Load-following power plant
A load-following power plant, regarded as producing mid-merit 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.
Base load and peaking power plants
s are dispatchable plants that tend to operate at maximum output. They generally shut down or reduce power only to perform maintenance or repair or due to grid constraints. Power plants operated mostly in this way include coal, fuel oil, nuclear, geothermal, run-of-the-river hydroelectric, solar, 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. 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, 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.
Load-following power plants
By way of contrast, load-following power plants usually run during the day and early evening, and are operated in direct response to changing demand for power supply. 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 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 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
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
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, 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. 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 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.
Coal-fired power plants
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.
Nuclear power plants
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. 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 the load-following mode became economical due to overall electricity demand fluctuating throughout the day.