Heat transfer characteristics have been determined for the circular finned and unfinned tubes in latent heat thermal energy storage (LHTES) systems using salt hydrates as a phase-change material (PCM). For intermediate-temperature LHTES systems, magnesium chloride hexahydrate ($MgCl_2$. $6H_2O$ = MCHH) with a quasi-congruent melting temperature of 116.7℃ has been investigated. The effects on the heat transfer characteristics have been determined of tube geometry (finned and unfinned) and inlet temperature and flow rate of air used as the heat transfer fluid (HTF). The thermal energy storage(TES) vessel in made of cylindrical Pyrex glass (55 mm i.d. × 140 mm high × 2.5 mm thick). The double-heat transfer tube was constructed from a stainless steel tube (outer tube : 9.4 mm o.d. × 100 mm long × 0.45 mm thick ; inner tube : 6.35 mm o.d. × 100 mm long × 0.45 mm thick). Five and ten circular fins of stainless steel in 30 mm over-fin dia × 9.4 mm fin-root dia × 0.7 mm thick were welded on the outer surface of the tube with 20 and 10 mm-axial pitch, respecively. The total weight of the PCM was 0.36 kg with a total heat-storage capacity of about 97 kJ including the sinsible heats of the liquid and solid phases. The inlet temperatures and flow rates of HTF were varied in the ranges of 40~90℃ and 20~50 1/mim in the heat recovery stage, 140~180℃ and 20~50 1/min in the heat storage stage, respectively.
The supercooling of magnesium chloride hexahydrate is reduced from 17. 1℃ to less than 1℃ by using 0.5 wt% of calcium oxalate ($CaC_2O_4$. $H_2O$) as a nucleating agent, and the phases separation cannot be observed over 50 thermal cycling experiments.
In the heat recovery stage, the PCM heat transfer coefficients in the unfinned-tube system are larger than the calculated values based on the theory of steady-state heat conduction due to the dendritical crystal growth of PCM. The ratio of the PCM heat transfer coefficient of the 5-finned- to the unfinned-tube systems is about 3.5 within the finned section and decreases gradually far from the finned section with an increase in crystal volume. The PCM heat transfer coefficients in the unfinned-and 5-finned-tube systems are 100~230 and 200~890 $W/m^2-k$, respectively. The total amounts of heat recovered in terms of the Fourier, Stefan, and Reynolds numbers. The thermal performance in the unfinned-tube system is found to be more affected by the inlet temperature than by flow rate of HTF, whereas in the 5-finned-tube system, the effect of the HTF flow rate on the thermal performance is more pronounced than that in the unfinned-tube system.
In the heat storage stage, the heat transfer from the heat-transfer tube to the PCM is largely influenced by natural convection at the melting liquid layer section. In the unfinned-tube system, the PCM heat transfer coefficients are much greater than the calculated values based on the theory of steady-state heat conduction due to the movement of natural convection in liquid PCM. The ratios of the PCM heat transfer coefficient of the 5-finned- and 10-finned-tube to unfinned-tube systems are about 1.7 and 2.1 within the finned section and decrease gradually to 1.1 and 1.2, respectively, far from the finned section. The PCM heat transfer coefficients in the unfinned-, 5-finned-and 10-finned-tube systems are 150~380, 280~400 and 340~480 $W/m^2-k$, respectively. The melting front velocity in the unfinned-tube system agrees well with the predicted value by moving boundary problem assuming that the sensible heat of melted liquid PCM is negligible. The total amounts of heat storage in three systems have been correlated in terms of the Fourier, Stefan, and Reynolds numbers. The melted volumes and thermal performances in three systems are found to be more affected by the inlet temperature than by the flow rate of HTF.
For low-temperature LHTES systems, sodium acetate trihydrate ($CH_3COONaㆍ3H_2O$ $\equiv$ SATH$) with a melting temperature of 58℃ has been investigated. The effects of inlet temperature and flow rate of water used as HTF and tube geometry (finned and unfinned) on the heat transfer characteristics have been measured. The TES vessel is made of cylindrical stainless steel .74 mm i.d. × 530 mm high × 1.0 mm thick). The heat-transfer tube was constructed from a double tube of stainless steel (outer tube : 19.05 mm o.d. × 480 mm long × 0.5 mm thick ; inner tube : 13.5 mm o.d. × 480 mm long × 0.5 mm thick). Twelve circular fins of a stainless steel (60 mm over-fin dia × 19.05 mm fin-root dia× 0.4 mm thick for thin-finned system, 3.0 mm thick for thick-finned-tube system) are welded on the outer surface of the tube with 40 mm-axial pitch. The total weight of the PCM was 2.6 kg with a heat-storage capacity of 530 kJ, considering the latent heat only. The inlet temperatures and flow rates of HTF were varied in the ranges of 26~50℃ and 200~800 g/min in the heat recovery stage, 68~92℃ and 200~800 g/min in the heat storage stage, respectively.
A mixiture of 2 wt% carboxymethyl cellulose-Na(CMC-Na) and 1 wt% super-absorbing polymer (SAP) is added in order to prevent the undesirable phase separation of SATH [Woo, 1991]. The supercooling of the thickened SATH is reduced from 23 to 6~10℃ by using 2 wt% potassium sulfate ($K_2SO_4$) as a nucleating agent.
In the heat recovery stage, the supercooling of PCM in the finned-tube system is larger than that in the unfinned-tube system due to the higher cooling rate by the presence of fins. The PCM heat transfer codfficient in the unfinned-tube system is in agreement with the calculated value based on the theory of steady-state heat conduction except the beginning stage of solidification with some degree of supercooling. The heat transfer process between the heat-transfer tube and PCM is significantly reduced by void cavities upon shrinkage in the finned-tube system the circular fins inhibit partially the downward movement of thickened liquid PCM. The enhancemener by the fins was found to be negligible in the thin-finned-tube system, whereas the PCM heat transfer coefficients in the thick-finned-tube system are approximately two times greater than in the unfinned-tube system. The PCM heat transfer coefficients in the unfinned-tube and thick-finned-tube systems are 45~150 and 90~250 $W/m^2-k$, respectively. The amounts of heat recovered in three systems have been correlated in terms of the Fourier, Stefan, and Reynolds numbers. The thermal performances in three systems are found to be almost affected by the inlet temperature of HTF, because the HTF heat transfer coefficient is increased greatly by the insert of the spiral silicone wire to the inside of heat-transfer tube and the effect of HTF flow rate on thermal performance becomes negligible.
In the heat storage stage, the PCM heat transfer coefficient in the unfinned-tube system agrees well with the calculated value by the heat conduction equation during whole experimental period. The heat transfer process between the tube and PCM was not enhanced by the fins in the thin-finnedtube system, whereas the ratio of PCM heat transfer coefficient for the thick-finned-tube to unfinned-tube systems in about 2.1 in the heat storage stage, as like in the heat recovery stage. The PCM heat transfer coefficients in the unfinned-tube and thick-finned-tube systems are 40~170 and 80~320 $W/m^2-k$, respectively. The amounts of heat storage in three systems have been correlated in terms of the Fourier, Stefan, and Reynolds numbers. The thermal performances are found to be almost affected by the inlet temperature of HTF, as like in the heat recovery stage.
