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

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 ($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. [Display omitted] •Explores integrating continuous RO and PRO with overlapping components•High salinity PRO with SWRO brine may be economical in niche market conditions.•Membrane properties create a trade-off for optimizing bimodal system performance.•Hybridization could allow RO to economically do demand response and energy storage.