Stirling engine performance enhancement by axial variation of the regenerator matrix structure – Experimental validation

The regenerator is the critical component for achieving high efficiencies in Stirling engines. One promising approach to improve the performance of regenerators and thus that of Stirling engines is the axial variation of the regenerator matrix structure, which was theoretically demonstrated by optimizing the metal felt regenerator of an experimental Stirling engine with a numerical model. This contribution now presents the experimental validation of the performance enhancement by this new regenerator design. A prototype matrix consisting of three sections, with a porosity of 70.5%–76.4%-85.8% and a fiber diameter of 177 μm–126 μm-55 μm viewed from the cold to the hot end, is realized and deployed in the engine. For assessing the thermal performance of the regenerator, a recently presented method for the direct determination of the thermal regenerator loss using high-speed gas temperature measurements is used. The engine is investigated over a wide range of operating points, varying the speed between 200 rpm and 383 rpm, the mean pressure between 20 bar and 38 bar and the heater temperature between 400 °C and 500 °C. Its performance using a regenerator with axial constant properties is compared to that with the optimized prototype. The thermal efficiency of the Stirling engine is increased by 0.88% while maintaining a constant mechanical power at the nominal operating point of the engine. This efficiency gain in the experiment even exceeds that of 0.38% predicted by the model. At operating points with smaller power densities, the experiments confirm the predicted decreasing efficiency gains compared to the nominal operating point. At the lowest power densities, the experimental efficiency is actually decreasing, whereas the numerical model still predicts small efficiency gains. These observations are analyzed and discussed in this contribution.