It is possible to extract energy from two solutions with different concentrations of solute when the two are mixed. This form of energy extraction is called OsmoticPower.
In general, OsmoticPower would use water and common salt as their solvent and solute, as these are common on earth and there is a constant supply of both fresh water and brine. The mechanism for extraction would be a OsmoticGenerator. My current best plan for an OsmoticGenerator is the OsmoticDumbell.