Heat Transfer Analysis of a Parabolic Trough Collector Prototype under Botswana`s Terrestrial Conditions

Botswana has about 84 % of its land area as part of the semi-arid Kalahari desert and it experiences over 320 clear sky days per annum, with a solar insolation of 21 MJ/m2 per day on a horizontal flat surface. This favors energy generation using concentrated solar thermal technologies, especially parabolic trough collector (PTC). PTC is the most mature technology and contributes up to about 90 % of installed concentrated solar thermal power in the world. A PTC prototype was fabricated and tested for the first time at the Botswana International University of Science and Technology (BIUST) campus in Palapye. A stainless steel sheet lined with an adhesive Mylar film of high reflectivity (≥ 0.94) was used as the collector. Two receivers, a coated copper tube of 0.015 m external diameter and an evacuated commercial receiver tube were used. The highest outlet temperature of 76.0 oC at a flow rate of 0.0026 kg/s was achieved for the coated copper tube, while 91.9 oC was recorded for the commercial receiver at a mass flow rate of 0.0020 kg/s. Thermal and Optical performances of the coated and the commercial receiver tubes were analyzed. Results showed better performance for the commercial receiver with a maximum thermal efficiency of 24.2 % in contrast to 22.5 % for the coated copper tube receiver. The effect of the wind speed, mass flow rate of the water and the irradiation on the Carnot and thermal efficiencies of the system were studied for the two receivers. When the average wind speed varied from 3.0 m/s to 4.0 m/s during experimental days, the Carnot and thermal efficiencies of the coated receiver decreased from 53.6 % to 38.5 % and 22.5 % to 14.1 %, respectively. At constant wind speed, a decrease in the flow rate from 0.0036 kg/s to 0.0012 kg/s resulted in an increase in the Carnot efficiency from 38.5 % to 54.1 % due to an increase in the outlet temperature, but the thermal efficiency dropped by almost half from 14.1 % to 8.2 %. For the commercial receiver, the effect of wind speed was eliminated because of the insulation around the tube. The thermal efficiency was observed to be 11.0 % and 24.2% for mass flow rates of 0.0012 kg/s and 0.0036 kg/s, respectively. These increase in the mass flow rate resulted in a drop of the Carnot efficiency from 56.4 % to 46.2 % as a result of a drop in the outlet temperature. An increase in the flow rate raises the volume of water that needs to be heated by the same amount of irradiation; which results in a lower outlet temperature. The average outlet temperatures were generally between 69 oC and 81 oC. This temperature range is suitable for industrial process heat applications such as water desalination, water heating, cooling and refrigeration.