reliability & Capacity
A central question with wind systems is: How much backup does it need? Wind capacity credit has been defined as the amount of conventional generation that can be displaced by adding wind while maintaining system reliability.
Conventional practice by the system operators and the published literature is to use an averaging technique. Averaging is a fundamentally flawed method because the wind system is not homogeneous. System reliability is driven by peak loads and averaging wind suppresses the peaks making the system appear to more reliable than it really is.
The following chart shows MISO daily peak loads for 4 years along with residual load peaks. Residual load is calculated by scaling wind data to 40% of annual average and then subtracting hourly wind from concurrent hour load. With no wind, conventional generators need to reliably manage the daily peaks. With wind conventional generators must reliably manage residual peaks. As we can see from the chart, wind reduces peaks by very little (awsc) which means wind provides very little system capacity.
These ideas were first presented at an ASME conference is 2014. A more rigorous paper (draft) investigates 12 grids and 67 years of data to find that wind system capacity varies little and averages about 2.5% of wind nameplate.