Thermal performance assessment of a Parabolic Trough Collector prototype system in Palapye, Botswana

Abstract

A parabolic trough collector (PTC) prototype was fabricated and tested at Palapye (22.59404 oS and 027.12455 oE) in Botswana. A stainless steel sheet lined with an adhesive Mylar film of high reflectivity (≥ 0.94) was used as the collector with area 3.001 m2 and a copper tube of 15 mm external diameter and length 2.440 m coated with silicone 250 selective coating as the receiver. The performance of the coated receiver tube was compared to that of a 2.0 m long commercial receiver without any modification of the collector. The highest outlet temperature of 75.8 oC was achieved, with inlet water and ambient temperatures of 26.1 and 28.0 oC, respectively. The effect of the wind speed, mass flow rate of the water and the irradiation on the thermal efficiencies of the system were studied. An increase in the average wind speed from 2.8 to 4.4 m/s resulted in both the Carnot and thermal efficiency decreasing from 61.1 to 54.1 % and 20.3 to 7.7 %, respectively. At constant wind speed, a decrease in the flow rate from 0.0036 to 0.0012 kg/s resulted in an increase in the Carnot efficiency from 43.9 to 54.1 % due to an increase in the outlet temperature, while the thermal efficiency dropped by half. The wind speed had no influence on the commercial receiver outlet temperature because of the vacuum insulation.