VELOCITY-CONTROLLED TRACKING OF THE SUN

For solar concentrator systems as well as for radiometric measurement of the solar radiation the tracking of the sun is necessary. Conventional trackers periodically update the orientation of the device to the actual position of the sun. This approach, however, has two major disadvantages. First, the orientation of the tracking device changes by jerks, which can lead to considerable torque loads on gears and drives of large structures such as heliostats. Second, the aiming error of the device is proportional to the time step chosen for updating the position. Thus for a maximum error of 10(-4) (i.e., 1 cm/100 m), the worst case time is 1.4 s. Here we present an approach controlling the angular velocity of the tracking device. Based on the present orientation and the orientation the device should have delta-t later, we calculate the mean angular velocity for the device to have the correct orientation after delta-t. At the time the velocity is readjusted. This method is of second order in delta-t. Thus for the maximum pointing error of 10(-4) the worst case time step delta-t is 6.4 min. This approach has been successfully implemented to track the 51.8 m2 glass heliostat of the solar furnace at the Paul Scherrer Institue. We use a standard servo unit to control the drives of the heliostat. The calculations of the orientation of the sun and the nominal velocity of the heliostat are performed on a PC. Other continuous motion tracking has been implemented for example by Centre Suisse d'Electronique et de Microtechnique (CSEM) for a sun tracker to carry an instrumentation panel.