Numerical study on the performance of an air—Multiple PCMs unit for free cooling and ventilation

For the present paper the potential of an air- multiple phase change materials (PCMs) unit is investigated for the free cooling and ventilation of buildings. Firstly, a CFD model has been developed to determine the charging and discharging temperatures of the PCM as well as the air outlet temperature of an air-PCM unit. The effective heat capacity method is applied introducing the C_p-T relationship obtained from the Differential Scanning Calorimetry (DSC) and has been validated by experimental results. Secondly, the validated model has been further extended for multiple PCMs to investigate the thermal performance enhancement incurred by using multiple PCMs for free cooling application. The model takes into account the daily temperature profile and therefore a variable temperature is coupled to the air inlet boundary condition. Thirdly, a parametric study has been conducted to investigate the effect of multiple PCMs within the panels, followed by the effect of geometry e.g. height, length and the charging and discharging air mass flow rates. The proposed best case design is a combination of RT20 and RT25 with geometric dimensions of H = 0.03 m and L = 1.5 m and an air mass flow rate of 0.25 kg/s. The unit presented a thermal energy varying from 0.4 to 1.46 kJ over 8 h, with an average thermal energy of 1.02 kJ. The average effectiveness of the unit is 0.5, remaining similar to this value for over 6 h. In order to make the proposed unit competitive in terms of energy consumption and running cost, the charging process of the PCM has to be improved during night time leading to high air mass flow rate requirement.