Gas Turbine Performance
The generation efficiency (the proportion of energy in the gas turbine fuel converted to electrical output at the generator terminals) can range from 20% to 35% (Gross basis), depending on the inlet temperature and pressure and any power-enhancing facilities employed. About 30% (Gross) is typical in practice. Gas turbines, however, need to be operated at or near their full rated output: the efficiency of smaller, single-shaft machines deteriorates markedly when operated below full rated output, although the larger twin-shaft units have a better part-load performance.
Gas turbines are available in a wide range of power outputs, from less than 1 MW to more than 200 MW.
However, it is not common in the current energy market to find cost-effective CHP applications for turbines of less than 5 MW.Turbine efficiency in the 1-5 MW range is lower than for larger machines – often below 25% – and, in comparison with larger turbines, the cost of supplying and installing smaller units does not fall pro rata with output. Micro turbines, in the 50-250 kW range are now being developed and are available as CHP units.
A gas turbine has to take in more air than is required solely for the combustion of its fuel.The presence of this excess air means that the exhaust gases contain sufficient residual oxygen for extra fuel to be burned in the exhaust stream before it enters the heat recovery unit.‘Supplementary firing’, as this is called, can have significant benefits in terms of fuel efficiency and plant operation. It can raise the overall heat to power ratio to 5:1 and offers valuable flexibility in meeting variable heat loads. It also enables the flue gas temperature to be raised to suit higher-temperature applications.
There are several other ways of increasing the power output and efficiency of gas turbines.The most significant are the addition of intercoolers, reheaters and regenerators.The first two improve the efficiency of the compressor and power turbines respectively, but require them to be split into two stages.The third reduces the primary fuel consumption but restricts the amount of heat available for site use.
Steam or water injection can also be used to enhance power output, although this does cause a minor reduction in CHP efficiency. High-pressure, high-quality steam is required for steam injection and, consequently, this practice is more commonly encountered on the larger machines.Water injected into a gas turbine must be of very high purity to avoid the risk of damage to the equipment. Injecting either water or steam can be used as an emissions reduction technique.
The use of machines incorporating any or all of these arrangements depends on the trade-off between increased complexity/capital cost and the benefits that can be realised.
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