Heat recovery potential from ventilated passive and active facades

A numerical study

Abstract

A ventilated air-space behind external claddings can potentially affect the thermal performance of the entire building structure. In particular, in the Building Integrated Photovoltaic (BIPV) facades, ventilated cavities are typically present between the PV panels and the walls of the building. The airflow in the cavity can remove the generated heat behind the active external cladding, which could be eventually used as an additional source for heat recovery. In this study, the heat recovery from a ventilated air-space behind passive (wood) and active (BIPV) facades are investigated using transient simulations. The numerical model used in this study is validated against experimental measurements carried out in a building prototype located in the Smart Living Lab in Fribourg (Switzerland). The original façade is made of wooden cladding that is separated from the wall core incorporating a ventilated cavity. To study the impact of façade type on the results, the external cladding is virtually replaced with typical polycrystalline PV panels. The analyses are performed for representative days in the winter and summer of 2021 using recorded weather data on the test building. The results are examined in terms of the temperature distribution of the layers in the wall assembly, heat flux through the indoor space, airspeed in the cavity, and heat flow in the air gap. The potentials for heat recovery per day of interest are also calculated and compared. It was shown that the heat recovery from the cavity behind the BIPV façade could become equal to 5341 kWh on a representative summer day, which is considerably higher compared to the value obtained for a passive cladding. The results highlight the potential for harvesting heat from the ventilated air gaps behind passive and active facades. The outcome of this study highlights the need for the integrated vision for energy-savings at the building scale.