Integration of biodiesel internal combustion engines and transcritical organic Rankine cycles for waste-heat recovery in small-scale applications

The aim of the work is the analysis of a novel integrated energy system for small-scale combined heat and power (CHP) generation. The system consists of a topping biodiesel-fired internal combustion engine (ICE) and a bottoming transcritical organic Rankine cycle (TORC) for waste-heat recovery. Specifically, the engine exhaust gas provides energy to the TORC sub-system, while the energy contribution of the ICE cooling circuit assures low-temperature heat generation. Furthermore, a thermal energy storage (TES) unit for the exploitation of the thermal surplus and an auxiliary boiler are present. A mathematical model is developed to evaluate the main performances at full and partial load in terms of thermal and electric production, efficiency, fuel consumption, and primary energy saving. A preliminary analysis is carried out to find the proper organic working fluid for the TORC sub-system. Afterwards, the biodiesel ICE-TORC combined system is adopted to satisfy thermal and electric requests of a commercial centre. To this purpose, hourly energy balances are evaluated, and a techno-economic analysis is performed on an annual basis. The investigated system guarantees a 16.7% primary energy saving and an 8.4 years payback time.