Pickens Plan Fails to Account for Limitations on Wind Power

Pickens Plan Fails to Account for Limitations on Wind Power
November 1, 2008

The Pickens Scare, being promoted by incessant advertising by billionaire investor T. Boone Pickens, sounds so familiar. Our consumption of foreign oil is increasing. We’re sending billions of dollars overseas to buy oil. “This is one problem we can’t drill our way out of,” says Pickens.

The Pickens solution sounds so simple: (1) Generate electricity with wind instead of natural gas. (2) Free up natural gas to use for transportation. (3) Voilá! We are now energy independent.

Never mind that we get most of the “foreign oil” from Canada, and secondly from Mexico.

Never mind that we get our natural gas by drilling, and that Pickens has said we can’t drill our way out of the “crisis.”

Never mind that the United States is not self-sufficient in Persian rugs or other consumer goods either, and that free trade allows us to import relatively inexpensive foreign goods to save U.S. consumers money.

And let us note that the United States imports natural gas (primarily from Canada) to make up for the shortfall in domestic production. Net natural gas imports are equal to 16.5 percent of domestic production.

Wind Foibles

In considering public policy toward wind power, it’s important to understand the potential and limitations of this energy source.

Most wind machines built today have nameplate capacities in excess of one megawatt, far larger than the 100 kw units of the 1970s. For just one example, consider the 1.5 mw unit made by General Electric. There are two versions, with diameters of 77 meters (253 feet) and 82.5 meters (271 feet). Hub heights are 80 meters (262 feet). The larger-diameter unit is for less-windy places.

The cut-in speed (the point at which the turbines start turning) is 3.5 meters per second (7.8 mph), and full power occurs at a speed of 14 m/s (31.3 mph) for the 77 m unit. At the low speed, the power produced is about 25 kw, less than 2 percent of the rated power.

I don’t mean to imply any failure of engineering here. It is simply that the power available in the wind is proportional to the cube—the third power—of the wind speed.

Between 14 m/s and 25 m/s (56 mph), the power produced by the wind turbine is constant at the nameplate value. Although there is more power available in the wind at the higher speeds, the system is designed not to overdrive the generators, by “trimming” the rotor blades to reduce the efficiency.

This circumstance is not very common because 14 m/s (31 mph) and faster winds are not all that common. When the wind speed reaches 25 m/s, the turbines are designed to shut off completely to avoid stresses that could tear the turbine apart.

The problematic wind speeds are those near 12 m/s (27 mph). The system produces full power at 14 m/s, but a drop to 10 m/s reduces the power from 1.4 mw to 1 mw, a sudden loss of 400 kw. It is necessary for the utility to maintain spinning reserve to handle those sudden fluctuations.

Spinning Reserve

The term “spinning reserve” refers to generators that are ready to produce electrical power at a moment’s notice. Whenever they are running, they are putting some power—perhaps one-third to one-half of their capacity—on the grid. Thus they can compensate for both increases and decreases in demand for power.

A utility generally has to have enough spinning reserve to handle the sudden loss of the largest generator online. Since the wind might stop or slow down over the entire area occupied by a wind “farm,” the largest generator may well be the entire wind farm.

It is often said that if we had enough wind machines in enough locations with enough transmission line capacity, then the wind power from one location could compensate for sudden decreases in another. But the general rule is that whenever a wind turbine is spinning, it is putting all the power it can muster onto the grid. In other words, wind turbines are not generally capable of serving the role of spinning reserve.

In theory, a sufficiently large array of wind turbines could occasionally take on the role of spinning reserve. It would only be necessary to trim the blades to reduce efficiency so that they produced only 50 percent of the power they could at that wind speed, so that more power could be quickly obtained by trimming the blades for higher efficiency. But doing so would cut the turbines’ paltry 35 percent capacity factor in half.

Capacity Factor

If a wind machine has a 1 mw (1,000 kw) generator, and produces that much power for every one of the 8,760 hours in a year, it produces 8,760,000 kwh and has a capacity factor (CF) of 100 percent. If it produces half that many kwh during the year, it has a capacity factor of 50 percent.

In other words, the capacity factor is the energy the machine produces during the year, compared with the energy it could have produced if it ran full-tilt, full-time. Alternatively, one can think of it as the average power produced compared to the nameplate power.

Wind turbines have improved considerably since the 1970s, not so much in efficiency but in both power output (typically 100 kw in the 70s to 2,000 mw presently) and mechanical reliability. The capacity factor has “improved” from about 22 percent to about 35 percent, but because of something no more profound than a changed philosophy of design.

There is a simple way to make a wind machine with any desired capacity factor. To get zero percent, attach a child’s pinwheel to a 1 mw generator. The average power would be zero watts. Divide that by 1 megawatt and you get zero.

To make a wind machine with a capacity factor approaching 100 percent is just as easy: Attach a 100-meter-diameter rotor to a 1 watt generator. A device that big should almost always be able to produce one watt, so the CF would be near 100 percent.

Both of these devices would be worthless, of course.

Deceptive Numbers

In other words, one can design a system to have any given capacity factor by establishing the ratio of generator capacity to blade diameter. These days, wind systems are designed to have capacity factors of about 35 percent.

With no change to the rotor blades, one could replace the generators on the 1970s-style wind machines still in operation with generators of lower nameplate capacity, and immediately improve the capacity factor. Big deal.

By comparison, the three-year capacity factors averaged over the nation’s 104 nuclear reactors exceed 90 percent.

Space Requirements

The power from wind obviously depends on location, because some places have stronger or more persistent winds than others. But there is a limit to how much power you can get from a certain amount of land.

The job of a wind turbine is to extract the energy—the kinetic energy—from the wind. Of course, it can’t extract all of the energy, because the air stream would stop and no more moving air could get to the wind turbine. The wind turbine slows the air considerably and spreads it out, leaving a low-velocity wake behind the wind turbine. Thus it would be folly to place one wind turbine close behind another.

Furthermore, there is always some turbulence associated with wind turbines. Just as it is unwise (read: fatal) for one jumbo jet to fly too close behind another, the turbulent wake of one wind turbine can damage a nearby turbine.

Accordingly, there are rules of thumb for placing wind turbines. Basically, it amounts to placing them roughly 10 diameters apart in each direction.

Now let’s look at the effect of diameter on power. The intercepted power is proportional to the cross-sectional area of the circle traced out. If we double the diameter, we quadruple the power.

But if we double the diameter of the wind turbines, we must also double the spacing between wind turbines in both directions, placing them on four times the land area.

Land Area

So there’s an important fact. If we quadruple the power and quadruple the land area, then the power per unit of land area remains the same. It simply remains to look at real data from real wind farms to see what the universal power per unit area is.

It is not always easy to find good data about wind farms. Most often missing is the land area occupied. A survey of many sites showed the best sites produce a year-round average power of about 12.5 kw/hectare (5 kw/acre).

Pickens cites a report from the Energy Efficiency and Renewable Energy (EERE) division of the U.S. Department of Energy about the future of wind, which—with irrational exuberance—claims wind will contribute 20 percent of our electrical energy by 2030.

By my calculations, it would require about 105,000 square km (38,000 square miles) to generate 20 percent of the expected 5.8 billion mwh U.S. electricity output in 2030. EERE gives a more optimistic figure of 50,000 square km (18,000 square miles), but with no justification whatsoever.

Howard C. Hayden (corkhayden@comcast.net) is professor emeritus of physics at the University of Connecticut and adjunct professor at Colorado State University at Pueblo. He writes a monthly energy newsletter available for $35 annual subscription at The Energy Advocate, PO Box 7595, Pueblo West CO 81007.