A free-piston Vuilleumier heat pump has pistons with a spring-driven resonance system to control the heat and cold energy output, the strokes of these two pistons being controlled. This is done by controlling the working gas temperature in a hot chamber. It can also be done by controlling the motor output signal while the operating frequency of the motor, assigned to at least one of the pistons, is held near the resonance frequency which is determined by the driving system of both pistons. Only with this sort of pump, which uses the spring-driven resonance system for reciprocal movement, is an exact output setting possible to match the hot and cold loads.
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