Windmills in a flat green landscape stretching to the horizon and beyond – the archetypical image of Holland. Windmills even provided the means of creating the Mondrianesque landscape with its straight cuts and dykes bordering rectangular sections of grassland. In the seventeenth century, windmills were the mainstay of the Dutch efforts to reclaim land from the sea and finally create a dry place to live in. Say windmills, and everyone will instantly think of panoramic views with rows of rotating sails cutting through the air. The open country, where the wind can go as it pleases, is the windmill’s natural habitat. Until recently, that is. In an overcrowded country like the Netherlands, space comes at a premium. Today, visual intrusion, environmental impact, and noise have become the hurdles to be surmounted before wind turbines can be installed in the open country. These problems are an incentive to look for creative solutions for putting the wind’s motional energy to good use. Of course, the wind can blow fiercely across the open Dutch landscape, but as anyone who has passed a high-rise building on foot or a bicycle on a windy day will tell you, conditions can be pretty blustery in built-up areas too. When moving air reaches a building, there are only two ways to clear the obstacle. It can go either around it, or over the top. In doing so, as we have all noticed, the wind picks up speed. Dick Sidler wondered whether the wind in built-up areas could not be put to good use with a wind turbine designed specifically for use in such a location. Following his graduation as an electrical engineer at TU Delft he had taken a job elsewhere. In 1993 he started his own energy technology consultancy company, already convinced that decentralised power generation would become the thing of the future. Instead of getting all our electricity from large, central power stations, we would be using an increasing number of local power sources. Not just wind power, but also solar energy and total energy plants. Forty percent of our energy consumption can be attributed to buildings, in other words, room for improvement. One thing he was certain of: the plan had to involve a wind turbine on a vertical axis rather than one on a horizontal axis like most wind turbines we see around us. The advantage of using a vertical axis in a wind turbine is that it will keep turning irrespective of the direction the wind. The classic windmill, with its horizontal axis and rotor blades turning in a vertical plane, needs some sort of guide vane or mechanism to yaw it in the right direction whenever the wind shifts. Dick Sidler realised that in a built-up environment, with the wind constantly, and often unpredictably, changing direction, it would pay to use a type of mill that would not need to waste energy by going in circles to follow the wind around. On top of that, a wind turbine with a vertical axis requires less maintenance since it has no mechanism to prevent cable twist and no system to turn it into the wind. However, first the drawbacks of existing vertical axis designs had to be tackled: the relatively low efficiency and high vibration and noise levels. Whisk Sidler started by surveying the market for vertical-axis turbines, and he came up with two possible candidates, the Windside and the Catavent. The Windside was his first choice, but it proved too expensive. According to Sidler, experimental readings showed that the yield was only about 25% of the manufacturer’s figure. Sidler realised that he required much more than his own knowledge of aerodynamics if he were to conduct the necessary wind turbine experiments successfully. So he contacted Dr. Gerard van Bussel of the Wind Energy Section of the TU Delft faculty of Civil Engineering (which has now become part of the faculty of Aeronautical Engineering). At the time, Ir. Sander Mertens was working on his post-doctoral research into the use of wind energy in built-up areas. Sidler conducted some experiments on the Catavent at the test site of the Wind Energy Section. The Catavent is a Canadian design that looked like a large air scoop that could turn itself into the wind by means of a set of guide vanes. As the air hit the device, it was diverted by a hornlike device onto a horizontal drum. Both the wind scoop and the drum rotated around the same axis. The air flowed into the drum through its open top, and could only escape through holes in the side. Vanes mounted at right angles to the outside were shaped so that the airflow was diverted in a tangential direction. This would cause the wheel to turn, driving a generator through a belt system. The lower end of the wind scoop featured a type of wind deflector designed to prevent the wind striking the vanes directly, and in addition to create a form of suction.