Browsing by Author "Malan, Karel Johan"

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    A heliostat field control system
    (Stellenbosch : Stellenbosch University, 2014-04) Malan, Karel Johan; Gauche, Paul; Treurnicht, J.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: The ability of concentrating solar power (CSP) to efficiently store large amounts of energy sets it apart from other renewable energy technologies. However, cost reduction and improved efficiency is required for it to become more economically viable. Significant cost reduction opportunities exist, especially for central receiver system (CRS) technology where the heliostat field makes up 40 to 50 per cent of the total capital expenditure. CRS plants use heliostats to reflect sunlight onto a central receiver. Heliostats with high tracking accuracy are required to realize high solar concentration ratios. This enables high working temperatures for efficient energy conversion. Tracking errors occur mainly due to heliostat manufacturing-, installation- and alignment tolerances, but high tolerance requirements generally increase cost. A way is therefore needed to improve tracking accuracy without increasing tolerance requirements. The primary objective of this project is to develop a heliostat field control system within the context of a 5MWe CRS pilot plant. The control system has to govern the tracking movement of all heliostats in the field and minimize errors over time. A geometric model was developed to characterize four deterministic sources of heliostat tracking errors. A prototype system comprising 18 heliostats was constructed to function as a scaled down subsection of the pilot plant heliostat field and to validate the chosen control method and system architecture. Periodic measurements of individual heliostats’ tracking offsets were obtained using a camera and optical calibration target combined with image processing techniques. Mathematical optimization was used to estimate model coefficients to best fit the measured error offsets. Real-time tracking error corrections were performed by each heliostat’s local controller unit to compensate for a combination of error sources. Experimental tracking measurements were performed using the prototype system. Daily open-loop RMS tracking errors below one milliradian were obtained, thereby satisfying the project’s primary objective. The thesis concludes that high tracking accuracy can be achieved using the control method proposed here. This could potentially lead to a reduction in heliostat cost, thereby lowering the levelised cost of electricity for CRS plants.