Optical performance of the reflective surface profile of a Heliostat

Micro gas turbine (MGT) central receiver systems offer advantages which could improve the techno-economic viability of the next generation of concentrating solar power (CSP) plants. This relatively young technology is not yet well understood, and the optimal configurations are yet to be determined. The high flux requirements and small modular configuration suggest that the heliostat field of a MGT CSP plant may have alternative parameter sensitivities than conventional systems. The objective of this thesis is to fundamentally understand the optics of a heliostat to develop methods, models and figures of merit as tools to improve the techno-economic viability of central receiver systems with particular emphasis on MGT CSP. A study of the fundamentals of heliostat optics shows that heliostat beam aberrations are statistically differentiated according to whether they occur consecutively or are path dependent. Three key factors – namely the sun shape, normal vector error aberrations and astigmatic aberrations – are found to dominate the dispersion of a heliostat beam and they are described analytically. This knowledge presents the principal components required to accurately describe heliostat field performance. The development and validation of a new analytical method to model flux distribution of a heliostat shows that it is possible to achieve suitable levels of confidence by appropriately accounting for these factors. The accuracy improvements offered by the method is particularly beneficial when used to model higher accuracy heliostats that would typically be used in MGT CSP. The flux distribution error and peak flux error of the proposed method are shown to be up to 60.6 % and 88.2 % lower than that of state of the art methods respectively. This method is applied in a techno-economic sensitivity study that illustrates that high accuracy optics result in lower levelised cost of energy. Both the cost breakdown and the alternative optical requirements show that MGT CSP does have alternative parameter sensitivities. The collective findings of this thesis suggests that small heliostats offer significant optical performance increases in the context of MGT CSP and potentially leads to cost minimum. ii Stellenbosch University https://scholar.sun.ac.za