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A WIND turbine that produces three-and-a-half times as much electricity as
conventional turbines has been tested by researchers in Croatia. The trick, they
say, is to enclose the turbine in a cowling.

Bernard Frankovic and Ivan Vrsalovic from the University of Rijeka tested
their 1-kilowatt prototype of the new design on the island of Lastovo, off the
Croatian coast. The key to the turbine鈥檚 efficiency is the shape of its cowling,
which increases the speed of the air flowing over the blades.

The cross section of the cowling has the same shape as an aerofoil, says
Frankovic. When it faces the wind the aerofoil shape creates a region of low
pressure inside the cowling, drawing more air over the turbine鈥檚 blades.

This has two consequences. First, it increases the output from the turbine at
normal wind speeds. Secondly, the turbine is able to generate electricity in
winds that are too light for conventional turbines to work.

Frankovic says that Lastovo would not normally be considered windy enough
for a wind farm. For 40 per cent of the year the breezes are less than 5 metres
a second鈥攚hich is normally reckoned to be the lower threshold for
conventional wind farms. But the minimum air speed for the Croatian turbine is
about 2 metres a second. Frankovic and Vrsalovic found that the time that the
prototype could produce useful electricity was 60 per cent more than a
conventional turbine.

The idea of improving the efficiency of wind turbines by encasing them in a
cowling is not new. It was first tried back in the 1970s. But wind power was
barely commercial then and the extra construction cost of the casing made it
completely unaffordable. 鈥淭he problem was economics,鈥� says Robert Thresher
director of the US National Wind Technology Centre in Golden, Colorado. 鈥淭he
materials pushed the cost up,鈥� he says.

But there were also considerable technical problems. High winds dramatically
increased the load on the turbine鈥檚 blades, forcing them to shut down to prevent
blades being broken. The efficiency of encased turbines also dropped
dramatically when the wind was gusty. 鈥淚f the wind jumps around a lot it just
screws up the airflow,鈥� says Thresher.

But Frankovic and Vrsalovic believe they have found a way round this. In
high winds, hydraulically actuated vents linked to a wind gauge bleed off air to
reduce the load on the turbine. The vents also help stabilise the turbine鈥檚
electricity output in variable winds. 鈥淭his represents a significant progress in
developing large wind power plants with safe and stable operation regardless of
the variations in wind speed,鈥� says Vrsalovic.

The Croatians are not the only ones to have shown renewed interested in the
idea of encased wind turbines. Vortec, a company based in New Zealand, has been
studying wind turbines with cowlings because of their potential for generating
power more efficiently.

鈥淐onstruction costs are still quite high,鈥� admits Frankovic. He says that a
1-kilowatt turbine will cost about $2600, which is about 75 per cent more
expensive than a conventional wind turbine. In a short-term economic analysis of
a wind farm using 1-kilowatt generators he says that their greater efficiency
improved profits by up to five times.