or with regular periodicity at the input heat exchanger 24 while the ambient low temperature heat source transfers thermal energy to the input passageway 44 of the cold level heat exchanger 40 cycling of the hot and cold displacers 14 16 then acts in accordance with the vuilleumier cycle to establish a thermal gradient along the length of the regenerator 30 the extreme levels are controlled in general terms by the lower temperature t c established by the ambient heat source at the passageway 44 and by the higher level t h controlled by the thermal energy source 28 the temperature level t m in the intermediate chamber 18 varies about an intermediate temperature level related to the temperature in mid region 32 of the regenerator 30 this intermediate level temperature is controlled by the temperature conditions at the output passageway 38 from the intermediate level heat exchanger 36 the tendency of the cold chamber 22 to go colder is limited by the low temperature ambient heat source and similarly any tendency of the hot chamber 20 to go colder is limited by the high temperature heat source at this point it can be seen that the overall structure defines a heat driven heat pump system and particularly that apart from the minor amount of mechanical work input incidental to movement of the displacers there is no need for a prime mover driving a separate cycling system for a thermodynamic process to.