It’s called the “duck curve,” and for utilities it’s a barrier to ramping up reliance on solar power. But a range of tactics are available to bring the duck curve’s operational dangers into line with a reliable renewable energy supply, Midwest Energy News reports.
The term refers to the shape of a graph first observed by California’s grid system operator representing “net load,” or “the difference between forecasted load [power demand] and expected electricity production” from “variable generation resources [like] wind and solar” over the course of a typical day. Put another way, net load is the amount of actual demand those renewable resources can’t meet at any given moment, and the graph shows what that difference is for each hour in an average 24.
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The resulting curve through 2020, which looks a little like a right-facing duck’s profile, suggests that by then renewable sources will require nearly 33 gigawatts of “top-up” power from conventional fossil fuel sources during the California dinner hour, but less than 22 GW in mid-afternoon. The difference of more than 10 GW must be made up by power plants that can be brought online in the short time between mid-afternoon and early evening—something coal and nuclear facilities can’t do, but natural gas plants can.
“So far, these challenges are quite manageable without major changes to grid operations,” said Meredith Fowlie of U.C. Berkeley’s Energy Institute. “But the duck of the future will present a more formidable challenge,” she added, noting that California wants to source fully half its electricity from renewables by 2030.
But several tactics are available to flatten out the duck’s problematic morning and evening peaks and afternoon slump.
One is the increasingly common practice of using variable rates to motivate consumers’ timing for powering up their appliances. “We’ve got a 125-year history of the utility industry producing power when people want it and varying the output of power to match customer demand,” said Jim Lazar of the Regulatory Assistance Project, which assists with clean energy transitions. “That’s changing. Utilities are putting programs in place to get customers to increase their usage in low-cost hours and decrease their usage at high-cost hours.”
Visionary operators also have their eyes on the humble home electric water heater, responsible for 9% of all U.S. electrical demand. MWEN cites a report in which the National Rural Electric Cooperative Association, an industry group, calls them “essentially pre-installed thermal batteries that are sitting idle in more than 50 million homes across the U.S.” Timing their operation to heat water for evening use during the duck’s afternoon low “belly” period, would also help flatten the troublesome curve.
Battery storage, meanwhile, is becoming an increasingly viable alternative to firing up back-up power plants, effectively allowing utilities to time-shift their solar production from low- to high-demand times of day.
“In Hawaii,” MWEN notes, “the Kaua’i Island Utility Cooperative (KIUC) and SolarCity are building what is believed to be the world’s first ‘dispatchable’ utility-scale solar plant using 52 MWh of Tesla Powerpack lithium-ion batteries.” The battery banks will lower the duck’s “head” by reducing the fossil power the co-op needs to meet customers’ evening demand.
