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Turning wind turbines into rain-making machines

May 23, 2002

MAKING rain sounds outlandish, and maybe it is. But audacious ideas are nothing new to Stephen Salter. If the wave-power pioneer thinks he can solve the world`s worsening water shortage by turning wind turbines into rain-making machines, there are plenty of people who`ll listen to him.
        
Salter, an engineer at the University of Edinburgh, became famous in the 1970s for inventing the "nodding duck" wave-power device, which spawned many of the wave-power designs now under development and in trials. He`s even tried his hand at designing a whirling remote-controlled machine to detonate landmines. At an international marine conference in Crete last week, Salter outlined his latest idea: a floating wind turbine that sprays water vapour high into the air, to increase evaporation from the ocean and precipitation over land.

        He says it could help defuse burgeoning conflicts over access to water, stop deserts spreading, improve soil quality, top-up water tables, save rainforests and neutralise the impact of climate change. Sceptics will doubtless ask if it can make the tea as well.
        Salter proposes using an existing design called a Darrieus turbine. This looks like a 40-metre-high egg beater, with slender aerofoil-shaped blades attached at top and bottom, which the wind sets spinning about the vertical axis.

        Although this design can generate electricity, it is not as efficient as the horizontal-axis machines. But Salter isn`t interested in generating power. He plans to use the centrifugal force generated by the rotating blades to pump water droplets into the atmosphere. In his scheme, seawater will be sucked up from the ocean into pipes within the blades. Nozzles at the ends of these pipes will turn the water into an aerosol and spray fine droplets from the trailing edges of the blades 5 to 20 metres above the sea surface into the turbulent wake of the rotor.

        This, Salter points out, will hugely increase the surface area of water that can be turned into water vapour. The turbine will also overcome one of the main brakes on evaporation from the ocean, he argues. The problem is a wafer-thin layer of stagnant, humid air that clings to the surface of the sea and prevents water molecules from escaping.

        Salter calculates that with a wind speed of 8 metres per second, each "spray turbine" could lift more than half a cubic metre of water a second to a height of 10 metres. Hundreds or even thousands of the machines in hot areas of the world could make enough rain to prevent droughts, he estimates. "The successful large-scale deployment of spray turbines could reduce the number of people who are short of water by several billion."
        About 100,000 turbines operating for 100 years would, theoretically, be enough to reverse the one-metre rise in sea level predicted as a result of global warming. But this assumes that a quarter of the resulting rainfall is retained underground in areas like the Sahara desert, where water tables are very low.

        Salter accepts that spray turbines present major technical challenges, such as how to make the spray fine enough, and how to filter the seawater to prevent marine life from clogging the pipes. "Engineers should always fear biology," he warns.

        But meteorologists doubt whether water vapour scattered on a vast scale several metres above the sea will mix sufficiently with air higher in the atmosphere to make any significant impression on cloud formation. There is also the obvious difficulty of predicting where any rain will actually fall.

        "It`s a very long shot," says Ian Brooks, a meteorologist at Leeds University. "It is an engineer`s solution to a problem that no atmospheric scientist would ever have dreamed of trying." But he agrees that it`s worth investigating.

Author: Rob Edwards, Edinburgh
http://www.newscientist.com">New Scientist issue 25th May 2002

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New Scientist