Water is a renewable source of energy already being exploited by hydroelectric plants. Further developments in its use as a fundamental source of fuel to create clean energy has given us the tried and tested concept known as 'blue energy'. Blue energy relies on the mixing of fresh water and sea water to create a pressure gradient that can be exploited to power devices.
Using a semi-permeable membrane to initally separate two incubators and obstruct the passage of salt ions, fresh water flows through the membrane and mixes with the denser, saltier water. The pressure fallout from this movement can be used to power turbines and generate electricity. In this effect, the mixing of water isn't the sought-after ends, just the pressure that's created and harnessed. The process is called 'pressure-retarded osmosis' or 'osmotic power'. Beyond the theory and early experimentation, scientists are trying to produce greater yields using different methods and substances, other than salt water, in order to initiate pressure-retarded osmosis. However, the processes are still too underdeveloped to be able to match the production capacity of other renewables.