Experimental study of overheating of an unglazed transpired collector façade under southern European summer conditions for four modes of operation

The use of unglazed transpired collector (UTC) façades for air preheating in buildings has been proved to be an energy saving solution for refurbishing old buildings. However, not all climates are appropriate for the installation of this type of façade, and in some cases their benefits in winter can be counterbalanced by the negative effects during summer. There is a risk of overheating and façade cooling load increase if the system is not operated appropriately in summer. In this study, a UTC façade cooling load increase was measured in real weather conditions in four different operating modes. Ambient temperature and solar radiation values were monitored. Surface and air temperatures were measured in the different layers, and the heat flux through the test cell wall was registered in two cases, both with and without UTC. The four operation modes combined mechanical or natural ventilation and air flow direction. Results showed that not ventilating the façade or using natural ventilation increased the façade cooling load by around 45%, whereas outdoor mechanical ventilation produced an increase of 23%. Ventilating with indoor air reduced heat transfer compared to a non-UTC façade but only when it is integrated into an existing ventilation system. Cooling load increase due to overheating in the UTC façade was low in all four modes of operation. UTC façades must be integrated into the building ventilation system to avoid a cooling load increase during summer, or they must be ventilated with outdoor air if there is no ventilation system to reduce the impact of overheating.