So-called "blue energy" has previously been branded too expensive for practical use. However, a new process - with no moving parts - could see its start-up costs plummet.
Pierre MarkuseWhen fresh water from a river meets the sea's saltwater - energy is being produced beneath the waves.
Researchers at Stanford University have developed a new method to harness the power of osmotic power - more commonly referred to as blue energy. Their method, which features no moving parts, produces no carbon and requires no initial power input, could revolutionise energy intensive industries such as waste water reclamation.
First theorised in the 1950s as a renewable energy source, blue energy has in fact been around since the dawn of time. The process, which is also known as salinity gradient power, occurs wherever waters with different salt levels, or salinity, mix together - for example at the mouth of the river. As the water mixes, and the salinity is equalised between the fresh and the salt water, energy is produced, as much as 2.2 kilojoules of power per litre.
Traditional osmotic power theories and processes rely on recreating this process in two tanks separated by a semipermeable membrane. As fresh water moves into the salt, the pressure in the tank is increased, which in turn powers a conventional turbine. Although experimented with since the 1970s, the adoption of osmotic power has been slow, primarily due to its expensive installation costs in comparison to fossil fuels. The first prototype osmotic power plant opened in Norway in 2009, but by 2013 it had been discontinued and labelled as uncompetitive.
Stanford University, however, have approached the issue from a different direction. Instead of relying on pressure and membranes, the team has developed a battery which gathers energy as freshwater and seawater is flushed over it.
The process, published in American Chemical Society's ACS Omega, first releases sodium and chloride ions from the electrodes of the battery into a wastewater solution - with the electrical current flowing from one electron to another. When the electrodes are then flushed with seawater, the electrodes start to reincorporate the sodium and chloride ions, reversing the current flow and generating electricity. This process requires no significant upfront energy investment and no need for charging - instead the battery is constantly discharging and recharging without any additional energy input required.
The Stanford system also features no moving parts, has a simpler design and consists of cheap and available materials - primarily the pigment Prussian Blue and polypyrrole, an organic polymer. Compared to the Norwegian prototype discussed above, it's start-up costs should be significantly cheaper. Furthmore, although its current energy production is small, the simplicity of the design means that it could be scaled up for use in larger plants.
Currently, the Stanford team does not see their blue energy innovation as a solution suitable for powering entire communities, but it could be a useful means of supporting coastal and wastewater facilities, allowing them to become energy independent.
Wastewater treatment accounts for around 3 percent of US power consumption currently, while the services it provides are essential to maintaining hygiene and community health. Additionally, wastewater plants are susceptible to power cuts and blackouts - potentially resulting in dire circumstances if separated from an energy source. The Stanford process for blue energy could provide a robust, cheap and clean way of maintaining these plants. Surplus energy could also be redirected to other nearby coastal installations.
Following the success of the prototype, the team now hopes to scale up the experiment, creating a bank of batteries to fully access its power potential.