A novel PCM accumulator for refrigerated display cabinet: design and CFD simulations.

In supermarkets, power outages, temperature fluctuations due to defrosting cycles and peak demands when loading warm items, contribute to reducing shelf life and quality of food products. The objective of the study is to investigate the integration of a phase change material (PCM) accumulator along the air circulation path in the lower part of a refrigerated display cabinet to maintain the product temperature within safe limits. The PCM accumulator consists of three fluids: PCM (pure water is used here), air to be cooled down and CO2 as cooling medium. The lower part of such cabinets offers limited volume and thus requires effective space use. Dimensioning a three-fluid PCM accumulator integrated into the air circulation path requires a detailed CFD design, given the dynamic nature of thermal loads and small temperature differences during charging and discharging phases. A generic geometry is designed to fulfil the purpose. In this first study, transient CFD simulations are performed for a series of simplified geometries to evaluate the influence of the distance between CO2 coils and air ducts with regards to freezing time, melting time and heat absorbed from the air. The results showed that a variation of a few millimetres in the distance between the air ducts for a given PCM
accumulator dimension yields considerably different charging and discharging times. The calculated integrated heat flux from all air ducts provided a useful indication to select the geometric configuration giving the best match to a given application, e.g. covering defrost cycles or night-time cooling.