A heliostat field control system

[摘要] ENGLISH ABSTRACT: The ability of concentrating solar power (CSP) to efficiently store large amounts ofenergy sets it apart from other renewable energy technologies. However, costreduction and improved efficiency is required for it to become more economicallyviable. Significant cost reduction opportunities exist, especially for central receiversystem (CRS) technology where the heliostat field makes up 40 to 50 per cent of thetotal capital expenditure.CRS plants use heliostats to reflect sunlight onto a central receiver. Heliostats withhigh tracking accuracy are required to realize high solar concentration ratios. Thisenables high working temperatures for efficient energy conversion. Tracking errorsoccur mainly due to heliostat manufacturing-, installation- and alignment tolerances,but high tolerance requirements generally increase cost. A way is therefore needed toimprove tracking accuracy without increasing tolerance requirements.The primary objective of this project is to develop a heliostat field control systemwithin the context of a 5MWe CRS pilot plant. The control system has to govern thetracking movement of all heliostats in the field and minimize errors over time. Ageometric model was developed to characterize four deterministic sources of heliostattracking errors.A prototype system comprising 18 heliostats was constructed to function as a scaleddown subsection of the pilot plant heliostat field and to validate the chosen controlmethod and system architecture. Periodic measurements of individual heliostats'tracking offsets were obtained using a camera and optical calibration target combinedwith image processing techniques. Mathematical optimization was used to estimatemodel coefficients to best fit the measured error offsets. Real-time tracking errorcorrections were performed by each heliostat's local controller unit to compensate fora combination of error sources.Experimental tracking measurements were performed using the prototype system.Daily open-loop RMS tracking errors below one milliradian were obtained, therebysatisfying the project's primary objective. The thesis concludes that high trackingaccuracy can be achieved using the control method proposed here. This couldpotentially lead to a reduction in heliostat cost, thereby lowering the levelised cost ofelectricity for CRS plants.

[发布机构] Stellenbosch University