Existing fossil fuel power plants will play a pivotal role in enabling the full transition to a near-zero-carbon electricity system in many countries. How can such a surprising and perhaps counterintuitive conclusion be reached? The key word is intermittency, in reference to the wide fluctuations of energy supply associated with solar and wind. Even if these two sources are, to some degree, complementary (with more wind at night and during winter, complemented by more sun at daytime and during the summer), the combination still carries a high degree of intermittency.
In this analysis, we have used data from Germany from 2012 to 2019, and scaled this up to a near 100% renewable system – assuming that the total capacity will be 160 GW, or three times the average consumption. In this system, there will still be 28 days where solar and wind combined produce less than 30% of the consumption. This happens typically during high-pressure weather systems during the winter months from November to February.
Moreover, there will on average be two extreme periods per year, with up to three days in a row when sun and wind will deliver less than 10% of Germany’s total energy consumption. Even with adjustments to imports and consumption levels, the country would still need some 50 GW of power to avoid blackouts (with 72 hours at 50 GW equating to 3.6 TWh). Total water pumping capacity today is 7 GW over four hours, or about 30 GWh. Assume this multiplies ten-fold by 2050, and assume that 45 million cars are battery electric vehicles with surplus capacity of 20 kWh each. This would deliver about 1.2 TWh in total, meaning the system would still need 2.4 TWh of power, or a continuous load of 33 GW.
During these periods, restarting old gas-fired power plants could be an economically rational way to deliver the power needed to keep the nation running as usual. The carbon footprint of this would be small – probably less than the footprint associated with constructing gigantic battery facilities for those few extreme cases. Germany presently has 263 gas power plants, with a total capacity of 25 GW.
Thus, finding a way to maintain these plants for emergency back-up capacity could be an enabler for an energy future based around solar and wind power. Capacity pricing rather than price per kWh is probably one of the commercial changes needed. This is the same pricing model that most people also have for home internet services, and should thus not be too difficult to implement.