Abstract: A vehicle power supply system comprises an internal combustion engine (1) and a steam generator (2) arranged in a “closed system”. Water, or another suitable working fluid, is supplied to the steam generator (2) whereupon it is heated using exhaust heat from the internal combustion engine (1). The steam that is generated is stored in a storage tank (3) until a later time, e.g. when the internal combustion engine (1) is not running, whereupon the steam is used to provide heat and/or power to vehicle. For example, the steam can be used to drive a generator (5), via an expander (4), to provide electricity for powering various electrical systems within the vehicle. Additionally, the hot exhaust (7) from the expander (4) can be supplied to a radiator (8) to provide heating for the vehicle cabin.