A load-following power plant, regarded as producing mid-merit or mid-priced electricity, is a power plant that adjusts its power output as demand for electricity fluctuates throughout the day. Load-following plants are typically in between base load and peaking power plants in efficiency, speed of start-up and shut-down, construction cost, cost of electricity and capacity factor.
Base load and peaking power plants Base load power plants are dispatchable plants that tend to operate at maximum output. They generally shut down or reduce power only to perform maintenance or repair or due to grid constraints. Power plants operated mostly in this way include coal, fuel oil, nuclear, geothermal, run-of-the-river hydroelectric, solar, biomass and combined cycle natural gas plants. Peaking power plants operate only during times of peak demand. In countries with widespread air conditioning, demand peaks around the middle of the afternoon, so a typical peaking power plant may start up a couple of hours before this point and shut down a couple of hours after. The duration of operation for peaking plants varies from a good portion of the waking day to only a couple of dozen hours per year. Peaking power plants include hydroelectric and gas turbine power plants. Many gas turbine power plants can be fueled with natural gas, fuel oil, and/or diesel, allowing greater flexibility in choice of operation- for example, while most gas turbine plants primarily burn natural gas, a supply of fuel oil and/or diesel is sometimes kept on hand in case the gas supply is interrupted. Other gas turbines can only burn a single fuel.
Load-following power plants By way of contrast, load-following power plants usually run during the day and early evening, and are operated in direct response to changing demand for power supply. They either shut down or greatly curtail output during the night and early morning, when the demand for electricity is the lowest. The exact hours of operation depend on numerous factors. One of the most important factors for a particular plant is how efficiently it can convert fuel into electricity. The most efficient plants, which are almost invariably the least costly to run per kilowatt-hour produced, are brought online first. As demand increases, the next most efficient plants are brought on line and so on. The status of the electrical grid in that region, especially how much base load generating capacity it has, and the variation in demand are also very important. An additional factor for operational variability is that demand does not vary just between night and day. There are significant variations in the time of year and day of the week. A region that has large variations in demand will require a large load following or peaking power plant capacity because base load power plants can only cover the capacity equal to that needed during times of lowest demand. Load-following power plants can be hydroelectric power plants, diesel and gas engine power plants, combined cycle gas turbine power plants and steam turbine power plants that run on natural gas or heavy fuel oil, although heavy fuel oil plants make up a very small portion of the energy mix. A relatively efficient model of gas turbine that runs on natural gas can also make a decent load-following plant.
Gas turbine power plants Gas turbine power plants are the most flexible in terms of adjusting power level, but are also among the most expensive to operate. Therefore, they are generally used as "peaking" units at times of maximum power demand or Combined cycle or cogeneration power plants where turbine exhaust waste heat can be economically used to generate additional power and thermal energy for process or space heating.
Diesel and gas engine power plants Diesel and gas engine power plants can be used for base load to stand-by power production due to their high overall flexibility. Such power plants can be started rapidly to meet the grid demands. These engines can be operated efficiently on a wide variety of fuels, adding to their flexibility. Some applications are: base load power generation, wind-diesel, load following, cogeneration and trigeneration.
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