A novel thermal energy storage based air conditioning system for rail transport

[摘要] Thermal energy storage system using phase change materials (PCMs) has a potential for a wide range of applications, particulary dealing with the mismatch between energy demand and supply. The work reported in this PhD thesis concerns the use of PCM in temperature regulation in cabins of rail transportation. The focus of the work is on: (a)enhancing the heat transfer from heat exchanger side by using extended surfaces(fins), and (b)PCM side by using additives. Air was used as heat transfer fluid as such a fluid can be used for air circulation. Both modelling and experimental work was carried out on the PCM based heat exchanger.The modelling results show that the heat transfer coefficient can be significantly improved by the presence of fins with the charging and discharging durations reduced by ~ 85% and 74% respectively. The modelling results revealed that the use of fins in airside is more efficient than that in PCM side. In the absence of the fins in the PCM side, the same level of enhancement of heat transfer can be achieved by increasing thermal conductivity of the model PCM by 4-5 times. The modelling results were validated by experimental data. Based on the experimentally validated modelling results, an air-conditioning system consisting of a PCM heat exchanger was designed, constructed and tested in the lab. The results show that, for both charging and discharging processes, the heat transfer performance of the system is significantly improved with fins used in both air and PCM sides, leading to the increased thermal comfort, decreased outlet temperature fluctuation, an excellent heat transfer rate, a large energy storage capacity and a long working time.Based on the above work, a novel compact thermal energy storage (TES) device containing a commercial PCM (RT 18 HC) was designed and experimentally investigated with an aim to improve thermal comfort and smooth cooling load of a rail air conditioning system. Two different types of fins were employed to in the TES device for the thermal performance enhancement, one is with serrated fins used in the air side and other is perforated straight fins employed in the PCM side. The time evolutions of PCM temperature during the discharging process of both in the axial and radial directions were presented. The discharging time, discharging depth, discharging power, thermal efficiency and exergy efficiency were studied as function of inlet air temperature and velocity. The results showed that hot air in the cabin could be cooled down to the desired temperature range of 16-20oC in seconds. Both the energy and exergy analyses revealed that the designed device performed excellent discharging depth, higher than 97%. The results also showed favourable air temperature around 18oC for extended period of time, a flexibility for cooling load adjustment and had an ability to smooth out the frequent fluctuations of cooling load of transport compartments.The charging behaviour was then investigated and the effects of charging time, transient charging rate, overall thermal efficiency and exergy efficiency at different inlet air temperatures and velocities were examined. The time evolutions of PCM temperature in both the axial and radial directions were also measured. The dynamic exergy efficiency was studied for the first time to assess the optimal charging depth and charging time. The experimental results showed that the designed storage device had a flexible charging rate with the maximum rate at 1.3 kJ/s, a high thermal efficiency of 87% and an overall exergy efficiency of 70%. Decreasing the inlet air temperature or increasing the air velocity shortened the whole charging time, which varied from 50 to 174.2 mins. The analysis of the dynamic exergy efficiency showed that the maximum exergy efficiency was around 90% with the optimal charging time varying from 10 to 52 mins. The corresponding optimal charging depth increased with increasing inlet air temperature or velocity, ranging from 46% to 58% under the studied conditions.Further analyses were performed on the heat transfer characteristics of the storage device for transport air conditioning systems. The charging and discharging times, the transient heat flux and the heat transfer coefficient were obtained. Comparison of the overall heat transfer coefficient for the charging and discharging processes was used to explain the difference between the charging and discharging kinetics. The variation in Nusselt number with Reynolds number under different Prandtl number for the discharging and charging processes were established.Such an empirical relationship was experimentally validated. Validations between the empirical and experimental Nusselt number were carried out. The average error for the charging and discharging processes between the empirical and experimental Nusselt is 2.33% and 1.65% respectively which shows a good reliability. This indicates that the obtained equations can be used for the design of such novel TES device for transport air conditioning application. Finally, the PCM based storage device integrates with a conventional air conditioner(AC). Comparisons were carried out between the AC and PCM-AC, with respect to the space temperature fluctuation, the coefficient of performance (COP), energy savings, and emergency ventilation/cooling. The experimental results show that, compared with the AC, space temperature fluctuation of the PCM-AC is decreased to 2.56°C; the ON-OFF cycles of the compressor in the PCM-AC are reduced by 27%; the overall COP is increased by 19.05%; the emergency ventilation and cooling time is prolonged by up to 9 times. What’s more, the economic evaluation shows that the electrical cost of the PCM-AC could be saved by up to 17.82%; the payback period is around 3.3 years and can be reduced to 1.83 years when using industrial grade PCM.

[发布机构] University:University of Birmingham;Department:School of Chemical Engineering

[关键词] T Technology;TD Environmental technology. Sanitary engineering