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

Abstract

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.