The high rate of oil consumption and air pollution problems have stimulated many activities directed toward the development of electric vehicles (EVs). Other than the propulsion system, the climate control system usually consumes more energy than the other accessories in electric vehicles, which, in turn, reduces the vehicle range, particularly in cold weather. For example, GM estimates that the Impact would lose 85% of its range in 0 F weather if no thermal management system were used (GM, 1994). To minimize the loss in range, the energy consumption from the battery must be minimized. The goal of the present work is to develop a thermal storage system using phase-change material (PCM) as a storage medium for EVs. The use of this system has the advantages of negligible use of battery energy, no tailpipe emissions, and less weight than batteries and resistance heaters.
The specific objective of this work is to design a space heating system for electric vehicles which has minimum weight, volume, cost and environmental impact relative to electric-resistance heaters and fossil-fueled heaters. The prototype of this system is to have sufficient capacity to meet the heating load of 800 W over a 5-hour period for an existing twenty-two-foot electric bus, which has been "winterized" with added insulation, multi-pane windows, air sealing, and other conservation measures.
Calcium chloride hexahydrate was chosen as the storage medium, and the phase change material was contained in high-density polyethylene (HDPE) bottles. Simple steady-state heat loss equations based on existing correlations were used to design a prototype of the heating system, which we manufactured and operated in an electric bus for several weeks. The prototype contains two shelves of PCM storage containers (with a third level for future expansion) as well as an electrical resistance coils and fans to circulate heat for charging at night or between runs. The unit has an aluminum shell 2 ft. by 2 ft. by 2 ft, has two inches of foam insulation, and can hold roughly 10 kWh of heat energy. The heater is about half the weight of the equivalent batteries for the same energy output.
Performance tests showed successful operation in general but indicated a need to have better control of the rate of heat output. Evidence was found of air bypassing contact with the PCM storage containers by leaking past one or two layers, thus diminishing efficiency.
A time-dependent, two-dimensional, finite-difference mathematical model of the heat-storage/delivery system was developed in order to improve the thermal performance of the proposed system. The model has been used to estimate storage bottle spacing and airflow rates needed to achieve different output heat rates. Also, to improve performance, we recommend a single-level PCM storage unit design to avoid air leakage from level to level in the existing system. The results of this study are useful not only for vehicle space heating for also for battery thermal management and in addition for solar energy storage systems in general.
For more information contact John_Duffy@uml.edu
