Osmotic power is the energy available from the difference in the salt concentration between seawater and river water. Two practical methods used are reverse electrodialysis and pressure retarded osmosis. Both processes rely on osmosis with membranes. The key waste product is brackish water. The technology of generating electricity by osmotic method was invented in 1973. The technology has been confirmed in the laboratory and are being developed for commercial use in Norway and the Netherlands. A membrane is usually used during the power generation process. Other technologies are being developed that could complement or replace membranes such as electric double layer capacitor technology and vapour pressure difference.
- Pressure Retarded Osmosis
- Reversed Electrodiaysis
- Capacitive Method
- Vapour pressure Differences
- Solar Pond
- Boron Nitride Nanotubes.
Pressure Retarded Osmosis
In this method seawater is pumped into a pressure chamber that is at a pressure lower than the difference between the pressure of saline water and freshwater. Freshwater is also pumped into the pressure chamber through a membrane, which increases both the volume and pressure of the chamber. As the pressure differences are compensated, it drives the turbine producing electricity.
Reverse Electro dialysis
This is another method that is being studied and developed. It is basically the creation of a salt battery. It has been described as an array of alternating anion and cation exchange membranes which can be used to generate electricity from the free energy of a river or seawater.
This method has so far only been tested in the laboratory. With this method energy can be extracted from the mixing of saline water and freshwater by cyclically charging up electrodes in contact with saline water, followed by a discharge in freshwater. Since the amount of electrical energy which is needed during the recharging step is less than what is gotten out during the discharging step each completed cycle effectively produces energy.
Vapour pressure differences.
Vapour pressure differences are of two types, open cycle and closed cycle. Both of these methods do not require membranes. The open cycle is similar to the open cycle in ocean thermal energy conversion. In a closed cycle the primary power source originates from a thermal difference, as part of a thermodynamic heat cycle. In this method water vapour is dissolved in a deliquescent salt water mixture using osmotic power as an intermediary.
This method does not harness osmotic power only solar power. Sunlight reaching the bottom of the pond is absorbed as heat. The effect of natural convection where heat rises is blocked using density differences between the three layers that make up the pond, in order to trap heat. It is made up of three zones, the upper convection zone, the stable gradient zone and the bottom thermal zone. The stable gradient zone acts as an insulator for the bottom layer as it blocks natural convection. This water from the lower layer the storage zone is pumped out and the heat generated is used to drive a turbine producing electricity.
Boron Nitride Nanotubes.
A research team built an experimental system using boron Nitride that produced much greater power than Statoil prototype. It used an impermeable and electrically insulating membrane that was pierced by a single boron nitride nanotube with an external diameter of a few dozen nanometers. With this membrane separating salt water and freshwater the team was able to measure the electric current passing through the membrane using two electrodes immersed in the fluid on the other side of the nanotube. The results show that the device was able to generate electric current on the order of a nanoampere.