Thermodynamic analysis and material design to enhance chemo-mechanical coupling in hydrogels for energy harvesting from salinity gradients

Abstract

The coupling between solution salinity and the mechanics of charged hydrogels presents an opportunity to harvest osmotic energy in a clean and sustainable way. By applying mechanical pressure to retard the swelling or deswelling of hydrogels in saline solutions, the free energy of mixing is converted to mechanical work. This study developed a theoretical framework and experimentally investigated the potential of hydrogels for energy production from salinity gradients. Mathematical modeling revealed the effect of parameters including the charge and elastic modulus of hydrogels, applied pressure, and the solution salinity on energy conversion using different thermodynamic cycles. With proper material design and process control, the thermodynamic efficiency of an ideal process was predicted to exceed 5% with 10 mM and 600 mM NaCl solutions. Experiments with poly (styrene sulphonate) (PSS) hydrogels verified the theoretically predicted trends and demonstrated more than 10% thermodynamic efficiency for moderate-salinity sources, due to the unique swelling-strengthened mechanical properties of the gels. The study suggests the potential of polyelectrolyte hydrogels for extraction of energy from low-to moderate-salinity sources and provides a framework for their design.

Energy Lost in a Hydrogel Osmotic Engine Due to a Pressure Drop

“…Part of the mixing energy is converted into elastic energy within the hydrogel during swelling (low salinities). This elastic energy is stored in the hydrogel and later released during deswelling (high salinities) . Accordingly, we have a process that is reversible when the hydrogels are exposed to low and high salinity solutions.…”

“…Zhang et al . demonstrated that the net change of Gibbs free energy of mixing during one deswelling/swelling cycle corresponds to the released mixing energy between the high and low salinity solutions, with no net change in the free energy of the gel (which starts and ends in the same state).…”

“…This elastic energy is stored in the hydrogel and later released during deswelling (high salinities). 34 Accordingly, we have a process that is reversible when the hydrogels are exposed to low and high salinity solutions. When the exchange of water and ions reaches equilibrium, the maximum available mixing energy is achieved, and the swelling volume of the hydrogels becomes constant.…”

“…Zhang et al 34 demonstrated that the net change of Gibbs free energy of mixing during one deswelling/swelling cycle corresponds to the released mixing energy between the high and low salinity solutions, with no net change in the free energy of the gel (which starts and ends in the same state). Accordingly, hydrogels can convert the mixing energy of two solutions of different salinities into mechanical energy without changing its own energy state.…”

“…Most studies focus on the energy recovered from the system, and the energy lost during the cycling process has not been explored for lab-scale energy production systems. Zhang et al 34 discussed the pressure drop of the supply pump for a small system, utilizing poly(styrene sulfonate sodium salt) hydrogels. The results indicated that the energy loss due to pumping was much smaller than the work output.…”