Characterisation of a Line-Axis Solar Thermal Collector for Building Facade Integration

The integration of concentrating solar thermal collectors into the structural envelope of buildings can significantly increase the cost effectiveness of solar thermal utilisation in the UK. The key, however, to their wide scale application is performance. Typically, most solar thermal collectors are mounted on inclined roof structures, thus presenting an optimal surface area for solar gain. Vertical building facades offer an alternative mounting surface and whilst they may have an overall lower level of incident solar radiation, the collector receives a more uniform annual distribution of solar radiation, reducing potential summer over heating problems. Furthermore, facade integration is beneficial to the building performance as the collector unit results in a higher U-value realising higher building heat retention. In concentrating solar thermal collector systems, the absorbing surface area is reduced relative to that of the aperture, leading to a reduction in the overall heat loss from the system, hence improving thermal efficiency. To maximise collection in a vertically mounted concentrating solar thermal collector however, the concentrator profile should be optimised to benefit solar collection relative to the mounting inclination. This paper presents the optical and experimental investigation of a low concentration line axis solar thermal collector employing symmetric and asymmetric CPC geometries. The potential for collected solar radiation when facade integrated has been investigated with the use of three-dimensional ray trace. Several prototype units were fabricated and experimentally evaluated. A series of fluid flow configurations (serpentine and parallel) using different flow velocities have been investigated and a range of slope angles (beta) considered. Results from this study have shown that this type of concentrating solar thermal collector has particular application for domestic hot water production and that the design can effectively operate in the vertical orientation and is suitable for building facade integration in Northern European locations.