Application of a perforated shadow band to the decomposition of global solar irradiance

Brooks, Michael John (2015-12)

Thesis (PhD)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: The earth’s atmosphere causes pronounced spatial and temporal variability in downwelling solar radiation at the planet’s surface. Since the characterisation of sun strength is important in solar resource assessment studies, and in the Earth sciences generally, more effective methods are sought to measure irradiance at ground stations. The general drive is towards greater spatial coverage, reduced instrument uncertainty, lower costs and higher temporal data resolution. This study investigates a new method of measuring the principle components of solar irradiance at 1-minute intervals using a single pyranometer and a novel shading structure. The perforated shadow band decomposes global horizontal irradiance (GHI) to obtain the diffuse horizontal and direct normal irradiance components (DHI and DNI). The design of the band and its positioning relative to the thermopile sensor of a radiometer are described. A ray trace-derived model of pyranometer exposure is presented as a function of the local hour angle. In operation, the band produces a composite output trace incorporating both global and diffuse fragments that require separation and reconstitution as independent time-series. DNI values can then be calculated from these components. Gaps between data fragments must be filled using appropriate interpolation techniques to lower statistical uncertainty. The structure of the trace is dependent on atmospheric turbidity and the nature of the prevailing cloud field. A test programme was run at the US National Renewable Energy Laboratory in Colorado to establish performance of the system relative to collocated reference instruments. The band functioned most effectively under clear sky conditions, where it produced GHI, DHI and DNI measurements with root mean square differences of 2.7%, 13.6% and 2.0% respectively. Mean bias differences were 0.1% for GHI, 7.9% for DHI and –0.3% for DNI. The presence of cloud introduces stochasticity to the perforated band output trace. In such a case the ray trace model of pyranometer exposure can be used to identify and separate GHI and DHI data. Uncertainties rise for GHI and DNI under partly cloudy conditions. As the inaugural study on perforated band performance, this work tested several approaches to filling measurement gaps, including numerical interpolation and data replacement by radiometric decomposition models. A key finding of the study is that uncertainties may be lowered by interpolating adaptively according to the prevailing clearness index. Tests run at a southern hemisphere ground station suggest that the system’s performance is not location-dependent. It may be concluded that the perforated shadow band system is most effective in sunny regions where the average daily clearness index remains above approximately 0.7. This would include large parts of continental Africa in the south-western and northern desert areas. The best potential for deploying the band is in existing sub-optimal measurement schemes utilising a single pyranometer, where it would enable the direct measurement of two radiometric components rather than one.

AFRIKAANSE OPSOMMING: Die aarde se atmosfeer veroorsaak beduidende ruimtelike en tydafhanklike veranderlikheid in afwellende sonstraling op die planeet se oppervlakte. Aangesien die karakterisering van sonsterkte belangrik is in hulpbronbeoordelingstudies, en in die aardwetenskappe in die algemeen, is doeltreffender metodes in aanvraag om bestraling by grondstasies te meet. Die algemene stukrag is in die rigting van groter ruimtelike dekking, verminderde instrument-onsekerheid, laer koste en hoër data-resolusie met tyd. Hierdie studie ondersoek ’n nuwe metode om die hoofkomponente van sonbestraling teen 1-minuut intervalle te meet deur ’n enkele piranometer en ’n nuutgeskepte skadubandstruktuur te gebruik. Die geperforeerde skaduband breek die globale horisontale bestraling (GHB) op om die diffuse horisontale en direkte normale bestralingskomponente (DHB en DNB) te verkry. Die ontwerp van die band en sy plasing relatief tot die termostapelsensor van ’n radiometer word beskryf. ’n Straalnavolgmodel van piranometerblootstelling word voorgestel as ’n funksie van die plaaslike uurhoek. In bedryf lewer die band ’n saamgestelde uitsetverloop wat beide globale en diffuse breukdele inkorporeer, wat skeiding en hersamestelling as onafhanklike tydreeks vereis. DNB-waardes kan dan uit hierdie komponente bereken word. Gapings tussen die data-breukdele moet gevul word deur geskikte interpolasietegnieke te gebruik om statistiese onsekerheid te verminder. Die struktuur van die verloop hang af van atmosferiese turbiditeit en die aard van die heersende wolkveld. ’n Toetsprogram is by die US National Renewable Energy Laboratory in Colorado bedryf om die vertoning van die stelsel te bevestig relatief tot aanliggende verwysings-instrumente. Die band het die doeltreffendste gewerk onder skoon lugtoestande, waar dit GHB-, DHB- en DNB-metings gelewer het met wortelgemiddelde kwadraat afwykings van 2.7%, 13.6% en 2.0% onderskeidelik. Gemiddelde afwykingsneigings was 0.1% vir GHB, 7.9% vir DHB en –0.3% vir DNB. Die teenwoordigheid van wolke bring wisselvalligheid in die geperforeerde band se uitsetverloop mee. In so ’n geval kan die straalvolgmodel van piranometerblootstelling gebruik word om die afsonderlike GHB- en DHB-data te identifiseer en te skei. Onsekerhede in GHB en DNB ontstaan onder gedeeltelik-bewolkte toestande. Synde die inleidende studie oor geperforeerde bandvertoning, toets hierdie werk verskeie benaderings vir die invul van meetgapings, insluitende numeriese interpolasie en datavervanging deur radiometriese dekomposisie-modelle. ’n Sleutelbevinding van die studie is dat onsekerhede verminder kan word deur aanpasbaar te interpoleer volgens die heersende helderheids-indeks. Toetse gedoen by die suidelike halfrond-grondstasie doen aan die hand dat die stelsel se gedrag nie afhanklik is van die ligging nie. Die gevolgtrekking kan gemaak word dat die geperforeerde-skaduband stelsel die effektiefste werk in sonnige streke waar die daaglikse helderheidsindeks bo ongeveer 0.7 bly. Dit sluit groot dele van kontinentale Afrika in die suidwestelike en noordelike woestynareas in. Die beste potensiaal vir die ontplooiing van die skaduband is in bestaande sub-optimale meetstelsels wat ‘n enkele piranometer gebruik, waar dit die direkte meting van twee radiometriese komponente moontlik maak, eerder as een.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97959
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