Progress in track-mounted heliostat

A novel concept is presented for a track-mounted heliostat for central receiver plants, intended to be manufacturable at low cost and to reach an installed cost of ~ $60 /m² when implemented in high volume. Cost reduction is achieved by using steel in the structurally most efficient form of spaceframe structures. These spread the load broadly, halving the quantity of steel per square meter of reflecting area compared to a conventional post /fishbone heliostat design . The structure is turned in azimuth on wheels on a large diameter ground track, eliminating the need for an expensive, heavy slewing bearing. A preliminary finite-element design of a 22 m² heliostat with spaceframe azimuth and elevation structures has a total steel mass of mass of 15 kg/m² supporting 10 kg/m² of glass reflectors and survives 145 km/h wind. Here we report of field tests of engineering performance of a first prototype of the lower, azimuth spaceframe mounted on a 7 m diameter concrete track. The moving structure with wheels and drive weighs 11 kg/m² and has a measured lowest resonant frequency of 7.5 Hz, when loaded with a simulated elevation structure. The prototype incorporates inclinometers which measure alignment of the azimuth and elevation axes to better than 0.2 mrad. Absolute azimuth rotation angle measured via an incremental encoder on the drive motor encoder and metal reference bars in the concrete track was found to be accurate to 0.35 mrad. Combining the tip/tilt, azimuth rotation and servo errors in quadrature gives a total of 0.49 mrad in tracker orientation, providing encouragement that our full heliostat system target of 1 mrad RMS pointing accuracy (double the orientation accuracy) should be achievable.