Blue energy is a way of generating energy by osmosis, that is, from the differences in salt concentration of two volumes of water. By separating these masses of water through a semipermeable membrane, a pressure difference is generated that can be used to drive a turbine. The turbine is connected to an alternator that produces electricity.
This difference can be used in places where fresh water flows into the sea.
The osmotic energy technique is still under development. Research is currently underway on whether it is possible to increase salinity by using waste heat to convert heat directly into electrical energy using thermoelectrochemical systems.
One of the advantages of blue energy is that it is a renewable energy source that does not generate greenhouse gas emissions. It is also considered clean energy because it does not generate polluting waste, it only discharges water with a certain concentration of salt into the sea.
How does blue energy work?
A saline solution contains energy that can be harnessed. The dissolution of salt is an endothermic process in which when dissolved, as is the case with most salts, the temperature of the water drops.
The energy required to evaporate water is greater with salt water than with fresh water. So if salt and fresh water cross while being at the same temperature, the temperature increases slightly. This increase in temperature generates pressure that can be used to drive hydraulic turbines to generate electrical energy.
Blue energy in nature
This osmotic process also occurs naturally. In this case, plant cells contain salts whose concentration increases with evaporation. The plant cell wall is a membrane for water to be transported from the soil upward through the process of osmosis.
Disadvantages of blue energy
The main drawbacks of this type of renewable energy is that osmotic energy has a limited capacity per square meter of membrane surface in addition to the price and resistance of the membranes.
Another problem is the appearance of contamination of the membranes, which are, after all, extremely fine filters. Both fresh river water and seawater contain algae, sand, and clay particles.
Methods to harness blue energy
Delayed pressure osmosis
The first technique, pressure retarded osmosis (PRO) is based on osmosis. It occurs when river water (fresh) is in contact with sea water (salty) through a membrane that allows the passage of water but not dissolved substances such as salt. Fresh water flows through the membrane to the salty side and flows into the sea. There, an osmotic pressure is generated that can be used as a source of energy.
The pressure theoretically increases to 28 bar at 10°C and a salt concentration difference of 3.5%. The actual pressure is lower due to the dilution that occurs. With the resulting pressure, a hydraulic turbine can be driven and electricity can be generated.
Brackish water is produced as a waste product; nothing more than what would have happened if the water had reached the sea without obstacles.
Reverse electrodialysis
A second method uses reverse electrodialysis. Flowing salt and fresh water are surrounded by ion-selective membranes, resulting in minimal tension between these membranes.
By connecting the membranes in series, a usable voltage is created and electricity is generated immediately. The problem remains too little capacity per m² of membrane surface.
Ammonia
A third method uses ammonia (NH3) in a fuel cell with ion-selective membranes. Low-calorie waste heat restores the required concentration difference and uses waste heat from, for example, industry (TRAB thermally regenerative ammonia battery).
This method to harness osmotic energy is still in the study phase.
