Experimental comparative analysis of finned-tube PCM-heat exchangers? performance

The use of phase change materials (PCMs) in energetic systems today is in large parts restricted by dynamic issues, or what could be termed the "rate problem"; i.e., how long will it take (heat transfer rate) to store, or recover, a given amount of thermal energy in a PCM-based thermal energy storage system (TES) for a specific application? There is no theory that can easily answer this question for a given PCM-TES. Therefore, an important long-term objective in the latent heat thermal energy storage community is to determine heat exchanger design rules for PCM-TES. To this end, it is imperative to study a large variety of heat exchange configurations: heat exchanger size and shape, tank size and shape, use of fins or not, various PCMs, charging and discharging conditions, etc. This paper presents a series of new controlled experimental characterizations of a specific PCM-heat exchanger (HX): a vertical finned tube-and-shell system designed in a way that the number of finned-tubes used in the HX can be varied. Experimental characterization was performed for systems having 4, 8 and 12 finned-tubes. The inlet heat transfer fluid (HTF) temperature, the initial system temperature, and the HTF flow rate were also varied consistently for each geometrical arrangement to cover a wide range of operating conditions. Results are first presented in terms of power curves, followed by two comparative metrics: mean power and normalized power. Determining and applying these two comparative metrics is the first step forward in the search for PCM-HX design theory and rules. These two possible metrics, the mean and normalized power, applied to this modular finned tube-and-shell PCM-TES shed light on their merit as comparative metrics as well as the impact of PCM-TES operating conditions.