A heliostat based on a three degree-of-freedom parallel manipulator

In this paper, we propose a three-degree-of-freedom spatial parallel manipulator to track the sun in central receiver tower based concentrated solar power systems. The proposed parallel manipulator consists of three 'legs' each containing a passive rotary(R) joint, an actuated sliding or prismatic (P) joint and a passive spherical (S) joint and is known in literature as the 3-RPS manipulator. In contrast to existing serial mechanisms with two degrees-of-freedom, firstly it is shown that the extra actuator and enhanced mobility helps in reducing spillage losses and astigmatic aberration. Secondly, due to the three points of support, the beam pointing errors are less for wind and gravity loading or, alternately, the weight of the supporting structure to maintain desired deflections of the mirrors are significantly lower. Finally, the linear actuators used in the parallel manipulator do not require the use of large, accurate and expensive speed reducers. In this paper, we model the 3-RPS manipulator and derive the kinematics equations which give the motion of the linear actuators required to track the sun and reflect the incident solar energy at a stationary target at any time of the day, at any day of the year and at any location on the surface of the Earth. Finite element analysis is used to determine an optimized design which can reduce the weight of the supporting structure by as much as 60% as compared to the existing tracking mechanisms. A proportional, integral plus derivative (PID) control strategy using a low-cost processor is devised and a detailed simulation study is carried out to show that the proposed parallel manipulator performs better compared to the current tracking algorithms. Finally, a prototype of the parallel manipulator is manufactured and it is demonstrated that it is capable of performing autonomous sun tracking with the above mentioned advantages.