Combustion Engine vs Gas Turbine: Derating due to Ambient Temperature
The increasing need for flexible power across the world, often in harsh climates, makes power plant performance under varying conditions an important consideration in technology selection. As combustion engines are less sensible to temperature and humidity, Wärtsilä power plants outperform gas turbines in hot conditions.
Depending on the technology and site conditions, a power plant’s actual electrical output, efficiency, and fuel consumption can be quite different than its performance at design conditions. Ambient conditions can vary dramatically with geographic location and by season. For example, summer temperatures in the Middle East and northern Africa frequently exceed 40°C (104°F), while large seasonal temperature swings of over 38°C (100°F) occur in some locations. As surging temperatures usually correspond to peak electrical demand, reduction in power output at high ambient temperatures can be problematic. Gas turbines in particular can experience significant performance derating in hot, humid conditions.
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.