Experimental and numerical study of a micro-cogeneration Stirling engine for residential applications

Micro-cogeneration Stirling engines are considered promising for residential applications. The present work covers the experimental and numerical analysis of a commercial Stirling unit capable of 8 kW of hot water and 1 kW of electricity. A previously concluded experimental campaign that focused on external measurements is extended here to include internal measurements. The scope is collecting useful data to validate a detailed numerical model. Three test cases are considered by fixing the temperature of the cogeneration water at the unit inlet at alternatively: 30, 50 and 70°C. Mass flow rate of the water is kept at the nominal value of 0.194 kg/s. This numerical model is an extension of the well-known work by Urieli and Berchowitz. The model is calibrated on the 50°C case and compared in the other two cases. Maximum deviations with respect to experiments are about 4% on net power output, whereas they remain below 1% on heat input and rejection. The Stirling unit has shown an electrical efficiency exceeding slightly 9% and a thermal efficiency of 90% (both based on the Higher Heating Value) if the cogeneration water inlet temperature is 30°C, which decreases down to about 84% with water inlet at 70°C. The Primary Energy Index is remarkably positive for all cases, ranging from 17% to 22% as the temperature of the water inlet goes from 70°C to 30°C.