Combustion Engine vs. Gas Turbine: Pulse Load Efficiency and Profitability
Natural gas-fired power plants are the most responsive and flexible power generating assets in electricity markets. Natural gas power plants are dispatchable and can adjust load daily, ramping up and down with demand and balancing the intermittent production of renewable energy sources. For short duration pulse load needed to balance solar and wind output, gas turbines are not profitable. Wärtsilä combustion engine power plants offer significant advantages over gas turbines, with higher pulse load efficiency and the capability to provide ultra-flexible and cost-efficient output.
Gas power plants are often ramped up and down, or cycled, in response to changes in demand and load. The time required to ramp up and down, efficiency at part load and overall duration or plant output determine the fuel consumption and economic performance of the power plant. Unlike baseload plants that operate continuously, flexible power plants can provide short-duration load, or pulse load. Pulsed loads are produced in response to the sudden loss of a power generator, reduced output from wind and solar sources, or spikes in demand. Pulse generation improves reliability and power quality by stabilizing the electric grid.
A recent assessment by the California Independent System Operator (CAISO) demonstrated the need for significant pulse generation over two hours during the morning peak (8,000 MW) and the end of the work day (13,500 MW) to meet demand when solar and wind are not producing , as shown in Figure 1. This challenge is not unique to California. Many countries are adopting energy policies to achieve 20 percent or more from renewable sources. Surges in wind and solar output in Germany recently peaked at 59 percent of electric generation during a single day, causing conventional power plants to ramp down significantly.
Figure 1: CAISO Load Profile Demonstrates Need for Pulsed Loads
Image: Combined Cycle Journal
Pulse Operation and Energy Consumption
Power plant owners want to optimize operation to minimize fuel consumption and maximize revenue. The startup time, time to ramp up output to full load, operating at a minimum load, and the efficiency at part load determine the amount of fuel consumed (energy input) and the length of time the plant operates each day. By producing output during peak demand times, the power plant earns revenue from high electric tariffs. Ramping up and down should be as fast as possible without risking damage to the power plant equipment.
Combined cycle gas turbine (CCGT) plants have technical constraints that affect their startup time, ramp rate, and minimum load for maintaining hot conditions. Hot start conditions maintain temperature and pressure in the steam portion of the combined cycle to allow quick ramping of the gas turbine. Cycling raises concerns about increased air emissions and thermo-mechanical stress on CCGT plant equipment. Wärtsilä engines have lower exhaust gas temperatures (360ºC) compared with a gas turbine (600ºC) and thus lower steam temperatures, enabling quicker startup and ramping to full load.
Typically, a load following CCGT ramps up slowly during early morning hours, operates at full load during morning peak demand, curtails output during midday hours to a minimum operating load at which hot conditions are maintained (40 to 50 percent of full load), then ramps back up again to full load for the evening peak. The plant is shut down at night and then ramped up again in the early morning hours the next day. This double pulse load profile, also called “two-shifting” or “two-cycling” for a typical CCGT compared with a Wärtsilä Flexicycle™ combustion engine power plant is shown in Figure 2. The duration of each pulse is 4 hours, with a curtailment in between pulses of 2 hours (4–2–4 pulse production). The shaded area in Figure 2 shows when the power plant would be receiving revenue from pulse production.