River-to-sea pressure retarded osmosis: Resource utilization in a full-scale facility

• A facility-scale simulation model for the power production of non-ideal river-tosea PRO was developed. • The model was linked to an optimization package to determine the system variables that produce a maximum net power for the facility. • Energy requirements of the components external to the PRO modules were quantified by analyzing state-ofart technologies. • In order to maximize resource recovery, a low power density must be accepted.

In this investigation, the net specific energy consumption of a riverto-sea PRO facility was evaluated using a novel simulation and optimization method that integrates parasitic loads and efficiencies of the PRO facility components. Although this PRO configuration represents a source of renewable energy, the overall specific energy that may be recovered is quite low at approximately 0.1 kWh per m3 of fresh water when all the parasitic loads are accounted for. Freshwater and seawater pretreatment and conveyance were found to be the main parasitic loads. These loads could perhaps be reduced with technological breakthroughs in membrane materials and antifouling properties [43-44] that would allow for lower energy requirements for pretreatment. Even with a “perfect” membrane, the maximum specific energy recovered could be approximately 0.4 kWh per m3 of fresh water. To put this in perspective, a household in the United States generates approximately 1 m3 of wastewater per day, and at the same time consumes approximately 30 kWh [45]. By contrast a photovoltaic panel of 0.40 m2 would generate 0.4 kWh in a day [46]. On the other hand, PRO could have niche applications when high salinity brines can be utilized, as the specific energy linearly increases as the osmotic pressure difference between the solution increases (i.e., double specific energy with double osmotic pressure difference). For example, PRO may help reduce seawater desalination energy consumption in the so-called reverse osmosis (RO)-PRO system [8,47]. This system, in addition to the elevated draw solution salinity would also have reduced parasitic loads compared to the river-to-sea configuration as the draw solution is already pretreated before RO and the turbine-generator is not employed. PRO may also have application in a closed-loop configuration to convert waste heat into useful work [48]. In this configuration, elevated draw solution salinity would be beneficial to convert energy from the waste heat at higher rates, making the system more compact and more efficient. Acknowledgments The authors thank the Humboldt State University Environmental Resources Engineering students from the Fall 2013 Senior Capstone Design course, T.K. Williams, the Humboldt Bay Municipal Water District (HBMWD), and the Humboldt Bay Harbor, Recreation & Conservation District for their help in the facility analysis. Lori Jones was partially funded by the USDA National Food and Agriculture Grant no. 201138422-31204.