Combustion Engine vs Gas Turbine: Ramp Rate
Power plant flexibility is recognized as a vital tool to manage variability in electric loads and provide grid support services. One measure of this flexibility is ramp rate – the rate at which a power plant can increase or decrease output. Wärtsilä engines can ramp at over 100%/minute, much faster than gas turbines, providing ultra-responsive power that is needed to integrate renewable energy.
Flexible generating units help provide stability to the electric grid by ramping output up or down as demand and system loads fluctuate. Because solar and wind generation can change within minutes, electric grid operators rely on power plants that can provide additional load (or curtail load) on the same timescale as variations in renewable output. The increase or reduction in output per minute in spinning mode is called the ramp rate and is usually expressed either as% per minute or MW per minute.
Ramp rates from spinning mode of most industrial frame gas turbine models are around 20% / minute and around 50% / minute for aeroderivative gas turbines. For combined cycle gas turbines, typical ramp rates are around 10 % / minute. Alternatively, ramp rates are sometimes expressed as MW / minute. Ramp rate depends on generating unit capacity, operating conditions (whether unit is just starting up or operating at a minimum load hold point) and optional technologies for reducing startup time and increasing ramp rate. The ramp rate of a power plant also depends on the number of units and configuration. For example, a ramp rate of 110 MW/min is based on multi-turbine plant designs with large unit capacity, such as a 2x1 combined cycle gas turbine (CCGT) plant
(net power output of 880 MW) where each gas turbine is rated to ramp at 55 MW/min. While ramp rate in MW/minute is a valuable metric, it is important to understand the operating conditions under which advertised ramp rates can be achieved.
Starting loading capability vs ramp rate
The starting loading capability is often quite different than the advertised ramp rate for gas turbines. Gas turbine ramp rates are typically achievable only after the unit has reached self-sustaining speed. Wärtsilä combustion engines have true quick start capability –reaching full load within 2 minutes from start command.
The starting load delivery of Wärtsilä power plants and typical gas turbines is compared in Figure 1.The fast startup time of Wärtsilä engines provides a significant operational advantage over gas turbines. As gas turbines are just producing output, both the Wärtsilä power plants have already reached full load.