Free as the Wind

The worst situation is windy days when demand is low. Wind turbines can actually make it necessary to discard energy to keep the grid from going to excess voltage. Wind farms sometimes have to pay for this service. Fortunately, hydroelectric power can be used like a giant battery to stabilize the grid. With pumped storage, excess power is used to pump water back into the reservoir to be released later when there is a shortage of power. Pumped storage is about 70-85% efficient and simply uses the wind power to run the pump motors when there is a surplus of energy created.

Denmark has the good fortune of having its grid connected to hydro-rich Sweden and Norway. When the wind gets really strong the excess power is simply stored behind [dams in Sweden and Norway] for future use. There were 9 occasions in 2003 when wind produced power in excess of 85% of installed capacity. And then there was one day in 2003 the wind stopped and the wind turbines actually consumed more energy than they produced.

Since Danish wind turbines are only expected to produce an average of approximately 20% of their stated capacities, the country is indeed lucky to have neighbors who are happy to level their load. With wind currently supplying 19% of total electrical load, the Danes are planning to go to 50% in the future.

Ontario, Canada actually publishes a chart of power output vs. capability hourly for each type of generation every day. It is very interesting to study a particular day and see the wind die down and gas powered or hydro plants kick in to support the load. It’s a complex problem that uses a kind of auction with highest prices during shortages and very low prices during surplus. The electricity into the grid must equal what is taken out at all times or the voltage will go unstable. Wind and solar are a special challenge because they can be so unpredictable.

You have probably heard that wind turbines kill birds. They certainly do, but so do coal power plants, houses, cars and anything else birds could run into. The Audubon society strongly supports properly sited wind power. Today’s giant wind towers have blades that are more like the wings of an airliner. Environmentally, wind is squeaky-clean and doesn’t even take up any space if developed in farming areas. Cows happily graze under the towers and the farmer gets a nice monthly check. Biomass might make even more sense as a crop to grow under the towers.

Wind is a real success story that didn’t start out well. Early windmills built in the 1970s had a kind of frenetic feel to them with fast spinning blades. Today’s giant towers are graceful, majestic and almost restful looking. They’re also much more reliable. Often the old wind farms had a large percentage of the blades broken or stuck in one position. Our modern turbines have electronic monitoring of blade condition and transmission particle count that can electronically signal for help before trouble develops. According to Vestas, the energy used in building a wind turbine can be paid back in the first 7-9 months of operation! Much better than the 2-3 years it takes for silicon photovoltaic panels to generate the power it took to make them.

Power output rises as the cube of wind velocity; so doubling wind velocity actually gives eight times the power output. Power output also increases as the square of rotor diameter so large turbines in good locations really pay off. We already have towers as tall as a 35-story high-rise. Expect to see them go even taller. As they get bigger they are more and more like a building. Instead of ladders, many now have elevators that lead to an equipment room inside the nacelle. Repairs can be done from inside the nacelle including generator replacement and gear repairs.

Gearboxes tend to wear out in about 5 years so some new designs are gearless. As the generators get larger its easy to get high velocities for efficient generation without gears. One direct-drive generator has 3,960 permanent magnets around the periphery. American Superconductor is developing a 10-MW generator that uses superconducting wire cooled by liquid nitrogen. Because of the zero resistance wire, a 10-MW generator is no larger or heavier than a conventional 5-MW unit. The blades and tower size are scaled up.

In windy areas small wind turbines can be more cost effective than solar power for off-grid generation. A Chinese maglev 300-watt turbine uses magnetic levitation for extremely low friction, allowing it to work on a wind speed of only 1.5 m/s. That’s nice to keep it turning but in order to achieve efficient wind power generation, high wind velocities are required. If you’re on the grid it’s probably best to let the giants generate the power rather than look to small wind. Bigger turbines are much more efficient.

Building integrated turbines on the tops of high-rise buildings are good PR but the economics are still questionable. Remember that power ratings in Watts are meaningless as they only apply at a very strong wind velocity. What really matters is average kilowatt-hours over the year. That’s what we pay for and that’s what should be used to estimate payback time.

Wind power is already cheaper than coal. The amazingly fast evolution of wind turbine design and cost effectiveness will continue, leaving coal power in the dust (literally!) Offshore wind promises another jump in reliable capacity factor. Possibly more pumped storage will have to be built to allow wind to continue to grow, but the cost of pumped storage is a drop in the bucket compared to the billions being spent trying to rescue the coal business with carbon capture and storage.