Synergetic clean energy production: solar power and low-grade heat assisted osmotic power generation from salt and fresh water

Abstract A clean and stable power generation system based on solar energy, low-grade heat, and salinity gradient energy is proposed. In the proposed system, fluctuating solar energy is used to enhance the natural concentration difference between fresh and salt water through a solar engine driven reverse osmosis process, and low-grade heat is further used to increase the temperatures of the fresh and concentrated salt water streams. An enhanced and stable salinity gradient power output is then generated by pressure retarded osmosis. Using validated mathematical models, the energy conversion performance of the integrated system under daily condition was systematically evaluated. The results show that the open-loop fresh and salt water configuration achieves a higher overall energy storage efficiency, with a maximum value of 19%. Under the optimal reverse osmosis operating pressure of 3.90 MPa and with sufficient membrane area, enhanced effluent solutions with stable concentrations of 0.8 M and 0.004 M can be produced during daytime operation. When the pressure retarded osmosis inlet solutions are further heated to 60 °C, the power output increases from natural 2.89 kW to enhanced 5.51 kW, with solar energy and low-grade heat contributing 22% and 25% of the total output respectively. Graphical Abstract