While 160 companies around the world have committed to use “100 percent renewable energy,” that does not mean “100 percent carbon-free energy.” The difference will grow as power grids become less reliant on fossil power, according to a new Stanford study published today in Joule. Entities committed to fighting climate change can accurately measure and boost the environmental benefits of their renewable strategies, the authors write.
Current methods of estimating greenhouse gas emissions use yearly averages, even though the carbon content of electricity on the grid can vary a lot over the course of a day in some locations. By 2025, the use of yearly averages in California could overstate the greenhouse gas reductions associated with solar power by more than 50 percent when compared to hourly averages, the paper shows. One finding of this analysis is that wind power – not solar – needs to be the next wave of investments for California. Similar analyses could suggest different options like nuclear power, geothermal energy, and long-range transmission in other locations.
“To guarantee 100 percent emissions reductions from renewable energy, power consumption needs to be matched with renewable generation on an hourly basis,” said Sally Benson, co-author of the paper and co-director of the Precourt Institute for Energy.
“Just purchasing more solar energy in a grid that already has lots of solar generation will not result in zero emissions,” Benson, professor in the Energy Resources Engineering Department in the School of Earth, Energy & Environmental Sciences (Stanford Earth), also said.
Annual vs. hourly accounting
Corporations that claim to be 100 percent renewable do not actually cover all their power use with renewables, as some acknowledge. Instead, they purchase or generate enough renewable energy to match 100 percent or more of their electricity use over the course of the year. For energy purchases dominated by solar power, an entity generates far more electricity than it uses during the afternoon and sells the excess. Then at nighttime it purchases power from the grid, which is much more carbon-intensive if generated by burning of fossil fuels.
The use of annual averages of the carbon dioxide associated with grid power is valid only when fluctuations in renewable generation are small, or when all excess renewables can be stored. Places like California, Hawaii and some European countries experience large fluctuations in carbon content due to existing renewables, and do not yet have enough storage capacity to capture all excess electricity. In California, intentional reductions in solar and wind production, or “curtailments,” reached 3 percent of total generated energy in two months last year, the paper cites. As a result, sometimes 100-percent renewable consumers in California were selling their excess renewable power only to cause another renewable generator to shut down, which had no climate benefit.