Solar power’s “duck-curve” problem – where solar production and demand peaks don’t align – is rearing its head where grid operators are at the cutting edge of clean energy, recent analysis shows.
The good news for utilities across the U.S. is that operators are exploring a wide range of potential fixes for the technical challenge of integrating renewables into the conventional grid.
The so-called duck curve has emerged in utility circles as shorthand for the midday surge in solar power supply, which distorts how and when fossil-fuel generators operate. It refers to a 2013 graph released by the California Independent System Operator (CAISO) that shows a sharp decline in demand for dispatchable, non-renewable power during midday when solar output is high, followed by a sharp spike in demand for those sources in the early evening as solar output plummets.
“The units that we have and the amount of capacity that have that ramping capability is limited,” says George Gross, a professor of electrical and computer engineering at the University of Illinois at Urbana-Champaign. “I can’t tell a solar unit to ramp up … [but] I can do that with gas-powered units.”
In May, Meredith Fowlie of the Energy Institute at University of California Berkeley’s Haas School of Business, crunched CAISO data for this year’s “duck season” (roughly March and April) and found that the projected declines in midday net loads were arriving more or less on schedule as increased amounts of solar has come online in California.
“So far, these challenges are quite manageable without major changes to grid operations,” Fowlie wrote in a blog post. “But the duck of the future – especially given California’s new target of 50% renewables by 2030 – will present a more formidable challenge.”
‘Teaching the duck to fly’
For Jim Lazar, the solution lies in “teaching the duck to fly.”
Lazar, a senior advisor at the Regulatory Assistance Project, a non-governmental organization that works on clean-energy transitions across the globe, wrote a paper in 2014 (updated in 2016) that offered a range of duck-curve fixes, extending the analogy derived from its fowl-like shape.
A real duck in flight, Lazar notes, flattens out its profile to minimize wind resistance. In essence, he says, utilities must follow suit. The power sector must do more to flex demand to meet the nature of today’s evolving power supply, Lazar says. It must flatten out the peaks and valleys to obtain a more streamlined energy profile.
“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. That’s changing,” Lazar says. “Utilities are putting programs in place to get customers to change their demands to increase their usage in low-cost hours and to decrease their usage at high-cost hours.”
These programs and strategies vary widely from increased use of electric vehicles to more widespread use of demand-response technologies. Some utilities are beginning to alter the price of electricity based on the balance of supply and demand.
Most ratepayers today pay the same flat rate regardless of what time of day they use power. But, as the duck curve illustrates, not all power is generated equally, and the economics of supply and demand can fluctuate throughout the day. If electricity costs more during peak hours (say, between 4 p.m. and 7 p.m.), Lazar and others say customers would be more efficient, thereby helping to flatten the late-day peak in demand.
Battery storage, meanwhile, can help utilities capture some of solar’s peak mid-day supply and transpose it a few hours later to help meet the post-work demand surge. In Hawaii, 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 batteries will “‘shave’ the amount of conventional power generation needed to meet peak demand in the evening from 5 p.m. to 10 p.m.,” KIUC said in a February press release.
Low-tech solutions
One of the simplest remedies for the duck curve may come in a technology that lies hidden in plain sight: electric water heaters.
Nine percent of U.S. household electricity goes to electric water heaters, according to a study issued in January by the National Rural Electric Cooperative Association, an industry group.
These simple devices “are essentially pre-installed thermal batteries that are sitting idle in more than 50 million homes across the U.S.,” the report concludes. In other words, water heaters can be flipped on when power is most readily available, and the hot water can be used at a later time.
Lazar is equally bullish on water heaters and devotes a whole chapter of his paper to the technology. He calculates that an additional 100,000 megawatts of variable wind and solar could be added in the U.S. if every electric water heater employed smart-grid technology to store energy and help reshape aggregate supply and demand.
And while the duck curve isn’t yet an issue in the Midwest, the use of electric water heaters as batteries has already gained a lot of ground in rural electric cooperatives – most notably at Great River Energy in Minnesota. There the cooperative says it has been able to store 1 gigawatt of energy each night by controlling the electric water heaters of its 65,000 members. That’s half the capacity of the Hoover Dam.
Back in California, Berkeley’s Fowlie makes the case that regional coordination will be key to taming CAISO’s duck. While California is connected to the broader western grid, in many ways it tends to operate in isolation. If CAISO were more integrated with neighboring grids, it might have more options for spreading out imbalances in supply and demand.
“A regionally coordinated western grid would integrate mandated renewables across a larger area, thus reducing the likelihood of over-generation,” Fowlie writes. “Coordination across balancing areas should also provide increased flexibility.”