Load following power plant

Load following power plant

Discussion
Ask a question about 'Load following power plant'
Start a new discussion about 'Load following power plant'
Answer questions from other users
Full Discussion Forum
 
Encyclopedia
A load following power plant is a power plant that adjusts its power output as demand for electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 fluctuates throughout the day. Load following plants are typically in-between base load
Base load power plant
Baseload is the minimum amount of power that a utility or distribution company must make available to its customers, or the amount of power required to meet minimum demands based on reasonable expectations of customer requirements...

 and peaking power plant
Peaking power plant
Peaking power plants, also known as peaker plants, and occasionally just "peakers," are power plants that generally run only when there is a high demand, known as peak demand, for electricity.-Peak hours:...

s in efficiency, speed of startup and shutdown, construction cost, cost of electricity and capacity factor
Capacity factor
The net capacity factor or load factor of a power plant is the ratio of the actual output of a power plant over a period of time and its potential output if it had operated at full nameplate capacity the entire time...

.

Base load and peaking power plants


Base load power plant
Base load power plant
Baseload is the minimum amount of power that a utility or distribution company must make available to its customers, or the amount of power required to meet minimum demands based on reasonable expectations of customer requirements...

s 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
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

, fuel oil
Fuel oil
Fuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. Broadly speaking, fuel oil is any liquid petroleum product that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, except oils having a flash...

, almost all nuclear
Nuclear power
Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...

, geothermal
Geothermal power
Geothermal energy is thermal energy generated and stored in the Earth. Thermal energy is the energy that determines the temperature of matter. Earth's geothermal energy originates from the original formation of the planet and from radioactive decay of minerals...

, hydroelectric
Hydroelectricity
Hydroelectricity is the term referring to electricity generated by hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy...

, biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....

 and combined cycle
Combined cycle
In electric power generation a combined cycle is an assembly of heat engines that work in tandem off the same source of heat, converting it into mechanical energy, which in turn usually drives electrical generators...

 natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

 plants.

Peaking power plant
Peaking power plant
Peaking power plants, also known as peaker plants, and occasionally just "peakers," are power plants that generally run only when there is a high demand, known as peak demand, for electricity.-Peak hours:...

s operate only during times of peak demand. In countries with widespread air conditioning
Air conditioning
An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...

, 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
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

 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 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
Fuel oil
Fuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. Broadly speaking, fuel oil is any liquid petroleum product that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, except oils having a flash...

, 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 at military facilities. This is because gas turbine generators typically have significantly higher heat 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 a troop support or line-of-sight station.
  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
    Combined cycle
    In electric power generation a combined cycle is an assembly of heat engines that work in tandem off the same source of heat, converting it into mechanical energy, which in turn usually drives electrical generators...

     or cogeneration
    Cogeneration
    Cogeneration is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat....

     power plants where turbine exhaust waste heat can be economically used to generate additional power and thermal energy for process or space heating.

Hydroelectric power plants


Hydroelectric
Hydroelectricity
Hydroelectricity is the term referring to electricity generated by hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy...

 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
Spillway
A spillway is a structure used to provide the controlled release of flows from a dam or levee into a downstream area, typically being the river that was dammed. In the UK they may be known as overflow channels. Spillways release floods so that the water does not overtop and damage or even destroy...

. Another factor is whether the plants have to release significant quantities of water downstream in order to maintain the stream habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...

. 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
Water turbine
A water turbine is a rotary engine that takes energy from moving water.Water turbines were developed in the 19th century and were widely used for industrial power prior to electrical grids. Now they are mostly used for electric power generation. They harness a clean and renewable energy...

, 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.

Boiling water reactors


Boiling water reactor
Boiling water reactor
The boiling water reactor is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor , also a type of light water nuclear reactor...

s (BWR) and Advanced Boiling Water Reactor
Advanced Boiling Water Reactor
The Advanced Boiling Water Reactor is a Generation III boiling water reactor. The ABWR is currently offered by GE Hitachi Nuclear Energy and Toshiba...

s can use a combination of control rod
Control rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...

s 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 reactor
Pressurized water reactor
Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...

s (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

  • Capacity factor
    Capacity factor
    The net capacity factor or load factor of a power plant is the ratio of the actual output of a power plant over a period of time and its potential output if it had operated at full nameplate capacity the entire time...

  • Energy storage
    Energy storage
    Energy storage is accomplished by devices or physical media that store some form of energy to perform some useful operation at a later time. A device that stores energy is sometimes called an accumulator....

  • Intermittent power source
    Intermittent power source
    An intermittent energy source is any source of energy that is not continuously available due to some factor outside direct control. The intermittent source may be quite predictable, for example, tidal power, but cannot be dispatched to meet the demand of a power system. Examples of intermittent...

  • Relative cost of electricity generated by different sources
    • Economics of new nuclear power plants
      Economics of new nuclear power plants
      The economics of new nuclear power plants is a controversial subject, since there are diverging views on this topic, and multi-billion dollar investments ride on the choice of an energy source...

      (for more cost comparisons)