Windmills in paradise

April 7, 2008
To drive regularly between Abilene and Lubbock, Tex., is to witness the industrialization of the Llano Estacado.

To drive regularly between Abilene and Lubbock, Tex., is to witness the industrialization of the Llano Estacado.

This Houston resident and his wife make the trip frequently thanks to daughters who attend Texas Tech University.

Each time, we see more wind turbines than we saw the trip before. Thousands of the structures now spike the landscape westward from just outside Abilene along Interstate 20 and northwestward along Highway 84 toward Lubbock.

They used to sit mostly atop the crooked red ridges that make the vast, mostly tree-free flatness out there interesting. Now they’re sprouting everywhere, in many places right next to the road.

My wife dislikes what the proliferation of wind turbines does to the view. It seems to me there’s plenty of view to go around in West Texas, but I don’t argue.

I just wonder why so many of the things don’t spin, even when the wind blows.

Wind research

These observations, voiced during a conversation in Houston, inspired Tom Standing, a San Francisco engineer and energy analyst, to conduct a little research.

Actually, a lot of research. Standing, who has written articles for Oil & Gas Journal, squeezes a subject hard when it gets his attention.

Standing says new and expanded wind ranches planned for the next few years will “add about 10,000 new turbines across the Texas prairie, all of which would be visible from up to 7 miles distant.”

A certain Houston lady won’t like hearing that. But Standing has different concerns: how much power all those propellered units actually will produce and variability of the power profile.

He points out that nameplate capacities of wind turbines are rated at wind speeds of 30-32 mph, that power output is proportional to the third power of wind speed, and that the capacity factor of a turbine is defined as instantaneous power output divided by the rated power output. Capacity factor can be calculated from the ratio of instantaneous wind speed divided by the wind speed at which the turbine is rated, raised to the third power.

Standing says a turbine needs a wind speed of 80% of the rated speed to register a capacity factor of 50%. At the 9-10 mph of wind needed to overcome inertia, a turbine generates power at about 3% of rated capacity.

Standing uses these relationships and wind-speed data from the University of Wyoming, interpolated to 80 m above the surface to account for the hub height of large turbines, to calculate capacity factors for hypothetical wind turbines in windy US locations.

He observes that the largest area with the strongest winds in the Lower 48 stretches between Amarillo and Dodge City, Kan. To the north and south, wind speeds decline gradually. To the east and west, they fall rapidly.

Standing’s calculated capacity factors are for windy March and April and calm August 2003 in seven cities in the high-wind corridor.

The highest capacity factor is 50% for Dodge City in April, the second 43% for Amarillo in the same month. That means a 100 Mw wind ranch near Dodge City would have produced 50 Mw of power, and a comparable facility near Amarillo, 43 Mw, when winds were strongest. Calculated capacity factors are much lower for Midland and Fort Worth, other cities demarcating the area of fast wind construction in Texas.

“Far more important than monthly averages is that capacity factors are erratic with respect to time, constantly gyrating from a low end of 0-10% when winds are light to brief periods at the high end when windy conditions give capacity factors between 50% and 100%,” Standing says.

Surges and lulls

Calculated capacity factors can remain at 0-20% for 2-3 days then jump to nearly 100% in barely 12 hr. Contrary to assurances of wind-power proponents, Standing adds, surges in some areas usually don’t offset lulls in others; the surges and lulls tend to happen at the same time across the windy corridor.

Between Amarillo and Midland, capacity factors can rise or fall by 60% within 12 hr. So by 2011, when Texas has about 10,000 Mw of installed wind capacity, power generation might rise or fall by 6,000 Mw in half a day, requiring the sudden start-up or shut-down of 15-20 peaking plants to balance load.

This might create more problems than the sight of windmills in rattlesnake pasture, but please don’t tell a certain Houston lady that you encountered that judgment here.