Development of an Improved PCM-to-Air Heat Exchanger for Building Envelopes - Advancement to Free Cooling Applications

With high energy consumption trends in buildings, the adoption of the thermal energy storage systems toward reducing cooling load has been increased in recent years. In this study, the utilization of phase change materials (PCMs) has been comprehensively discussed for building free cooling applications. The potential and limitations of using PCM-to-Air Heat Exchangers (PAHXs) for free cooling applications were analyzed. Referring to the local climatic conditions, one of the most important operational challenges that face PAHX applications is the insufficient cooling charging energy during the PCM solidification phase. An improved PAHX type was developed in this study for building envelopes by applying the concept of thermal radiation losses to the sky during night-time to increase the cooling potential of the system. Two real-scale prototypes were designed and set-up to monitor the effect of radiative cooling on the thermal performance of the PAHX system. The experimental results indicated that exposure to the clear sky, as a cooling source, during PCM solidification increases the cooling potential of the system due to the maximized thermal losses by radiation. A 2D numerical model was developed considering the PCM thermal storage, short and long waves radiation, and convection phenomena. New thermal boundaries of long-wave radiation were developed between the system elements and the sky temperature. In addition, the 2D model considered various forms of convective heat transfer phenomenon. The experimental data and inter-model comparison were applied to validate the proposed model. The application of the developed PAHX system in the hot desert climate was assessed to evaluate its thermal performance. It was concluded that the PAHX system outlet air could be directly supplied to the indoor spaces through a direct free cooling application to satisfy the cooling loads, also it could be introduced to the mechanical cooling unit to minimize the energy consumption. The results showed that the thermal comfort levels were enhanced by up to 18% (during the direct free cooling application), and the energy consumption for cooling purposes was \nreduced by 18.5% (during the energy savings mode) saving an amount of energy up to about 11 kWh/week.