Modelling and simulation of Stirling engine for micro-cogeneration

Abstract

Micro-cogeneration, specifically the simultaneous generation of thermal and electrical energy in residential buildings below 10 kW_e provides an attractive option to reduce the environmental burden. In particular Stirling engines, originally invented in the year 1816, are emerging on the market to challenge conventional oil- or gas-powered heating systems. Characteristic strengths of external combustion engines are their high energy efficiency, low emissions, fuel flexibility, and operation with low noise and vibration. However, viable operation regarding primary energy demand, C0₂ emissions, and economic costs requires a carefully optimised operational strategy that is sensitive to the energy mix, building type, and climate. A whole-building-oriented simulation model is typically needed to discover the most energy-efficient system topologies.

In this work the IDA-ICE building simulation program is employed to assess a Stirling engine micro-CHP device following the model specifications of IEA/ECBCS Annex 42. The simulation routine implemented, which has been validated through inter-program comparison, accounts for the dynamic effects of micro-cogeneration devices, such as warm-ups and shutdowns. The study contributes to the improvements obtained by adaptation to hourly changes in the energy generation mix and the utilisation of thermal exhaust through heat recovery.

The results suggest that a Stirling engine for micro-cogeneration can most viably be operated in a cold climate together with a fossil fuel-based energy mix. However, the attractiveness of a micro-CHP plant can only be preserved with efficient exhaust gas heat recovery. With the aid of optimally chosen operational strategies, exhaust gas heat recovery, and relevant thermal storage, the micro-cogeneration system generates savings of 3 - 5 % in primary energy consumption and C0₂ emissions. The configuration investigated is, moreover capable of creating annual operational savings for any combination of fuels and electricity prices between 0.05 euros/kWh and 0.15 euros/kWh. Financially, a Stirling engine for micro-cogeneration can most viably compete against electric and oil-powered furnaces for hydronic heating systems. The results are not yet able to be generalised for buildings in warmer climates due to the increased imbalance between the electrical and thermal energy demand.