Browsing by Author "Brooks, Michael John"

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    Application of a perforated shadow band to the decomposition of global solar irradiance
    (Stellenbosch : Stellenbosch University, 2015-12) Brooks, Michael John; Von Backstrom, Theodor W.; Van Dyk, E. Ernst; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    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.
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    Performance of a parabolic trough solar collector
    (Stellenbosch : University of Stellenbosch, 2005-03) Brooks, Michael John; Harms, T. M.; University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    Parabolic trough solar collectors (PTSCs) constitute a proven source of thermal energy for industrial process heat and power generation, although their implementation has been strongly influenced by economics. In recent years, environmental concerns and other geopolitical factors have focused attention on renewable energy resources, improving the prospects for PTSC deployment. Further work is needed to improve system efficiencies and active areas of research include development of advanced heat collecting elements and working fluids, optimisation of collector structures, thermal storage and direct steam generation (DSG). A parabolic trough collector, similar in size to smaller-scale commercial modules, has been developed locally for use in an ongoing PTSC research programme. The aim of this study was to test and fully characterise the performance of the collector. Specialised logging software was developed to record test data and monitor PTSC performance in real-time. Two heat collecting elements were tested with the collector, one unshielded and the other with an evacuated glass cover. Testing was carried out according to the ASHRAE 93-1986 (RA 91) standard, yielding results for the thermal efficiency, collector acceptance angle, incidence angle modifier and collector time constant. Peak thermal efficiency was 55.2 % with the unshielded receiver and 53.8 % with the glass-shielded unit. The evacuated glass shield offered superior performance overall, reducing the receiver heat loss coefficient by 50.2 % at maximum test temperature. The collector time constant was less than 30 s for both receivers, indicating low thermal inertia. Thermal loss tests were conducted and performance of the trough’s tracking system was evaluated. The measured acceptance angles of 0.43° (unshielded) and 0.52° (shielded) both exceeded the tracking accuracy of the PTSC, ensuring that the collector operated within 2 % of its optimal efficiency at all times. Additionally, experimental results were compared with a finite-volume thermal model, which showed potential for predicting trough performance under forced convection conditions.