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Since Gibbs free energy change (ΔG) can judge the maximum ability of the system to do other useful work besides expansion work, ∆G was introduced into the general thermodynamic model of the ORC, the relationships between work, heat, exergy and ∆G were derived to judge the thermodynamic performance of ORC. And the ORC using pure fluid of R245fa and zeotropic mixture of R245fa/R227ea were selected as the study cases, analyzed the influence of different evaporation pressures, condensation pressures, heat source outlet temperatures and mass fractions of R245fa on the ∆G of the four thermodynamic processes to judge the ORC thermodynamic performances, the judgment results were compared with the calculation results. The results indicate that the change trends of net output power, thermal and exergy efficiencies judged by the ΔG are consistent with calculation results. The ability of ORC to do work increases with increase of Gibbs free energy change of heat absorption process (∆Ge), but the evaporator exergy destruction is affected by ∆Ge and working fluid entropy increase, its change depends on the specific working conditions; Output power is improved with increase of Gibbs free energy change of expansion process (∆Gt), but it will increase the turbine exergy destruction.

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