Solar by the Numbers - ORIGINAL CONTENT

The Administration goal of a fossil free grid by 2035 would require that the renewable portion of grid energy supply be supported by additional renewable generation plus electricity storage. The hourly, daily, monthly and seasonal variability of renewable generator output would no longer be supported by conventional fossil generation.

The US Energy Information Administration (US EIA) Electric Power Monthly reports an average solar photovoltaic capacity factor for calendar year 2020 at 24.2%, with a monthly average range from 7.1 – 33.3%. Monthly average capacity factors for 2021 through October range from 6.3 – 30.2%. Capacity factors are highest in the Summer and lowest in the Winter.

A 2.5 Megawatt (MW) solar collector array would have produced at an average rate of 0.605 MW (2.5 * 0.242) per hour, or 14.52 MWH per day in 2020, with a monthly average rate ranging from 0.1775 – 0.8325 MW, or 4.26 – 19.98 MWH per day. These averages mask the fact that solar output could range from 0 – 2.5 MW uncontrollably throughout the day and from day to day and would be zero at night. Therefore, on an annual basis and applying a typical utility capacity reserve margin of approximately 20%, a 2.5 MW solar array could be relied upon to provide approximately 0.15 MW (0.1775/1.2) if combined with storage capacity capable of storing electricity at a rate of up to 2.5 MW and discharging electricity at a rate of approximately 0.15 MW during a typical day.

A 2.5 MW solar array would also require storage capacity of approximately 15 MWH for each low/no solar day which might be experienced at the solar array location. The recent “solar drought” in the UK and parts of the EU lasted for approximately 10 days. Using this experience as guide, a 2.5 MW solar array would require storage of approximately 150 MWH capable of continuous discharge at a rate of 15 MWH per day. This storage would have to be recharged at the end of the period of low/no solar. However, the output of the solar array would be required to meet contemporaneous grid demand, so additional generating capacity would be required to recharge storage. Assuming that recharging the storage over the same number of days over which it was discharged would be acceptable, another 2.5 MW solar array would be required. More rapid recharging would require additional solar array capacity.

The availability of long-term, low-loss storage would permit the reliable capacity of the solar array and storage system to be increased from the 0.15 MW calculated above to approximately 0.51 MW [(0.605/0.1775) * 0.15)]. However, such long-term, low-loss storage is not currently commercially available and its likely cost, based on current technology, would exceed the cost of the additional solar array capacity required to increase output in the lowest output month of the year to the average annual output of the 2.5 MW solar array.