Grid load is never constant — it rises in the morning, peaks in the evening, and drops at night. To handle this fluctuation, grids do not rely on a single type of plant, but on different roles that work together.
The Daily Load Curve
If you plot the grid load over the course of a day, you get a winding curve: a load that never drops below a certain value even during the quietest hours of the night (base load), which rises gradually during the day (intermediate load), and reaches a short peak during certain hours (peak load).
The Three Roles
| Role | Description | Suitable Plant Type |
|---|---|---|
| Base Load | The continuous minimum level of load around the clock | Plants with a high capacity factor: nuclear, large hydroelectric, large steam plants — slow to start and stop, but economical for continuous operation |
| Intermediate Load | Changes gradually and predictably throughout the day | Combined-cycle plants — reasonable flexibility and efficiency |
| Peak Load | Short, fast-appearing and fast-disappearing peaks | Fast-response plants: simple gas turbines, hydroelectric, pumped-storage plants |
Why Not Use Base-Load Plants Alone for All Roles?
- Response speed: a large nuclear or steam plant needs hours to change its output safely — it cannot "keep up" with a peak that lasts only minutes.
- Economics: operating an expensive-to-build plant for only a short period (a peaking role) sharply raises the cost of each unit of electricity it produces — see capacity factor.
- Frequent start-stop cycling: some large plants are technically and economically unsuited to frequent daily start-stop cycling.
Why Not Use Peak-Load Plants Alone for Everything?
Peaking plants (such as simple gas turbines) are usually less efficient than combined-cycle or large steam plants during continuous operation — running them 24 hours a day is costly in fuel per unit of electricity compared to a base-load plant designed for continuity.
Every plant has its "optimal position" on the load curve — the question "which plant is better?" is incomplete without asking "better for which role?" This integration of roles is the essence of the generation mix concept that concludes this encyclopedia's journey.
A common mistake is associating "peak load" with importance because it is numerically the largest. What matters operationally is that it is the shortest in duration and the fastest-changing — and this is what determines the suitable plant type for it (response speed), not its size alone.
Sample answer: The three roles are: base load (the continuous minimum level of load around the clock, covered by high-capacity-factor plants such as nuclear and large hydroelectric), intermediate load (changes gradually and predictably during the day, covered by flexible plants such as combined-cycle), and peak load (short, fast-appearing peaks, covered by fast-response plants such as simple gas turbines and pumped-storage plants). A single plant cannot cover all roles because base-load plants are slow to respond and uneconomical for short intermittent operation, while peaking plants are less efficient during long continuous operation — each type has its optimal position on the load curve.
Striving to build a single "comprehensive" plant that efficiently covers all roles. No single design combines economy in long continuous operation with instant response to short peaks — the solution is always a mix of specialized plants, each for its own role.
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