Researchers at Stanford University have developed a “mixing entropy battery” (MEB) that can harness energy from the mixing of fresh and salt water. The energy created this way is sometimes called “blue energy.” According to a Stanford news release, the team’s objective is to apply the technology to coastal wastewater treatment plants and to use the electricity generated to make the plants energy-independent and carbon-neutral.

“Blue energy is an immense and untapped source of renewable energy,” said Kristian Dubrawski, a postdoctoral scholar in civil and environmental engineering at Stanford. “Our battery is a major step toward practically capturing that energy without membranes, moving parts or energy input.”

Development of a mixing entropy battery (MEB) depends upon the motion of sodium and chlorine ions from seawater into and out of inexpensive electrode materials. (Image source: Stanford)

Electrochemistry

The Stanford battery isn’t the only technology available to capture blue energy, but it’s the first to use battery electrochemistry instead of pressure or membranes. The present work is based on earlier research at Stanford that tapped into salt gradients to produce electricity, but that effort required an expensive electrode made from silver, and an initial energy input to begin the process.

The new Stanford battery floods a tank with salt-free water (which can be wastewater effluent. The tank contains electrodes which release sodium ions (Na ) and chlorine ions (Cl–) from the electrodes into the solution. This motion of ions also causes a current to flow from the anionic electrode to the cationic electrode. Then, a rapid exchange of the wastewater effluent with seawater allows the electrodes to reincorporate the sodium and chloride ions, reversing the electric current flow. Energy is recovered during both the freshwater and seawater flushes. This means that the battery is constantly discharging and recharging without needing any input of energy. As reported in a paper in the journal ACS Publications, energy is recovered during both the freshwater flush (43.6% of the total energy recovered) and the seawater flush (56.4% of the total energy recovered), with no upfront energy investment.

Unlike the earlier effort, that used expensive materials as the electrodes, this new MEB is cost-effective. The electrodes in the new MEB are made with Prussian Blue, a material widely used as a pigment and medicine, that costs less than $1 a kilogram, and polypyrrole, a material used experimentally in batteries and other devices, which sells for less than $3 a kilogram in bulk. The materials are relatively robust and a polyvinyl alcohol and sulfosuccinic acid coating protects the electrodes from corrosion when in contact with seawater.

Wastewater a Good Starting Point

Wastewater treatment is a good starting point for a practical application of the Stanford MEB study. The water treatment process is energy-intensive, accounting for about three percent of the total US electrical load. If sufficient blue energy could be generated by an MEB system, a wastewater treatment plant could be self-sufficient and operate off the grid.

According to the Stanford news release, “The researchers tested a prototype of the battery, monitoring its energy production while flushing it with alternating hourly exchanges of wastewater effluent from the Palo Alto Regional Water Quality Control Plant and seawater collected nearby from Half Moon Bay. Over 180 cycles, battery materials maintained 97 percent effectiveness in capturing the salinity gradient energy.” The team also reported that every cubic meter of freshwater that mixes with seawater produces about .65 kilowatt-hours of energy – enough to power the average American house for about 30 minutes. If the 68% efficiency achieved in a small prototype MEB can be achieved at full-scale, the energy produced would be sufficient to meet much or even all of the electrical energy demands for a conventional wastewater treatment plant.

“It is a scientifically elegant solution to a complex problem,” Dubrawski said. “It needs to be tested at scale, and it doesn’t address the challenge of tapping blue energy at the global scale – rivers running into the ocean – but it is a good starting point that could spur these advances.”

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Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.

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