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 250 MW/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 is called the ramp rate and is usually expressed as megawatts per minute (MW/min).
Ramp rates of most industrial frame gas turbine models are advertised as 10 MW/min up to 100 MW/min, with an average of about 25 MW/min. 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 100 MW/min is based on multi-turbine plant designs, such as a 2x1 combined cycle gas turbine (CCGT) plant (net power output of 750 MW) where each gas turbine is rated to ramp at 50 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 of 35 to 50 MW/min are achievable only after the unit has reached self-sustaining speed. The fastest loading gas turbine models produce 30% load delivery after 7 minutes and take nearly 30 minutes to reach full output under hot start conditions. Wärtsilä 34SG combustion engines have true quick start capability – an effective ramp rate of 50% per minute, reaching full load within 2 minutes. For a 200 MW plant, this equates to 100 MW/min.
The starting load delivery of Wärtsilä power plants and gas turbines is compared in Figure 1, showing the percentage of load delivered 7 minutes after startup. This assumes optional gas turbine technology for enabling fast loading and is based on manufacturer-published ramp rates. 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 34SG and 50SG engines have already reached full load.