Numerical and experimental investigation of a compound parabolic concentrator-capillary tube solar collector

The paper reports numerical and experimental investigation of a compound parabolic concentrator-capillary tube solar collector (CPC-CSC). The capillary tube having an outer diameter 4 mm is used as the absorber, which reduces the size of the solar collector and achieves a concentration ratio of 4.22 in comparison with the same thickness 75 mm of a flat-plate solar collector. The horizontal CPCs separate the air layer into smaller cells, resulting in decrease in heat loss through the air layer. Details of the design and work principle of the new collector are described. Both experimental and numerical studies were carried out for the purpose of operating characteristics and thermal performance of the solar collector. The effect of solar irradiation intensity and inlet water temperature were tested based on the standard of GB/T 4271-2007. A three-dimensional model of the collector is developed and its performance is numerically simulated using FLUENT. The influence of solar irradiation intensity, velocity of ambient air, glass thickness, insulation thickness, inclination angle, mass flow rate and inlet temperature of water are analyzed. The results of numerical were verified by experimental data and the maximum deviation is found to be 15%. The numerical results suggest that: 1) the main heat losses of the CPC-CSC is due to the heat leakage through the air layer between glass cover and CPC; 2) the collector efficiency decreases as the convective heat transfer of the air layer increases; 3) when the solar irradiation intensity rises, the surface temperature of collector tubes increases, enhancing air layer convection and thus reducing collector efficiency; 4) with the increase in mass flow rate of water in the pipe, the heat absorption of water in the pipe increases and thus the collector efficiency increases and 5) the velocity of ambient air, glass thickness, insulation thickness, and inclination angle have little effects on the performance of the solar collector.