An inlet air cooling system installed in a desert-dry area to increase turbine power output

Turbine inlet air cooling 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.^[1] This technology is widely used in hot climates with high ambient temperatures that usually coincides with on-peak demand period.^[2]

Principles

Effect of inlet air cooling on power output

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 for a given shaft speed (rpm). Thus the air mass flow in is directly related to the density of air, and the introduced volume.

${\displaystyle {m}={\rho }{V}}$,

where ${\displaystyle m}$ is the mass, ${\displaystyle \rho }$ is the density and ${\displaystyle {V}}$ is the volume of the gas. As the volume ${\displaystyle {V}}$ is fixed, only density ${\displaystyle \rho }$ of the air can be modified to vary air mass. The density of the air depends on the relative humidity, altitude, pressure drop and temperature.

${\displaystyle \rho _{\,\mathrm {humid~air} }={\frac {p_{d}}{R_{d}T}}+{\frac {p_{v}}{R_{v}T}}={\frac {p_{d}M_{d}+p_{v}M_{v}}{RT}}\,}$^[3]

where:

${\displaystyle \rho _{\,\mathrm {humid~air} }=}$ Density of the humid air (kg/m³)

${\displaystyle p_{d}=}$ Partial pressure of dry air (Pa)

${\displaystyle R_{d}=}$ Specific gas constant for dry air, 287.058 J/(kg·K)

${\displaystyle T=}$ Temperature (K)

${\displaystyle p_{v}=}$ Pressure of water vapor (Pa)

${\displaystyle R_{v}=}$ Specific gas constant for water vapor, 461.495 J/(kg·K)

${\displaystyle M_{d}=}$ Molar mass of dry air, 0.028964 (kg/mol)

${\displaystyle M_{v}=}$ Molar mass of water vapor, 0.018016 (kg/mol)

${\displaystyle R=}$ Universal gas constant, 8.314 J/(K·mol)

The performance of a gas turbine, its efficiency (heat rate) and the generated power output strongly depend on the climate conditions, which may decrease the output power ratings by up to 40%.^[4] To operate the turbine at ISO conditions^[5] and recover performance, several inlet air cooling systems have been promoted.

Applied technologies

Filter-house modified to place the heat exchanger after the filtering stage.

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.

Vapour compression chiller

Turbine inlet air cooling filter-house modification to place the cooling coil coming from ammonia compression chiller plant

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.^[16] 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.

The majority of such systems involve more than one chiller unit and the configuration of the chillers can have a great bearing on the system parasitic power consumption. The series counterflow configuration can reduce the compressor work needed on each chiller, improving the overall chiller system by as much as 8%.^[17]

Other options such a steam driven compression are also used in industry.^[18]

Benefits

In areas where there is demand cooling, daily summer on-peak periods coincide with the highest atmospheric temperatures, which may reduce the efficiency and power gas turbines. With the vapor mechanical compression technologies, cooling can be used during these periods so that the performance and the power output of the turbine may be less affected by ambient conditions

Another benefit is the lower cost per extra inlet-cooling kilowatt compared to newly installed gas turbine kilowatt. Moreover, the extra inlet-cooling kilowatt uses less fuel than the new turbine kilowatt due to the lower heat-rate (higher efficiency) of the chilled turbine. Other benefits may include the incrementation of steam mass flow in a combined cycle, the reduction of turbine emissions (SOx, NO_x, CO₂),^[21] and increase in power-to-installed volume ratio.

Calculating the benefits of turbine air cooling requires a study to determine payback periods, taking into consideration several aspects like ambient conditions, cost of water, hourly electric demand values, cost of fuel.^[22]

See also

• Psychrometrics
• Load duration curve
• Thermal energy storage
• Vapor-compression refrigeration
• Demand response
• Heat recovery steam generator
• Absorption refrigeration
• Water injection (engine)

References

External links

• Turbine Inel Cooling Association
• Heat Rate Basics
• On Peak Demand Techniques
• ASHRAE American Society of Heating, Refrigerating and Air Conditioning Engineers
• International Energy Agency