Thermo-ecological assessment of Stirling engine with regenerator fed with cryogenic exergy of liquid natural gas (LNG)

The paper presents a thermo-ecological analysis of a liquefied natural gas regasification system equipped with a Stirling engine. The proposed system includes an alpha configuration machine, working as a cold engine with the heat sink acting as the evaporator for the LNG and the upper heat source being at ambient temperature (sea water) or moderately higher. The engines are modelled using two approaches developed by the authors, that is a CFD model and a second-order one-dimensional model. First a small engine is modelled by both CFD and the second order approach to compare results and after confirming their similarity, due to a lower computational cost a larger machine is modelled with the second order model. Due to the heat transfer conditions being crucial for a Stirling cycle machine's performance, those are investigated. It is shown that the internal, convective heat transfer coefficients are much higher in cryogenic conditions than in typical ones. The results of engine assessment are used for the Thermo-ecological cost (TEC) analysis. Within the thermo-ecological analysis, the influence of selected parameters is demonstrated. Additionally the TEC of electricity generated in cold power plant is presented for different configurations. In the case of analysed Stirling systems savings of natural gas reach the level of only 0.5%. However, when TEC of electricity is calculated, the savings are at the level of 72%. The proposed theoretical investigations are the base for the acquisition of new knowledge in the field of possible ways of converting cryogenic exergy of LNG into electricity and optimization of this cycle with strong focus on ecological effects, including the TEC.