Optical and thermal performance analysis of a micro parabolic trough collector for building integration

Medium temperature thermal energy (100-400oC) is widely used in industrial processes and in large buildings. The presented work investigated the feasibility of a new solar collector which was designed to harvest solar energy from factory rooftops for industrial process heat applications. The proposed solar collector was comprised of parallel micro-parabolic troughs, vacuum tube receivers, and an internal tracking mechanical contained in a glazed box which can be easily mounted on large buildings. The system does not require external rotational tracking and achieves a concentration ratio of ~4.2 in a ~150 mm height, so it can be easily integrated with buildings. The optical performance of the presented collector is analysed and modelled theoretically and numerically, considering the effect of shading, inclination, and orientation. Furthermore, a transient thermodynamic model is developed to calculate its thermal efficiency and stagnation temperature, along with the effect of vacuum pressure, beam radiation, and emissivity of selective coatings. The theoretical analysis, verified by TRNSYS simulations and an outdoor experiment, revealed that the annual optical efficiency of the system was about 66.7% and the thermal efficiency was about 59.3% at 200 oC, if the collector was inclined to local latitude angle. These results reveal that the proposed design is competitive with evacuated flat plates (i.e., the TVP collector which has a thermal efficiency of ~36% at a normalized temperature difference of 0.2). Further, if the collector were to be installed on the vertical façade of a building, the theoretical model estimated that the optical and thermal efficiencies would be 44.1% and 37.5%, respectively. An economic analysis indicated that a levelized cost of heat energy of 0.51 $/kWh can be obtained. Overall, since the proposed collector has a simple structure and a low-profile, this study indicates it is promising for medium temperature solar thermal heat production for industrial processes and/or for multi-effect absorption chillers.