Heat transfer characteristics in a coolness storage system was investigated using a phase change material. A 2 wt % hydrated sodium sulfate aqueous solution was selected for this study as one of the most appropriate storage materials based on its low cost and excellent thermophysical properties. It was also suggested, however, that several other salts could be good candidates of coolness storage materials.
The cold storage system consists of 90 tubes arranged in 20 staggered rows with 5 and 4 tubes per row alternating. A metal box (0.22 m wide x 0.42 m high x 0.75 m long) encloses the copper tubes (32 mm I.D. x 420 mm high) which contain the cold storage material. The total weight of storage material is 32 kg, giving a latent heat storage capacity of 3 kw-hr. The etylene glycol (30 wt % solution) is used as a heat transfer fluid. The affects of the inlet temperature and inlet flow rate of heat transfer fluid on thermal performances both in horizontal and in vertical geometrics were determined. The effects of the surrounding fluid temperature, subcooling temperature and tube diameter on the freezing and melting characteristics of the phase change material were also predetermined in a single tube system as a preliminary study.
The main results of this study are as follows. First, a hydrated sodium sulfate solution has the advantage of better supercooling behavior, fast crystal growth rate and little volume charge on fusion over water as a phase change material. Second, energy transfer is facilitated vice natural convection during the melting. The initial subcooling of the solid substantially reduces the rate of melting, and consequent decrease in the heat transfer for melting. The flow of melted PCM and the melting charactistics are significantly affected by the density difference due to tube wall temperature. Third, In the coolness storage process, the performance is more affected by the temperature than by the flow rate of the heat transfer fluid in the horizontal system. However, the performance is more affected by the flow rate than by the temperature of the heat transfer fliud in the vertical system. The vertical arrangement of tubes has better performance than the horizontal system in view of heat transfer rate and supercooling temperature. Take account of supercooling temperature of coolness storage material and the coefficient of performance (COP) reduction of refrigerator, the suitable inlet temperature of heat transfer fluid is found to be -6℃. Forth, In the Coolness recovery process, the heat recovery ratio is ranged from 80 to 85% of total amount of heat stored, with the heat transfer coefficient of 100 ~ 200 kcal/h m K. The outlet temperature is ranged 4~10℃ which is lower than the inlet temperature at the present experimental condition.
