Experimental and Numerical Study of a Building Retrofitting Solution Combining Phase Change Material Wallboards and Night Ventilation

The interest in phase change materials (PCMs) as a solution for thermal energy storage has been growing for the last decades. It is clear that PCMs are promising for reducing the summer heat peaks without increasing the energy demand for cooling. A new modular, reversible, lightweight retrofitting system was developed and integrated in a real size experimental test cell. The basic idea of the concept is to develop a retrofitting solution for office buildings which can be rapidly and reversibly installed as an inside layer on the existing structure, like a “box in a box” to replace air-conditioning by night cooling. It combines ventilated PCM panels with controlled shutters to manage incoming solar energy. Ventilation has two goals: decrease indoor air temperature, and solidify PCM. During the night, the PCM recovers its storage potential, which limits the increase in temperature the day after. The experimental analysis covers 8 months measurement campaign in real climate conditions. They show that the night ventilation of the PCM panels is efficient and allows a decrease in the inside temperature of about 3°C during summer nights. Average panel temperature is associated with a value of the thermal capacity of the PCM, thanks to the theoretical curve Cp(T) deduced from the technical data of the PCM. From the thermal capacities evaluated every minute, an average daily thermal capacity is calculated. Results of the paper show that the box in the box concept is able to replace air-condition by free cooling in temperate climates. Nevertheless, wise combination with space ventilation is crucial to obtain competitive results for hot periods. The simulations show that a PCM changing phase at 27 °C can decrease the “integrated thermal discomfort level over 26°C” (ITDL26) by 65% compared to a PCM changing phase at 23°C. Numerical analysis also emphasizes that rise the ventilation rate of the developed panels from 4 ACH to 8 ACH reduce the ITDL26 by 50%.