Poly(acrylic acid)-based hydrogels can swell up to 100-1000 times their own weight in desalinated water due to osmotic forces. As the swelling is about a factor of 2-12 lower in seawater-like saline solutions (4.3 wt% NaCl) than in deionized water, cyclic swelling, and shrinking can potentially be used to move a piston in an osmotic motor. Consequently, chemical energy is translated into mechanical energy. This conversion is driven by differences in chemical potential and by changes in entropy. This is special, as most thermodynamic engines rely instead on the conversion of heat into mechanical energy. To optimize the efficiency of this process, the degree of neutralization, the degree of crosslinking, and the particle size of the hydrogels are varied. Additionally, different osmotic engine prototypes are constructed. The maximum mean power of 0.23 W kg-1 dry hydrogel is found by using an external load of 6 kPa, a polymer with 1.7 mol% crosslinking, a degree of neutralization of 10 mol%, and a particle size of 370-670 µm. As this is achieved only in the first round of optimization, higher values of the maximum power average over one cycle seem realistic.
Keywords: energy recovery; hydrogels; osmotic engine; poly(acrylic acid); polyelectrolytes.