An intermediate duct (108) is connected between first and second positive displacement machines (104, 106). An inlet duct (107) is connected to the first positive displacement machine (104). An outlet duct (109) is connected to the second positive displacement machine (106). A heater (102) raises the temperature and pressure of a gaseous working fluid in the intermediate duct (108). There is a kinematic connection (111) between the first and second positive displacement machines (104, 106) and the arrangement is such that, in operation, the first positive displacement machine (104) causes the working fluid to flow through the intermediate duct (108) to the second positive displacement machine (106), the heated working fluid drives the second positive displacement machine (106), and the second positive displacement machine (106) drives the first positive displacement machine (104) via the kinematic connection (111). The positive displacement machines include at least one orbiting piston. The heater (102) is preferably constituted by a condenser in a heat pump circuit (101-103).