Performance investigation of a combined heat and power system with internal and external combustion engines

In this paper, a new model of a combined heat and power system with hybrid prime mover including an internal combustion engine and an external combustion engine (Stirling engine) at different rotational speeds of the internal combustion engine is investigated. The recommended system can be used in building applications. Therefore, an internal combustion engine is used as the main prime mover of the system and for optimal using of the heating value of fuel, dissipated heat from its exhaust gas is used as the heat source of Stirling engine. Also, for estimating the performances of internal combustion and Stirling engines, a mathematical model called zero-dimensional single-zone and a numerical model called non-ideal adiabatic are used, respectively. Also, the combined production system has been modeled from viewpoints of thermal efficiency, overall efficiency, primary energy consumption, carbon dioxide emission and fuel consumption costs at different rotational speeds of the internal combustion engine and analyzed based on internal combustion engine (solely) and based on two engines (hybrid prime mover). Moreover, this system has been compared with a conventional energy production system for the building. Furthermore, a payback period in various working conditions for the system is calculated. Modeling results showed that using the hybrid prime mover led to a considerable increase in power output, thermal efficiency, and primary energy saving. Also, by using the hybrid system, the carbon dioxide emission and payback period had a dramatically reduction. In addition, for the combined production system with the internal combustion engine at rotational speed of 3500 rpm and operation of 9 h per day, the payback period was 10.5 years and for hybrid prime mover at this operation condition, it reduced to 7.5 years.