The performance of power plants at partial load has become a significant operational consideration for electric power grids worldwide, especially since operating regimes of thermal power plants are changing from pure base load to balancing of variable renewable energy. This technical comparison examines the range of output and the part load efficiency of combustion engines and gas turbines, and how Wärtsilä power plants deliver enhanced flexibility.
Figure 1: Dispatch curve shows cycling of coal and gas power plants to accommodate large load fluctuations from wind and solar sources in Germany in May 2021. Image: Wärtsilä Energy Transition Lab
Near-zero variable operating costs for renewable energy often result in fossil-fueled generation curtailing output to allow for wind and solar loads. This periodic adjustment of output is called cycling. For example, Wärtsilä Energy Transition Lab, based on Entso-E Transparency Platform, illustrates how thermal power plants in the EU change their loading levels in response to pricing signals driven by high amounts of variable renewable energy sources.
Simple cycle gas turbines have traditionally served as peaking units because they can be started within minutes and ramped up and down quickly to meet spikes in demand or sudden changes in electric system loads. They also have lower efficiencies – typically less than 40 percent even on full load – so they operate only when electric demand peaks and the price of electricity is high. With the expanding need for more flexible power, capacity that was designed for continuous, baseload operation is often being used to provide load-following and even peaking electric service. This is particularly true for combined cycle gas turbines (CCGTs) which can respond to changes in load faster than conventional steam power plants.
The cycling of CCGT plants presents other issues however, including increased thermal and mechanical stress on plant components and load turndown limitations. The performance of cycling power plants at part load is an important consideration for minimizing power system emissions, maintaining efficiency, and maximizing operational flexibility. The part load technical limitations and efficiency performance of combustion engines compared with gas turbines are explored below.
A technical constraint for partial load operation of gas turbine power plants is the minimum environmental load, also called the minimum emissions-compliant load. This is the lowest output at which the generating unit can operate and still meet environmental limits for nitrous oxides (NOx) and carbon monoxide (CO) emissions. The minimum environmental load for most gas turbines is about 50 percent of full output because operation at lower loads can result in reduced combustion temperature, less conversion of CO to CO2 and potential emissions permit exceedances. In combined cycle plants, the gas turbine outlet temperature must also be kept high to produce sufficient steam to power the steam turbine.
To facilitate a wider range of gas turbine output, manufacturers have introduced control systems designed to extend emissions-compliant turndown while minimizing efficiency impacts at part load. While the exact methods for turndown optimization vary from manufacturer to manufacturer, the control systems use variable guide vanes to decrease compressor mass flow and sequential firing (reheat) to produce higher combustion temperatures at low loads. Higher combustion temperatures not only enhance the conversion of CO to CO2 but also boost steam production and thus output from the steam turbine, improving overall part-load plant efficiency.
As a result, some gas turbine models can achieve emissions-compliant turndown to about 40 percent of baseload power. Site-specific conditions including the environmental permit requirements, plant configuration and post-combustion emissions control systems will ultimately dictate the exact emissions-compliant turndown limit.
For all practical purposes, combustion engine power plants do not have minimum load limitations and can maintain high efficiency at partial load due to modularity of design – the operation of a subset of the engines at full load.
Gas turbine manufacturers boast efficiencies of 55 percent or greater for combined cycle power plants, but this is the efficiency at full, or baseload power. In reality, CCGT power plants often cycle frequently. Operation at partial load and turndown limitations can restrict the flexibility of CCGT plants. To compare the performance of CCGTs, simple cycle gas turbines, and Wärtsilä combustion engines at varying load, efficiency data was produced using GT PRO. The gas turbines chosen for comparison were based on popular heavy frame industrial models well-suited for combined cycle operation that could also be used in simple cycle operation as peaking units.
Similar sized units are compared, with capacities of approximately 180 – 275 MW in simple cycle, and 235 – 310 MW when running in combined cycle mode (depending on ambient conditions). This assumes a 1x1 CCGT configuration (one gas turbine and heat recovery steam generator supplying one steam turbine), air-cooled condensers and a bypass stack to isolate the steam generating portion of the plant from the gas turbine.
Figure 2 shows efficiency curves for plants operating at summer ambient conditions of 25ºC (77ºF). The efficiencies of CCGTs drop below 50 percent between 55 to 65 percent of full load. In simple cycle mode, the degradation of gas turbine efficiency is more pronounced, with gas turbines dropping to less than 30 percent efficiency at half load. The minimum environmental load of 50 percent for typical GT turndown and 40 percent for extended turndown is noted in Figure 2. For a 300 MW combined cycle plant, this means that the minimum emissions-compliant output is between 120 to 150 MW.
Unlike gas turbines, Wärtsilä engine power plants have near full range capability of emissions-compliant turndown. Minimum load per engine can be as low as 10%, allowing for feasible operation on spinning reserve markets. When plant efficiency must be optimised while load is decreased, individual engines within the generating set are shut down to reduce output. The engines that remain operating can generate at full load, retaining high efficiency of the generating set. As a result, engine power plants provide a much wider range of output flexibility than gas turbines without the constraints of turndown limitations or efficiency impacts.
Figure 3: Net efficiency at different loads
* A power plant with 5x W31SG can operate at 2% of nominal plant output
** Technical limits allow GT operation at 20% load, in reality environmental regulations limit min load to 40-50%
Figure 4: Unit minimum stable load*
* Gas turbine minimum loads with typical emission limits