Optimization of small-scale solar nanofiltration systems for water purification and energy production

Low population density in remote areas makes universal provision of electricity and potable water from central infrastructure economically infeasible. Alternatively, off-grid energy and water purification can be provided by solar-nanofiltration (SNF) systems designed to serve individual or co-located homes. Here, we present a cost analysis for optimized SNF systems to desalinate water and provide excess energy. System design and operation were numerically simulated and optimized to minimize system costs while satisfying a series of constraints on water production and reserved energy at two locations (Tucson, AZ and Seattle, WA) during winter. Water production constraints ranged from 0 to 300 gal per day, and daily excess energy provision for nighttime energy use from 0 to 2 kWh. Present value costs proved more sensitive to the constraint of nighttime energy than water production requirements, primarily due to the high cost of energy storage and the proportionality between energy storage and energy demand. At a water production target of 200 gpd (0.76 CMD) and excess energy demand of 2 kWh, the unit cost of water produced was $0.47/100 gal ($1.24/m³) in Tucson and $0.54/100 gal ($1.43/m³) in Seattle. This suggests that SNF is an economically feasible alternative for water purification in areas where centralized water treatment and energy provision are unavailable.