The challenge of making sun and wind power more reliable
The New York Times: MATTHEW L. WALD - November 19, 2009
http://www.wind-watch.org/news/2009/11/21/the-challenge-of-making-sun-and-wind-power-more-reliable/
Technology and policy are coming
together to promise electricity as abundant as sunshine and as freely available
as the breeze — and about as fickle.
The challenge for power management experts is how to deal with thousands of
new generators, often in inconvenient places, that will not respond to electronic
commands to produce more when needed. These are mostly windmills and fields
of solar cells, which produce close to 20 percent of California’s electric
needs.
In a nondescript building with hardly any exterior signage, experts at the California
Independent System Operator are moving beyond their traditional job of balancing
California’s constantly changing electric demand against the output of
1,400 generators in the state and others scattered around the western half of
the continent.
Consider it a dress rehearsal for America. Gov. Arnold Schwarzenegger has ordered
the electrical system to find 33 percent of its needs from alternative sources
by 2020. Meanwhile, Congress is contemplating a national “renewable energy
portfolio standard.”
Managers here discuss new tools, like Doppler radar on remote hilltops to detect
the speed of the wind hitting the windmill blades and to forecast what it will
be in 15 or 30 minutes, so natural gas plants can be started in time to meet
demand when the wind stops, or shut down before the gusts come through and overload
the system.
Managers also explore how they might forecast haze and dust, which change the
output of solar cells. They worry about rogue clouds that will disrupt solar
fields. They talk about batteries of an unprecedented scale that will go from
fully charging to fully discharging in one second, to smooth out the herky-jerky
output of mammoth new renewable developments.
The idea is to convert a power chart that looks like an EKG into something smoother
like, say, a ski slope. A two-megawatt version — a battery big enough
to run two Super Wal-Marts — is being tested.
A massive shift to renewable energy is “a huge societal issue in terms
of the benefits, but also the challenges,” said Yakout Mansour, the president
and chief executive of the California Independent System Operator. “The
benefits are understood; the challenges are way understated.”
The grid experts are often reticent about their concerns, because when they
speak up, “we get perceived as we are not supportive of renewables,”
Mr. Mansour said. Electric planners are hesitant to draw the ire of the politically
powerful environmental movement, or the solar or wind lobbies, as Congress contemplates
a national “renewable energy portfolio standard.”
To hit the goal of 33 percent, there will be hours when California must run
at about 50 percent renewable energy — mostly wind — to balance
out the hours when the wind does not blow. That is hard, because the wind blows
best at night, when demand is low, and already there are night hours when the
system cannot use all the energy available, because there is so little demand.
Planners talk about the challenges of trying to keep power production no higher
than the level of demand, even with generators they cannot shut down. One idea
is to run air-conditioning and heating units extra hard at night and make buildings
warmer or cooler than needed, so demand will be less during the day.
The public may expect that with a proliferation of solar panels and wind machines,
companies will shut plants that run on coal and natural gas. But all of it will
be needed if the system is to remain stable and ride through periods of low
wind, said Mr. Mansour.
While California is at the vanguard, the problems exist all over. “The
transmission grid in its current state wasn’t designed for the new energy
frontier,” said Ken Salazar, the secretary of the interior, in a conference
call with reporters in October.
One early sign of what may lie ahead came in Texas, which is the leader in wind
generation. One night in February 2003, an expected cold front pushed demand
up so sharply that electric clocks throughout the state began running about
one quarter of 1 percent slower. Had the slowdown lasted an hour, Texas would
have been nine seconds behind the rest of the world.
Power-grid operators did not let it go nearly that long. Facing unexpectedly
strong demand and weak generation, they unplugged some big industrial customers
until the system was back in balance. But the cause of the problem was disputed.
Some people called it a “wind event;” the American Wind Energy Association
insisted it was a “reliability event” and pointed out that the shortfall
in conventional generation was larger than the drop in wind generation. In the
wind lobby’s view, the system worked as intended: industrial users who
had volunteered to be unplugged on short notice in an emergency, in exchange
for lower rates, earned their discount.
The intermittent nature of wind power and grid constraints already affect power
generation. Turbine operators sometimes have to shut down their machines because
there is no space for their electricity on the transmission lines. Thousands
of megawatts of additional wind power are on the drawing board, but cannot be
added until more lines are built.
Experts say 15 percent to 30 percent of total energy generation from alternative
sources is not impossible, but the system will have to be run differently to
accommodate more intermittent sources.
The problem is that the electric system runs on two different resources, energy
and capacity. Energy can be thought of as accomplished work; pumping a thousand
gallons of water from the basement to the roof, for example, is an energy problem.
Capacity means having enough electricity whenever it is demanded. Stepping out
of the shower and turning on a hair dryer without calling the electricity company
to ask for permission is an example of the need for capacity.
Wind adds complications. The wind industry added more than 1,600 megawatts of
capacity in the third quarter this year, bringing its total to 31,000 megawatts,
enough to power nine million homes. At times, those thousands of wind machines
will be producing enough electricity for many more millions of households; at
times, they will make hardly any.
SO how do places with a large number of wind machines calculate their capacity
value, the amount they can count on at a peak demand time? That depends on location,
said Mark G. Lauby, manager of reliability assessments at the North American
Electric Reliability Corporation, or Nerc, a nonprofit corporation designated
by the government to develop and enforce rules for operating on the electric
grid.
“There’s a variety of ways people calculate that capacity range,”
he said. “It goes from zero to 37 percent, based on historical performance.”
If the wind machines were operating at 37 percent of maximum capacity in the
most recent peak hour, they may be relied on for that much, he said, but the
number is usually lower, because most utilities reach peak demand on summer
afternoons, which are typically low-wind periods.
In Colorado, where Xcel Energy is under orders from state regulators to increase
the portion of electricity from renewable sources, the company sometimes produces
more electricity at night from wind alone than its service territory is using.
During the day, when demand is high, it is sometimes producing none. The solution
in Colorado and elsewhere is gas turbines, which run on natural gas and are
not terribly efficient, but can vary their output relatively quickly.
Solar power has its own problems. While the angle of the sun in the sky can
be easily predicted, the sun can go behind a cloud even faster than wind can
die down. At a recent symposium on the electric grid, Steven Chu, the secretary
of energy, presented a graph that showed how a solar power farm lost 81 percent
of its output in five minutes.
California utilities have contracted for solar power farms that will cover hundreds
of acres with thousands of megawatts, but solar advocates say stray clouds are
manageable. “It turns out that when the field gets large enough, the kind
of single clouds that can cause the 80 percent drop in 90 seconds are no longer
a problem,” said Ken Zweibel, director of the Solar Institute at George
Washington University. “Why? Because they only turn off part of the field.”
The variability problem in both sun and wind could be mitigated by a better
grid; the existing system resembles a sheet cake in a flimsy box, which will
hold together as long as it is supported from many points. The variability would
average out if the solar cells and the wind turbines were in many different
places, but they would have to be tied together more tightly, experts say.
Such a system would look very different. The North American Electric Reliability
Corporation reported in October, for example, that a requirement that 10 percent
of electricity demand be met with renewables by 2018 would mean that one-quarter
of all generators on the system be renewable, or mostly wind.
Transmission would be another problem. For the last two decades, power-line
construction has been trending lower, to about 4,000 miles now from about 8,000
miles a year in the early 1990s, largely because it is so difficult to get permission
to build. To integrate the new renewables would require doubling the pace of
construction, the group said.
A potential solution is to reverse the pattern of making supply meet demand.
Instead, demand could be tailored to meet supply, through “demand-side
response,” in which utilities selectively unplug their customers under
a prearranged deal where the customer gets lower rates for interruptible service.
Kelly Ziegler, a Nerc spokeswoman, said, “we’ve always been a proponent
of demand response as a dance partner for wind.”