Energy, exergy and economic (3E) analysis of solar thermal energy assisted cascade Rankine cycle for reverse osmosis

Abstract

A novel cascade Rankine cycle is proposed for treating brackish groundwater using a reverse osmosis system. The cascade RO system is arranged in a loop with a steam Rankine cycle (SRC) at the top and an organic Rankine cycle (ORC) at the bottom to provide high recovery, electricity-free, and scalable options. The system comprises a solar Scheffler dish for heat input, steam turbine and expander for work output, evaporative condenser and ORC condenser for heat rejection, SRC pump, ORC pump and RO high-pressure pump for work input devices, and RO module for water desalination. This study evaluated the thermodynamic design of the system along with energy, exergy, and economic analyses performed by considering eight working fluids such as R245fa, HFO1336mzz(Z), R1225ye(Z), R1224yd(Z), R1233zd(E), R1243zf, R1234yf, and R1234ze(E). The highest overall efficiency was found 14.08% with R1234yf, and the highest RO permeate flow rate was found with R245fa, HFO1336mzz(Z), and R1233zd(E), which require the lowest mass flow rate of ORC working fluid. The highest exergy destruction was found in the solar collector and RO unit. The treated water cost is estimated to be 0.89–0.924 $/m3 of permeate water.