A service technician inspects the rotor blades of a wind turbine. (Photo by Christoph Soeder/picture ... [+]
Wind and solar may introduce concerns about variability into the power grid, but in some cases they can resolve those concerns—according to an analyst from the National Renewable Energy Laboratories—better than fossil-fuel power plants can.
Renewables can provide many of the ancillary services needed to ensure the reliability of the electric system, said Paul Denholm, a principal energy analyst with NREL’s Grid Systems Group, if they’re managed strategically.
“That's always been my biggest concern: making sure that wind and solar at a minimum can address any increased uncertainty and variability that it (renewable energy) imposes on the system,” Denholm said. “Let it solve its own problems.”
Renewables are better positioned to provide some services, said Denholm, who focused on wind energy. A wind turbine can ramp up to 1oo percent or down to zero percent almost immediately. Thermal and hydro plants have a more limited range, and it takes them minutes to hours to ramp up or down.
“Wind does have some advantages here,” Denholm said in an NREL webinar. “It can operate over its entire range of output, and it can have ramp rates that are much higher than a conventional generator. So for two of those elements of providing reserves, wind can actually be superior to a thermal or hydro plant.”
Wind, of course, is constrained by nature, just as thermal plants are constrained by fuel. Because nature is variable, the grid needs to change before wind can effectively replace thermal plants.
“The ability of wind by itself to replace traditional thermal capacity is relatively low,” Denholm said. “You know, typically 10, 20, 30 percent. So if you've got a 100 megawatt wind farm you'd only expect to be able to replace maybe 20 or 30 megawatts of conventional capacity. So that's one of the reasons why folks are interested not only in storage, so you can ship the generation of wind to other periods, but seeing what other things wind can do to increase its value to the system.”
Among those other things, wind could take advantage of its ability to rapidly ramp up and down to quickly meet changes in demand or frequency.
“You know many of these are market procured and compensated services,” Denholm said, “and in many places wind can provide them.”
To do that, producers would “pre-curtail” wind farms, dispatching less power than they’re capable of dispatching.
“So if you've got a wind turbine, and the wind conditions are blowing such that it would produce one megawatt, you can actually control the output of that wind turbine anywhere between zero and one megawatts and you could do it at a very rapid rate,” Denholm said. “So if that wind availability is one megawatt on that individual turbine, we might back the turbine down to 500 kilowatts and that way we can provide 500 kilowatts of upward reserve.”
There’s little reason for wind producers to do that, however, if the system only pays them for production and not for curtailment—for capacity reserves—or for other ancillary services. Similarly, thermal power producers have complained in the past about fly-ball governors that ramp plants up and down without compensation.
“Technically wind can pretty much do most of what conventional generators can do, with a big caveat that the wind has to be blowing at the time when you need that service.”
And when the wind stops blowing, said Denholm, wind power would be better served by eight-hour batteries than the current four-hour norm.
Watch Denholm’s webinar:
