Multi-objective optimisation of a geothermal-liquefied natural gas combined organic Rankine cycle system under different distribution pressures

Multi-objective optimisation is drawing more and more attention in the organic Rankine cycle research field due to its comprehensiveness in demonstrating optimum results from multiple perspectives. For this reason, a comparative two-objective optimisation is performed on a combined organic Rankine cycle system liquefied with geothermal among three typical distribution pressures: 1 MPa, 4 MPa and 7 MPa. Eight operational parameters, 11 working fluids, and 16 possible configurations are included during optimisation. The optimisation result is divided and analysed on the basis of the thermodynamic weight (W 1). The results reveal that the recuperative organic Rankine cycle configuration is preferred when W1 is higher, while the basic organic Rankine cycle configuration is preferred when W 1 is lower. Two typical cases are further analysed in detail: the balanced case (W 1 = 0.5) and the maximum case (W 1 = 1). The system is able to yield the maximum energy efficiency of 32.78% with the working fluid Propane under the maximum case with a distribution pressure of 1 MPa. Compared to the maximum case, the balanced case is capable of achieving considerable economic benefits while incurring minimal thermodynamic losses. When the distribution pressure is 7 MPa, the balanced case can achieve as much as 82.94% of the energy efficiency compared to the maximum case, requiring only 44.17% of the electricity production cost.