THERMAL PERFORMANCE OF EVACUATED TUBULAR COLLECTORS UTILIZING SOLAR RADIATION FROM ALL DIRECTIONS

摘要

A prototype collector with parallel-connected evacuated double glass tubes is investigated theoretically and experimentally. rnThe collector has a tubular absorber and can utilize solar radiation coming from all directions. The collector performance is measured in an outdoor test facility and an efficiency expression for the collector is determined. Further, a theoretical model for calculating the thermal performance is developed. In the model, flat plate collector performance equations are integrated over the whole absorber circumference and the model determines the shades on the tubes as a function of the solar azimuth. Results from calculations with the model are compared with measured results and generally there is a good degree of similarity between the measured and calculated results. However, the comparison shows that the model is suitable only for vertical placed pipes. rnThe model is used for theoretical investigations on vertically placed pipes at a location in Denmark (Copenhagen, lat. 56°N) and at a location in Greenland (Ummannaq, lat. 71°N). For both locations, the results show that to achieve the highest thermal performance, the tube centre distance must be about 0.2 m and the collector azimuth must be about 45°- 60° towards west. Further, the thermal performance of the evacuated solar collector is compared to the thermal performance of the Arcon HT flat plate solar collector. The Arcon collector is the best performing collector under Copenhagen conditions, whereas the performance of the evacuated tubular collector is highest under the Ummannaq conditions. The reason is that the tubular collector is not optimally tilted in Copenhagen but also that there is much more solar radiation “from all directions” in Ummannaq and this radiation can be utilized with the tubular collector. It is concluded that the collector design is very promising – especially for high latitudes.