Thermal management system and process

A battery pack thermal management system and a process for providing cooling and/or heating to a battery pack are described. The battery pack thermal management system 100 comprises i) an evaporator 120 for evaporating liquid ammonia; ii) an absorber 126 for absorbing the evaporated ammonia into aqueous solution to provide a first aqueous ammonia solution 130; and iii) a heat exchange portion 102 for receiving the first aqueous ammonia solution and configured to provide cooling and/or heating to a battery pack 104 by heat exchange with the first aqueous ammonia solution. The thermal management system is configured to recycle ammonia 106 evaporated from the first aqueous ammonia solution to provide liquid ammonia to the evaporator and to recycle a second aqueous ammonia solution 132 derived from the evaporation of ammonia from the first aqueous ammonia solution to the absorber. The evaporator and absorber are preferably in thermal communication so that the evaporator can cool the absorber by heat exchange. A condenser 108, preferably a heat exchanger or compressor, may be provided in the ammonia recycling circuit. A liquid ammonia store 113 may be provided. A reactor vessel for the system, comprising absorber, evaporator and bypass chambers, is also described.