Performance and heat transfer analysis of uncovered photovoltaic-thermal collectors with detachable compound

Concerning the manufacturing of uncovered, liquid-based photovoltaic-thermal (PVT) collectors, design requirements or low expenses have higher priority than the all-out optimization of the thermal efficiency. The specific impact of inevitable thermal resistances caused by wavy surfaces and imperfect contacts between PV modules and heat exchangers has not been comprehensively investigated yet. This paper focuses on the performances of uncovered PVT collectors with detachable mounting of PV modules on heat exchangers and the effect of macroscopic air gaps. Two prototypes with thin film modules were built without using silicon glue, other adhesives or heat conductive paste. Measured in sun simulator in electrical open-circuit mode, the thermal zero-loss efficiencies for the adhesive-free collectors are η0,M = 66.8% (copper pipe meander laser-welded on aluminium sheet) and η0,M = 69.2 % (aluminium roll-bond) based on gross areas. These results are similar or better than the efficiencies of commercially available collectors. Due to the thermal resistance of air gaps, we calculated temperature differences of 12 K and higher between absorbing sheets and heat carrier, which have to be considered while estimating the electrical yield by means of system simulations. For comparison, we built a prototype with a roll-bond heat exchanger and a very thin adhesive layer and thereby an optimized heat transport (η0,M = 76.8 %). Using numerical, two-dimensional models, we can verify the efficiency measurements and determine the influence of design and boundary conditions on heat transfer mechanisms in the collectors. Simulations show good agreement with experimental results and prove to be an effective tool for design optimization.