Ammonia, R717, has been considered as a useful working fluid of the organic Rankine cycle (ORC) in the OTEC. The aim of this study is to investigate the potentials of R717 blends on thermodynamic and economic performances to surpass pure R717 used in an OTEC. The environmentally friendly refrigerants of R32 and R1234YF are considered and selected as the components of R717 blends. The effects of mass fraction ratios for R717 blends and four pinch points of conditions for the ORC are applied to evaluate the net power output and energy cost of the OTEC.
The performances of OTEC system with R717/R32 and R717/R1234YF are evaluated and compared with pure R717. In thermodynamic improvement, the maximum net power outputs of R717/R32 and R717/R1234YF are higher than pure R717 by 18.9% and 12.23% with mass fraction ratios of 0.21/0.79 and 0.41/0.59, respectively. In addition, R717/R32 and R717/R1234YF blends with mass fraction ratios of 0.4/0.6 and 0.62/0.38 possess higher optimum economic performance than pure R717 by 3.45% and 2.1%, respectively. Furthermore, the optimum mass fraction ratios for economics are both higher than those for thermodynamics as R717/R32 and R717/R1234YF used in OTEC.