12Passive thermal management systems using phase change materials (PCMs) 13 provides an effective solution to the overheating of lithium ion batteries. But this 14 study shows heat accumulation in PCMs caused by the inefficient cooling of air 15 natural convection leads to thermal management system failures: The temperature in a 16 battery pack operating continuously outranges the safety limit of 60 o C after two 17 cycles with discharge rate of 1.5C and 2 C. Here a hybrid system that integrates 18PCMs with forced air convection is presented. This combined system successfully 19 prevents heat accumulation and maintains the maximum temperature under 50 o C in 20 all cycles, even with 7 o C rise in the ambient temperature. Study on airspeed effects 21 reveals that thermo-physical properties of PCMs dictate the maximum temperature 22Recently, passive thermal management system using phase change materials 47
Improper operating temperature will degrade the performances of electronic components, Li-ion batteries and photovoltaic (PV) cells, which calls for a good thermal management system. In this paper, specific attention is paid to the thermal management systems based on phase change materials (PCMs). Performances of the PCM-based thermal management systems for for each kind of these three devices along with the type of PCM used, thermal properties of that kind of PCM, like phase change temperature, enthalpy of phase change and thermal conductivity are discussed. Discussion in detail on techniques to improve the thermal conductivity of PCMs are made because of its crucial influence. Advanced-structure heatsinks with multi-layer PCMs and hybrid passive heatsinks combined with active cooling are also introduced. The PCM-based thermal management system is powerful in ensuring electronic devices, Li-ion batteries and photovoltaic cells working safely and efficiently.
a b s t r a c tThis work studies factors that affect the thermal conductivity of an organic phase change material (PCM), RT44HC/expanded graphite (EG) composite, which include: EG mass fraction, composite PCM density and temperature. The increase of EG mass fraction and bulk density will both enhance thermal conductivity of composite PCMs, by up to 60 times. Thermal conductivity of RT44HC/EG composites remains independent on temperature outside the phase change range (40-45°C), but nearly doubles during the phase change. The narrow temperature change during the phase change allows the maximum heat flux or minimum temperature for heat source if attaching PCMs to a first (constant temperature) or second (constant heat flux) thermal boundary. At last, a simple thermal conductivity model for EG-based composites is put forward, based on only two parameters: mass fraction of EG and bulk density of the composite. This model is validated with experiment data presented in this paper and in literature, showing this model has general applicability to any composite of EG and poor thermal conductive materials.
The thermal management systems using EG-based phase change materials(PCMs) can provide power batteries with a proper operating temperature, slow temperature rise rate and uniform temperature distribution. In this study, a systematical investigation on the effects of thermo-physical properties of the used PCMs on the performance of the systems has been conducted. A series of paraffin/expanded graphite(EG) composites have been applied to a simulative battery thermal management system and to find out the PCM with the best thermal properties. The performances of PCMs varying with the kind of paraffin used, the paraffin mass fraction in composites and the packing density of the composites have been compared.
Solar water evaporation is a promising strategy to extract freshwater from seawater or wastewater directly by solar energy. Employing cheap substrates capable of reducing light reflection to prepare photothermal layers helps to develop high-efficiency and lost-cost evaporation systems. Herein, pyramid polyurethane sponge (PPUS), a black sound-absorbing material, was first explored as the substrate for loading graphene (GE) to prepare a novel photothermal layer. The porous microstructure, wettability, and optical property of the obtained GE/PPUS photothermal layer were characterized, measured, and compared with those of the flat polyurethane sponge (FPUS) loaded with GE. It is shown that the modifications of PPUS and FPUS with poly(vinyl alcohol) (PVA) endow the two photothermal layers with good wettability. PPUS exhibits higher optical absorption than FPUS, thereby making the GE/PPUS photothermal layer reaches a solar absorptance of 98.5% in the wavelength range of 200−2500 nm, higher than 97.0% of GE/ FPUS. Consequently, an evaporation efficiency of 85.27% under 1 kW m −2 is achieved by the system composed of GE/PPUS, 9.23% higher than that of the system based on GE/FPUS. The enhancement in evaporation performance of GE/PPUS as compared with GE/FPUS is attributed to the pyramid-shaped projection units and thus can provide larger evaporation area, reduced light reflection, and induced thermal energy recovery by a spatial temperature distribution. A portable evaporate device employing GE/PPUS as the photothermal layer was fabricated, which can provide an ion rejection rate of 99.9% and excellent cyclic stability and long-term salinity durability.
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