Dutch research institutes ECN and TU Delft are considering proposals from Groningen based consultancy Adviesbureau RIES for the development of a "hovering wind turbine." The brainchild of Dutch engineer, Anderies van Maldegem, the "zweef" machine comprises a lightweight wind turbine mounted inside a tubular, helium filled balloon which is tethered to the ground by a reinforced-fibre strengthened high voltage cable some 10 cm in diameter. Suspended two to three hundred metres above the ground, the zweef turbine would double or triple the yield of a conventional turbine in the same location, claims Van Maldegem. The increased output being due partly to the higher wind speeds at this altitude, but also to the fact that the aerodynamically shaped balloon will act as a wind tunnel. Van Maldegem's calculations have been confirmed by Stefan Gruijters, a postgraduate student at the TU Delft. Safety is not a problem, says Van Maldegem. In the event of storm the cable can be used to bring the unit safely to earth. Because helium is inert there is no risk of explosion and should a unit break free of its mooring, gas can be released to allow a controlled descent. Van Maldegem, who began work on his design in November, conceived the zweef turbine as a response to the problems facing ground based turbine technology in the Netherlands. Wind turbines need to be at least 100 metres above the ground before their energy can compete with fossil fuels he argues. The 10 cm thick cable of the zweef turbine is invisible at 100 metres: you will look straight through them he says. The zweef turbine would be particularly well-suited to offshore developments as it could be tethered to a conventional buoy, thus obviating the need for complicated and expensive foundation structures. He is currently looking to find funding for a prototype and hopes the technology will be operational in three years.