Power generation through polyelectrolyte hydrogels in osmotic engines via salinity gradients

Energy from fossil fuels is causing a number of environmental issues. Exploiting the salt gradient between sea and river water to generate mechanical energy is a promising option as it is available in many regions on ocean-front river deltas and is less subject to periodic fluctuations than wind and solar energy. The global amount of salinity gradient energy capacity is estimated to be 3.1 TW.

Energy can in this case be generated by e.g., an osmotic engine, using polyelectrolyte hydrogels (also referred to as superabsorbent polymers (SAPs)). The swelling of ionic hydrogels in salt solutions is lower than in fresh water due to differences in chemical potential and changes in entropy. SAPs can absorb up to 1000 g of distilled water and up to 100 g of salt solution per gram of polymer depending on the ion concentration and valency. The mechanical motion caused by cyclic swelling in deionized water and shrinking in saline solution can potentially be used to move a piston in an osmotic motor to directly generate mechanical power. 

My research is mostly focusing on design of osmotic engines and optimization of hydrogel synthesis. The directions of new engine design are: 1) drainage system, so that the liquid can be transported through hydrogels faster than diffusion; 2) automatization, then saline and freshwater can be introduced separately and controlled by magnetic valves, and the piston movement can be recorded by a laser; 3) sturdy construction, so more weights can be lifted by the engine without collapsing. Currently there are 4 engines with different volumes and structures in our lab.

As for the hydrogel synthesis, the monomer is basically acrylic acid which has the most intensive charge density and so with the best swelling capacity. Two methods are employed to prepare different samples: polyelectrolyte particles though microfluidic device and 3D printed bulk materials from digital light processing. The parameters for spherical particles such as crosslinking degree, particle diameter and neutralization degree have been explored to find out the best ones for the engines. However, during expansion the mutual collision of particles may cause gel-blocking which will reduce liquid transportation rate. In this way 3D printing is utilized to produce bulk materials directly to fit every engine. The structures and porosity in 3D printing samples have also been adjusted to increase output power. The maximum power we can produce until now is 13.30 W/kg, which is similar to the power produced by human muscles.