Promote optical performance of linear Fresnel micro-concentrator by an offset-axis mirror layout in building-integrated PV/T application

Abstract

In building integrated PV/T applications, the utilization of linear Fresnel micro-concentrators could promote solar energy harvesting efficiency. For the consideration of low wind load and building aesthetics, the height of the concentrator is invariably kept in a small range, resulting in severe optical blocking between mirrors. In this paper, two offset-axis mirror models are introduced to promote the optical efficiency of the micro-concentrator. The rotating axis of the mirror in the former model is not located on the symmetrical axis of the mirror but is offset to one side of the mirror. The later model is a rod offset-axis mirror with the axis offset to the outside of the mirror by a rod. These offset-axis mirrors allow the mirrors farther away from the receiver to possess a higher position for larger angular rotation, thus reducing the blocking between mirrors. Thereafter, mathematical models were established to obtain the key parameters of each mirror at different axis positions, and the influence of the axis position on the optical performance of the micro-concentrator was analyzed. The results revealed that both kinds of offset-axis mirrors could significantly improve the ground utilization ratio of the micro-concentrator and realize a more compact micro-concentrator design. In comparison with the layout scheme with the conventional central-axis mirrors, the ground utilization ratio of micro-concentrator with a hybrid layout of side-axis mirrors and rod offset-axis mirrors was improved by 28%. The optical simulations based on the Monte Carlo method revealed that a micro-concentrator with a hybrid layout of side-axis mirrors and rod offset-axis mirrors could promote the solar harvesting efficiency by 56.8% compared to the conventional central-axis mirror layout, regardless of direct normal sunlight or large angle inclined sunlight. The concept and calculation methodology of offset-axis mirror presented in this paper could reduce the thickness of linear Fresnel micro-concentrators in the design and improve their concentrating efficiency, which would lay the solid foundation for the wide application of micro-concentrators in the building domain.