A framework for blue energy enabled energy storage in reverse osmosis processes

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Reverse osmosis (RO) is perhaps the most promising desalination technology, but it is facing growing economic challenges when coupled to an intermittent energy supply, such as renewable energy-based grids. A technology with similar components to reverse osmosis is pressure retarded osmosis (PRO), which produces energy from differences in salt concentration (blue energy). However, with the increasing cost-competitiveness of wind and solar photovoltaic renewable energy, PRO faces severe technoeconomic challenges as a stand-alone energy technology. To address cost and energy challenges of both RO and PRO, we propose a framework to intermittently operate RO with an instantaneous, electricity-producing PRO mode by taking advantage of the component overlap and existing infrastructure. Here, we analyze the process and component efficiencies associated with a hybridized RO-PRO system and outline the associated technoeconomic limitations. This approach considered operating PRO by mixing seawater RO brine with either RO permeate or a cheap low salinity water source. The results show that a hybridized system may make PRO viable in a niche subset of conditions, including a combination of relatively cheap water (<$1/m3), pricier electricity (>$0.15/kWh), and with electricity pricing schemes having large differences between lowest and highest prices. This hybridization may effectively allow a hybrid PRO-mode system to act as a cheaper, yet less efficient, salinity gradient energy storage mechanism. However, the economics are extremely challenging unless there is inexpensive, or even free, low salinity water source (e.g. treated wastewater) available.