Energy performance of an office cooling system with phase-change material tank

Abstract

This work presents results from a cooling system in a UK office building served by two no. 241·3 kW chillers and two tanks containing hydrated salt phase-change material. The tanks are intended to save electrical energy by pre-cooling chiller return water. Accumulated heat is dicharged at night when chiller coefficients of performance increase. Chiller power and system temperatures were monitored for 6 days. The electrical energy required for an identical tankless system was calculated using tank temperatures and chiller performance curves for varying ambient temperatures. Differential scanning calorimetry results were used to calculate the tanks’ thermal capacity. The system incurred a 687 kWh energy cost (10·6% of total cooling energy), as compared to a tankless system, because night chiller and pump power outweighed daytime savings. Only 28% of the tank capacity was utilised because insufficient heat was discharged to the chilled water at night. This is partly because the freeze temperature of the phase-change material appears too low for sufficient heat rejection to the chilled water.