A gas turbine aeroengine must remain light and compact so it is not possible to add to it significantly in order to improve its performance. The stationary turbine for power generation does not suffer this restriction. Taking advantage of this greater freedom, engineers have explored a number of strategies that can be applied to stationary gas turbines in order to provide significant performance enhancements.
In large steam-turbine-based power plants it is traditional to split the turbine into separate sections, one handling high-pressure steam, one handling medium-pressure steam and a third handling low-pressure steam. By splitting the turbine in this way, efficiency gains can be made through matching the individual turbine sections to operate under a narrower range
Figure 4.3 Block diagram showing advanced gas turbine cycles: (a) reheating, (b) intercooling and (c) recuperation. LP: low pressure; HP: high pressure
Figure 4.3 Block diagram showing advanced gas turbine cycles: (a) reheating, (b) intercooling and (c) recuperation. LP: low pressure; HP: high pressure of steam pressures. Further, once the turbine has been split into separate sections, additional efficiency gains can be made by reheating the steam when it exists the high-pressure turbine (where it will have cooled) and before it enters the medium-pressure turbine. This is a common feature of the steam turbines used in coal-fired power plants.
A gas turbine can also be split in a similar way, though normally only two separate sections, called spools, are used. But again, once the turbine has been split into sections, it is possible to introduce a second combustion stage to reheat the air between the higher-pressure and the low-pressure section of the turbine. Using reheating makes the turbine more efficient, just as in the case of the steam turbine.
Reheat is already making an appearance in gas-turbine-based power plants. A 1000 MW plant in Monterrey in Mexico uses four gas turbines in which the hot gas is passed through a second combustor after the first stage of turbine blades before passing through the remaining four sets of blades.
It is possible to go a stage further with a gas turbine, by splitting the compressor into two sections: a low-pressure compressor section and a high-pressure compression section. And like the reheating of the air between the two sections of the turbine, it is possible to improve efficiency by cooling the air between the two sections of the compressor. (Compressing air tends to heat it and hot air occupies a larger volume. Cooling it reduces the volume so the compressor actually has less work to do.) This is called intercooling.
Intercooling a high-performance aeroderivative gas turbine (that is, a gas turbine for power generation based directly on an aeroengine) will boost its efficiency by around 5%, double its power output and substantially reducing the cost per kilowatt of generating capacity.7
Yet another strategy for increasing the efficiency of an aeroderivative gas turbine is to inject water vapour into the compressed air before the gas turbine combustion chamber. This system, called the humid air turbine cycle (HAT cycle), has a history dating back to the 1930s but it was only during the 1980s that an effective way of building such a turbine was devised.
The HAT cycle works because it requires less work from the compressor to deliver the same mass of gas into the turbine. The mass of water added to the compressed air tips the balance. It has been estimated that a 11 MW cascaded HAT cycle (CHAT cycle) unit incorporating humid air, intercool-ing and reheat could achieve an efficiency of 44.5%.8 More striking still, a 300 MW CHAT turbine system would have an estimated efficiency of 54.7% and could prove cheaper than a gas-turbine combined cycle plant.
One disadvantage of HAT and CHAT cycle power units is that they release a considerable amount of water vapour into the environment. In situations where water is scarce it may be necessary to recover the water from the exhaust gas.
A fourth strategy for improving the performance of a gas turbine is to use heat from the turbine exhaust to partially heat the compressed air from the compressor before it enters the combustion chamber. This process, referred to as recuperation, results in less fuel being needed to raise the air to the required turbine inlet temperature.
Effective recuperation systems have been under development for several years. At the end of 1997, the US company Solar Turbines introduced a 3.2 MW gas turbine for power generation applications with a claimed efficiency of 40.5% using recuperation. This unit was developed under the US Department of Energy (DOE) Advanced Turbine Systems (ATS) programme. Other companies involved in the ATS programme include Pratt and Whitney which is developing a high-efficiency small gas turbine and GE Power Systems and Siemens-Westinghouse, both of which are working on high efficiency, large base-load combined cycle units. These are expected to yield overall efficiencies of 60% combined with low emissions.
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