As California's grid operator looks toward a future with 33 percent renewables on the grid, it has adopted something of a mascot that sums up the challenge it faces: a duck.
The “duck curve” graph, as it’s known in energy circles, shows the gap between the total load a utility serves and what that load looks like after wind and solar generation serve some of that load. The graphic interpretation of that scenario looks somewhat like a sitting duck. It represents an average day, rather than a peak demand day.
The problem for utilities is twofold: the risk of over-generation in the afternoon and an increased need for ramping as solar drops off in the late afternoon.
Some, like former California PUC Commissioner Mark Ferron, believe the duck concept is overblown. But it's clear that utilities are facing some pretty serious changes to their traditional balance between load and demand.
So what to do about it?
“This challenge is causing some utilities to raise questions about the long-term suitability of variable energy resources,” wrote Jim Lazar, senior advisor for the Regulatory Assistance Project, in a recent report, Teaching the “Duck” to Fly.
Flattening out the curve so there is less ramping needed from traditional generation will be key for utilities. Lazar’s study identifies ten strategies that can achieve the result -- none of which require entirely new technology.Target energy efficiency to the hours when load ramps up sharply
. The study finds that lighting is a significant portion of the load from 4 p.m. to 7 p.m. (although AC might drive more of peak during summer days), and therefore the switch to LEDs -- in both homes and businesses -- could have a notable impact on the curve.Orient solar panels to the west.
Solar panels are often fixed on rooftops to maximize the total energy production, but if the goal is matching demand curve, they might need to be tilted westward. A study from Pecan Street Research Institute found that west-facing rooftop solar panels produced 49 percent more electricity during peak demand compared to south-facing panels.Substitute solar thermal with a few hours of thermal storage in place of some projected solar PV generation.
Since solar PV is most of the generation represented in the duck curve, swapping a fraction of the solar PV to solar thermal would allow the energy to be used later. Arizona is at the forefront of CSP and storage with its Solana Generating Station. Storage tied to solar PV could be another option, and one that could come faster than building solar thermal plants.Implement service standards allowing the grid operator to manage electric water heating loads.
Water heaters have been used for decades as simple load control devices by many utilities. But it’s time to expand that even further, according to Lazar. He says that supercharging water heaters could conservatively offer about 150 megawatts in any hour to California for storage.Require new air conditioners to include two hours of thermal storage capacity under grid operator control.
Air conditioning load is responsible for most of the peak use during the summer, and any solution should tackle both commercial and residential AC units. Lazar suggests units that include a few hours of thermal storage, such as the technology from Ice Energy. More sophisticated and automated demand response, however, may be a more feasible way in the short term to match AC cycling with solar load while reducing peak demand.Retire inflexible generating plants with high off-peak must-run requirements.
Older coal and nuclear plants just don’t play as well with renewables as do newer gas-fired coal plants. However, the move to retire these plants is being driven by state and federal regulations, as well as by low gas prices and increased energy efficiency, not necessarily by their limited ramping capability.
There is a lot of talk about time-of-use pricing and demand charges, which are increasingly being levied on commercial customers. But instead of just charging customers more when demand is highest, rates should also match the periods when ramp is highest, argues Lazar.Deploy electrical energy storage in targeted locations.
Flywheels, batteries, compressed air storage, pumped hydro and even electric vehicles could be deployed in areas with a lot of renewables or nuclear to take advantage of surplus generation.Implement aggressive demand-response programs.
The U.S. is already a world leader in demand response, both to curb peak and to provide ancillary services like grid balancing. But there may be another 3 percent that could easily be squeezed out of demand response programs, concludes Lazar.Use interregional power exchanges to take advantage of diversity in loads and resources.
The power profile of the hydro-rich Northwest is vastly different from sunny Southern California. Regional planners have already started to take advantage of on-peak power exchanges, but there could be a whole lot more, according to Lazar. In the West, there are more than 8,000 megawatts of interregional transfer capability using the existing transmission system.
So what would happen if some of these solutions were implemented? The graph shows an example of how it might change the load profile of a southern California utility:Graphic: Regulatory Assistance Project
Reducing the need for ramping is just one challenge that renewables are creating for traditional utilities.To learn more about technology solutions being adopted by leading utilities in response to distributed energy resources, join Greentech Media at