Whether it’s charging a phone, using electric heating, or keeping food refrigerated, steady and reliable power is a large part of modern life. As most people use electricity to some degree every day, understanding how electricity is generated and managed is important for everyone, not just engineers and utility providers.

Sources of Electricity

Electricity can be generated from a number of different energy sources, including fossil fuels, hydro, solar, wind, and nuclear. These sources of energy are converted into electricity, which is distributed to homes and businesses. However, not all energy sources are equally reliable or predictable. This leads to the distinction between intermittent and dispatchable generation, both of which are essential in supplying power to an electric power grid.

Maintaining Grid Balance

In any power system, the electricity generated must continuously match the electricity demand to maintain system stability. This delicate balance ensures that when you turn on a light or plug in an appliance, you receive high-quality power. As energy demand fluctuates throughout the day, different types of generation may be used to meet it. A combination of intermittent and dispatchable generation sources can be used to meet current demand, which typically peaks in the mornings and evenings.

Dispatchable Energy Generation

Dispatchable generation technologies are those that can respond quickly to meet demand. Examples include hydroelectric systems, diesel and natural gas generators, and battery energy storage. These resources offer controllable, on-demand power when operating, making them vital for ensuring system reliability. However, dispatchable energy resources vary in startup time. Nuclear and steam turbines require hours due to reactor startup and steam generation, necessitating planning based on load forecasts. In contrast, diesel and hydroelectric generation can start within minutes, while battery energy storage responds in milliseconds. These generation sources allow power operators to quickly add or remove large or small amounts of generation to meet changing demand. This change in generation can be achieved by increasing or decreasing the output of dispatchable generators that are currently operating (i.e., reducing the flow of water through a hydro dam turbine), or by turning on more sources (i.e., starting up another diesel generator), or turning off sources (i.e., shutting down a diesel generator).

Ramping up and down generation is like pressing the accelerator pedal in a car. When you add more gas, you get more power; when you let off the gas, you get less. Dispatchable generation sources play a similar role in the power grid, as they can respond to rapid changes in demand. To maintain stability (i.e., avoid brownouts and blackouts), power grids must keep voltage and frequency stable, which can change dramatically in less than half a second. As every electrical device connected to the power grid constantly changes the stability of the grid’s frequency and voltage, dispatchable resources are needed to maintain the grid’s voltage and frequency, as they can provide the rapid power adjustments needed to address the fast-changing nature of demand.

Examples of dispatchable generation: Yukon Energy, diesel generator (left) and natural gas generators (right) in Whitehorse, YT
Source: Yukon Energy and Whitehorse Star

Intermittent Energy Generation

Intermittent generation is a source with an unpredictable power output. Common examples of intermittent generation sources are wind and solar because they depend on environmental conditions, and therefore, their output cannot be reliably predicted. Solar panels, for example, only generate electricity when sunlight is available, and clouds greatly reduce the output power. Similarly, wind turbines rely on wind speed, which is difficult to predict and constantly changes, affecting power output. The variability of these sources means they cannot be relied upon to meet demand independently, requiring that dispatchable energy sources be available to quickly fill the gap without warning. Having dispatchable generation available ensures that if a cloud passes over a solar panel, the grid can promptly use fast-reacting dispatchable resources to compensate for the loss of generation. The use of dispatchable generation in this instance ensures that system voltage and frequency are maintained and that customers receive high-quality electricity.

Source: Examples of intermittent generation: North Klondike Solar Array (Left) and Haeckel Hill-Thay T’äw Wind Energy Project (Right) in Whitehorse, YT
Source: NEC/Green Cat Renewables and Solvest

Spinning Reserve

To have fast-reacting dispatchable energy, a dispatchable generation asset must be running and ready to step in to meet demand. This method of holding dispatchable assets at the ready is called spinning reserve and is common practice among utilities. However, holding dispatchable assets in spinning reserve wastes resources while they are not supplying power to the grid. This can incur fuel, resource, operation, and maintenance costs for the utility. To ensure the grid can maintain balance while still using unpredictable, intermittent energy, many utilities limit the amount of intermittent generation on the power grid to 20-30% of total generation to minimize the large fluctuations inherent to intermittent generation.

Conclusion

As generation sources in power grids become increasingly diverse, the interaction between intermittent and dispatchable sources is becoming increasingly important for understanding modern power systems.

Dispatchable generation, such as hydro, diesel, natural gas, and battery storage, is essential for responding quickly to real-time changes. These resources provide the flexibility needed to maintain frequency and voltage, ensuring system stability and power quality when intermittent resources fluctuate. In this way, dispatchable sources act as a reliable backbone to support the dynamic behaviour of intermittent generation.

Intermittent generation is increasingly important as the world shifts toward sustainable energy systems and reduced dependence on fossil fuels. However, the variability and unpredictability of these resources, such as wind and solar, mean they must be complemented by dispatchable resources.

As power grids must continuously match generation with demand, coordinating and managing intermittent and dispatchable resources becomes more critical. To support the addition of renewable, intermittent generation, power grids must maintain adequate dispatchable capacity to be ready for periods of low generation or sudden output changes from intermittent generation.

Achieving a stable, resilient grid depends on a well-balanced mix of generation types. By understanding and optimizing the roles of both dispatchable and intermittent generation, power grids will be better able to meet all customers’ needs.

Contributors

Author: Simon Kerkhof
Reviewed By: Trent Gardiner and Mackenzie Smith