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Winter in California reveals a hidden Grid problem!

By Ziad Alaywan P.E.

January 10, 2023

We all know that wind and solar are intermittent resources, with energy production dependent upon the sun shining and the wind blowing. And in response to the duck curve, California regulators have incented the development of four-hour energy storage resources to shift energy production from solar to non-solar hours. We’ve seen the pitfalls of reliance on limited duration storage during the past two summers as batteries have not always been used to shift generation to the evening hours. What we have not seen until this past December is the magnitude of renewable generation intermittency during solar hours.

The recent much needed rains in California and the West left me curious as to how the CAISO’s solar and battery heavy portfolio was performing in the face of an unrelenting atmospheric river.

So, I downloaded some data from the CAISO. This is what I found.

  1. 4,345,468 MWh of renewable generation served 22.8% of the CAISO’s load in December 2022. Renewable generation ranged between 1,811 MW and 15,232 MW.

  2. Battery operation ranged between 2,333 MW of charging and 2,588 MW of discharging mostly used not always been used to shift generation to the evening hours

  3. During the CAISO’s peak December load of 31,677 MW, renewable resources were producing 4,895 MW or 15.5% of the CAISO’s load while batteries were discharging 1,164 MW. When the CAISO’s load dipped to its December minimum of 19,762 MW, renewable resources were producing 2,188 MW or 11.1% of the CAISO’s load while batteries were charging 4 MW.

  4. Natural gas generation for the month was 105,580,835 MWh or 46.4% of the electricity generated. Net energy imports of 37,097,342 MWh accounted for 16.3% of the energy supply portfolio.

  5. We have seen natural gas generation during solar hours run even higher than summer months.

The wide range of renewable energy production during a rainy winter month is a result of mother nature but it brings many challenges. Average renewable energy production for December was 5,841 MWh/hour but the variation was huge, ranging between 31% and 261% of average production during solar hours. Second, although battery energy storage provides some operational flexibility, it is still very small and not nearly enough to compensate for renewable energy variability, which is one of its primary functions, although not during the day when these batteries are being charged in anticipation of the CAISO’s evening four-hour net peak load period. Third, the grid workhorse is still natural gas as it was the predominant energy resource type representing 46.4% of energy in December. Fourth, California continues to rely heavily on net energy imports, although some days and during solar hours while gas was producing over 10,000 MW, California was a net exporter during solar hours. I found this to be interesting!!

Conclusions. California is making progress toward its clean energy goals. However, there is a long way to go to hit the 60% renewable target by 2030, as natural gas resources are still the energy workhorses and act as a safety net to grid reliability to cover the variations in renewable generation. If we have more renewables penetration as planned, the variation in terms of MW will be even greater during the solar hours in winter months. Would gas remain a workhouse to supply demand in the evening and to smooth variations in renewable generation during the solar hours in the winter months? Or do we get smarter and plan the missing pieces such as hydro pump storage and dispatchable geothermal resources, both of which have proven technology and have greater value to reliability but vastly under=developed. Otherwise, there are no substitutes for natural gas, and I bet that we will rely more on natural gas as renewables penetration increases.

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