run for a short or highly variable time, it does not make economic sense to make it as efficient as a base load power plant. In addition, the equipment and fuels used in base load plants are often unsuitable for use in peaker plants because the fluctuating conditions would severely strain the equipment. For these reasons, nuclear, waste-to-energy, coal and biomass are rarely, if ever, operated as peaker plants. Renewable energy As countries trend away from fossil fuel-fired base load plants and towards renewable but intermittent energy sources such as wind and solar, there is a corresponding increase in the need for grid energy storage systems, as renewable alternatives to building more peaking or load following power plants. Another option is broader distribution of generating capacity, through the use of grid interties, such as the WECC Intertie Paths. Types Peaker plants are generally gas turbines or gas engines that burn natural gas. A few burn biogas or petroleum-derived liquids, such as diesel oil and jet fuel, but they are generally more expensive than natural gas, so their use is limited to areas not supplied with natural gas. However, many peaker plants are able to use petroleum as a backup fuel, as storing oil in tanks is easy. The thermodynamic efficiency of simple-cycle gas turbine power plants ranges from 20 to 42%, with between 30 and 42% being average for a new plant. For greater efficiency, a heat recovery steam generator (HRSG) is added at the exhaust. This is known as a combined cycle plant. Cogeneration uses waste exhaust heat for process, district heating or other heating uses. Both of these options are used only in plants that are intended to be operated for longer periods than usual. Natural gas and diesel generators with reciprocating engines are sometimes used for grid support using smaller plants. Another option for increased efficiency and power output in gas turbines is installing a turbine inlet air cooling system, that cools down the inlet air temperature increasing mass flow ratio. This option, in combination with a thermal energy storage tank, can increase the turbine power output in on-peak periods up to 30%. Hydroelectric dams are intentionally variable; they can generate less during off-peak and quickly respond to peak demands, consequently hydroelectricity may function as load following or a peaking plant and with sufficient water, a base-load plant. Natural gas turbines or pumped storage are often used