<p>A system in which objects (6) are raised to an elevated height by buoyancy in a liquid contained in a tower (2) and are transferred to a receiving device (17) operable to lower the objects (6) by the force of gravity acting on the objects (6) for return to the tower (2). The receiving device (17) is provided with a power take-off device for generating electricity or operating mechanical devices. The objects (6) are introduced into a depressurised airlock chamber (24) and a pressuring gas introduced to equalise the pressures in the airlock chamber (24) and a pressure chamber (23) communicating with the lower end of the tower (2) such that the objects (6) can be inserted into the lower end of the tower (2) via the pressure chamber (23).</p>
