Power Plant Efficiency
In general terms, efficiency is the output of a process compared to the input. In the context of
a power plant we have three types of efficiencies.
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Economic efficiency
Operational efficiency
Energy efficiency
Economic efficiency of a power plant
Economic efficiency is essentially the specific cost of producing useful output and tends to be
the main driver behind shaping process plant design and operation.
Operational efficiency of a power plant
Operational efficiency is generally called “capacity factor” or sometimes “load factor” and
measures the actual output from a process compared to the potential maximum output.
Energy efficiency of a power plant
Energy efficiency refers to the ratio of useful work output to the heat input, so it may be more
correct to use “energy conversion efficiency” when considering mixed inputs and outputs.
Heat rate
Energy efficiency is closely related to the 'Heat rate'. Heat rate is the quantity of heat required
to produce an unit of useful output and therefore a lower heat rate is more efficient and gives
a higher percentage energy efficiency. The relationship most frequently used to describe heat
rate and efficiency in respect of electrical power generation is:
Energy efficiency = 3600 / heat rate
Where heat rate is measured in kJ/kWh, MJ/MWh or GJ/GWh.
Energy related analysis often refers to terms such as thermal efficiency, anergy, exergy.
Thermal Efficiency
"Thermal efficiency” is strictly defined as the useful output energy for a given quantity of
gross input heat energy and is therefore subtly different from energy conversion efficiency,
which might include both heat and power as inputs and outputs.
Exergy
“Exergy” is defined as a measure of the potential of a system to do work. the exergy 'B' of a
system with respect to a reservoir is the maximum work done by the system during a
transformation which brings it into equilibrium with the reservoir. Exergy analysis is used in
the field of industrial ecology as a tool to both decrease the amount of exergy required for a
process and to use available exergy more efficiently. Exergy efficiency is also called
second-law efficiency because it computes the efficiency of a process by considering the
anergy.
Anergy
“Anergy”, is the complementary part of the (heat) energy that cannot be converted into work.
Taking second law of thermodynamics into account. The energy E and exergy B balances of
a process are:
Einput = Ein product + Eloss
Binput = Bin product + Bloss + Bdestroyed
The efficiency η of the process may be described using the thermodynamic potentials E or B.
Efficiency is the fraction of the potential that makes its way into the product.
The efficiency quoted by equipment suppliers is usually the energy conversion or first-law
thermodynamic efficiency. This indicates how well the particular appliance converts one
form of energy into another, but it does not indicate how the equipment compares to an
alternative energy conversion process.
An example which can be used to illustrate the difference between energy conversion
efficiency and exergy efficiency is the combustion of natural gas solely to heat water. While
this has a high first-law efficiency, it has a low second-law efficiency, and wastes a large
amount of high-temperature, high-quality energy to heat the relatively cold water.
Typical Power Plant Efficiencies
Efficiencies of Different types of Power generation sources:
Type
Efficiency
Operating Condition
Sub Critical Thermal plant
35-38%
170 bar , 570˚C
Super Critcial Thermal
Plant
42%
220 bar, 600˚C
Ultra Super Critical
45-48%
IGCC
45-55%
Hydro Power Plant
85-90%
Wind turbine
30-45%
Solar Thermal System
12% annually
Geo Thermal System
35%
Nuclear Power Plant
0.27%
Diesel Engines
35-42%
300bar,600˚C
Efficiency in a Coal Fired Power Plant
Coal fired power plant operates on modified Rankine cycle. Its efficiency is decided by
parameters of this cycle. The changes of efficiency of different power plant attributes mostly
to different temperature , pressure, superheat steam and Reheat steam temperatures.
Conventional power plants operate at 170 bar and 570˚C superheat steam parameters. Their
efficiency lies between 35-38% depending upon nature of coal. Super Critical thermal power
plant operates 220 bar and 600˚C and their efficiency is generally 42%. While Ultra super
critical plant parameters are 300bar and 600˚C and efficiencies are around 45-48%.
2nd type of coal fired power plant are IGCC ( Integrated Gasification Combined Cycle) and
these plants have efficiency between 45-55% depending on the coal quality and process opted
for gasification.
Efficiency in a Renewable Energy Power Plant

Hydro power plant has the highest efficiency. The potential head of water is near to
turbine so there are no losses only some turbine losses occurs. The efficiency lies
between 85-90%.
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Wind turbines has an overall conversion efficiency of 30-45%.

Solar thermal system can achieve upto 20%. But changes in weather and moving
path of sun makes it to achieve 12% efficiency annually.
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Geo thermal systems uses the rankine cycle with steam temperature at saturation
point. It usually achieves around 35% efficiency.
Efficiency in a Nuclear Plant
The efficiency of nuclear plant is little bit different. On the steam turbine side they use the
rankine cycle with steam temperature at saturation point. This gives them efficiency aroung
38%. Energy release in a nuclear fission is extremely high, and very small amout of it goes to
heat up the steam. So the overall efficiency generally comes over at 0.27%.
Efficiency of power generated by Diesel Engines
The large capacity industrial diesel engines operates between 35-42% efficiency.