Linear Stirling Engine with a Buffer Tube

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Domestic CHP systems are an obvious way of both generating electricity with a high efficiency and reducing strain on the grid and local distribution systems. The move to electric and plug-in hybrid vehicles will place additional demand on the grid and overload the local electrical distribution system that can currently only cope with about 10% of households recharging vehicles. With domestic CHP systems a different mind-set is needed as it is necessary to consider the electricity generation to be a by-product of the heating demand, as the electricity can be exported. Although a small domestic boiler might have a rating of 12 kW the heating demand in the summer is of course much smaller and this leads to a lower power (but high efficiency) requirement for the Stirling engine. Consider the following example which assumes a baseline efficiency for a conventional boiler of 90%:For an electrical output of 1 kW the 'indicated power' of the Stirling engine would need to be 1.3 kW (to allow for losses that are mostly electrical losses in the generator and power electronics). With a pessimistic 28% efficiency assumption (Net W[e] out/Heat in), this will require a heat input of 3.6 kW, with 4 kW of fuel energy. The waste heat from the engine will provide 2.3 kW for domestic heating, and in a conventional boiler this would have required 2.6 kW of fuel energy. So, 1kW of electricity has been generated from an increased fuel energy consumption of 1.4 kW (= 4.0 - 2.6); an overall electrical efficiency of 71% assuming the heat is needed. This is about double the efficiency of a conventional power plant, once allowance is made for the grid transmission efficiency.The ultimate aim is for a Stirling engine with an electrical output of at least 1 kW, but as a demonstration unit the current work will produce a Stirling engine with an electrical output of 100 W. This smaller size has been chosen because we have a moving magnet motor of this rating that can be used as a generator. This will avoid the need to scale-up the motor design and will give a significant reduction in the project cost. This 100 W system will be large enough to install pressure transducers, thermocouples and displacement transducers, and the experimental data can be used to validate the modelling, so that there will be confidence in the model predictions of the larger engines. The smaller size will also reduce the manufacturing costs. Electrical heating will facilitate accurate measurements of the heat input, and avoid the need to develop a combustion system. Longer term, a catalytic combustion system would operate at a sufficiently low temperature so as to make NOx emissions negligible, and be suitable for a range of gaseous fuels.The attraction of CHP systems has already led to small linear Stirling generators being developed (e.g Sunpower/Microgen and Infinia/QEnergy systems). Although the idea has been well demonstrated these technologies have not been successful due to high ownership costs and reliability issues. The low cost manufacture of conventional displacer configurations is extremely challenging. A very significant benefit of the research proposed here will be the demonstration of a new engine configuration that radically simplifies the design and manufacture of the displacer - a key component. The cost reductions possible will greatly enhance the prospects of Stirling CHP systems.The US Department of Energy has recently funded several Stirling engine projects for domestic CHP (https://arpa-e.energy.gov/?q=news-item/department-energy-announces-18-new-projects-accelerate-technologies-efficient-residential). Although the mass market is envisaged to be domestic CHP there are other niche markets for silent power generation that can be exploited, and these would support greater costs associated with small scale manufacture. Examples of this include auxiliary power generation on yachts and military applications.