Performance comparison of two eutectic solder based latent heat storage systems during discharging

Abstract

Latent heat storage systems consisting of phase change materials (PCMs) offer the advantage of a large thermal energy storage density as compared to sensible heat storage systems. Most recent work has focussed on organic and inorganic PCMs which have problems of subcooling and phase segregation. Metallic PCMs are a recent innovation for medium to high temperature applications due to their high thermal conductivities, high volumetric storage capacities and their low degrees of sub-cooling during the release of latent heat. For medium temperature applications like cooking of food, very limited work has been done on metallic PCMs for energy storage. Solder based PCMs have rarely been investigated for medium temperature applications thus it is necessary to carry out an experimental study on the use of a solder as PCM candidate. No work has ever been reported using the eutectic solder (Sn63/Pb37) as a PCM. Its use is justified since it is low cost and locally manufactured solder worldwide. Another recent innovation is cascaded thermal energy storage (TES), whereby two PCMs with different melting temperatures are used in a single storage tank to improve the efficiency of energy storage. No work has also appeared in recent literature involving metallic solders in cascaded systems. In a bid to investigate the suitability of the eutectic solder as a PCM for medium temperature applications, two eutectic solder (Sn63/Pb37) based systems are experimentally evaluated during discharging cycles. The first system is a single PCM system composed of a packed bed of spherically encapsulated eutectic solder capsules. The other system is a two PCM cascaded system comprising of eutectic solder spherical capsules at the top and erythritol spherical capsules at the bottom in a storage ratio of 50 %: 50 %. Discharging experiments are carried with three different discharging flow-rates to investigate the effect of the flow-rate on the thermal performance. The three discharging flow-rates used are 4 ml/s, 6 ml/s and 8 ml/s. For both storage systems, an increase in the discharging flow-rate increases the peak discharging energy and exergy rates. For both storage systems, an increase in the discharging flow-rate increases the peak discharging energy and exergy rates. The single PCM system shows higher energy and exergy rates for most of the discharging duration compared to the cascaded system. An increase in the flow-rate increases the peak energy and exergy discharging rates for the single PCM and the cascaded PCM storage systems. The single PCM system out-performed the cascaded system during the discharging tests possibly due to the lower melting temperature and lower thermal conductivity of the bottom PCM (erythritol) in the cascaded system. However, for the higher flow-rates (6 ml/s, 8 ml/s), the cascaded system shows a slightly better or comparable performance at the start and at end of discharging.