Electric Energy Storage

The operation of the electric grid requires continuous balance between supply and demand. Electric demand fluctuates throughout the day and these fluctuations are significantly different between weekdays and weekend days. Electric powerplant outputs are adjusted as required to match demand. The recent addition of limited grid scale electric storage facilities assists the grid operators in responding to demand fluctuations. However, these storage facilities are primarily intended to assist grid operators in responding to supply fluctuations resulting from the intermittent nature of renewable energy sources such as wind and solar, whose outputs can change very rapidly.

The principal tool of grid supply management is the natural gas combined cycle powerplant, since its output can be adjusted rapidly over a broad range. However, as the percentage of renewable generation feeding the grid increases and the percentage of fossil fueled generation decreases to reduce CO2 emissions, the availability of demand-responsive fossil fuel generation to match the expected rapid changes in renewable supply would decrease.

The intended replacement for demand-responsive generation is grid-scale storage. This storage would be configured to meet short term fluctuations in renewable generator output, intermediate term reductions resulting from multi-hour periods of overcast skies or still air conditions, overnight periods of zero solar generation and even multi-day periods when either solar or wind or both are unavailable or dramatically reduced.

The ability to use grid-scale storage to supplement renewable generation is dependent upon the availability of surplus renewable generation to charge and maintain the storage batteries as well as to compensate for the losses which occur during battery charging and discharging. The capacity of the storage system is a function of the percentage of renewable generation supplying the grid, the number of days during which there might be low or non-existent renewable generation and of the capacity factor of the renewable generation fleet.

The 30 GW offshore wind generation target established by the Biden Administration for 2030 can be used to illustrate this issue. The generators to be deployed to meet this target were assumed in the previous commentary to have a 60% capacity factor. This would be achieved over a range of possible operating scenarios from 100% capacity operation 60% of the time to 60% capacity operation 100% of the time. In the case of 100% capacity operation 60% of the time, approximately 40% of the power generated would have to be stored for use during the 40% of the time the generators were not producing power. In the case of operation at 60% capacity 100% of the time, storage requirements would be reduced, with the reduction determined primarily by the demand curves of the served customers.

Onshore wind generation has a lower capacity factor, which peaks at 40-45% in the best locations and would decrease as additional generators were installed in sub-optimal locations. Solar generators have an even lower capacity factor, which peaks at approximately 30% in optimal locations. As the capacity factor of the renewable generator fleet declines, the storage capacity required to supply grid demand during periods of low or no renewable generation increases, as does the renewable generator capacity required to meet current grid demand while providing sufficient additional generation to recharge the battery storage.