The performance of thermal energy storage systems of the salt hydrates $[Na_2S_2O_3·5H_2O; CH_3COONa·3H_2O; Na_2CO_3·10H_2O]$ and eutectic mixtures $[Urea/KNO_3/NH_4NO_3; Urea/NaNO_3/NH_4NO_3; Urea/NH_4Cl/NH_4NO_3; Urea/NH_4SCN; Urea/NH_4NO_3; Urea/NaNO_3/NH_4NO_3]$ have been investigated. A super-absorbent polymer made from an acrylic acid copolymer is found to be an effective thickener to prevent the undesirable phase separation of the salt hydrates. Also, the most preferred thickeners for $Urea/KNO_3/NH_4NO_3$ and the low hydrate salts are CMCs and natrosal. Various nucleators($K_2SO_4, Na_2SO_4, SrSO_4, Al, C$) have been evaluated for the different storage medium to overcome supercooling of the thickened PCMs.
The thermal cycling tests in a test tube have been performed to upgrade the performance of thermal energy storage of a sodium acetate trihydrate(SAT=$CH_3COONa·3H_2O$). A mixture of 2wt% carboxy-methyl cellulose and 1wt% acrylic acid copolymer is proposed as an effective thickener to prevent the undesirable phase separation of SAT. The supercooling of the thickened SAT is reduced from 20 to 2~3℃ by using 2wt% potassium sulfate as a nucleating agent. From the calorimetric measurements of the mixture of SAT, thickener and nucleator, the latent heat, thermal conductivity and heat capacities are found to be 292J/g, 0.55W/m-K, 2.1J/g-K in the solid and 3.7J/g-K in the gelled state, respectively. Thus, this mixture is a promising latent heat storage material for residential heating from solar energy.
Three flow paths of the heat transfer fluid have been studied in 2.3kWh using the above material to find an optimum one to maximize the thermal performance an integrated system with a solar collector. Also, the effects of the flow rate(6~12ℓ/min) and inlet temperature of heat transfer fluid in the heat storage(65~80℃) and in the heat recovery stages(10~40℃) on the thermal performance have been determined in a heat exchange system.
The heat storage material has phase changed at 56~58℃ in the heat storage and 55~56℃ with a minor supercooling of heat recovery stage, thus this material can be utilized in the actual application. The heat transfer process of the heat storage unit is significantly affected by the thermal stratification of the heat transfer fluid at lower Reynolds numbers(124~250). In the heat storage stage, the outlet temperature in the top to bottom flow mode is lower than that of the bottom to top flow mode. It is confirmed by the fact that the temperature profile is uniform in the bottom to top flow mode by high mixing intensity. However, the temperature in the top to bottom flow mode increases with an increase in height of the storage unit and the thermal stratification is well developed. In the heat recovery stage, although the bottom to top flow mode has much higher heat transfer rate during the sensible heat recovery stage than that of the top to bottom flow mode, it reduces greatly during the latent heat recovery and consequently this trend is reversed. The ratio of total recovery to the maximum available value is about 80% during 2 hours operation in bottom to top flow, whereas it is above 90% in top bottom flow mode. The heat transfer rate and the heat recovery efficiency of the center to center flow mode have the smallest values among three flow modes. Subsequently, the top to bottom flow mode is superior to any other flow modes in both heat storage and heat recovery stages with overall heat transfer coefficients in the range of 100~120 $W/m^2-K$.
The heat storage and recovery amounts to the theoretical value are correlated in terms of Fourier, Stefan and Reynolds numbers. The inlet temperature of heat transfer fluid has larger influence on the heat transfer than that by flow rate in heat storage and the flow rate of heat transfer fluid has less influence on heat transfer than that by the inlet temperature in heat recovery stages.
