Reaping the wind

Though windmills form a miniscule fraction of energy harvesting in India, it is an industry that is fast gaining momentum and popularity, writes S Ananthanarayanan

Wind power is fast becoming a compelling alternative to reduce the use of fossil fuels and pollution. Windmills have been seen across the world from centuries ago. In fact, the traditional windmills of Holland may have even predated airplanes in employing aerodynamics in their functioning. 

After the oil crisis of 1973, Denmark developed expertise in building and installing wind-driven electric generators. It is now the world leader in wind power and the share of wind power in Denmark is over 30 per cent, with their target being a 100 per cent by 2050.

Special design

The arm of a windmill can be likened to an airplane wing, which is fixed upright. It is the shape or profile of the wing that reacts with the stream of air, to create an upward force, on each wing, which lifts the load in between. 

This same force which arises when air moves over a curved surface similarly acts on the vanes of the windmill when the wind blows, and it acts in the same sense on each vane, to turn them round in a vertical circle. 

The modern application of wind energy is no longer used just in a machine to drive a grain mill, but it is to drive the rotor of an electric dynamo. The arrangement is called a wind turbine. 

The vanes are no longer made of wood and canvas, but are giant structures of fibreglass reinforced plastic, with struts and supports of sophisticated composite materials. And the arrangement has to bear a huge load, with the transmission and generator that work for weeks and months without a break. 

Denmark, in fact, had a head start in wind turbine research as it had got into the field as early as 1897, with facilities which included a wind tunnel.

In the same way as an airplane wing is specialised for different loads and wind speeds, the wind turbine vanes are also specially designed, based on dimensions and wind conditions. 

As the outer ends of the vanes move faster than the inner part, the profile of vanes, which can be as long as 82 m, needs to optimised along the length to present the best surface to the wind at each point and to get the best conversion of energy. 

The material and construction of the vanes needs to be good enough at all places and the connection to the central hub needs to be designed to withstand huge loads. Poul (Poho) Hummelshoj of the Technical University of Denmark, during a presentation to journalists at the Euroscience Conference at Copenhagen said that the force at the central hub was like that of a motor car pressing down through a lever that was seven km long! 

The research facility, which is housed near a now unused nuclear facility that had been set up under the renowned physicist Niels Bohr, has separate departments which study the entire spectrum of the science of wind turbines. 

They use sophisticated micro and macro material science to arrive at the most suitable materials, laser beams, their scattering pattern, analysis of air dynamics and design of the placement of turbines in a farm of turbines.

What wind turbines need to function is just the engineering and a reasonably continuous stream of air, at least 4-5 m/s and not more than 25-30 m/s.

Most coastal areas and hilltops have ample wind and are good places to set up several wind turbines, to act together as a wind farm. The usual wind turbines have been smaller and rated at 225 kilowatt, of which one would need four for a megawatt. 

But, the present wind turbines have span of 164 m and capacity of 8 MW. A farm of these giants, each one larger than three football fields standing at end, can generate power in thousands of megawatts, larger than a nuclear plant. 

A farm known as the East Anglia Array has been planned for generating 7,200 MW, using 1,028 meter turbines. 

India now has wind turbines in many states and the capacity installed is approaching 20,000 MW, which is 8.5 per cent of the total capacity, although the actual generation is only 1.6 per cent. 

This is far below the capacity installed in other countries, but India still ranks fifth worldwide and the Ministry of New and Renewable Energy has assessed the potential of wind resources at more than 1,00,000 MW.

Impediments

The negatives associated with wind power are usually the cost, the noise from the generators and the fast-moving arms, and danger to birds. A technical solution to reduce noise from generators is to install the turbines far from cities. 

There is also a trend now to install the farms out at the sea, as off-shore facilities, either on steel and concrete stilts or even on floating platforms. 

These, of course, have higher costs for installation, maintenance and transmission of electricity. As for the cost, the investment is generally being made by the state electricity authorities, with a growing market of private investment, mostly by pension funds.

“Inviting people in an area to invest in the wind farms that serve them would also help reduce resistance, as this would give the users the comfort of ownership,” said Poho. As for the danger to birds, Poho said that there had been a number of studies and the risk from wind turbines was found to be no more than the usual dangers that birds faced.