By now our readers should know about Rule 21, but if you missed it, Rule 21 describes the Interconnection, operating and Metering requirements for Generating Facilities to be connected to (PG&E) Distribution System over which the CPUC has jurisdiction. On September 8th, there has been yet another change to Rule 21.
Moving forward, RULE 21 will allow PG&E to curtail certain solar arrays and parts of the grid, where they would otherwise lose money. What? PG&E losing money? Yes, let us introduce “The Duck Curve”. Negative pricing can be caused by various factors, but it is increasingly due to renewable energy sources like solar.
California is seeing increasing durations of negative pricing during the day when solar production occurs. The figure to the right shows the daily grid electricity demand curve, with demand plummeting during the day when a large amount of solar power is produced from existing solar plants around the state.
Negative pricing happens because California’s grid generation assets can’t all be turned down or off as solar production ramps up. Some baseload must run all the time. And as solar plants come on-line in amounts that exceed the baseload generation plus the solar power, some power must be curtailed or sold at negative prices. As the duck’s “belly” gets fatter (lower in the chart) there will be more and more negative-priced power.