The costs of conventional sources of electric power have been the cost of the inputs and the penalty of pollution. The main cash cost to the producer has been the cost of inputs, usually coal and oil.

In the case of hydroelectric, solar or wind-powered plants, on the other hand, the sources are nature-given and the costs have been only the cost of the installation and maintenance. While hydroelectric plants are restricted to just a few locations, the use of solar and wind-powered plants has been increasing. As wind farms grow more in number, however, farms that are upwind of other farms have started affecting the working of the downwind farms.

JK Lundquist, KK DuVivier, D Kaffine and JM Tomaszewski from the University of Colorado, the National Renewable Energy Lab, Colorado and the Denver Sturm College of Law, in a paper in the journal, Nature, examine the practical effects of crowding of wind farms, as well as the economic and legal consequences, in the case of wind farms in the State of Texas.

Texas happens to be the state in the US with the greatest number of wind farms – 12,077 turbines, over 121 farms, with capacity of 21 GW. In comparison, India has an installed capacity of 34 GW. India has a surface area of 3.3 million square km, compared the 0.7 million square km of Texas. The coverage in India is hence about a third of the coverage in Texas, but this is comparable and the study is relevant to India.

The principle is that when wind strikes the vanes of a collection of turbines, energy is transferred to the vanes, which are set rotating, and energy is lost by the wind. The wind field on the lee side of the turbines is thus less energetic than on the windward side. If another set of turbines is located close behind the first lot, there would be less energy to be transferred and the second set would work less efficiently.

A similar effect is found when a flock of birds are flying. When birds move through the air, they face air resistance, which amounts to a wind. Now, it is found that the conical space behind the leading bird is at a lower pressure and the birds that follow face less air resistance. Birds are thus adapted to fly in formation, so that they use the least energy. The space behind the leading bird, or a set of turbines, is called the “wake” and in the case of the turbine, the turbines that lie in the “wake” of turbines that are upwind are starved of strong winds to set their vanes in motion.

The study considers a group of three wind farms, Roscoe, Loraine and Champion, in the state of Texas, to see how exactly they affect each other. Roscoe was set up in March 2008 and the Loraine farm was set up in November 2009, at a nearby location, within 10 km, and upwind of Roscoe.

Champion is a similar wind farm, set up in March 2008, the same time as Roscoe, but outside the influence of Loraine. The study examined the available data of production efficiency and the time of the year, of the three farms, between 2008 and 2015, from the time Roscoe and Champion started working, and through the time when Loraine was set up and could be expected to affect the working of Roscoe.

The method of study was to consider the daily or available record of production during the year and a half that Roscoe and Champion were in operation before Loraine was set up and then the production at Champion for the whole period, as well as the production at Loraine. The data of Champion and Loraine, as well as national, hourly wind speed data bases and other sources of information indicated the actual wind conditions, under the seasonal and other conditions, during 2009-15. From this, as well as simulations, the production at Roscoe that should have been, during 2009-15, had Loraine not been set up, was calculated.

The actual production at Roscoe during 2009-15 showed that there was a reduction by five per cent, on the average, ever since Loraine came up. The output graphs of Roscoe and Champion are seen to keep pace till November 2009, when Loraine started operation. From that point on, the graphs of Roscoe and Champion diverge, showing graphically the effect that Loraine has on Roscoe. The highest losses in the downwind farm were seen to occur during specific conditions, rising as high as 15 per cent (production of 270 MW against potential 315 MW) and wind speeds could drop by two meters per second, in the short range, and by 0.5 meters per second as far as 50 km away.

The paper then analyses the economic costs of this drop in the efficiency of downwind farms. The wind farm occupies a large area and the sites are carefully selected after a wind survey that lasts at least a year. Apart from optimum wind conditions, it is also important that the farm be well located to get connected to the electric grid and also approachable for maintenance.

The wind conditions, in fact are studied in great detail, using lasers and the Doppler Effect to detect air movement, so that the dimensions and the section of the wanes are optimally designed. The care and attention is important, for getting the best return on the large investment made and because every extra mill watt generated amounts to huge savings, over time, in coal burned and CO2 released by conventional electricity generation.

With all this care, for a working wind farm to find that its output is hit to the extent of five per cent because another farm is set up at an upwind location, is tragic twice over. As the first plant was surely installed at the best place the terrain has to offer, the site for the second farm has been chosen with some compromise. And then, the output of the first farm is reduced. No doubt, the total power produced by the two farms is more that what could come from either of them, but when both farms are working below capacity, it shows that regulation and planning could have allowed both farms to stay optimal.

The study observes that the regulation of new wind farms are mostly the zoning and architectural restrictions that are relevant to civil constructions in general. There appears to be no regulation, specific to wind farms, either by the State nor any means for owners of wind farms to prevent other farms from coming up and affecting the working of the earlier farms.

The study notes the legal position with respect to a land owner’s rights to the benefits of what lies beneath his land. If it were a source of oil, the same source could be tapped by wells sunk in a neighbouring tract of land, and this could deplete the oil in the source. There is now legislation to protect the interests of the first oil well, and also water sources, the paper says. Regulation of wind farm spacing, however, is still to start.

The paper also notes that the effect that wind farms have on each other, indicates that there is limit to the power that can be extracted, the world over, from the wind. It is hence important that optimum positions for wind farms be regulated so that the best possible wind power is extracted, and in the most economical way.

The writer can be contacted at [email protected] science.in