Load-following power plant

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 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 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 near their full capacity on a continuous basis.

Where hydroelectric dams or associated reservoirs exist, these can often be backed up, reserving the hydro draw for a peak time. This introduces ecological and mechanical stress, so is practiced less today than previously. Lakes and man-made reservoirs used for hydropower come in all sizes, holding enough water for as little as a one-day supply (a diurnal peak variance), or as much as a year's supply (allowing for seasonal peak variance). A plant with a reservoir that holds less than the annual river flow may change its 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 its normal rate (either 50 or 60 hertz). Hydroelectric power plants can be utilized for making extra revenue in an electric grid with erratic grid frequency. When grid frequency is above normal (e.g. Indian grid frequency is exceeding the rated 50 Hz for most of the duration in a month/day[3]), 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.[4]

Example of daily peak load (for the Bonneville Power Administration) with large hydro, base load thermal generation and intermittent wind power. Hydro is load following and managing the peaks, with some response from base load thermal. Note that total generation is always greater than the total BPA load because most of the time BPA is a net exporter of energy. The BPA load does not include scheduled energy to other balancing authority areas.[6]

Large size coal fired thermal power plants can also be used as load following / variable load power stations to varying extents, with hard coal fueled plants typically being significantly more flexible than lignite fueled coal plants. Some of the features which may be found in coal plants that have been optimized for load following include:

• 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 down 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.

Historically, nuclear power plants were built as baseload plants, without load following capability to keep the design simple. Their startup or shutdown took many hours as they were designed to operate at maximum power, and heating up steam generators to the desired temperature took time.[2] Nuclear power generation has been also portrayed as inflexible by anti-nuclear activists and the German Federal Environment Ministry, while others claimed "that the plants might clog the power grid".[7]

Modern nuclear plants with light water reactors are designed to have maneuvering capabilities in the 30-100% range with 5%/minute slope, up to 140 MW/minute.[7] 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 program 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.[8] A more efficient solution is to maintain the primary circuit at full power and to use the excess power for cogeneration.[9]

While most nuclear power plants in operation as of early 2000's were already designed with strong load following capabilities, they might have not been used as such for purely economic reasons: nuclear power generation is composed almost entirely of fixed and sunk costs so lowering the power output doesn't significantly reduce generating costs, so it is more effective to run them at full power most of the time.[10][11] In countries where the baseload was predominantly nuclear (e.g. France) the load-following mode became economical due to overall electricity demand fluctuating throughout the day.

Boiling water reactors (BWRs) can vary the speed of recirculation water flow to quickly reduce their power level down to 60% of rated power (up to 10%/minute), making them useful for overnight load-following. They can also use control rod manipulation to achieve deeper reductions in power. A few BWR designs do not have recirculation pumps, and these designs must rely solely on control rod manipulation in order to load follow, which is possibly less ideal.[12] In markets such as Chicago, Illinois where half of the local utility's fleet is BWRs, it is common to load-follow (although potentially less economic to do so).

Pressurized water reactors (PWRs) use a combination of a chemical shim (typically boron) in the moderator/coolant, control rod manipulation, and turbine speed control (see nuclear reactor technology) to modify power levels. For PWRs not explicitly designed with load following in mind, load following operation isn't quite as common as it is with BWRs. However, modern PWRs are generally designed to handle extensive regular load following, and both French and German PWRs in particular have historically been designed with varying degrees of enhanced load following capabilities.[12]

France in particular has a long history of utilizing aggressive load following with their PWRs, which are capable of (and used for) both primary and secondary frequency control in addition to load following. French PWRs use so called "grey" control rods which have lower neutron absorption capability and are used for fine-tuning reactor power, as opposed to "black" control rods in order to maneuver power more rapidly than chemical shim control or conventional control rods allow.[2] These reactors have the capability to regularly vary their output between 30–100% of rated power, to maneuver power up or down by 2–5%/minute during load following activities, and to participate in primary and secondary frequency control at ±2–3% (primary frequency control) and ±3–5% (secondary frequency control, ≥5% for N4 reactors in Mode X). Depending on the exact design and operating mode, their ability to handle low power operation or fast ramping may be partially limited during the very late stages of the fuel cycle.[12]

Modern CANDU designs have extensive steam bypass capabilities that allow for a different method of load following that does not necessarily involve changes in reactor power output. Bruce Nuclear Generating Station is a CANDU pressurized heavy water reactor that regularly utilizes its ability to partially bypass steam to the condenser for extended periods of time while the turbine is operating to provide 300 MW per unit (2400 MW total for the eight-unit plant) of flexible (load following) operation capabilities. Reactor power is maintained at the same level during steam bypass operations, which completely avoids xenon poisoning and other concerns associated with maneuvering reactor power output.[13][14][15]

Concentrated solar power plants with thermal storage are emerging as an option for load-following power plants.[16][17] They can cater the load demand and work as base load power plants when the extracted solar energy is found excess in a day.[18] Proper mix of solar thermal storage and solar PV can fully match the load fluctuations without the need of costly battery storage.[19][20]

Hydrogen based fuel cell power plants are perfect load-following power plants like emergency DG sets or battery storage systems. They can be run from zero to full load within few minutes. As the transportation of hydrogen to the far away industrial consumers is costly, the surplus hydrogen produced as byproduct from various chemical plants are used for power generation by the fuel cell power plants.[21] Also they do not cause air and water pollution. In fact they clean the ambient air by extracting PM2.5 particulates and also generate pure water for drinking and industrial applications.

The decentralized and intermittent nature of solar and wind generation entails building signalling networks across vast areas. These include large consumers with discretionary uses, and increasingly include much smaller users. Collectively, these signalling and communication technologies are called the "smart grid". When these technologies reach into most grid-connected devices the term Energy Internet is sometimes used, though this is more commonly considered to be an aspect of the Internet of Things.

In 2010, US FERC Chairman Jon Wellinghof outlined the Obama administration's view that strongly preferred smart grid signalling over dedicated load-following power plants, describing following as inherently inefficient. In Scientific American he listed some such measures:

• "turning off the defrost cycle on the refrigerator at a given time...the grid could signal...As long as that refrigerator got defrosted at the end of the day, you, as a consumer, wouldn't care but ultimately the grid could operate more efficiently."
• "...if you didn't do that with the refrigerator you would have do that with the coal plant or combustion turbine running up and down, and doing that makes that unit run much more inefficiently."

At the time, electric vehicle battery integration into the grid was beginning. Wellinghof referred (ibid) to "these cars now getting paid in Delaware: $7 to $10 a day per car. They are getting paid over $3,000 a year to use these cars to simply control regulation service on the grid when they are charged".