Study on collection and retention efficiencies of a new integrated collector storage solar water heating system using a planar thermal diode: an experimental study.

Thermal diodes are a novel method to rectify the heat transfer mechanism and help reduce heat losses in solar thermal collectors during non-collection periods. The current study introduces and analyzes a new planar thermal diode integrated collector storage (ICS) solar water heating system using an experimental approach. This thermal diode ICS system has a simple, affordable structure composed of two parallel plates. Water serves as a phase change material to transfer heat through evaporation and condensation inside the diode. Three scenarios were considered to assess the dynamics of the thermal diode ICS: atmospheric pressure, depressurized thermal diodes, and P = 0, - 0.2, and - 0.4 bar. The water temperature reached 40 °C, 46 °C, and 42 °C in P = 0, - 0.2, and - 0.4 bar, respectively. The heat gain coefficients are 38.61, 40.65, and 39.26 W/K, while the heat loss coefficients are 9.56, 5.16, and 7.03 W/K in P = 0, - 0.2, and - 0.4 bar, respectively. The optimum heat collection and retention efficiencies are 45.3% and 33.5% in P =  - 0.2 bar. Hence, there is an optimum partial pressure to achieve the best performance, equal to - 0.2 bar. The acquired results illustrate the robustness of the planar thermal diode in reducing heat losses and rectifying the heat transfer mechanism. Furthermore, despite the simple structure of the planar thermal diode, its efficiency is as high as that of other types of thermal diodes analyzed in recent studies.