Turbine inlet air cooling
Turbine inlet air cooling (TIAC) is a group of technologies and techniques consisting of cooling down the intake air of the gas turbine. The direct consequence of cooling the turbine inlet air is power output augmentation. It may also improve the energy efficiency of the system. This technology is widely used in hot climates with high ambient temperatures that usually coincides with on-peak demand period.
Gas turbines take in filtered, fresh ambient air and compress it in the compressor stage. The compressed air is mixed with fuel in the combustion chamber and ignited. This produces a high-temperature and high-pressure flow of exhaust gases that enter in a turbine and produce the shaft work output that is generally used to turn an electric generator as well as powering the compressor stage.
As the gas turbine is a constant volume machine, the air volume introduced in the combustion chamber after the compression stage is fixed at certain shaft speed (rpm). The quantity of fuel introduced in the mixture depends on the oxygen that the air contains. The air mass in is directly related to the density of air, and the introduced volume.
where is the mass, is the density and is the volume of the gas. As the volume is fixed, only density of the air can be modified to vary air mass. The density of the air depends of the relative humidity, altitude, pressure drop and temperature.
- Density of the humid air (kg/m³)
- Partial pressure of dry air (Pa)
- Specific gas constant for dry air, 287.058 J/(kg·K)
- Temperature (K)
- Pressure of water vapor (Pa)
- Specific gas constant for water vapor, 461.495 J/(kg·K)
- Molar mass of dry air, 0.028964 (kg/mol)
- Molar mass of water vapor, 0.018016 (kg/mol)
- Universal gas constant, 8.314 J/(K·mol)
In summary, the performance of a gas turbine, its efficiency (heat rate) and the generated power output strongly depend on the climate conditions, decreasing the output power ratings by around 20 to 40%.
Different technologies are available in the market. Each particular technology has its advantages and inconveniences according to different factors such as ambient conditions, investment cost and payback time, power output increase and cooling capacity. The criteria to select the best alternative should be studied independently.
This cooling system is based on fogging nozzles and a high pressure pump system. Fogging reduces inlet air temperature by evaporating a spray of water after the filter stage but early enough that moisture cannot reach turbine compressor blades. Usually moisture eliminators are installed before the compressor stage to reduce the possibility of moisture carrying-over, which could produce serious damage in the turbine. Cooling capacity is limited by ambient conditions, and wet bulb temperature is theoretically the lowest limit. Typical fog system performance is around 80-95%, and its effectiveness is limited by the difference between dry bulb and wet bulb temperatures that depends on the relative humidity in the area.
This technology is a low-cost solution, with simple operation and low maintenance. However, this technology also presents some disadvantages: cooling beyond wet bulb temperature is impossible; performance is highly dependent on relative humidity changes; de-mineralized water usage is necessary; there is some risk of erosion to blades of the first stages in the compressor; corrosion may occur due to incomplete atomisation of injected water.
The evaporative cooler is a wetted rigid media where water is distributed throughout the header and where air passes through the wet porous surface. Part of the water is evaporated, absorbing the sensible heat from the air and increasing its relative humidity. The air dry-bulb temperature is decreased but the wet-bulb temperature is not affected. Similar to the fogging system, the theoretical limit is the wet bulb temperature, but performance of the evaporative cooler is usually around 80%. Water consumption is less than that of fogging cooling.
Cooling systems based on latent heat as the water evaporates are preferred in dry/desert climates not near the sea where the relative humidity is low, and where the system can boost the turbine output by nearly 12%. The problem is that for a desert climate, a large amount of water is a limiting factor. For warm and humid climates the evaporative-kind of air cooling system may not increase the turbine output by more than 2-3%.
Vapour compression chiller
In a mechanical compression chiller technology, the coolant is circulated through a chilling coil heat exchanger that is inserted in the filter house, downstream from the filtering stage. Downstream from the coil, a droplet catcher is installed to collect moisture and water drops. The mechanical chiller can increase the turbine output and performance better than wetted technologies due to the fact that inlet air can be chilled below the wet bulb temperature, indifferent to the weather conditions. Compression chiller equipment has higher electricity consumption than evaporative systems. Initial capital cost is also higher, however turbine power augmentation and efficiency is maximized, and the extra-cost is amortized due to increased output power.
Other options such a steam driven compression are also used in industry.
In vapor-absorption chillers technology, thermal energy is used to produce cooling instead of mechanical energy. The heat source is usually leftover steam coming from combined cycle, and it is bypassed to drive the cooling system. Compared to mechanical chillers, absorption chillers have a low coefficient of performance, however, it should be taken into consideration that this chiller usually uses waste heat, which decreases the operational cost. 
Combination with TES TANK
A thermal energy storage (TES) tank is a naturally stratified thermal accumulator that allows the storage of chilled water (or ICE) produced during off-peak time, to use this energy later during on-peak time to chill the turbine inlet air and increment its power output. A TES tank reduces operational cost and refrigerant plant capacity. One advantage is the production of chilled water when demand is low, using the excess of power generation, which usually coincides with the night, when ambient temperature is low and chillers have better performance. Another advantage is the reduction of the chilling plant capacity and operational cost in comparison with an on-line chilling system, which produce delays during periods of low demand.
Benefits of TIAC
World energy demand is increasing. One of the main reasons is the increase of cooling loads during the summer season, which coincide with the decrease of the power output in gas turbine power plants. Daily summer on-peak periods coincide with the highest temperatures, which sternly affect the efficiency and the power generation of the gas turbine. Furthermore, the price per kilowatt produced in peaking plants during on-peak periods is increased by 4 times the off-peak time. With the vapor mechanical compression technologies, the performance and the power output of the turbine will not be affected by ambient conditions during the cooling period, coinciding with on-peak demand period. This means full turbine output availability, covering the demand and avoiding the necessity to buy the shortfall energy at higher prices. Another TIAC benefit is the cheaper cost per extra TIAC kilowatt compared to new installed gas turbine kilowatt. Moreover, the extra TIAC kilowatt uses less fuel than the new turbine kilowatt due to the lower heat-rate (higher efficiency) of the chilled turbine. Other benefits are the increase of steam mass flow in a combined cycle, the reduction of turbine emissions (SOx, NOx, CO2), and reduction/elimination of the necessary space for a new installed generation plant. Use of TIAC also results in grid-wide fuel savings and emissions reduction because its use in combined cycle systems, which are more efficient than simple cycle system, eliminate the equivalent need to operate simple cycle peaking systems. Likewise, use of TIAC on simple cycle systems eliminate the need to operate lesser efficient peaking systems.
To calculate the economical and technical benefits of TIAC, a deep study should be performed to determine payback periods, taking into consideration several aspects like ambient conditions, cost of water, hourly electric demand values, cost of fuel, etc. In any case, TIAC represents a cost-effective, environmental solution for power generation, with low payback periods and short installation time.
- Load duration curve
- Thermal energy storage
- Vapor-compression refrigeration
- Demand response
- Heat recovery steam generator
- Absorption refrigeration
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- Equations - Air Density and Density Altitude
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