◊ This is part of the ‘Electricity Generation’ series of articles ◊
Supply may be functionally described by the demand it fulfills with the exception of fluctuating energy production.
Base generation operates at a fixed output. Its output does not follow the load. If the demand falls below the generator set point, the output will not stay within operating limits and must be taken off-line.
An example of base generation is nuclear. Nuclear generation provided by processes such as the CANDU operate at a constant output with a high capacity factor.
Intermediate generation has the capability to adjust its output to follow changes in load.
An example of intermediate generation is combined cycle natural gas. Fossil fuel plants are quite versatile and are well suited for load following.
Peak generation is used during periods of high demand for short periods of time. Peak generation can ramp up or down quickly and follow demand fluctuations.
An example of peak generation is simple cycle natural gas. Simple cycle generation can start up in a matter of minutes and provide short term energy for the daily peak demand.
The distinction between base, intermediate and peak demand is important since different generator capabilities are required to suit each of those categories.
Fluctuating energy production
In an ideal situation, energy capacity would always be available for dispatch and controlled by a central authority to meet demand. The reality is far more complex. A portion of energy production is not dispatched by a central authority. Instead, it operates when the primary energy source is available. This is how some types of renewable resources feed energy to the grid. Solar energy is only available when the sunlight intensity is sufficient to produce power. Wind energy is only available when wind velocity is within specific limits. Run-of-the-river is only available when water resources are sufficient.
Renewable resources are preferred over fossil fuel generation for environmental reasons, however they pose operational challenges. The grid must operate reliably with fluctuating renewable energy production. The terms used to describe these resources are variable and intermittent.
Variable supply
The term variable may apply to generators with different characteristics depending on context. Variable may refer to the generator’s ability to modulate its output, however, it is more likely to describe the behaviour of a generator whose output varies as a result of the availability of its primary energy source. Local planning and operating authorities may have specific definitions and requirements pertaining to variable generation as it poses challenges for load/generation balancing. Ref. the IESO Market Rules, Chapter 11.
Examples of variable generators are solar PV and wind turbines.
Intermittent supply
The terms intermittent and variable are sometimes used interchangeably to describe generators, however, they are not exactly the same. It depends on the context in which the terms are used and the definition from the local authority having jurisdiction.
Intermittent generation is typically from renewable sources. It is a supply which depends on the availability of a renewable source such as wind, solar or run-of-the-river hydroelectric. Intermittent renewable supply is used to displace fossil fuel generation and reduce emissions. It doesn’t follow demand but operates when the primary fuel source is available.
Note: According to the IESO, an intermittent generator operates ‘on an intermittent basis as a result of factors beyond the control of the generator’ – IESO Market Rules, Chapter 11 section 1, Definitions.
There is no recognized international standard which defines intermittent and variable generation. There is a view that supports distinguishing between the two based on predictability. Intermittent can be used to describe an unpredictable resource and variable to describe a predictable one.
Additional information describing what generators fit in each category is provided in the ‘generator type’ article in this series.
previous article > Supply planning
next article > Advanced concepts of grid supply
Derek
