Intermediate Replacement - ORIGINAL CONTENT

Replacement of conventional intermediate load generators with renewable generation plus storage is similar to, but not identical to, replacement of baseload generation. Intermediate load generators must be able to deliver their full capacity to the grid during periods of high demand, at or near the system peak. Therefore, they require sufficient storage capacity to deliver full capacity in the absence of active wind or solar generation. However, the full capacity requirement exists for only a portion of the day, so the storage capacity required is less than that required when replacing a baseload generator.

However, like the baseload generators, the intermediate load generators must be able to perform their function over the maximum number of days for which wind and solar generation might not be available. The long duration storage capacity required would be a function of the expected load duration for that intermediate load generator on a day when demand was at or near the system peak. For wind generation, this situation might occur near the summer peak, during a period of hot, still days. For solar generation, the situation might occur near the winter peak, during a period of overcast skies, or during and after a snowstorm.

Like baseload generators, intermediate load generators must have sufficient capacity to recharge storage used during a period of low/no wind or solar generation while still meeting the contemporaneous demand of the grid. The required capacity would be determined by the number of low/no wind and solar days and the likelihood of these days occurring at or near grid system peak demand.

It has been common for electric utilities to maintain a capacity reserve margin of approximately 20% relative to peak demand to allow for the possible unavailability of one or more conventional generators. Conventional intermediate load power plants are capable of operating at design capacity for extended periods in the event of a generator failure, though they typically operate in load-following mode. Some portion of the intermediate load renewable plus storage generation would likely also be designed to be capable of baseload operation. However, the percentage of intermediate load renewable generation designed for this purpose might be reduced significantly, since the generating capacity of renewable plus storage generators is typically far lower than the capacity of a single conventional generator, so the loss of a single generator would have less effect on system capacity.

Short “needle peak” demand events would likely be served primarily from additional storage capacity at some portion of the intermediate load renewable generators, since storage would likely be capable of very rapid response to changes in grid demand. This storage capacity would likely not be kept fully charged for most of the year, but would be charged in anticipation of high demand on the grid.

The uncontrolled variability of the output of wind and solar generators will present growing grid management challenges as the renewable fraction of grid generation increases and the dispatchable fraction of grid generating capacity decreases.