On Sunday, at 1:55 PM Pacific Time the California Independent Systems Operator (CAISO) reported that greenhouse gas emissions necessary to serve its demand (~80% of California’s electricity demand on an annual basis), was measured at a rate -16 metric tons of CO2 per hour. Five minutes later, the value was -2 mTCO2/h, before it crept back up to 40 mTCO2/h at 2:05 PM PST. At 2:10 PST though it fell back to -86 mTCO2/h and stayed negative until 3:05 PM PST.
This information was brought to the attention of pv magazine via tweet from eagle eye Jon Pa after CAISO’s site first noted the negative values:
The region was still generating CO2 though, as natural gas, biogas, biomass, geothermal and even coal plants were running and pumping out emissions. CAISO’s Greenhouse Gas Emission Tracking Methodology, December 28, 2016 (pdf) notes the below calculations to create the value what it terms, “Total GHG emissions to serve ISO demand”:
Of importance to note is that to get to the net negative value, CAISO considered all electricity imports and exports. And as can be noted in the image below the CO2 intensity of imports during the day rapidly declined as the sun came up, first going negative around 9:05 AM PST, and mostly staying so until just before 6 PM PST.
During this same weekend, other records were noted (reiterating that we’re in record setting season) such as a new electricity export record of greater than 2 GW and total renewable electricity as part of total demand at greater than 70%.
At the peak negative moment of 2:15 PM PST, -112 mTCO2/h seen below, the total amount of clean instantaneous generation being used in the power grid region was 17 GW, with renewables giving 76% of the total, hydro 14%, nuclear 13% and imports of -12% countering the CO2 coming from just over 1.4 GW of gas generation.
Also of importance are a few layers of nuance in the electricity demand charts. First off we’re in the shoulder seasons of California – nice cool weather before the warmth of summer drives air conditioning demand. Additional the weekend electricity demand is always lower, as well, Easter Sunday might have had an affect.
Lastly to note was the amount of electricity from solar and wind generation being curtailed. And while the Sunday numbers weren’t available yet, the below image noted Saturday with 10 GWh in total being curtailed (pdf) – peaking at over 3.2 GW of instantaneous mostly solar power in the hours of 2 and 3 PM PST. On an annualized basis, less than 2% of total potential solar electricity was curtailed in 2018.