Blue energy is the energy that comes from osmosis, that is, from the differences in salt concentration of two bodies of water. By separating these bodies of water through a membrane, electricity can be extracted both directly and indirectly. Such a difference can be used in places where fresh water flows into the sea.
It is also investigating whether salinity can first be increased using low-calorie residual heat to directly convert heat into electrical energy (thermo-electrochemical systems), blue energy is also completely free of greenhouse gas emissions. This renewable energy source is a good alternative to non-renewable energy derived from fossil fuels. Therefore, it is one more option to reverse climate change.
How does blue energy work?
A saline solution contains energy. For example, the dissolution of common salt (NaCl) is an endothermic process during dissolution, as is the case with most salts. Water temperature drops. The evaporation of salt water requires more energy than fresh water. If salt and fresh water with the same temperature intersect, the temperature rises slightly. Consequently, a pressure is generated that can be used to drive hydraulic turbines as if it were another source of hydraulic energy.
Blue energy is a method to convert (partially) this released energy into an electric current. In fact, blue energy is already present in nature. Plant cells contain salts. The salt concentration increases with evaporation. The wall of the plant cell is a membrane so that water is transported from the ground upwards through the osmosis process.
Disadvantages of blue energy
The main drawbacks of blue energy are the limited capacity per square meter of membrane surface, the price and the resistance of the membranes.
Another problem is the appearance of contamination of the membranes, which are, after all, extremely fine filters. Both freshwater from the river and seawater contain algae, sand and clay particles.
Methods to harness blue energy
Pressure delayed osmosis
The first technique, pressure delayed osmosis (PRO) is based on osmosis. When the fresh water of the river and the salty water of the sea are separated from each other by a membrane that allows the passage of water, but not of dissolved substances such as salt, the fresh water flows through the membrane to the salty side. There, a pressure is generated that can be used as an energy source.
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 operated 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 or RED
A second method uses reverse electrodialysis. Flowing salt and fresh water are surrounded by selective ion 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 a too small capacity per m² of membrane surface.
A third method uses ammonia (NH 3 ) in a fuel cell with ion selective membranes. The low-calorie residual heat restores the required concentration difference and uses the residual heat of, for example, the industry (TRAB thermally regenerative ammonia battery).
This method to harness blue energy is still under study.