Water-Based Salinity Gradient and Waste Heat Technologies

Many different technologies have been developed to harness energy from salinity gradients or waste heat. Through the links below (or from the menu) you can learn about each technology. For salinity gradient energy (SGE) technologies, either from natural occurring salinity differences (saltwater vs fresh water or river water), or from engineered gradients (using for example ammonium bicarbonate and waste heat), the following can be used:

  • Pressure retarded osmosis (PRO)
  • Reverse electrodialysis (RED)
  • CapMix: Three different technologies based on capacitance, Donnan potentials, or battery electrode reactions (pseudo-capacitive)
  • Hydrogel expansion (HEx)

For waste heat technologies, these can be organized as:

  • SGE technologies using thermolytic salts such as ammonium bicarbonate, where conventional technologies such as distillation are used to prepare high- and low-concentration solutions, which include primarily PRO and RED. CapMix and HEx are less explored
  • Thermal regenerative batteries (TRB) which have been classified based on the ligands complexing the metal: with an ammonia ligand, these are called thermally regenerative ammonia batteries (TRABs); with ethylenediamine, these are thermally regenerative ethylenediamine batteries (TREnBs).

For more information, click on either the name of the technology to navigate to a specialized web page.



Pressure retarded osmosis (PRO) can generate electricity based on the pressure produced in the salt water chamber due to the fresh water crossing the water-permeable membrane (not ion permeable). A turbine must be used to make the electricity from the pressured water.


Reverse electrodialysis (RED) can be used to generate electricity based on the electrochemical potential established by separating river water and seawater solutions alternating anion and cation exchange membranes. There can be tens to hundreds of paired membranes, and the electricity is directly generated from two electrodes on either side of the stack of membranes.


CapMix is a general term used to describe electrode-based electricity generation. Using this technology a pair of electrodes directly generates electrical power based on ion accumulation on or in the electrodes from capacitive charges, battery-like electrode reactions (psuedocapacitive), or Donnan potentials from membranes or ionic exchange polymers on the electrodes. These systems operate on cycles of alternating fresh and salt water.


The hydrogel technology is based on the swelling of a hydrogel in freshwater, and shrinking in seawater. This expansion in freshwater is used to drive a piston and thus the technology is based on mechanical conversion of energy to electricity.


Thermal regenerative batteries (TRBs) use identical metal electrodes inserted into identical salt solutions, with the battery “charged” by adding a ligand such as ammonia to only one side. Waste heat is used to remove the ammonia after the battery is discharged, and to add it to the other side, which the electricity generated due to metal dissolution from one electrode and deposition on the other electrode in a cyclical discharge process.


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