Research on tracking precision of the heliostat

The tracking precision and manufacture cost of the heliostats is obviously crucial to the system efficiency and its investment scale in a tower power plant. An open loop control system according to astronomic formula of high precision was proposed in this paper. The astronomical formulae to calculate the solar zenith and azimuth angles with uncertainties of +/-0.005 mrad ( milli-radian). This System was built in 2005 at De Zhou China. The structure of this control system is 'PC-PLC-PLC'. Taking account of the inevitable error occurred during the heliostat's installation, a mathematic model was established in this paper. The tracking control precision of this system is 0.17 mrad. Total error stack-ups for tracking and beam quality in calm winds is about 3 mrad. Drive systems with large gear ratios were designed to give sufficiently rapid movement in high wind condition with sufficient precision to meet the focusing requirement which minimizing motor power and manufacturing costs. After the analysis on the gear down ratio of the drive system in a heliostat unit, it came to the conclusion that having one inverter connected to two common motors with time-sharing control can not onlymeet the tracking precision requirement but achieve low cost.