Combustion Engine vs Gas Turbine: Derating due to Ambient Temperature

How do ambient conditions affect power plant output and efficiency?

Ambient temperature, altitude and humidity affect the density of air. Hot and humid air is less dense than dry, cooler air. In gas turbines, power output is dependent on the mass flow through the compressor. As the density of air decreases, more power is required to compress the same mass of air. This reduces the output of the gas turbine and decreases efficiency. Studies have found that gas turbine efficiency deteriorates by one percent for every 10 degree rise in temperature above ISO conditions¹. This translates into a power output reduction of 5 to 10 percent, depending on the type of gas turbine. Gas turbine manufacturers use various techniques to cool inlet air and boost turbine output, including evaporative coolers and mechanical chillers. However, inlet air cooling requires additional power consumption, and the efficacy of cooling systems is highly dependent on the ambient humidity. Combustion engines are less sensitive to temperature and humidity, retaining their rated efficiency and power output over a broader range of ambient conditions.

The performance of simple cycle gas turbines, combined cycle gas turbines (CCGT) and Wärtsilä combustion engines at varying ambient conditions was assessed using data from GT PRO. Popular model heavy frame industrial gas turbines were compared with similarly sized Wärtsilä engines, with capacities of 200 – 275 MW in simple cycle, and approximately 300 MW in combined cycle (see fact box at end of article for full load output of the specific models compared). For combined cycle operation, a 1x1 CCGT configuration was assumed with air-cooled condensers and a bypass stack to isolate the steam generating portion of the plant from the gas turbine. Figure 1 presents the net power plant output at varying ambient temperatures ranging from 10°C to 40°C (50°F to 104°F) for gas turbines and Wärtsilä combustion engines operating in combined cycle. CCGT output decreases by 15 to 18 percent at 40°C compared to ISO reference conditions, while the Wärtsilä Flexicycle™ plant output decreases by only 8 percent compared to reference conditions.

The impact on plant efficiency is shown in Figure 2 for both combined cycle and simple cycle operation. At an ambient temperature of 40°C, CCGT efficiency decreases by 3.5 percent compared to ISO conditions. In a Flexicycle™ power plant using combustion engines, efficiency only drops by 1.1 percent at 40°C. All values represent net efficiency at the high-voltage grid side at sea level pressure. In simple cycle operation, Wärtsilä power plants demonstrate significant efficiency advantages over gas turbines. While simple cycle efficiency of a gas turbine is approximately 35 percent at 40°C, Wärtsilä efficiency is over 45 percent. The impact of ambient temperature on efficiency becomes even more pronounced when the plant is operating at part load. As a result, Wärtsilä power plants offer a wide range of operational flexibility and reliable performance, even in harsh ambient conditions.