Miscellaneous Process Design

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.

  • 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% 300bar,600˚C
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%

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%.
  • Wind turbines has an overall conversion efficieny 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.
  • 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.

 

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