Electric Supply Capacity

Depending on the circumstances in a given electric supply system, energy storage could be used to defer and/or to reduce the need to buy new central station generation capacity and/or purchasing capacity in the wholesale electricity marketplace.

The marketplace for electric supply capacity is evolving. In some cases, generation capacity cost is included in wholesale energy prices (as an allocated cost per unit of energy). In other cases, market mechanisms may allow for capacity-related payments.

Technical Considerations

Storage System Size Range: 1 – 500 MW
Target Discharge Duration Range: 2 – 6 hours
Minimum Cycles/Year: 5 – 100

The operating profile for storage used as supply capacity (characterized by annual hours of operation, frequency of operation, and duration of operation for each use) is location-specific. Consequently, it is challenging to make generalizations about storage discharge duration for this service. Another key criterion affecting discharge duration for this service is the way that generation capacity is priced. For example, if capacity is priced per hour, then storage plant duration is flexible. If prices require that the capacity resource be available for a specified

duration for each occurrence (e.g., five hours), or require operation during an entire time period (e.g., 12:00 p.m. to 5:00 p.m.), then the storage plant discharge duration must accommodate those requirements.

The two plots in Figure 2 illustrate the capacity constraint and how storage acts to compensate the deficit. The upper plot shows the three weekdays when there is need for peaking capacity. The lower plot shows storage discharge to meet load during those three periods and also shows that the storage is charged starting just before midnight and ending late at night during the times when system load is lower.