A gas bearing for the axially reciprocating displacer and power piston members in a free piston Stirling engine includes a bearing sleeve having a pair of interconnected annular pressure plenums formed by ribs on the outer surface of the sleeve which form a seal with the inner walls of a cylinder in which the sleeve is seated. The outer side of the raised ribs provide a second set of interconnecting drain plenums. A source of a high pressure gas, pressurized internally by the engine itself, is connected to the pressure plenum which communicates with the axially reciprocating members through a multitude of narrow orifices. The bearing gas is drained into the second or drained plenum through a large number of larger sized orifices.

Stirling engine displacer gas bearing

A free-piston Stirling engine usable as a heat pump has a closed vessel filled with helium working gas which is heated at the bottom end and cooled at the top end. The vessel contains a displacer supported for axial reciprocal oscillation on a gas spring post mounted on the vessel. The displacer shuttles the working gas from end to end in the vessel, alternately heating and cooling the gas. The vessel is sealed with a flexible diaphragm which flexes in response to the pressure wave generated in the vessel as the working gas is alternately heated and cooled. When the diaphragm flexes, it displaces hydraulic fluid in a hydraulic chamber and drives a power piston for driving a linear alternator and a gas compressor. A gas spring operating on a second hydraulic cylinder on the other side of the power piston stores part of the energy of the piston stroke and returns it for the return stroke. Controls are provided for balancing and controlling the hydraulic fluid pressure, for starting the Stirling engine, and for modulating its power output.

Diaphragm coupling between the displacer and power piston

Stirling engine control system

A paper presented at the 1974 Conference (Ref 1) described the development of a Stirling-cycle thermo-mechanical generator (TMG) for providing small amounts of electrical power continuously over long periods, while requiring much less fuel than other power sources running from hydrocarbon fuel or radio-isotopes. Two of these 25-Watt generators, fuelled by propane, have been used to power the UK National Buoy on two successive missions. A total of more than three years experience at sea has now been accumulated. In addition, a 60-watt version has provided the power for a major lighthouse for more than a year. An early development version of the Thermo-mechanical Generator, adapted to run from the heat of a radio-isotope source, was loaded with strontium-90 titanate in October 1974 and has run continuously in the laboratory ever since. The Paper describes the improvements and changes found necessary in the course of 90,000 generator-hours of running time, and outlines the improvements in operational performance and reliability which have resulted.

Small Stirling-cycle Power Sources in Marine Applications

The paper deals with primary power sources for remote or inaccessible installations, requiring continuous power of tens or hundreds of watts electrical. It is shown that the relatively high conversion efficiency from heat to electricity which is provided by the Stirling-cycle TMG can reduce the weight of fuel required at the site to about a quarter of the weight of equivalent primary batteries, with resultant operating economies. Two installations are in service in the field, and operating experience is being accumulated. The TMG can also be used with radio-isotope heat sources, to provide power unattended for many years, as may, for example, be required by undersea repeaters. Again the relatively high efficiency of the TMG offers advantages of economy over the radioisotope thermoelectric generators used hitherto in undersea applications.

Stirling-Cycle Thermomechanical Power Sources for Remote or Inaccessible Communications Sites

The thermomechanical generator uses a thermomechanical oscillator to convert heat efficiently into a mechanical oscillation which in turn excites a suitable transducer to generate alternating electricity The thermomechanical oscillator used is based on the Stirling cycle, but avoids the need for rotary motion and for sliding pistons by having a mechanically-resonant, spring- suspended displacer, and by using an oscillating metal diaphragm to provide the mechanical output The diaphragm drives an alternator consisting of a spring- suspended permanent magnet oscillating between fixed pole pieces which carry the electrical power output windings Because a thermomechanical generator is much more efficient than a thermo-electric generator at comparable temperatures, it is particularly suitable for use with a radioisotope heat source The amounts of radioisotope and of shielding required are both greatly reduced A machine heated by radioisotopes and delivering 107W ac at 80Hz began operating in October, 1974 Operating experience with this machine is reported, and these results, together with those obtained with higher-powered machines heated by other means, are used to calculate characteristics and performance of thermo- mechanical radioisotope generators capable of using heat sources such as the waste-management $sup 90$Sr radioisotope sources becoming available from the US nuclear waste management programme Amore » design to use one of these heat sources in a 52-W underwater generator is described (auth)« less

F.W. Yeats

Papers

Efficient thermo-mechanical generation of electricity from the heat of radioisotopes