field, C Steam turbine, D Generator, and E Transmission system
Using Solar Thermal Energy To Generate Electricity
BASIC WORKING PRINCIPLE
CSP uses various mirror configurations to focus the sun’s radiation on to a receiver.
The receiver absorbs thermal energy, comprising mainly infrared rays in variable
wavelengths, from the focused radiation. This energy or heat is used to drive a series
of steam turbines connected to generators, thus producing electricity. Figure 1 depicts
the various methods that can be used for focusing the sun’s radiation on to a receiver.
TECHNOLOGIES USED
There are four basic technologies which are currently in use:
ƒƒ Dish Stirling
ƒƒ Central Receiver Plants
ƒƒ Fresnel’s Reflector
ƒƒ Parabolic Trough
The Solana
Generating Station,
Arizona, is the
largest parabolic
trough plant in
operation. It has a
total capacity of 280
MW which is enough
to power 70,000
homes, thus avoiding
around 475,000
tonnes of CO2.
Different technologies produce different peak temperatures and have
different efficiencies, due to differences in the way they track the sun and focus the
the solar radiation.
Dish Stirling
A Dish Stirling system uses a large reflective dish (has a similar shape as that of a
satellite television dish) to focus all the sun’s radiation/sunlight striking the dish up onto
the reflector’s focal point. The tracking system of the reflector tracks the sun along two
axes. It has a receiver at the focal point, which consists of a working fluid, capturing
the heat and transforming it into electricity using a stirling engine. Dish Stirling is
known to have the highest efficiency of all solar technologies. It has an efficiency of 30
per cent as compared to solar Photo Voltaic (PV) technology, which has an efficiency
of approximately 15 per cent. The world record for solar to electric efficiency was set
at 31.25 per cent by Stirling Energy Systems (SEC) dishes at the National Solar Thermal
Test Facility (NSTTF) in New Mexico.
Dish Stirling
Central Receiver Plants
The Central Receiver Plant, also known as ‘Central Tower’ power plant, uses a central
receiver placed on top of a tower to receive the focused sunlight. It uses an array of
flat, movable mirrors (called heliostats) to focus the sun's rays on a collector tower
(the target). The receiver placed at the top of the tower consists of fluid deposits,
generally seawater, which is heated to about 500–1,000 °C. It is then used as a heat
source for power generation. The Solar Two in Daggett, California, and the CESA-1 in
Plataforma Solar de Almeria, Spain, are the representative demonstration plants. The
Planta Solar 10 (PS10) in Sanlucar la Mayor, Spain, is the first commercial utility-scale
central tower plant in the world.
Central Receiver Plant
Fresnel’s Reflector
A linear Fresnel’s Reflector power plant uses a series of long, narrow mirrors with
shallow-curvature (or even flat) to focus thermal energy from the sun’s radiation onto
one or more linear receivers positioned above the mirrors. These mirrors are capable
of concentrating the sun’s energy to approximately 30 times its normal intensity. The
reflectors are located at the base of the system and converges the sun’s rays into the
absorber. On completion of phase 1 of the Rajasthan 125 MW CSP Project in 2013, it
will be the largest linear Fresnel Reflector CSP power plant in Asia.
Fresnel’s Reflector
April 2014 | Akshay Urja |
33
RE Feature
Parabolic Trough
Photo 4: Parabolic Trough
Parabolic
trough plants
primarily consist of
three parts — the
solar field,
the storage system,
and the power plant
block.
A parabolic trough is a type of solar thermal collector that is straight in one dimension
and curved as a parabola in the other two dimensions. It is lined with polished metal
mirrors. The solar radiation is captured and concentrated by long rows of Parabolic
Troughs onto a receiver positioned along the reflector’s focal line. The reflector uses
a single axis tracking system to track the sun’s movement. The heat generated in this
way is enough to produce the required steam. The Solana Generating Station near Gila
Bend, Arizona, is the largest parabolic trough plant in operation. It has a total capacity
of 280 MW which is enough to power 70,000 homes, thus avoiding around 475,000
tonnes of CO2.
Among all the above mentioned systems, the parabolic trough systems are the
most developed and widely used. This article will therefore focus on the working of
parabolic trough plants.
POWER GENERATION USING PARABOLIC TROUGH PLANTS’ COMPONENTS
Parabolic trough plants primarily consist of three parts — the solar field, the storage
system, and the power plant block (Figure 2).
Solar Fields
The Solar Field consists of arrays of parabolic-shaped concave mirrors known as
parabolic. These parabolic troughs are around six meters high and several hundred
metres long. These mirrors are made from silver-coated white glass which are about
4 to 5 mm thick. In spite of their humongous size, they are aligned with millimetre
precision. The rows run in a north–south direction and track the sun from east to
west during the course of the day. Over 98 per cent of the solar radiation that falls on
the mirrors is reflected onto the absorber tube, which is fixed along the focal line of
the mirrors. Thermax India Pvt. Ltd, First ESCO India Pvt. Ltd, Soltigua, TSK Flagsol,
etc., are some of the manufacturers of parabolic trough collectors in India and around
the world.
Absorber Tube
The Absorber Tube consists of a heat transfer medium, heated to around 400 °C by
concentrated sunlight. The absorber tube is made of an aluminum alloy metal which is
surrounded by a glass tube. Aluminum alloy possesses high temperature and pressureresisting capabilities. The vacuum created between the metal tube and glass tube acts
as an insulating medium, thus reducing heat loss. Almeco Solar is a manufacturer of
absorber tubes.
Storage System
A properly
designed plant can
produce electricity
just by using solar
energy.
34
| Akshay Urja | April 2014
The Storage System consists of a cold tank comprising molten salt maintained at
about 280 °C and a hot tank containing molten salt at a higher temperature of about
380 °C. Nitrates of potassium and sodium generally form the primary composition of
molten salt.
Power Plant Block
The Power Plant Block consists of all the components and equipment necessary for
generating electricity. It mainly comprises steam turbines, generators, transformers,
transmission lines, etc.
Using Solar Thermal Energy To Generate Electricity
OPERATION
Infrared radiation, consisting
of most of the thermal
energy present in solar
radiation, is focused
along the focal line using
parabolic-shaped mirrors,
where the absorber tubes
that are intended to be
heated are positioned. The
heat is then transferred to the
heat transfer medium inside
the tube (generally oil) by
convection. Then this heated oil is sent to the heat exchangers where steam produced
is used to run the steam turbines, thus generating electricity. A properly designed plant
can produce electricity just by using solar energy.
The process of generating electricity during a 24 period can be divided into four parts:
Figure 2: Layout of a typical
parabolic trough plant
Notes: A Solar field, B Hot tank, C Heat
exchanger, D Cold tank, E Re-heater, F
Super-heater, G Steam turbine, H Generator,
J Vaporizer, L Economizer, K Condenser and
cooling tower
ƒƒ After sunrise (6 am to 9 pm)
ƒƒ During the day (9 pm to 3 pm)
ƒƒ In the evenings (3 pm to 6 pm)
ƒƒ During night ( after 6 pm till sunrise )
After sunrise: After sunrise, the parabolic troughs start tracking the sun and reflect
the sun’s energy onto the absorber tube, thus heating the oil inside it. This heated
oil is then transferred to a heat exchanger, where steam is generated from water
supplied from the cooling tower or the reservoir. This steam is used to drive the turbine
connected to the generator to generate electricity. Since the sun’s intensity is less
during this period, there is only enough solar energy to power the steam turbine.
During the day: As the day progresses, the sun’s intensity starts increasing gradually.
The surplus energy received from the sun is stored in the form of molten salt. A part
of the heated oil from the solar field is sent to a heat exchanger in the storage block
where molten salt pumped from the cold storage tank is further heated from 280 °C
to 380 °C, and is then sent to the hot storage tank. The rest of the heated oil is sent
to the heat exchanger in the power block to generate steam. Thus, solar energy is
simultaneously used for generating electricity as well as for heating the molten salt in
the storage system.
In the evenings: During this period, the energy to drive the turbine comes from the
solar fields and the storage system. The molten salt from the hot tank is transferred to
the cold tank through heat exchangers where the oil is heated and is sent to the power
block.
During night: After sunset, thermal energy is exclusively provided by the storage
system. If the storage system and solar fields are designed accordingly, the plant can
operate for 24 hours. These solar plants can run in conjunction with other sources
of energy, such as combustion of gas, biomass, etc. Such plants are known as hybrid
plants.
EFFICIENCY OF a PARABOLIC TROUGH
The efficiency and cost of the parabolic trough is influenced by structural stiffness, choice
of materials, assembly tolerances, mirror cleanliness, and wear. Thus, high efficiency
Currently, India
has only 52.5 MW of
CSP in operation
(of which, 50 MW
is contributed
by the Godawari
solar thermal
plant, located
in Northwest
Rajasthan).However,
there are seven
projects of 470 MW
aggregate capacity
scheduled under
the first phase of
the Jawaharlal
Nehru National
Solar Mission
(JNNSM).
April 2014 | Akshay Urja |
35
RE Feature
CSP has numerous advantages as it has zero fuel costs,
gives predictable 24*7 power, is environmental friendly as it
is pollution free, and has no global warming effects.
can be obtained by proper designing and maintenance. Current performance estimate
of the solar trough optical field efficiency is 54.2 per cent.
BENEFITS OF CSP
CSP has numerous advantages as it has zero fuel costs, gives predictable 24*7 power,
is environmental friendly as it is pollution free, and has no global warming effects. It
can be installed in deserts or other remote areas where the land is otherwise wasted,
and thus can power neighbouring towns and villages.
ISSUES AND CHALLENGES
Apart from the high costs involved in the implementation of a plant, the other limiting
factors include large water requirements for cooling purposes, location and size
limitations, long gestation periods, etc. In order to be profitable, the plant should be
operational for at least 25 years. Future technologies, therefore, have a high probability
of making CSP obsolete. The plant may also pose ecological and cultural issues.
CONCLUSION
Solar power can play a significant role in creating a secure and diversified energy
future. This technology is becoming commercially viable and is quickly expanding
around the world. Currently, India has only 52.5 MW of CSP in operation (of which,
50 MW is contributed by the Godawari solar thermal plant, located in Northwest
Rajasthan). However, there are seven projects of 470 MW aggregate capacity
scheduled under the first phase of the Jawaharlal Nehru National Solar Mission
(JNNSM). Private firms in India are planning to build a 250 MW solar thermal plant
(largest in Asia) in Dhursur, Rajasthan, within the next few years. Within the next
decade, solar thermal power plants will provide a significant contribution towards an
efficient, economical, and environmentally benign energy supply in India. It is estimated
that it could account for up to 25 per cent of the world's energy needs by 2050. AU
Nakul Varier and Rutuja Londhe are students from Fr. C Rodrigues Institute of Technology, Vashi, Navi Mumbai.
Email: [email protected]
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