Cold Storage and Release Characteristics of a Thermal Battery Unit Using Solid Nitrogen and Solid Nitrogen Impregnated in Metal Foam
Large cold storage capacity is very important to maintain the stable working condition of a superconducting magnet. Solid nitrogen is frequently used for cold storage at cryogenic temperatures. In this paper, we report the experimental investigation of the cold storage and cold release characteristics of a thermal battery unit using solid nitrogen, as well as the combination of solid nitrogen and copper foam under different conditions. In order to enhance the heat transfer without significantly compromising the cold storage capacity, the copper foam is adopted to enhance the effective thermal conductivity of solid nitrogen. It is found that the copper foam can effectively enhance the cold release process, making the cold energy stored effectively utilized.
Effective thermal conductivity of open-cell metal foams impregnated with pure paraffin for latent heat storage
Abstract The thermal conductivity of phase change material (PCM) significantly affects the thermal performance of latent heat thermal energy storage (LHTES) system, which is attractive for energy conservation and waste heat utilization. Metal foam can be applied to enhance the low thermal conductivity of pure PCM. In the present study, copper foam and nickel foam with various porosities and pore sizes were impregnated with pure paraffin with vacuum assistance. A steady-state test system which considered the thermal contact resistance (TCR) between the specimen and adjacent surfaces was constructed to measure the effective thermal conductivities of the composite PCMs. The thermal conductivities were also theoretically calculated based on the correlations and models from the literature. The results showed that the thermal conductivities measured with steady-state method showed good agreement with the theoretical predictions, and the thermal conductivities of the composite PCMs were drastically enhanced, e.g., the thermal conductivities of the paraffin/copper foam composite PCMs fabricated by the copper foams with the porosities of 96.95%, 92.31%, 88.89% and pore size of 25 PPI were about thirteen, thirty-one, forty-four times larger than that of pure paraffin, respectively. The ratios of TCR to the total thermal resistances of the composite PCMs with the thickness of about 20.0 mm were in the ranges of 15.0–50.0%.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.