Product Code Database
A load-following power plant, regarded as producing Merit order or mid-priced electricity, 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.
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. The duration of operation for peaking plants varies from a good portion of the waking day to only a couple of 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, fuel oil, and/or Diesel fuel, allowing greater flexibility in choice of operation- for example, while most gas turbine plants primarily burn natural gas, a supply of fuel oil and/or diesel is sometimes kept on hand in case the gas supply is interrupted. Other gas turbines can only burn a single fuel.
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 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 generator 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.
Some applications are: base load power generation, wind-diesel, load following, cogeneration and trigeneration.
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, 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.
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.
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. Nuclear power plants in France 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.Nuclear Development, June 2011, page 10 from http://www.oecd-nea.org/ A more efficient solution is to maintain the primary circuit at full power and to use the excess power for cogeneration.
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. 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.
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" 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.
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.
Rechargeable battery storage as of 2018, when custom-built new for this purpose without re-using electric vehicle batteries, cost $209 per kWh on average in the United States. 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 stabilization as an on line spinning reserve.
New studies have also evaluated both wind and solar plants to follow fast load changes. A study by Gevorgian et al has shown the ability of solar plants to provide load following and fast reserves in both island power systems like Puerto Rico and large power systems in California.
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:
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".
Such batteries are often repurposed in home arrays which primarily serve as backup, so can participate much more readily in grid stabilizing. The number of such batteries doing nothing is increasing rapidly, e.g. in Australia where Tesla Powerwall demand rose 30 times after major power outages.
Home and vehicle batteries are always and necessarily charged responsively when supply is available, meaning they all participate in a smart grid, because the high load (one Japanese estimate was over 7 GW for half the cars in Kanto) simply cannot be managed on an analog grid, lest "The uncoordinated charging can result in creation of a new peak-load" (ibid).
Given the charging must be managed, there is no incremental cost to delay charging or discharge these batteries as required for load following, merely a software change and in some cases a payment for the inconvenience of less than complete charging or for battery wear (e.g. "$7 to $10 a day per car" paid in Delaware).
Rocky Mountain Institute in 2015 listed the applications of such distributed networks of batteries as (for "ISOs / RTOs") including "energy storage can bid into wholesale electricity markets" or for utility services including:
RMI claimed "batteries can provide these services more reliably and at a lower cost than the technology that currently provides a majority of them thermal power plants (see above re coal and gas)", and also that "storage systems installed behind the customer meter can be dispatched to provide deferral or adequacy services to utilities", such as:
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