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 start up and shut down, 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. 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 fuelled 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 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 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 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. 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 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.

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 as many plants do not have enough water to operate anywhere near their full capacity on a continuous basis.

Lakes and man made reservoirs used for hydropower come in all sizes, holding enough water for as little as a one day supply, or as much as a whole year's supply. A plant with a reservoir that holds less than the annual river flow may change it's operating style depending on the season of the year. For example, the plant may operate as a peaking plant during the dry season, as a base load plant during the wet season and as a load following plant between seasons. A plant with a large reservoir may operate independently of wet and dry seasons, such as operating at maximum capacity during peak heating or cooling seasons.

When electrical generation supplying the grid and the consumption or load on the electrical grid are in balance, the frequency of the alternating current is at it's normal rate (either 50 or 60 cycles per second). Hydro electric power plants can be utilized for making extra revenue in an electric grid with erratic grid frequency. When grid frequency is above normal (ex; Indian grid frequency is exceeding the rated 50 Hz for most of the duration in a month/day), the extra power available can be consumed by adding extra load (say agriculture water pumps) to the grid and this new energy draw is available at nominal price or no price. However, there may not be a guarantee of continued supply at that price when the grid frequency falls below normal, which would then call for a higher price.

To arrest the fall of frequency below normal, the available hydro power plants are kept in no load/nominal load operation and the load is automatically ramped up or down strictly following the grid frequency (i.e. the hydro units would run at no load condition when frequency is above 50 Hz and generate power up to full load in case the grid frequency is below 50 Hz). Thus a utility can draw two or more times energy from the grid by loading the hydro units less than 50% of the duration and the effective use of available water is enhanced more than twice the conventional peak load operation.

Coal based power plants

Large size coal fired thermal power plants can also be used as load following / variable load power stations. These power plants are generally incorporated with following features to achieve this flexibility techno economically.

• Sliding pressure operation: Sliding pressure operation of the steam generator allows the power plant to generate electricity without much deterioration in fuel efficiency at part load operation up to 75% of the nameplate capacity.
• Over loading capability: The power plants are generally designed to run at 5 to 7% above the name plate rating for 5% duration in a year
• Frequency follow governor controls: The load generation can be automatically varied to suit the grid frequency needs.
• Two shift daily operation for five days in a week: The needed warm and hot start up of these power stations are designed to take lesser time to achieve full load operation. Thus these power plants are not strictly base load power generation units.
• HP/LP steam bypass systems: This feature allows the steam turbo generator to reduce the load quickly and allows the steam generator to adjust to the load requirement with a lag.

Nuclear power plants

Load following is the potential for a power plant to adjust its power output as demand and price for electricity fluctuates throughout the day. In nuclear power plants, this is done by inserting control rods into the reactor pressure vessel. This operation is very inefficient as nuclear power generation is composed almost entirely of fixed and sunk costs; therefore, lowering the power output doesn't significantly reduce generating costs and the plant is thermo-mechanical stressed. 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 manoeuvring capabilities. Nuclear power plants in France and in Germany operate in load-following mode and so participate in the primary and secondary frequency control. Some units follow a variable load programme with one or two large power changes per day. Some designs allow for rapid changes of power level around rated power, a capability that is usable for frequency regulation. A more efficient solution is to maintain the primary circuit at full power and to use the excess power for cogeneration.

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 made of boron steel, 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.

Solar and wind power plants

For countries that are trending away from coal fired baseload plants and towards intermittent energy sources such as wind and solar, there is a corresponding increase in the need for peaking or load following power plants and the use of a grid intertie.

The infirm or unreliable secondary power from the renewable energy such as solar and wind power plants can be used to follow the load or stabilise the grid frequency with the help of battery storage units economically. When the grid frequency is below the desired or rated value, the power being generated (if any) and the stored battery power is fed to the grid to raise the grid frequency. When the grid frequency is above the desired or rated value, the power being generated is fed or surplus grid power is drawn (in case cheaply available) to the battery units for energy storage. The grid frequency keeps on fluctuating 50 to 100 times in a day above and below the rated value depending on the type of load encountered and the type of generating plants in the electrical grid. Recently, the cost of battery units, solar power plants, etc. have come down drastically to utilise secondary power for power grid stabilisation as an on line spinning reserve.

See also: Availability-based tariff, Solar inverter, and Tesla Powerwall

See also

• Grid energy storage
• Relative cost of electricity generated by different sources
• Economics of new nuclear power plants (for more cost comparisons)
• Base load power plant
• Peaking power plant