Solar collector with asymmetric compound parabolic concentrator for winter energy harvesting and summer overheating reduction: Concept and prototype device

Abstract

A major technical barrier of solar water heating systems for space heating and domestic water usage is the mismatch between the seasonal availability of solar energy and demand loads. The resulting low solar fraction in winter and high risk of overheating in summer reduces the competitiveness of this technology. Aiming to solve this problem, a novel collector module which combines asymmetric compound parabolic reflector with horizontally aligned evacuated tubular absorber is proposed in this paper. The special configuration enables the reflector to realize the seasonal dual function which performs as a concentrator in winter and a shading device in summer. This collector design can achieve high thermal performance during winter when demand is high and lower the efficiency in summer when demand is low. Based on the design principle, a prototype device was fabricated. Its photo-thermal behavior was investigated by the numerical approach as well as the experimental approach under real outdoor conditions on summer and winter days. Both theoretical and experimental results revealed that the prototype device has a high average optical efficiency on clear December days of 0.70, whereas it maintained low average optical efficiency of 0.39 on clear August days. In addition, the daily thermal efficiency remained above 0.5 for a large temperature difference between the fluid and ambient air at 77 K during the testing days around the winter solstice. These results confirmed that this solar unit achieves satisfactory performance in winter and also reduces the overheating risk in summer.