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.

Factors that affect CCGT ramp rate

Ramp rates of CCGTs are limited to prevent thermal stress in the heat recovery steam generator (HRSG) components and steam turbine. In conventional CCGTs, the gas turbine is ramped to hold points (where the gas turbine is held at steady load) to allow steam temperatures and pressures to rise slowly within allowable material limits. Of particular concern are thick-walled components such as the high pressure steam drum, which could experience thermal fatigue if temperature and pressure increases too rapidly. However, recent CCGT advances including improved boiler designs, bypass systems to isolate the steam turbine, and attemperators that maintain steam temperatures within appropriate limits allow the gas turbine to ramp independently of the steam turbine. Technologies such as ultra-low NOx combustion systems and stand-by HRSG heating are used to reduce emissions while ramping. These improvements have resulted in reduced startup times and higher ramp rates, but such rapid cycling imposes increased CCGT maintenance costs.

Actual operating experience

Wärtsilä power plants are perfectly suited to cycling, as the ability to quickly ramp up and down in load does not affect the maintenance schedule. In addition to a fast startup time, Wärtsilä engines can stop within one minute and have lower emissions due to lean-burn technology. Even in combined cycle, Wärtsilä engines retain their loading responsiveness because the steam portion of the power plant is designed for low gas temperatures and pressures which can be maintained with only a small number of engines running. Due to the modular design of Wärtsilä power plants, the engines can be loaded and unloaded individually, providing the high plant efficiency even at part load.

Once running and at nominal operating temperatures, Wärtsilä power plants can adjust output up or down rapidly. Wärtsilä power plants can ramp from 10% to 100% load (or down) in just 42 seconds. This means that a power plant comprised of 12 Wärtsilä 50SG engines has an effective operational ramp rate of 288 MW/minute. Operational ramp rates of Wärtsilä engines and the most popular industrial frame gas turbine models of similar size (200 MW) are compared in Figure 2, with ramp rate expressed in two metrics – MW/minute and percent of full load per minute.

Figure 2: Wärtsilä engines have a significantly higher operational ramp rate than gas turbines of similar size.

The difference in performance between gas turbines and Wärtsilä power plants is evident in Figure 3, which presents a screen shot from an actual dispatch center in Colorado, U.S. The Wärtsilä Plains End power plant (red and white load curves) were used to compensate for a drop off in wind output, rapidly starting and ramping to full load within minutes. By contrast, gas turbines (purple load curve) ramped up at a much slower rate. This illustrates the flexibility provided by Wärtsilä power plants and underscores that standard measures of ramp rate do not always reflect operational capability.

Combustion Engine vs Gas Turbine Ramp Rate (3) 

Figure 3: Screen shot from a dispatch center shows the drop off in wind generation (green line) and rapid ramp up of Wärtsilä's Plains End power plant to compensate. Compared to the fast ramping of the Wärtsilä plant, gas turbine output (purple line) increases more slowly.

Further reading:
Annual Technology Baseline (2019). Prepared for the National Renewable Energy Laboratory (NREL), U.S. Department of Energy.

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