Performance evaluation of cascaded latent heat storage systems for domestic medium temperature applications

Abstract

The charging and discharging thermal performances of three packed bed cascaded latent heat thermal energy storage (LHTES) systems for medium temperature applications are experimentally evaluated and compared to a single PCM packed bed LHTES of eutectic solder capsules. Cascaded system 1 comprises of eutectic solder PCM capsules at the top, and erythritol PCM capsules at the bottom in equal storage volumes. Cascaded system 2 consists of eutectic solder PCM capsules at the top and adipic acid PCM capsules at the bottom in equal storage volumes. Cascaded system 3 consists of three PCM capsule layers of eutectic solder at the top, adipic acid in the middle, and erythritol at the bottom in equal storage volumes. The charging thermal performance characteristics of the four systems are evaluated in terms of the charging temperature profiles, charging energy rates and charging exergy rates. The effects of the flow rate and the heater set charging temperature are investigated in the experiments. Three different charging flow rates (4 ml/s, 6 ml/s, 8 ml/s), and three different heater set temperatures (260 oC, 280 oC and 300 oC) are used in the experimental tests. The effect of flow rate on the charging performance is more pronounced than the effect of the heater set charging temperature. Cascaded system 3, with 3 PCMs shows the best overall thermal performance as it possesses higher energy and exergy rates for most of the experimental conditions due to the 3 phase change transitions. The single PCM system shows the worst thermal performance, but its performance becomes better with an increase in the charging flow rate. The discharging thermal performance characteristics are also presented in terms of discharging temperature profiles, discharging energy and exergy rates. The effect of flow rate and the influence of the charging heater set temperatures is also investigated. To investigate the effect of the flow rate, three different flow rates (4 ml/s, 6 ml/s and 8 ml/s) are used during the discharging cycles. To investigate the effect of the final charging temperature on the discharging characteristics, discharging is carried out with a flow rate of 6 ml/s after charging with set temperatures of 260 oC, 280 oC and 300 oC. The final charging temperature shows an insignificant influence on the discharging performance. The increase in flow rate increases the rate of heat transfer which causes the energy rate, exergy rate and discharging inlet temperature peak values to occur earlier with increasing peak values. The single PCM system shows the best discharging characteristics with the least widening of the

thermal gradient as compared to other systems. This is an evidence of good heat transfer in the single PCM system. Cascaded system 2 and cascaded system 3 show comparable thermal performances during discharging, while cascaded system 1 shows a slightly worse thermal performance with greater temperature reversals in the storage tank. The overall storage performance is evaluated in terms of the energy and exergy storage efficiencies. Cascaded system 2 shows the highest energy and exergy storage efficiencies, which seem to increase with an increase in flow rate. However, the energy and exergy storage efficiencies of cascaded system 3 and cascaded system 2 are comparable. The energy and exergy efficiency of the single PCM is the worst, possibly due to the longer charging time. The higher energy and exergy efficiencies of cascaded system 2 in almost all the experiments when compared to cascaded system 3 is due to a greater volume of adipic acid in cascaded system 2. Adipic acid has a slightly higher thermal conductivity and a higher melting temperature as compared to erythritol at the bottom of cascaded system 2. The results also suggest that the performance of a cascaded system may depend on the PCM properties irrespective of the number of stages since a two PCM system with a higher bottom melting temperature shows better overall performance than the three PCM system. Cascaded system 1 shows lower energy and exergy storage efficiencies as compared to the other two cascaded systems.