Variability of Renewable Energy Sources
Wind and solar energy are referred to as variable generation sources because their electricity production varies based on the availability of wind and sun. However, they are not the only source of variation in a power system. The demand for electricity, or load, also varies, and the power system was designed to handle that uncertainty. Short-term changes in load (over seconds or minutes) are generally small and caused by random events that change demand in different directions. Over longer periods (several hours), changes in load tend to be more predictable. For example, there is a daily pattern of morning load pickup and evening load drop-off highly correlated with human endeavors. The key difference is that load variations are better understood than wind and solar variations. Uncertainty arises when levels and variabilities of power supply and demand must be sequentially matched.
Some aspects of renewable energy variation are easily predicted. For example, the electricity production of an individual wind turbine is highly variable. But the aggregate variability of multiple turbines at a single site is significantly less variable. The aggregation of multiple wind generation sites over a large geographic area results in even less variability. Harnessing the "law of large numbers," variability smoothing over large areas, yields enhanced prediction. Variability also decreases as the timescale decreases. The variability of large-scale wind power over seconds or minutes is generally small. Over several hours, however, it can be great.
Similarly, some aspects of solar variability are predictable (for example, sunrise and sunset). Other aspects, such as intermittent cloud cover, are much less so. However, the same reduction in variability is observed for the aggregation of solar photovoltaic plants over a broad geographic area.
All types of variability must be managed by the electric power system operator. With low penetrations of variable generation, the related impact and response are small because the wind and solar variability is much less than the load variability. At high penetrations, however, the renewable variability may be more challenging to respond to.
NREL is addressing variability issues on the transmission system through its work on:
- The Western Wind and Solar Integration Study
- The Eastern Renewable Generation Integration Study
- Energy imbalance markets
- The FESTIV model.
For additional information, see Long-Term Wind Power Variability.