The member states of the regional Greenhouse Gas Initiative are continuing their efforts to transition their electric systems toward renewable generation systems. Electricity demand growth and the shuttering of existing fossil fueled generation are moving them toward “tipping points” beyond which renewable generation would have to be dispatchable to maintain grid stability and reliability. Renewable generation, which has so far merely displaced output from conventional generation, would be required to replace that generation and to provide additional dispatchable generation.

The primary focus of this transition has been on existing coal generating stations, many of which provide baseload power to the grid. However, the renewable replacement for a 500 MW coal generating station is a very different facility. A solar-based facility with a 25% capacity factor would require generating capacity of 2,000 MW and storage capacity of 9,000 MWh with an output capability of 500 MW, assuming that full sun was available every day to provide output to the grid and recharge storage. Design to continue providing 500 MW on subsequent days without solar generation would require additional storage capacity of 12,000 MWh plus additional generation of 500 – 600 MW to recharge storage rapidly. Each additional day without solar generation would require further generation and storage expansion.

Intermediate load generation varies its output throughout the day to follow varying grid demand. A 500 MW intermediate load solar generating facility with a 25% capacity factor would require 500 MW of generating capacity to meet daytime load plus an additional 750 MW of generating capacity to meet nighttime loads approximately 50 % of daytime loads, as well as approximately 4,500 MWh of storage. Design to meet similar loads on subsequent days without solar generation would require an additional 7,500 MWh of storage plus an additional 300 – 400 MW of generation to recharge storage rapidly. Each additional day without solar generation would require further generation and storage expansion.

Peak loads could be met with output from storage sized to meet expected peak magnitude and duration, provided with dedicated generation to maintain sufficient storage charge.

Potential maximum duration of period of low/no solar generation output is a critical renewable system design factor. Conventional generation operation continues through extended periods. However, renewable generation systems must rely on storage to continue to power the grid through weather related interruptions or curtailments of generation output Recent RGGI state experience with solar output interruption of 10 days resulting from winter storm Fern suggest that significant additional storage capacity and additional generation capacity to rapidly recharge storage would be required to avoid blackouts or grid failure during such storms.

The greatest concern for the RGGI member states is winter storms because the capacity factor of solar generation systems is lowest in winter when the likelihood of multi-day generation interruptions is increased by snow accumulation on solar collectors. Blackouts or grid failure present far greater danger of adverse health effects during periods of protracted cold.