BIPV enhancement using thermal diodes and integrated heat storage

Electrical efficiency and overall energy yield from Building Integrated Photovoltaics (BIPV) can be significantly improved by cooling cells and utilising surplus thermal energy to satisfy local heat demands. Like conventional solar thermal systems, Building Integrated Photovoltaic Thermal (BIPV/T) systems often produce hot water to charge a thermal store, in addition to generating electricity. Despite BIPV/T offering clear energetic and space saving advantages compared to separate solar thermal and BIPV, similar overheating problems occur when no thermal demand exists, resulting in reduced electrical yields, stagnation damage to PV laminates, and excessive pressure in fluid flow systems. Overheating can be avoided by ensuring continuous fluid flows on hot sunny days but the corresponding parasitic energy requirements (eg for pumps and/or heat rejection fans) typically far exceed modest gains in electrical yields and the ancillary equipment (large thermal stores and/or heat rejectors) occupies valuable space.
An alternative approach combines BIPV and Integrated Collector-Storage Solar Water Heater concepts (ICSSWH) to achieve the same functionality as BIPV/T with a completely passive means of waste heat rejection leading to lower parasitic electrical loads, less demand for floor space, and minimal risk of overheating during stagnation. The use of a novel device known as a Planar Liquid-Vapour Thermal Diode (PLVTD) enables effective thermal coupling between the PV and ICSSWH elements and regulation of thermal conductance to overcome problems of excessive overnight heat losses. This presentation describes the BIPV-PLVTD-ICSSWH concept and presents an overview of experimental testing and theoretical modelling work undertaken at Ulster University to explain the benefits of the proposed approach. A key finding of the work is that the proposed concept reduces the maximum PV cell stagnation temperature by around 20°C compared to conventional BIPV/T systems.