Full Disclosure - Storage - ORIGINAL CONTENT

Wind and solar are intermittent resources, so both wind and solar generation are intermittent and require backup. This backup is currently provided by conventional generation. However, with the evolution of a fossil-free grid, storage capacity would be required to provide backup. Additional wind and solar generating capacity would also be required to recharge storage when wind and solar again become available.

Storage must be capable of functioning under several different conditions. Some storage is currently being used with both wind and solar installations to smooth the output of the generators when the wind is fluctuating or when the sun is partially obscured by clouds. This storage has relatively limited capacity and responds quickly to fluctuations.

Storage is now beginning to be installed with solar arrays to deal with the typical increase in demand and consumption at the end of the day, when solar output has decreased or ceased. This “four-hour storage” has approximately half the capacity of the solar array during a normal day. This storage must be recharged the next day during periods when grid demand is less than solar array capacity, or must be recharged overnight or during the next day from other generation sources. This storage has a current cost of approximately $350 per kWh and is projected to decline in cost to between $150 and $100 per kWh by 2050 according to the National Renewable Energy Laboratory (NREL).

As the transition to a fossil-free grid progresses, there will be a growing need for long-duration storage capable of storing the output of wind and solar generators in sufficient quantities to power the grid for multiple hours to multiple days when either wind or solar, or both, are unavailable or significantly diminished. The “wind drought” which affected the United Kingdom and parts of Western Europe in the fall of 2021 is an example of a situation requiring such long-term storage. In that event, storage capable of powering the grid for 10 days would have been required, had conventional generation not been available.

Another situation requiring long-term storage is those periods when wind and solar output are diminished seasonally. EIA reports that, in 2021, solar photovoltaic system output was below 25% of rating plate capacity in the period from October through February, while (onshore) wind capacity was below 35% in the period June through October and in January and February. The January through February period is of particular concern because both wind and solar capacity are diminished. Long-duration storage would be required to make up the generation deficit for a period of several months.

Clearly, additional wind and solar generating capacity would be required to charge long-duration storage to supplement both wind and solar generation through the Fall and Winter, as well as to support the grid through a “drought” period. This long-duration storage is not currently commercially available. Technology capable of approximately 150 hours is currently under development, but would be insufficient to support the grid through a 10-day “drought” or through a multi-month period of insufficient generation.

The cost of such long-duration storage is currently unknown, but the storage capacity required to support a fossil-free grid would be very large and very expensive. However, the cost of not having long-duration storage in sufficient quantities would be substantially greater.

The expected service life of current grid-scale batteries is approximately 10 years, compared to the expected 40 year service life of utility assets. This difference dramatically increases depreciation expense and system ownership costs.