A techno-enviro-economic assessment of a biomass fuelled micro-CCHP driven by a hybrid Stirling and ORC engine

Stirling engines (SE) offer good part load performance and high heat sink temperatures which make it a suitable candidate to serve as a prime mover in micro-combined cooling, heating and power (mu-CCHP) applications. In this study, a novel mu-CCHP configuration hybridising a SE prime mover with an ORC to utilise the waste heat from the SE to produce additional power is proposed. Additional waste heat was recovered from the flue gas to dry the biomass feedstock, fire a thermal chiller and produce hot water. Further, a non-ideal thermal model was formulated and implemented in MATLAB to model the SE prime mover while the models of the other subsystems were implemented in Aspen plus (R). Also, the control of the subsystems of the mu-CCHP was achieved in MATLAB by establishing a connection between the software and Aspen plus (R). A detailed sensitivity analysis was conducted to study the influence of cooling and heating loads, rotational speed of the prime mover and quality of the biomass fuel on the energy utilisation factor, primary energy savings (PES), CO2 emissions reduction (CO2ER) and exergy efficiency of the mu-CCHP system. It was found that hybridising SE and ORC increased the power output and thermal efficiency of the standalone SE by 66% and 63.4%, respectively at its operating speed of 2500 rpm, and also improved the performance at high rotational speeds. Further, the deployment of hybrid prime movers in the design of the mu-CCHP yielded high PES and CO2ER of 55% and 43%, respectively when the system utilised woodchips fuel containing 10% moisture. The proposed energy system performs better than conventional energy systems producing only one energy vector over a wide range of engine frequencies, cooling ratios and woodchips compositions.

Automated summary

The study proposed a novel mu-CCHP configuration that hybridizes a Stirling engine with an ORC to utilize waste heat for additional power generation. Through detailed sensitivity analysis, it was found that the hybridization of SE and ORC significantly improved energy utilization and reduced CO2 emissions in the system.