Load-following power plant

A load following power plant, also known as mid-merit, 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 startup and shutdown, construction cost, cost of electricity and capacity factor.

Base load and peaking power plants

Base load power plants operate at maximum output. They shut down or reduce power only to perform maintenance or repair. These plants produce electricity at the lowest cost of any type of power plant, and so are most economically used at maximum capacity. Base load power plants include coal, fuel oil, almost all nuclear, geothermal, hydroelectric, 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. 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.

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 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, diesel and gas engine 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. Gas turbines find only limited application as prime movers for power generation; one such use is power generation at remote military facilities, mine sites and rural or isolated communities. This is because gas turbine generators typically have significantly higher heat loss rates than steam turbine or diesel power plants; their higher fuel costs quickly outweigh their initial advantages in most applications. Applications to be evaluated include:

1. Supplying relatively large power requirements in a facility where space is at a significant premium, such as hardened structures.
2. Mobile, temporary or difficult access site such as isolated communities, isolated mine sites, or troop support or line-of-sight stations.
3. Peak shaving, in conjunction with a more-efficient generating station.
4. Emergency power, where a gas turbine’s lightweight and relatively vibration-free operation are of greater importance than fuel consumption over short periods of operation. However, the starting time of gas turbines may not be suitable for a given application.
5. 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 powerplants 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: baseload power generation, wind-diesel, load following, cogeneration and trigeneration.

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 depends heavily 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 releasing only 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.

Nuclear power plants

Older Nuclear (and coal) power plants may take many hours, if not days, to achieve a steady state power output. In general it is not economical for large thermal installations such as nuclear power plants to practice load following.

"Modern nuclear plants with light water reactors are designed to have strong maneuvering capabilities. Nuclear power plants in France and in Germany operate in load-following mode, i.e. they participate in the primary and secondary frequency control, and some units follow a variable load programme with one or two large power changes per day.

The minimum requirements for the maneuverability capabilities of modern reactors are defined by the utilities requirements that are based on the requirements of the grid operators. For example, according to the current version of the European Utilities Requirements (EUR) the NPP must at least be capable of daily load cycling operation between 50% and 100% of its rated power Pr, with a rate change of electric output of 3-5% of Pr per minute.

Most of the modern designs implement even higher maneuverability capabilities, with the possibility of planned and unplanned load-following fast power modulations in the frequency regulation mode with ramps of several percent of the rated power per second, but in a narrow band around the rated power level."

Boiling water reactors

Boiling water reactors (BWR) and Advanced Boiling Water Reactors can use a combination of control rods and the speed of recirculation water flow to quickly reduce their power level down to under 60% of rated power, making them useful for overnight load-following. In markets such as Chicago, Illinois where half of the local utility's fleet is BWRs, it is common to load-follow (although less economic to do so).

Pressurized water reactors

Pressurized water reactors (PWR) use a chemical shim in the moderator/coolant (see nuclear reactor technology) to control power level, and so normally do not load follow. (In most PWRs, control rods are either fully withdrawn or fully inserted - variable control is difficult, partly due to the large bundle sizes.)

In France, however, nuclear power plants use load following. French PWRs use "grey" control rods, in order to replace chemical shim, without introducing a large perturbation of the power distribution. These plants have the capability to make power changes between 30% and 100% of rated power, with a slope of 5% of rated power per minute. Their licensing permits them to respond very quickly to the grid requirements.

See also

• Relative cost of electricity generated by different sources
  • Economics of new nuclear power plants (for more cost comparisons)