Vertical evacuated tubular-collectors utilizing solar radiation from all directions

A prototype collector with parallel-connected evacuated double glass tubes is investigated theoretically and experimentally. The collector has a tubular absorber and can utilize solar radiation coming from all directions. The collector performance is measured in an outdoor test facility. Further, a theoretical model for calculating the thermal performance is developed. In the model, flat-plate collector's performance equations are integrated over the whole absorber circumference and the model determines the shading on the tubes as a function of the solar azimuth. Results from calculations with the model are compared with measured results and there is a good degree of similarity between the measured and calculated results. The 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 (Uummannaq, 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 the west. Further, the thermal performance of the evacuated solar-collector is compared to the thermal performance of the Arcon HT flat-plate solar-collector with an optimum tilt and orientation. The Arcon collector is the best performing collector under Copenhagen conditions, whereas the performance of the evacuated tubular collector is highest under the Uummannaq 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 Uummannaq and this radiation can be utilized with the tubular collector. It is concluded that the collector design is very promising--especially for high latitudes.