The two traditional excuses for not adding more wind and solar into grid operations have been that they could not be called on (“dispatched”) when energy was needed and they were too expensive (“uneconomic”). But grid operators around the U.S. have begun to discover that neither is necessarily true.
“The New York ISO was the first to implement dispatchable intermittent resources in allowing wind to participate in economic dispatch and congestion management. It was thought of then as crazy to use wind for grid reliability,” said National Renewable Energy Lab (NREL) Senior Research Engineer Erik Ela. “Now almost every ISO in the U.S. uses wind this way. And we can go further and use more of the flexibility wind has to provide more support to the grid.” Ela said utility-scale solar can eventually do the same.
Researchers at NREL, the Electric Power Research Institute (EPRI) and the University of Colorado are working to demonstrate how the unique characteristics of wind and solar can, much like traditional generation, perform Active Power Control (APC) to support the grid. Researchers are especially focusing, Ela said, on two areas of APC, regulating reserves and primary frequency control.
“APC is control of power output to balance generation and load,” Ela explained. “Providing dispatch is one way of doing that. It can even do that in the faster time scales.” APC can support the grid during specific events such as when a large generator goes offline and the lost supply has to be replaced, Ela said. It can also steady the grid as load and renewables vary and require balancing.
“Grid operators would like to have more support for transmission line overload, voltage drops, and frequency variations,” Ela said, but don’t typically ask that renewables provide it, and regulators have not imposed the requirement to use renewables in that way.
Wind turbine manufacturers say they can provide the capability for renewables to provide APC services but see no demand for it from renewables developers.
Renewables developers don’t see any point in driving up costs by building in the capability to provide a service for which there is no demand from grid operators.
Regulators see no point in requiring grid operators to buy a service from renewables they may get for free from traditional generators.
“We are doing research on ways to connect these perspectives,” Ela said.
Regulating reserves “correct for the energy imbalance within economic dispatch intervals,” Ela explained. “It is a little faster than normal dispatch. It uses a signal that comes directly from the system operator on how a generator’s output should be changed on a minute-to-minute time frame.” All U.S. balancing areas, RTOs/ISOs, and vertically integrated utilitieshave “a certain amount of capacity they use as regulating reserve,” Ela explained.
Studies have shown wind plants can provide this service “faster and in some ways more accurately on these time scales than conventional generation,” Ela said, because “conventional generation has thermal time constants that slow down this kind of response whereas wind is all power electronics: You need that response, you get it.”