Might wind turbines one day provide a large fraction of the electric power on the grid?

How much can wind turbines be relied on without causing havoc when the breeze stops?

Advocates for wind energy tend to cast the answer in terms of the amount of energy being provided to the grid. If the "penetration" of wind energy is less than some threshold, they argue, the system can manage it at negligible cost, just as it handles the unavoidable variability in the load. Beyond some modest penetration level, however, electric utilities are bound to get into trouble if they don't carefully plan how to integrate wind into the mix of power sources. Judging the degree of penetration that can be accommodated is tricky, but wind-energy proponents like to point out that Denmark currently gets about a fifth of its electrical energy from the wind.

Joseph F. DeCarolis, of the Atmospheric Protection Branch of the U. S. Environmental Protection Agency's Office of Research and Development, takes issue with this view of the situation. DeCarolis studied the problem in depth for his Ph.D. in engineering and public policy. He says that when he began his research at the turn of the millennium, most people were relying on nothing more than "back-of-the-envelope calculations." He points out that some analysts had concluded that the intermittency of the wind didn't much matter if the turbines were distributed over a sufficiently wide geographic area. But that notion "didn't really sit right" with DeCarolis and his thesis advisor, David W. Keith, so they created a numerical model to put the assertion to the test.

From their model, they concluded that there is always some intermittency cost, even if wind provides just a tiny fraction of the total power on the grid. DeCarolis estimates that the penalty would, in general, be something between one and two cents for each kilowatt-hour of energy produced in this way. That's a small but still significant component, given that the price of wind-generated electricity would otherwise be something like five cents per kilowatt-hour.

Denmark has managed to integrate a large number of wind farms into its grid because, as DeCarolis says, "there's a ton of hydro on it, and it's very easy to dispatch hydro to make up the difference." In particular, Denmark's electric grid is connected to that of Norway and Sweden, where hydroelectric plants can easily cut back on production when Danish winds are strong and boost their output when the breezes to the south abate. Balancing wind and hydroelectric power generation in this way allows the two renewable-energy sources together to provide cheap and reliable power to the grid at all times.

But what can an electric utility do with wind if its grid lacks river-spanning hydroelectric-power plants like those found in Norway and Sweden? Plenty. It can, for example, turn to what's known in the industry as "pumped hydro," whereby water generates power as it flows through turbines from a reservoir at high elevation to a lower one. The same water can be pumped back upward using the electricity available at times of lessened demand, allowing the excess energy to be stored.

Another well-known technique for dealing with variability on the grid is called compressed-air energy storage. The basic strategy is to use excess electric power to run compressors that inject air into underground storage fields. The air is held there in a manner similar to the way natural gas is routinely stored underground. Then, when electricity is needed, this compressed air is fed into turbine generators that burn natural gas. Such gas turbines normally expend much of the energy in their fuel compressing the air prior to combustion. Using pre-compressed air allows them to save anywhere from one-half to two-thirds of their fuel costs.

"In today's environmental world, you don't have much luck finding a place where you can bulldoze the top of a mountain to create a reservoir," explains Kent Holst, former general manager of the electric utility company in Traer, Iowa. Holst is currently working with the Iowa Association of Municipal Utilities to develop a hybrid facility where the energy from a wind farm will be used to compress air for later use in gas turbines. This "Iowa Stored Energy Plant," as it is dubbed, is still in early planning stages, but Holst reports that feasibility studies suggest it will have good economic return. And it will help Iowa utilities in a way that is less easy to put a dollar value on: "We feel a strong obligation to do what our customers want—and our customers want greener energy."

In other countries, proposals for storing the energy from the wind sometimes take a completely different shape. For example, managers of the Sorne Hill Wind Farm, which was erected in Ireland in 2004, are planning to install a huge battery to help even out variations in generation. The device they plan to use is known as a vanadium redox flow battery, which in some ways resembles a giant fuel cell, except that it can be charged and discharged repeatedly. A recent  study suggests that the building-size battery, which will be able to hold 12 megawatt-hours of energy, should pay for itself within 10 or 15 years.

Although DeCarolis is skeptical that such batteries could solve the problem at the largest scales, he is convinced that the variation in the output of wind turbines is something that can be managed.

Source: American Scientist

Interesting debate in terms of how much can wind intermittency can the grid support, without having excessive amounts of new peaking power plants held in hot standby to support renewables

I was very interested in the VRB battery system proposed for Sorne Hill, unfortunately it eventually did not go ahead, at around the same time the VRB company development in Canada folded - perhaps the two events are not unrelated, considering the battery development investment was proposed to run into millions for Sorne Hill!

I understand that there are moves to take up VRB battery development where the last company left off. It seems like a technology that promises a lot. If anyone has any opinions on the technology it would be good to read them. The company seemed to have problems taking the technology from demonstration projects to a commercial production basis. Technological flaws did not seem to be at the root of the foldup. The fact that this technology is scalable and does not rely on large use of land to store either air or water is a big plus - anyhow keep up the good work