Performance of the one-ended evacuated tubes as medium temperature solar air heaters at low flow rates

Abstract

Performance of a simple one-ended, evacuated tube solar collector was experimentally studied as a compact unit-cell for air heater at low steady flow rates. The investigated unit-cell was used to obtain medium temperature rise. In order to boost the temperature rise, one of these unit-cells was integrated with a single-axis sun-tracking solar parabolic concentrator (SPC) and simultaneously examined on clear-sunny days. The achieved instantaneous temperature rises were found to be independent of flow rate but sensitive to variation of solar intensity. This feature indicates a short response time of these unit-cells. In case of the simple unit-cell, a temperature difference above of 60 °C was achieved for moderate solar intensity of 710 W/m2. On the other hand, in case of the unit-cell with SPC a temperature difference of 100 °C was obtained for low-grade solar intensity of 600 W/m2. Generally, very low pressure drop of less than 29.2 Pa was measured across the unit-cell providing low consumed blowing power. The investigated unit-cell could be the basis for a large solar air heater with remarkable enhancement of the performance parameters compared to that reported on flat-plate solar air heaters.