Optical performance of the reflective surface profile of a Heliostat

Landman, Willem Adolph (2017-03)

Thesis (PhD)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: 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.

AFRIKAANSE OPSOMMING: Mikro gasturbine (MGT) sentrale ontvanger stelsels bied voordele wat die tegno-ekonomiese lewensvatbaarheid van die volgende generasie sonkragkonsentrerende kragstasies kan bevorder. Hierdie relatief nuwe tegnologie word nog nie goed begryp nie, en die optimale konfigurasies moet nog bepaal word. Die hoë termiese vloeddigtheid vereistes en klein modulêre konfigurasie dui daarop dat die heliostaatveld van ’n MGT sonkragkonsentrerende kragstasie moontlik alternatiewe parameter sensitiwiteite as konvensionele stelsels kan hê. Die doel van hierdie tesis is om fundamentele begrip te verwerf van die optika van ’n heliostaat ter ontwikkeling van metodes, modelle en maatstawwe as werktuie om die tegno-ekonomiese lewensvatbaarheid van sentrale ontvanger stelsels mee te verbeter, met bepaalde klem op MGT sonkragkonsentrerende kragopwekking (gekonsentreerde sonkrag – GSK). Bestudering van die grondbeginsels van heliostaat optika toon dat heliostaatstraal afwykings statisties onderskeidend is afhangende daarvan of dit opeenvolgend voorkom of koers gebonde is. Drie kernfaktore - te wete die son se vorm, normale vektorfoutafwykings en astigmatiese afwykings - is bewese dominante verstrooiers van ’n heliostaatstraal en word analities beskryf. Hierdie kennis behels die hoofkomponente vir die akkurate beskrywing van heliostaatveld prestasie. Die ontwikkeling en stawing van ’n nuwe analitiese metode om vloeddigtheid verspreiding van ’n heliostaat te modelleer, toon dat dit moontlik is om geskikte vlakke van vertroue te bewerkstellig deur gepaste berekening van hierdie faktore. Die verbeterde akkuraatheid wat hierdie metode bied is veral voordelig by die modelering van hoër akkuraatheid heliostate wat tipies gebruik sou word in MGT GSK. Dit is bewese dat die vloeddigtheid distribusie fout en piek vloeddigtheid dwaling van die voorgestelde metode onderskeidelik 60.6 % en 88.2 % laer is as die van die voorloper metodes. Hierdie metode word toegepas in ’n tegno-ekonomiese sensitiwiteitstudie wat toon dat hoë akkuraatheid optika tot laer vergelykbare elektrisiteitskoste lei. Beide die koste uiteensetting en die alternatiewe optiese vereistes toon dat MGT GSK wel alternatiewe parameter sensitiwiteite bevat. Die gehele bevinding van hierdie tesis dui daarop dat klein heliostate beduidende optiese prestasiestygings binne die konteks van MGT GSK bied en potensieel tot minimalisering van koste lei.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101210
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