The application of phase change materials to cool buildings

Abstract

Five projects improve understanding of how to use PCM to reduce building cooling energy. Firstly, a post-installation energy-audit of an active cooling system with PCM tank revealed an energy cost of 10.6% of total cooling energy, as compared to an identical tankless system, because PCM under%cooling prevented heat rejection at night. Secondly, development of a new taxonomy for PCM cooling systems allowed reclassification of all systems and identified under-exploited types. Novel concept designs were generated that employ movable PCM units and insulation. Thirdly, aspects of the generated designs were tested in a passive PCM sail design, installed in an occupied office. Radiant heat transfer, external heat discharge and narrow phase transition zone all improved performance. Fourthly, passive PCM product tests were conducted in a 4.2 m3 thermal test cell in which two types of ceiling tile, with 50 and 70% microencapsulated PCM content, and paraffin/copolymer composite wallboards yielded peak temperature reductions of 3.8, 4.4 and 5.2 °C, respectively, and peak temperature reductions per unit PCM mass of 0.28, 0.34 and 0.14 °C/kg, respectively. Heat discharge of RACUS tiles was more effective due to their non-integration into the building fabric. Conclusions of preceding chapters informed the design of a new system composed of an array of finned aluminium tubes, containing paraffin (melt temperature 19.79 °C, latent heat 159.75 kJ/kg) located below the ceiling. Passive cooling and heat discharge is prioritised but a chilled water loop ensures temperature control on hotter days (water circulated at 13 °C) and heat discharge on hotter nights (water circulated at 10 °C). Test cell results showed similar passive performance to the ceiling tiles and wallboards, effective active temperature control (constant 24.6˚C air temperature) and successful passive and active heat discharge. A dynamic heat balance model with an IES% generated UK office’s annual cooling load and PCM temperature%enthalpy functions predicted annual energy savings of 34%.