Solar Thermal Power Plant operating in Simple

Rankine Cycle

Jenico.J

Department of Mechanical Engineering

Alagappa Chettiar College of Engineering & Technology

Karaikudi-630004

jenico.kennedy@gmail.com

Ragav.N

Department of Mechanical Engineering

Alagappa Chettiar College of Engineering & Technology

Karaikudi-630004

ragavn257@gmail.com

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. Super-

heated 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.

I. INTRODUCTION

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

annually.

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.

II. OVERVIEW

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.

III. THEORETICAL RANKINE CYCLE

ANALYSIS

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.

IV. CONSTRUCTION

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

also.

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.

V. ADVANTAGES

a) 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

days.

b) 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.

c) 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

Equipment.

d) 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 .

VI. FUTURE SCOPE

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.

VII. LIMITATIONS

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

wildlife.

c) Limited Locations and Size

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

plant.

VIII. CONCLUSION

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.

REFERENCES

[1] http://en.wikipedia.org/wiki/Concentrated_solar_power

[2] http://en.wikipedia.org/wiki/Solar_power_tower

[3] http://mpoweruk.com/solar_power.htm

[4] http://science.howstuffworks.com/environmental/green-

tech/energy-production/solar-thermal-power.htm

[5] http://csirosolarblog.com/about/

[6] “Thermal Engineering” by Er. R.K. Rajput