Path planning of a 3-UPU wrist manipulator for sun tracking in central receiver tower systems

Heliostats capable of tracking the sun as it moves across the sky and focusing the incident solar energy on to a central receiver tower requires a two degree-of-freedom (DOF) mechanism which can orient the mirror in the desired manner. Existing two-DOF mechanism, such as the Azimuth-Elevation (Az-EL) and the Target-Aligned (T-A), have two actuators in series. It is known that during certain times of the day, the T-A configuration has less spillage losses and astigmatic aberration while at other times the Az-El configuration is better. In this paper, we propose a three-DOF parallel manipulator which can be used as a heliostat. The proposed 3-UPU, three-DOF parallel manipulator, has a fixed point about which the mirror can rotate about three axes. Since only two DOF are required to track the sun, the 3-UPU is a redundant system. We propose a strategy to use this redundancy and electronically reconfigure the 3-UPU to achieve the Az-El and T-A configurations thus achieving the advantages of both. As the motion of the sun is precisely known for any geographical location on earth, time and day of the year, numerical simulations done a priori provide the conditions for switching.