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With so many sources of renewable generation now on the grid, and lots more on the way, it’s important to gain an understanding of how future dispatchable resources will need to behave in order to accommodate increased renewable generation. A recent study shed some light in this area. What is clear from the findings is that combined cycle plants are due to play a greater role than simple cycle plants in stabilizing the grid in the presence of supply fluctuations from renewable sources.
Modern combined cycle plants are designed to be flexible while providing lower greenhouse gas emissions and a lower cost of generation compared to simple cycle technologies. Simple cycle plants are easier to maintain in a start-ready condition and have advantages for non-spinning reserve. But study projections show that such capabilities are not broadly needed for renewable integration.
Combined cycle plants use waste exhaust heat from the gas turbine exhaust to power a steam cycle and produce more electricity with no additional fuel. This requires the presence of a heat recovery steam generator (HRSG) and a steam turbine, and results in a combined cycle having much higher plant efficiency and lower operating costs, but higher capital costs than a simple cycle with the same gas turbine.
Lower-efficiency simple cycle plants, with lower capital cost, have typically been used as low-dispatch plants, available to cover high peaks in demand, while higher-efficiency combined cycle plants have traditionally been used for base- or intermediate-load duty. Modern combined cycles, such as Siemens Flex- Plant, can start up quickly and load follow over a wide output range, offering the flexibility to react to fluctuations in renewable generation.
Efficiency directly impacts the cost of generation, and consequently the frequency and duration of dispatch. For U.S. grid operations, the first plant dispatched is the one which can meet the demand at the lowest cost. Combined cycle plants enable a much lower levelized cost of electricity (LCOE) than simple cycle plants and will therefore be dispatched first and have greater overall use.
California Independent System Operator (CAISO) oversees a grid that needs to accommodate 33% renewables by 2020, according to the state’s aggressive Renewable Portfolio Standard (RPS). This will amount to as much as 5.5 GW of solar and 4.2 GW of wind, and perhaps another 3 GW of imported solar and wind resources.
Grid balancing authorities, such as CAISO, need to maintain a constant and precise balance between supply and demand. To deliver significant quantities of renewable energy and maintain a stable grid without curtailing demand, the rapid changes in the energy supplied by renewables must be balanced by controllable resources. These balancing resources need to be flexible to match system demand.
A recent CAISO study used computer simulation to forecast the energy flow and balancing requirements for the year 2020. From that simulation, CAISO determined that at least 5.3 GW of available resources would need to be sufficiently flexible to maintain grid reliability.
CAISO simulated the economic dispatch of a specific set of replacement resources that would be needed to satisfy requirements. In the model, the heat rate and startup costs of simple cycle plants closely matched a 100 MW aeroderivative gas turbine and the combined cycle plant characteristics are consistent with 2x1 F-class triple pressure technology.
These comparisons revealed the superiority of combined cycle over simple cycle technology. The study projected that with increased renewable generation, combined cycle plants would be well utilized. This is driven by the expectation that they will be dispatched based on favorable economics and will be online to supply balancing services, such as rapid ramping.
The modeled combined cycle plant averaged an output of between 700 MW and 800 MW out of its capacity. Its range of available output allowed it to provide upward load following and ancillary services between 100 and 150 MW each hour on average, and downward load following and ancillary services of 200 to 230 MW each hour. The CAISO study concluded that combined cycle plants were more economical than simple cycle plants and will be used more, thus allowing them to amortize their higher capital costs.
CAISO dispatch data was combined with economic information generated by the California Energy Commission (CEC). Plant cost rates from a CEC study were combined with the heat rates and capacity factors from the CAISO study to determine LCOE. The estimated LCOE for the modeled simple cycle plants ranged from $345 to $580 per MWh, while LCOE for combined cycle plants ranged from $135 to $140 per MWh (Figure).
In addition to enabling a lower cost of electricity, higher efficiency results in lower greenhouse gas generation. A 500 MW plant which dispatches 30% of the time using high efficiency simple cycles produces in excess of 400 million pounds more CO2 than generating the same power from a combined cycle with an efficiency of 58%. Flex-Plants with Clean-Ramp technology also offer the ability to maintain low NOx and CO while load following.
Modeling projects that the vast majority of future load balancing services can be met by combined cycle plants. They are projected to be dispatched for longer periods of time and to stay online for virtually the entire day, meeting ramping demands through load changes, offering lower cost electricity, and producing lower emissions than simple cycle options for the same generation.