• Energy Research
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The dynamic performance is a very important evaluation index of proton exchange membrane (PEM) fuel cells used for real application, which is mostly related with water, heat and gas management. A commercial PEM fuel cell system of Nexa module is employed to experimentally investigate the dynamic behavior and transient response of a PEM fuel cell stack and reveal involved influential factors. Five groups of dynamic tests are conducted and divided into different stage such as start-up, shut-down, step-up load, regular load variation and irregular load variation. It is observed that the external load changes the current output proportionally and reverses stack voltage accordingly. The purge operation benefits performance recovery and enhancement during a constant load and its time strongly depends on the operational current level. Overshoot and undershoot behaviors are observed during transience. But the current undershoot does not appear due to charge double-layer effect. Additionally, magnitudes of the peaks of the voltage overshoot and undershoot vary at different current levels. The operating temperature responds fast to current load but changes slowly showing an arc-like profile without any overshoot and undershoot events. The air flow rate changes directly following the dynamic load demand. But the increased amount of air flow rate during different step-change is not identical, which depends on the requirement of internal reaction and flooding intensity. The results can be utilized for validation of dynamic fuel cell models, and regarded as reference for effective control and management strategies.

This study investigates the thermal performance of composite ultra-thin heat pipes (UTHPs). UTHPs are fabricated by flattening cylindrical heat pipes with outer diameter of 2 mm. The thickness and width were 0.8 mm and 2.7 mm, respectively. The composite wick structure is made of sintered copper foam-mesh wick (CFMW). CFMW combines the good heat transfer performance of copper foam and the high mechanical strength of mesh. The manufacturing process of UTHP was studied and the thermal performance of UTHP samples were investigated experimentally. The results indicate that the optimum filling ratio of UTHPs is 100% and the maximum temperature difference is 3.7 °C under the maximum heat transport capacity of 5 W. The thermal resistances of UTHPs increase gradually with the heat power before drying out. Too low or too high filling ratios will reduce the heat transfer efficiency of UTHPs by increasing the thermal resistances. With the optimum filling ratio of 100%, the evaporation thermal resistance of UTHP is found to be 0.29 K/W and the condensation thermal resistance is 0.45 K/W at the heat load of 5 W.

Abstract The thermal performance of a battery thermal management system (BTMS) can be enhanced by cooling strategies, which are seldom taken into account in the study of heat pipe-based BTMS (HP-BTMS). The effects of coolant flow rate, ambient temperature, coolant temperature and start-up time on the thermal performance of HP-BTMS are crucial for the development of cooling strategies and are experimentally investigated in the present study. Results show that the thermal performance of HP-BTMS increases slightly with the decrease of ambient temperature as it is under 25 °C. When the ambient temperature is under 35 °C, the thermal performance of HP-BTMS can be kept nearly unchanged by reducing coolant temperature. The enhancement is little when ambient temperature is under 25 °C. Additionally, a drastic rise in the non-uniformity of battery temperature is observed at the moment of HP-BTMS initiation if HP-BTMS starts operating after battery temperature exceeds equilibrium value. Finally, intermittent cooling and constant cooling can achieve similar battery cooling performance, which indicates that the power consumption can be reduced by decreasing running time of HP-BTMS.

This study proposes three composite wick structures (copper power or mesh sintered on grooved tube), namely, single arch-shaped sintered–grooved wick (SSGW), bilateral arch-shaped sintered–grooved wick (BSGW), and mesh–grooved wick (MGW), to improve the thermal performance of ultra-thin heat pipes (UTHPs). Phase-change flattening technology is employed to fabricate UTHPs. The morphologies of the wick structures after flattening are observed. An experimental apparatus is setup to investigate the thermal performance of UTHP samples under incremental heat loads. The heat transfer limits of UTHP are theoretically and experimentally analyzed. Capillary limit is found to be the main heat transfer limit, and the theoretical values of the samples with SSGW and BSGW are in good agreement with the experimental results. Results indicate that the maximum heat transport capacities are 12 W, 13 W and 14 W, under the corresponding optimum filling ratios of 70%, 70%, and 80%, for the SSGW, BSGW and MGW UTHPs, respectively. Evaporation and condensation thermal resistances of UTHP samples increase with the increase in the filling ratio before the occurrence of dry-out. UTHPs with SSGW have the least evaporation thermal resistance whereas UTHPs with MGW have the least condensation thermal resistance.

A mathematical model of evaporation and condensation heat transfer in a copper-water wicked heat pipe with a sintered-grooved composite wick is developed and compared with experiments. The wall temperatures are measured under different input power levels and working temperature conditions. The results show that the heat transfer in the condenser section was found to be only by conduction. In the evaporator, however, either conduction or boiling heat transfer can occur. The experimental data for the boiling heat transfer are well correlated by the theory of Stralen and Cole. Higher heat load drives the heat pipe to spend more time achieving the equilibrium state during the transient start-up process. The response curves of the evaporator thermal resistance are overlapped, and the condenser thermal resistance increases more sharply at the beginning. The total thermal resistance of the heat pipe ranges from 0.02 to 0.56 K/W.

This paper presents the novel designs of a concentric cylindrical thermoelectric generator (CCTEG) and an annular thermoelectric module (ATEM). The simulations are carried out to compare the performance of ATEM and the conventional square-shaped thermoelectric module (STEM). The heat pipe technology is introduced into the heat sink system in order to enhance the heat transfer in the radial direction of exhaust gas flow. A new index termed as the heat transfer filling factor ff has been introduced which quantities the level of space utilisation for thermoelectric modules (TEMs). The correlation between the coolant flow rate and TEM performance is also carried out. Experimental work is also carried out to demonstrate the viability of using the heat pipes for heat transfer enhancement as well proving the viability of the design. The simulations indicate that the open circuit electric potential of the ATEM is 17% more than that of the STEM. The experimental results show that the CCTEG system performs well under various conditions. This results also demonstrate that the concept of adding heat pipes to the heat sink system is a practical solution to achieve higher thermoelectric generator (TEG) performance while maintaining the compactness of the TEG system. A heat transfer filling factor of 0.655 is achieved for the CCTEG system which is higher compared to the existing TEG systems. Moreover, a higher coolant flow rate contributes to obtaining a better performance of the TEG system. It is important to note that the introduced index can give guidance for further optimisation design of TEG systems.

Abstract Cylindrical heat pipes with sintered-grooved composite wicks are manufactured by more than 20 processes. Essential to their thermal performances are the working fluid filling and vacuuming processes. In this work, the effects of various process parameters on the thermal performance of a composite heat pipe were examined experimentally by conducting transient and steady-state tests. Under the conditions of the first vacuuming process, the effective working length showed a more remarkable effect on the start-up performance of the heat pipes than the first vacuuming time and filling ratio. The isothermal performance demonstrated sensitivity to the filling ratio. Under the conditions of the second vacuuming process, the second vacuuming temperature showed a remarkable effect on the isothermal performance. The thermal resistances were less than 0.02 K/W at the evaporator and less than 0.09 K/W at the condenser with respect to those less than 0.16 K/W after the first vacuuming process.