The generator used by Turby is a compact direct-drive system, i.e. one without a gearbox, designed by Dr. Ir. Henk Polinder of TU Delft. The rotor drives the generator directly without the intervention of a gearbox. A gearbox generator would have been cheaper, but the direct-drive system requires less maintenance, which was a design prerequisite. According to Sidler the maintenance-free period is about twenty years. The ring of permanent magnets forms part of the rotor. The motional energy is transformed into electrical energy, which is then electronically converted into alternating current of the same voltage and frequency as supplied by the mains grid. The generator not only handles the start-up sequence, but also acts as a brake and a safety monitor on the rotating turbine. Dick Sidler has had the first prototypes of Turby made through his company Turby B.V.. Interested parties from all over Europe and as far away as China, Japan, Australia, New Zealand and the U.S. have already been enquiring about the sustainable urban roof-top wind power system. Sander Mertens now works for the construction and industry section of DHV Engineering Consultants in The Hague. “Having done all this research, I can now advise others on what can be done with wind power in a built-up area, all the way from wind nuisance to wind usage. Architects come to consult me about the integration of wind turbines in between buildings and on top of them. I advise them about the shape of buildings, and discuss the kind of yield they can expect.” Wind power in a built-up area can be used in three fundamentally different ways, as explained by Mertens in his Ph.D. thesis, due to be published within a few months. Firstly there are wind turbines inside or on top of existing buildings, like Turby. For future use, Mertens is also contemplating buildings with integrated wind turbines inside a duct that connects the high-pressure and low-pressure sides of a building. A third option is to have wing-shaped buildings. Mertens: “Whereas a rectangular building produces an increase in wind speed at roof-top level of about twenty percent, a high cylindrical building could theoretically generate up to 200 percent along the sides of the building. The ultimate goal is to work with architects to devise a building that will enable a wind turbine to produce power as often as possible and as efficiently as possible. Unfortunately, many aerodynamic designs are not very practical as buildings, so the challenge will be to find the right balance of innovation between optimum aerodynamics and practical applicability.” Meanwhile at the Wind Energy Section of Dr Gerard van Bussel at Delft University another post-doctoral student is to continue the research into the aerodynamics of Turby-like generators for the built-up areas.