Solar Thermal Power Plant

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Solar Thermal Power Plant operating in Simple
Rankine Cycle


Department of Mechanical Engineering
Alagappa Chettiar College of Engineering & Technology
[email protected]

Department of Mechanical Engineering
Alagappa Chettiar College of Engineering & Technology
[email protected]

Abstract— Sun is the source of life on earth. It is
the ultimate fusion reactor producing 3.9 x 1026
joules of energy per second. This paper explores
a way to harness a tiny fraction of this energy in
producing electricity. Electricity is primarily
produced by burning fossil fuels, nuclear fission,
hydro power, etc. Burning fossil fuels leads to
global warming and also the world’s oil reserves
are depleting rapidly. The concerns surrounding
nuclear fission fail to go away. In this scenario,
the focus is now on using renewable sources of
energy to produce electricity. With windmills
having limited applications and as conversion of
solar radiation directly to electricity using
photovoltaics have dismally low output levels;
we’ll look at a form of energy that can be used to
produce electricity that’s often overlooked: Solar
Thermal Energy. Functional concentrated solar
power plants were established in 1900s but their
prominence was lost when the cost of natural gas
fell. As the world is searching for a novel source
for producing electricity, it’s time to think back,
learn from the past and start developing
concentrated solar power plants.
Right from coal thermal power plants to nuclear
fission reactors, the energy obtained from the
fuel is used to heat water to produce superheated
steam. This steam in turn rotates a prime-mover
(turbine) which is connected to an electric
generator thus producing electricity. This is the
most effective method of power production
known to mankind. The same schematic is used
in the Solar Thermal Power Plant which we’re
about to design and analyze.
The main components of this power plant are:

1. Pump
2. Boiler: A solar power tower surrounded by
flat mirrors on the ground.
3. Turbine
4. Condenser
First, water is pressurized in the pump and is
sent to boiler. The boiler consists of a zigzag
network of pipes and a tank of molten salt which
acts as a Thermal Energy Reservoir. This
network is placed in a tower surrounded by flat
mirrors on the ground. Different mirrors focus
on different parts of the network. This ensures
that the water is heated evenly & superheated
steam is produced when it leaves the boiler.
Super-heated steam is expanded in the turbine &
is condensed and fed back to the pump to repeat
the cycle. The possibility of feeding back the heat
rejected in the condenser to the boiler is also
explored. This cycle operates between the
pressure levels: 0.3166 bar & 4.5 bar. Superheated steam is produced at 150oC. If we
consider the Rankine cycle to be ideal with 100%
turbine & pump efficiencies; we get output
energy of 421.68 kJ per kg of water supplied.
The primary source of electricity in the world
today is fossil fuels. Scientists have pointed out that
crude oil resources will be exhausted soon, so
technical experts in the field of renewable energy
have come to the fore to develop a sustainable
model of electricity generation that can be used for
centuries to come without any hassles and damage
to the environment. Solar thermal energy systems

are different to solar photovoltaic cells, which are
the ‘solar panels’ you might be most familiar with.
Solar thermal and photovoltaic systems are two
separate methods of putting the energy from sunlight
to good use.
With solar thermal energy, the only part of the
sunlight that’s used is its heat. When you dry your
clothes on the line, or heat water with a solar hot
water system, you’re using solar thermal energy. But
it’s also possible to generate higher temperatures by
using a lens or mirror to focus the sunlight onto a
smaller area. Its large scale utility came to human
knowledge in 212 BC when Archimedes used an
arrangement of mirrors to focus sunlight on the
invading Roman fleet and burning their ships, thus
repelling them. With the right type of focusing
system, the sunlight can be turned from something
capable of merely drying clothes, to something hot
enough to boil water to run a large steam turbine that
powers a town – or do many other useful things
besides that. Most of us don't think much about
where our electricity comes from, only that it's
available and plentiful. Electricity generated by
burning fossil fuels such as coal, oil and natural gas,
emits carbon dioxide, nitrogen oxides and sulfur
oxides - gases scientists believe contribute to climate
change. Solar thermal (heat) energy is a carbon-free,
renewable alternative to the power we generate with
fossil fuels like coal and gas. This isn't a thing of the
future, either. In 1980s, the United States built nine
such power plants and today they continue to
provide a combined capacity of 354 megawatts
In the 1990s when prices of natural gas & crude
oil dropped, so did interest in solar thermal power.
And today it’s high time the technology makes a
comeback. There are two main ways of generating
energy from the sun. Photovoltaic (PV) and
concentrating solar thermal (CST), also known as
concentrating solar power (CSP) technologies.
PV converts sunlight directly into electricity.
These solar cells are usually found powering devices
such as watches, sunglasses and backpacks, as well
as providing power in remote areas.

Solar thermal technology is large-scale by
comparison. One big difference from PV is that
solar thermal power plants generate electricity
indirectly. Heat from the sun's rays is collected and
used to heat a fluid. The steam produced from the
heated fluid powers a generator that produces
electricity. It's similar to the way fossil fuel-burning
power plants work except the steam is produced by
the collected heat rather than from the combustion of
fossil fuels.
A coal fired thermal power plant contains a
source of water, pipelines to transport water &
steam, a pump, boiler, turbine and condenser. Here
coal is burnt and the heat produced is utilized by the
boiler to produce steam. In the proposed solar
thermal power plant, water is transported atop a
tower which is surrounded by flat mirrors. These
mirrors concentrate the sunlight incident on it and
focus it on the apex of the solar tower. This solar
thermal energy is utilized to boil the water and
produce superheated steam.
In the power plant which is to be constructed, the
initial temperature of water is taken as 25oC and
initial pressure is 1 bar. One prerequisite considered
while modeling this plant is that minimal pump
work is to be used to pressurize water before boiling
it. The optimum pressure at which the water can be
converted to superheated steam and also low is
4.5 bar. After the water is pressurized, it is sent to
the boiler – the solar power tower where the
pressurized water is heated using solar thermal
energy up to a temperature of 150oC. Super-heated
steam is produced at this particular temperature and
pressure. This steam is expanded in a turbine to
pressure of 0.3166 bar & the turbine is coupled to
an electric generator to produce electricity.
The enthalpy and entropy values of super-heated
steam at various states in the Rankine cycle are
taken from the steam tables. Using this, the various

energy changes, work input & work output involved
in the cycle is calculated.

Fig.1 – T-s diagram used for theoretical analysis
of the proposed Rankine Cycle. The information
obtained from this analysis is:
1. Pump work required: 4.2796 J/kg
2. Heat required to produce superheated
steam: 2451.1 kJ/kg
3. Heat rejected in condenser: 2029.42 kJ/kg
4. Work done on Turbine: 421.68 kJ/kg
Hence, the energy output of this cycle is
421.68 kJ provided the energy supplied to the pump
& boiler and the energy rejected in the condenser
match the aforementioned values.
The most suitable devices that can be used to the
four main components of the power plant are
discussed in this section.
A. Pump: A boiler feedwater pump is a
specific type of pump used to pump
feedwater into a steam boiler. The water
may be freshly supplied or returning
condensate produced as a result of the
condensation of the steam produced by
the boiler. These pumps are normally
high pressure units that take suction from
a condensate return system and can be of
the centrifugal pump type or positive
displacement type. A minimal hp pump

is enough to provide energy of about 5J
to pump 1kg of water to the boiler.
B. Boiler: The boiler which is to be used in
this proposed solar thermal power plant
is generally known as Solar power tower.
The solar power tower, also known as
'central tower' power plants or 'heliostat'
power plants or power towers, is a type
of solar furnace using a tower to receive
the focused sunlight. It uses an array of
flat, movable mirrors (called heliostats)
to focus the sun's rays upon a collector
tower (the target).
The proposed design utilizes molten salts
(40% potassium nitrate, 60% sodium
nitrate) as the working fluids placed in
the apex of the solar power tower. This
system is known as thermal energy
reservoir or Thermal energy storage
system. These working fluids have high
heat capacity, which can be used to store
the energy before using it to boil water to
drive turbines. These designs also allow
power to be generated when the sun is
not shining.
The upper atmosphere receives a power
of 1367 Watts per square metre (W/m2).
This value is known as Solar Constant
G0. Taking into account cloud cover,
diffraction of sun’s rays and other such
factors, the average power incident on
the surface of the earth is 342 W/m2.
In order to utilize this power to heat up
the molten salts, linear mirrors covering
an area of 14340m2 are required. This is
area of roughly one and a half football
fields. These linear mirrors are arranged
in such a way that the sun rays are
focused at the thermal energy reservoir
which is situated approximately 120m
from the ground level. This design is
considerably sufficient to harness 2452kJ
of energy required to boil one kg of water
assuming a boiler efficiency of 50%.
C. Turbine: A Parson’s reaction turbine
which is used in small-scale steam power
plants can be used in this power plant

D. Condenser: A surface condenser which is
generally used in thermal power plants is
also used here. The coolant or condensate
to which the latent heat of the steam is
transferred is fed back to the thermal
energy reservoir where the heat is
transferred to the molten salts. Thus a
part of the heat required by boiler is
supplied by the condenser. The exact
quantity of heat which can be fed back is
highly difficult to analyze theoretically
and goes beyond the scope of this paper.




No Fuel Cost - Solar Thermal Energy
does not require any fuel like most other
sources of renewable energy. This is a
huge advantage over other fossil fuels
whose costs are increasing at a drastic
rate every year. Electricity prices are
increasingly rapidly in most parts of the
world much faster than general inflation.
Price shocks due to high fuel costs are a
big risk with fossil fuel energy these
Predictable - 24/7 Power -Solar Thermal
Energy can generate power 24 hours a
day. This is made possible as solar
thermal power plants store the energy in
the form of molten salts etc. Other forms
of Renewable Energy like Solar PV and
Wind Energy are intermittent in nature.
The electricity supply is much more
uniform and reliable.
No Pollution and Global Warming
Effects – Solar Thermal Energy does not
cause pollution which is one of the
biggest advantages. Note there are costs
associated with the equipment used to
build and transport Solar Thermal Energy
Using Existing Industrial Base - Solar
Thermal Energy uses equipment like
solar thermal mirrors and turbines which
is made in large scale at low cost by the
existing Industrial Base and requires no
major changes in equipment and
materials unlike new technologies .

A study done by Greenpeace International,
the European Solar Thermal Electricity
Association, and the International Energy
Agency's SolarPACES group investigated the
potential and future of concentrated solar
power. The study found that concentrated solar
power could account for up to 25% of the
world's energy needs by 2050. The increase in
investment would be from 2 billion euros
worldwide to 92.5 billion euros in that time
period. Spain is the leader in concentrated solar
power technology, with more than 50
government-approved projects in the works.
Also, it exports its technology, further
increasing the technology's stake in energy
worldwide. Because the technology works best
with areas of high insolation (solar radiation),
experts predict the biggest growth in places like
Africa, Mexico, and the southwest United
States. It indicates that the thermal storage
systems based in nitrates (calcium, potassium,
sodium,etc) will make the CSP plants more and
more profitable. The study examined three
different outcomes for this technology: no
increases in CSP technology, investment
continuing as it has been in Spain and the US,
and finally the true potential of CSP without
any barriers on its growth.
Scientists at the National Renewable Energy
Laboratory (NREL) and SkyFuel teamed to
develop large curved sheets of metal that have
the potential to be 30% less expensive than
today's best collectors of concentrated solar
power by replacing glass-based models with a
silver polymer sheet that has the same
performance as the heavy glass mirrors, but at
much lower cost and weight. It also is much
easier to deploy and install. The glossy film
uses several layers of polymers, with an inner
layer of pure silver.
a) Water Issue – Solar Thermal Plants use
lots of Water which is Major Problem in
Desert Areas. Using non-water cooling
raises the cost of CSP projects too much.
While using Sea Water has been

proposed it remains to be seen if it
possible to implement this solution as
this would imply building Plants very
near the Coastline.
b) Ecological and Cultural Issues – The
Usage of Massive Arrays of Mirrors is
noted to heavily impact the Desert
Wildlife endangering the endangered
species. California has already seen a
massive fight on this issue with Project
Developers curtailing the size of their
Plants and spending money to move the
c) Limited
Limitations – Solar Thermal Energy can
only be built in places which have the
high amount of solar radiation. They can
be built in deserts mostly and require a
large land area. This means its not
possible to build them in populated areas.
Solar Thermal Energy also can only be
built in large sizes which are at least 30
MW in size to be economical. This
contrasts to Solar PV which is sold in
sizes as low as 5 Watts.
d) Long Gestation Time Leading to Cost
Overruns – The Gestation Time for
permitting, financing, drilling etc. can
easily take 5-7 years to develop a
concentrated solar thermal power plant.
Compare this to 6 months for a small

wind farm or 3 months for a Solar PV
This paper reinforces the claim that Solar Thermal
Power Plants having Solar Power Towers are a
commercially viable & sustainable method of
harnessing green energy. If the aforementioned
energy interactions are achieved successfully, it
implies that considering a generator efficiency of
80% if 100 kg of water is supplied at the rate of
1 kg/s about 33 MW of power can be produced.
Ivanpah Solar Power Facility in California which
produces 377MW net annual output is a testament
to that. The earth receives more energy from the
Sun in just one hour than the world's population
uses in a whole year. If we could point out one thing
that hasn’t changed since the inception of earth, it is
Sun rises in the east and sets in the west every
single day. So, let’s make use of this for the
betterment of mankind and humanity.
[6] “Thermal Engineering” by Er. R.K. Rajput

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