Load following power plant
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A load following power plant is a power plant that adjusts its power output as demand for electricity fluctuates throughout the day. Load following plants are in between base load and peaking power plants in efficiency, speed of startup and shutdown, construction cost, cost of electricity and capacity factor.
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[edit] Base load power plants
Base load power plants operate continuously at maximum output. They only shut down to perform maintenance or if something breaks. They produce electricity at the lowest cost of any type of power plant. Base load power plants include coal, fuel oil, nuclear, geothermal, hydroelectric, biomass and combined cycle natural gas plants.
[edit] Peaking power 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. However, the duration of operation for peaking plants varies from a good portion of the waking day to only a couple 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 or diesel. Most plants burn natural gas, but a supply of diesel is sometimes kept on hand in case the gas supply is interrupted. Other gas turbines can only burn either diesel or natural gas.
[edit] Load following power plants
Load following power plants run during the day and early evening. 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 online 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 also 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 and/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 include hydroelectric 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.
[edit] Gas turbine power plants
Gas turbine power plants are the most flexible in terms of adjusting power level, but are also among the most exepensive to operate. Therefore they are generally used as "peaking" units at times of maximum power demand.
[edit] Hydroelectric power plants
Hydroelectric power plants can operate as base load, load following or peaking power plants. They have the ability to start within minutes, and in some cases seconds. How the plant operates is heavily dependent on its water supply. Many plants do not have enough water to operate anywhere near their full capacity on a continuous basis. Plants that have a large amount of water may operate as base load or as load following power plants. Those that have limited amounts of water may operate as peaking power plants. Also, the plants may change their operating style depending on the time of year. For example, the plant may operate as a peaking power plant during the dry season, and as a base load or load following power plant during the wet season. This is often done when the reservoir frequently reaches full capacity and water either has to be used for electricity generation or be released through the spillway. Another factor is whether the plants have to release significant quantities of water downstream in order to maintain the stream habitat. Many plants have a base load capacity that is generated with the water released to maintain the stream habitat. For example, a 100-MW hydroelectric plant may generate 5-MW when it is only releasing enough water for downstream habitat. Except when it is undergoing maintenance and the water is bypassed around the turbines, the plant will always be generating at least 5-MW. Some plants have a small turbine for these releases because it is inefficient to run a little bit of water through a large turbine. Run of the river hydroelectric plants do not have any water storage. They simply divert water from a stream, run it through the turbines and then return it to the stream. For this reason, they are always base load plants. However, they may be forced to shut down or reduce the amount of diverted water when the streamflow is insufficient to provide habitat for aquatic organisms while providing water for electricity generation.
[edit] Nuclear power plants
Some nuclear power plants (ABWRs and older BWRs) have the capability to go from 100% of rated power down to 50% of rated power in about one hour. This makes them useful for overnight load-following.
