Construction of a Combined Cycle Power Plant

A combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turbine (CCGT) plant. The same principle is also used for marine propulsion, where it is called a combined gas and steam (COGAS) plant. Combining two or more thermodynamic cycles improves overall efficiency, which reduces fuel costs.

The principle is that after completing its cycle in the first engine, the working fluid (the exhaust) is still hot enough that a second subsequent heat engine can extract energy from the heat in the exhaust. Usually the heat passes through a heat exchanger so that the two engines can use different working fluids.

By generating power from multiple streams of work, the overall efficiency of the system can be increased by 50–60%. That is, from an overall efficiency of say 34% (for a simple cycle), to as much as 64% (for a combined cycle). This is more than 84% of the theoretical efficiency of a Carnot cycle. This can be done because heat engines can only use part of the energy from their fuel (usually less than 50%). In an ordinary (non-combined cycle) heat engine the remaining heat (i.e., hot exhaust gas) from combustion is wasted.

Efficiency of CCGT plants

By combining both gas and steam cycles, high input temperatures and low output temperatures can be achieved. The efficiency of the cycles add, because they are powered by the same fuel source. So, a combined cycle plant has a thermodynamic cycle that operates between the gas-turbine's high firing temperature and the waste heat temperature from the condensers of the steam cycle. This large range means that the Carnot efficiency of the cycle is high. The actual efficiency, while lower than the Carnot efficiency, is still higher than that of either plant on its own.

The electric efficiency of a combined cycle power station, if calculated as electric energy produced as a percentage of the lower heating value of the fuel consumed, can be over 60% when operating new, i.e. unaged, and at continuous output which are ideal conditions. As with single cycle thermal units, combined cycle units may also deliver low temperature heat energy for industrial processes, district heating and other uses. This is called cogeneration and such power plants are often referred to as a combined heat and power (CHP) plant.

In general, combined cycle efficiencies in service are over 50% on a lower heating value and Gross Output basis. Most combined cycle units, especially the larger units, have peak, steady-state efficiencies on the LHV basis of 55 to 59%.
Difference between HHV and LHV

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To avoid confusion, the efficiency of heat engines and power stations should be stated relative to the Higher Heating Value (HHV) or Lower Heating Value (LHV) of the fuel, to include or exclude the heat that can be obtained from condensing the flue gas. It should also be specified whether Gross output at the generator terminals or Net Output at the power station fence is being considered.

The LHV figure is not a computation of electricity net energy compared to energy content of fuel input; it is 11% higher than that. The HHV figure is a computation of electricity net energy compared to energy content of fuel input. If the LHV approach were used for some new condensing boilers, the efficiency would calculate to be over 100%. Manufacturers prefer to cite the higher LHV efficiency, e.g. 60%, for a new CCGT, but utilities, when calculating how much electricity the plant will generate, divide this by 1.11 to get the real HHV efficiency, e.g. 54%, of that CCGT. Coal plant efficiencies are computed on a HHV basis since it doesn't make nearly as much difference for coal burn, as for gas.

The difference between HHV and LHV for gas, can be estimated (using US customary units) by 1055Btu/Lb * w, where w is the lbs of water after combustion per lb of fuel. To convert the HHV of natural gas, which is 23875 Btu/lb, to an LHV (methane is 25% hydrogen) would be: 23875 – (1055*0.25*18/2) = 21500. Because the efficiency is determined by dividing the energy output by the input, and the input on an LHV basis is smaller than the HHV basis, the overall efficiency on an LHV basis is higher. Therefore using the ratio of 23875/21500 = 1.11 one can convert the HHV to an LHV.

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