By: Donald Jones, retired nuclear industry engineer, 2019 October 15.
Combined Cycle Gas Turbine (CCGT) power plants produce electricity very efficiently with steady state baseload and intermediate load overall efficiencies of over 60 percent for newer units. Most presently operating units would have somewhat lower efficiencies. Unfortunately on most power grids with significant wind and/or solar generation they rarely operate at steady state (reference 1).
Operating CCGT plants at part load and stopping and starting the gas turbine(s) leads to lower plant efficiency and increased emissions of greenhouse gases per megawatt-hour generated as well as wear and tear on the units. Every time a CCGT is warming up gas is being burned while the gas turbine is slowly increasing power and warming up the heat recovery steam generators (HRSG), and the steam turbine when steam becomes available. This increases the heat rate giving higher kg GHG/MWh. If a CCGT had a bypass stack the gas turbine could be delivering power very quickly but at high heat rate (low overall efficiency) since it would be operating simple cycle and giving higher kg GHG/MWh. When increasing power (manoeuvring/ramping) in the operating range the HRSG and steam turbine metal have to be warmed up which takes away gas for no useful power output, increasing kg GHG/MWh.
Ontario has a very low carbon intensity electricity grid averaging 40 g CO2e/kWh. In 2018 more than 93 percent of the electricity generated in Ontario came from non-GHG emitting resources, predominantly nuclear and hydro with some wind and solar. Nuclear provided 61 percent of generation in 2018. There is about 10000 MW of gas-fired and oil-fired generation connected to the transmission grid mostly CCGTs burning natural gas and just under 5000 MW (nameplate capacity) of wind and solar. CCGTs are used to meet peak load demands and provide operational flexibility.
If the load on the grid can be partly met by CCGTs operating at full power at their best efficiency point in steady state this will result in minimum GHG emissions. If it is met from more CCGTs operating at fluctuating lower powers, higher heat rate (lower overall efficiency) and consequently a higher kg GHG/MWh, in order to get ramping capacity and operating reserve capability on the grid, the absolute GHG emissions would be higher. Ontario’s Independent Electricity System Operator (IESO) makes use of the flexibility of its transmission connected wind and solar generation, when it is available, to replace the ramping and balancing capacity of the CCGTs so that fluctuating part load operation can be avoided and GHG emissions minimized. As the IESO puts it in its Media Centre’s, 2018 Electricity Data, “The IESO’s variable generation (VG) dispatch allows the system operator to harness the flexibility of wind and solar to help balance the electricity system. In mild weather conditions, the VG dispatch also helps avoid nuclear shutdowns during periods of surplus baseload generation (SBG).” If wind and solar were not available the variability of the grid would be reduced anyway and CCGTs could operate more in steady state.
There will always be a need for some steady state part load operation of the CCGTs so how does CCGT part load operation affect GHG emissions. For any particular CCGT provided there is not a precipitous drop in overall efficiency at steady state part load operation, which is normally the case with CCGTs, there will be a decrease in absolute GHG emissions relative to steady state full load because of the power reduction itself. How much less depends on the part load power level and the overall efficiency at that power level.
GHG absolute emissions at steady state part load could be more or less than those at full load depending on the reduced power level and the GHG emissions per unit of output at the reduced power level that depend on the overall efficiency at the reduced power level. The break even point is when the absolute emissions, the product of power (MW) and emissions per unit of output (kg GHG/MWh), are the same at the reduced power level as they are at full power. For example what is the break even point for a CCGT power plant operating at a reduced power level of 50 percent full power. To make the product of power and emissions per unit of output the same at reduced power as it is at full power means the emissions per unit of power at 50 percent of full power must be two times those at full power meaning the efficiency at 50 percent full power must be half that at full power. If it is less than two, meaning efficiency is greater than 50 percent of the efficiency at full power, the absolute emissions will decrease from those at full power and if it is more than two, meaning efficiency is less than 50 percent of that at full power, the absolute emissions will increase. So, break even point at part load of 50 percent full power is when efficiency is 50 percent of that at full power. If efficiency at 50 percent full power is same as at full power the absolute emissions at 50 percent full power would be half those at full power. Similarly at 25 per cent power the break even point is when the emissions per unit of output at 25 percent power is four times more than that at full power meaning when the overall efficiency is four times less than at full power.
The IESO is making optimum use of the rapid response of its wind and solar generation resources, that were foisted on it by a previous government, to reduce GHG emissions. Starting in 2013 September wind generation could be curtailed when flexible nuclear manoeuvring (from Bruce Power) reached its limit and before any nuclear unit had to be shutdown and go off line with subsequent increases in GHG emissions. Starting early 2016 the rules changed again allowing flexible wind generation and solar generation to be curtailed before manoeuvring down nuclear units. Now, nuclear refurbishment (Darlington and Bruce) and closures (Pickering) over the coming years will require increased use of the CCGT resources and result in increases in the carbon intensity and in absolute GHG emissions. To minimize GHG emissions the CCGTs need to be operated in steady state, without load changes, at their best efficiency point (full power) as much as possible. This also leads to less wear and tear on the units and more dependable operation. The rapid response to load change demands of the transmission connected wind and solar generators allow this to be done. If there were no wind and solar available there would be less, not no, need to manoeuvre the CCGTs with consequent effect on efficiency and emissions, in the first place. The longer term solution of course is to replace the CCGTs with flexible nuclear.
1. Duke Energy application points finger at solar for increased pollution, 2019 August 14, https://nsjonline.com/article/2019/08/duke-energy-application-points-finger-at-solar-for-increased-pollution/