Hydrogels, such as poly(acrylic acid), are commonly used in absorbent materials (like baby diapers) as they swell in water to many times their size when dry (Figure 1). However, they also swell to larger sizes in fresh water (little salt) and shrink in saltier water. The amount of swelling is reversible, and thus they can expand and contract when exposed to fresh or salt waters.

Figure 1. Photos of poly(acrylic acid) hydrogels in dry, after swelling in 35 g/L NaCl, and after swelling in 0.35 g/L NaCl. (Photograph credit: Xiuping Zhu and Xiaoyuan Zhang)

Based on this idea of the hydrogels swelling in non-salty and shrinking in salty water, we invented a cyclical process to extract energy from salinity differences between river water and seawater through mechanical processes we call the “Hydrogel Expansion” (HEx) process (Figure 2). The feasibility of this new approach was tested using hydrogel particle diameters of different diameters, and varying the mass of the gel used, the solution salinity ratios, solution flow rates, and the weights of the external load. Based on these tests we were able to extract as much as 124 mJ of energy in a 1 h test (0.36 g of hydrogel, particles 300 to 600 μm in diameter, salinity ratio of 100, external load of 210 g, water flow rate of 1 mL/min). Although these energy recovery rates were relatively lower than those typically obtained using PRO, RED, or CapMix, the costs of hydrogels are much lower than those of membranes used in PRO and RED. In addition, fouling might be more easily controlled as the particles can be easily removed from the reactor for cleaning. Further development of the technology and testing of a wider range of conditions should lead to improved energy recoveries and performance.

Figure 2. (A) Schematic of HEX and (B) a photo of the HEX reactor. (Source: Zhu, X., W. Yang, M.C. Hatzell and B.E. Logan. 2014. Energy recovery from solutions with different salinities based on swelling and contraction of hydrogels. Environ. Sci. Technol. 48(12):7157-7163.)