Passive cooling through phase change materials in buildings. A critical study of implementation alternatives

Global warming is gradually increasing the cooling energy demand of buildings. Phase change materials (PCM) offer high potential to passively reduce cooling energy consumption and overheating by absorbing heat gains in the daytime through their melting process, and releasing the heat at night while solidifying by taking advantage of free cooling through natural ventilation. However, the effectiveness of PCM-based solutions highly depends on the implementation techniques, material properties, environmental conditions and occupants' behaviour. This paper analyses the performance of PCM-based solutions towards passive and low-energy cooling through a parametric study carried out in TRNSYS, in order to identify main design criteria for their optimal implementation. Two PCM implementation alternatives are assessed: a conventional passive application based on a PCM layer attached at the ceiling in contact with the indoor space, in which the heat transfer between PCM and air is based on natural convection; and an optimised low-energy application designed as a PCM layer integrated inside the suspended ceiling, in which the air flow is forced by a fan to enhance the heat transfer between PCM and the air. Both solutions are studied with and without the simultaneous operation of air-conditioning systems. A dwelling in a multi-family building was selected as a reference scenario. The results show that in the scenario with no participation of air-conditioning systems, the optimised PCM-based solution could reduce discomfort hours by 65% regarding the adaptive comfort model, and up to 83% through additional improvements in order to increase the heat transfer between PCM and air. On the other hand, the simulations reflect that both PCM-based solutions do not provide benefits in scenarios with an intermittent operation of air-conditioning systems. This study concludes with design criteria and strategies for an optimal implementation of PCM towards low-carbon buildings.