A thermally driven gas resonance device comprises a resonance tube (3) which expands in cross-section along its length from one end to the other, a heat source (2) located at the one end of the resonance tube, and means (2, 4) to trigger oscillations in a gas in the resonance tube. The heat source (2) is preferably a pulsed heat source having a pulse repetition frequency corresponding to a resonant frequency of the gas resonance tube (3). The mechanical energy produced in the oscillating gas in the gas resonance device may be used to operate a pressure swing gas separator by including a bed (16) of molecular sieve material in the other end of the resonance tube (3). Alternatively, the mechanical energy may be used to drive a heat pump (19). In this case a heat sink (21) is located at the other end of the resonance tube (3), a regenerator (20) is also located adjacent the other end, and means (8) on the side of the regenerator (20) towards the heat source (2) effect heat exchange between the gas in the resonance tube (3) and a source of low grade heat.
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