Precooling of the Inlet Air
Evaporative cooling boosts the output of the gas turbine by increasing the density and mass flow of the air. Water sprayed into the inlet air stream cools the air to
Table 1.2 Typical Combined Cycle Plants
Source: Ref. 9.
Near its wet bulb temperature. The effectiveness of the evaporative cooling systems is limited by the relative humidity of the air. At 95°F dry bulb temperature and 60% relative humidity, an 85% effective evaporative cooler can alter the air inlet temperature and moisture content to 85°F dry bulb and 92% humidity, respectively This boosts the gas turbine output and the HRSG steam generation (due to the larger gas mass flow). The incremental cost of this system is about $180/kW. The cost of treated water, which is lost to the atmosphere, must also be considered in evaluating this system. The effectiveness of the same system in less humid conditions, say 95°F and 40% relative humidity, is much higher. The same evaporative cooler can reduce the inlet air temperature to 75°F dry bulb and 88% humidity. The combined cycle plant output increases by 7%, and the heat rate by about 1.9%. With evaporative coolers, the air cannot be cooled below the wet bulb temperature, so chillers are used for this purpose.
Chillers can be mechanical or absorption systems. Water is the refrigerant, and lithium bromide (LiBr) is the absorber in single-effect LiBr absorption systems. A low grade heat source such as low pressure steam drives the absorption process, which produces chilled water. Absorbers draw little electrical power and are well suited to cogeneration plants where steam is readily available. Sometimes the HRSG generates the low pressure steam required for chilling, or it can be taken from some low pressure steam header. Unlike mechanical chillers, the efficiency of an absorber is unchanged as its load is decreased. Chilled water output is limited to around 44°F, yielding inlet air at 52°F.
A mechanical chiller can easily reduce the temperature of GT inlet air from 95°F to 60°F dry bulb and achieve 100% humidity. This increases the plant output by 8.9% but also degrades the net combined cycle heat rate by 0.8% and results in a 1.5 in. WC inlet air pressure drop due to the heat exchanger located at the chilling section. Costs could be about $165/kW. Absorption systems are more complex than mechanical chillers.
Off-peak thermal storage is another method of chilling inlet air. A portion of the plant’s electrical or thermal output is used to make ice or cool water during lean periods. During peak periods, the chilling system is turned off and the stored ice is used to chill the inlet air.
The performance of HRSGs with varying ambient temperatures is discussed later. One can appreciate from the example why inlet air cooling is necessary, particularly in locations where ambient temperatures are very high.