Numerical analysis on the advantage of using PCM heat exchanger in liquid-flow window

Liquid-flow window is a multi-glazing system with a flowing liquid layer in the window cavity. Thermal transmission is largely restricted and warm water can be produced. These lead to considerable energy saving in buildings with daytime hot water demand. For service extension, it is necessary to enhance the thermal storage capability of the system as solar energy is an intermittent heat source. In this study, the potential advantage of applying PCM onto the double-pipe heat exchanger of liquid flow window was evaluated for different situations, including the unfavorable case with night-time hot water demand. The enthalpy-based method was adopted for precise modelling of the heat transfer process in the PCM layer. The validity of the numerical model was confirmed by experimental comparison with published data. Numerical studies were then conducted based on different design positions of the PCM layer at the heat exchanger, and for use in residential and office buildings. The results show that comparatively, the design with PCM located at the outermost layer of the heat exchanger has the best energy storage performance. For the case with PCM sealed in the middle annular space, the inner PCM layer is heated up rapidly, while the outer PCM layer remains at low temperature because of the continuous heat release to the cold water stream. With the use of PCM, 31.4% and 11.4% more hot water can be harvested during off-work hours for residential use in typical summer and winter weeks. And as a whole, the energy saving potential is greater in summer than in winter.