Numerical study of a PCM-air heat exchanger's thermal performance

Abstract

-Design of Experiments applied to numerical simulations: the study case of a PCM-air heat exchanger for temperature maintenance in rooms P Dolado, A Lazaro, J M Marin et al. Abstract. In this paper, the use of PCMs in HVAC applications is investigated by studying numerically the thermal performance of a PCM-air heat exchanger. The PCM used in this study is dodecanoic acid. A symmetric 3D model, incorporating conductive and convective heat transfer (air only) as well as laminar flow, was created in COMSOL Multiphysics 5.0. Simulations examined the dependence of the heat transfer rate on the temperature and velocity of the incoming air as well as the size of the channels in the heat exchanger. Results indicated that small channels size lead to a higher heat transfer rates. A similar trend was also obtained for high incoming air temperature, whereas the heat transfer rate was less sensitive to the incoming air velocity. IntroductionSeveral studies of phase change material (PCM) -air heat exchanger designs for thermal energy storage in heating ventilation and air conditioning (HVAC) systems are found in the literature. These studies cover different heat exchanger configurations: parallel PCMs slabs with rectangular air flow channels in-between [1-11]; cross-flow heat exchangers where one [12] or multiple PCMs [13] filled tubes and air is driven across the tubes bank; single [14] or modular [15,16] heat exchangers based on a shell and tubes design where the PCM is on the shell side and air is flowing either across the cylinder containing the PCM or through the tubes; and a cylindrical reactor where air passes through packed layers of spherical PCM capsules [17].Among these various heat exchanger geometries, the one with PCM slabs and air gaps is the most studied for thermal energy storage in building ventilation systems. It has been the subject of intensive experimental development where real-scale prototypes have been built and tested [2,5,6,9], and the results have been used to validate numerical analysis [5][6][7]9] for further optimization of the heat storage system.In this paper, a PCM-air heat exchanger configuration inspired in part from this previous work was modelled. However, a cellular structure where cubic PCM and air channels were alternated was considered. This configuration was adapted from the air-to-air heat exchanger design, typically used in heat recovery ventilation systems [18]. The main objective was to assess the thermal performance of such a heat exchanger design as a function of its geometrical properties and its operating conditions; the thermal performance in this case being centred on the heat transfer rates since most storage applications using PCM are facing a "rate problem", i.e. heat transfer rates too low for the system to be fully optimised [19]. To this end, a three-dimensional numerical model taking into account heat transfer by