The Allure of Blue Energy

The mouths of the world’s rivers hold an immense amount of unused energy from the chemical process that occurs when freshwater meets seawater.

When a river empties into the sea, freshwater is attracted to the salt in the seawater because salt is hydrophilic, meaning it loves water. This creates a natural flow of energy which can be harnessed to generate electricity, hence the name blue–or osmotic–energy. From the Nile to the Mississippi, from the Jordan to the Yangtze, their deltas have enough energy potential to meet the majority of their country’s electrical needs, if properly utilized. 

This was first realized in 1954 by Dr. Pattel, but the first serious plans were not created until an Professor Loeb came up with the concept of pressure retarded osmosis in 1973 while watching the Jordan empty into the Dead Sea. While his idea could certainly produce energy, as demonstrated by its application in a Norway power plant from 2009 to 2013, its economic feasibility remains dubious. 

Fortunately, the field has continued to progress, with a handful of new ideas making blue energy within reach.

What Is Pressure Retarded Osmosis?

Professor Loeb’s idea was to place a power plant where freshwater meets saltwater, the former of which would be placed in one container and the latter in another. Between these containers would be a semipermeable membrane to allow for the regulated, natural flow of freshwater into the saltwater, resulting in an increase in pressure in the saltwater container. This pressure would then be used to spin a turbine, creating electricity. 

Way back in 1831, Michael Faraday realized that moving magnets can be used to create an electric current. Now known as Faraday’s law of induction, he found that when magnets are placed in a loop around a wire, they excite the electric field perpendicularly when rotated. Together with Lenz’s law, which predicts the direction of the current, this principle makes the modern world possible. 

How? 53 years after Faraday’s discovery, Michael Parsons invented the steam turbine, which converts mechanical energy into electric energy. It does this by converting a natural flow of energy into the mechanical energy necessary to rotate magnets around a wire. For example, power plants use coal, natural gas, biomass, or uranium to heat water into steam, which rises and rotates a turbine. Even the wind, flowing water, and ocean waves have been harnessed to turn turbines. All of these forms of energy creation are possible by taking advantage of naturally flowing air or fluid from an area of high energy to an area of low energy. 

In a similar fashion, pressure retarded osmosis turns a turbine by taking advantage of the natural flow of freshwater to saltwater. This was put into practice by Statkraft, a state-owned Norwegian company, in the first osmotic power plant, built into a repurposed paper pulp manufacturer. On the bank’s of Oslo’s fjord, it produced only 2-4 kW of energy, and was shut down after 4 years of operation due to cost concerns, although this was merely a proof of concept and not intended to sustain itself for the long haul. Nevertheless, it demonstrated that such a method could actually generate electricity.

What is Reversed Electrodialysis?

Reversed Electrodialysis uses the same principle of osmosis, although the end goal is not to turn a turbine. Instead, the idea is to place one container of freshwater next to a container of saltwater, followed by another container of freshwater next to another container of saltwater, and so on, forming a chain of containers with different solutions. The membranes between each container alternate between those that allow positively charged ions through in one direction and those that allow negatively charged ions through in the other direction. The net result creates a movement of positively charged ions in one direction and an opposite movement of negatively charged ions in the other, creating an electric current. 

This concept is being employed in Harlingen, the Netherlands. The freshwater is supplied by a lake called IJsselmeer and the salt water by the Wadden Sea, which is part of the Baltic. It is capable of producing around 50 kW of electricity, with plans to scale it up to 200 MW.

Better Membranes Means Greater Efficiency

Improving both methods described above involves increasing the efficiency of the flow between fresh and saltwater, which depends on the membranes. 

A team from several universities in China produced membranes with a tweakable electric charge. In their paper, they describe creating a Janus membrane with varying hole size, which has asymmetric properties on each side that can be tuned with an electric current or chemical gradient. The authors claim that “Experiments and theoretical calculation demonstrate that abundant surface charge and narrow pore size distribution benefit this unique ionic transport behavior in high salt solution.” In other words, the membrane can be customized to the salinity of each solution, thus maximizing the amount of the correctly charged ions flowing through the membrane. 

Likewise, researchers from Switzerland showed that natural light can increase the efficiency of membranes to sort ions. When photons from the sun bombard the membrane, electrons are freed which accumulate on the surface, in turn increasing the surface charge, which has a direct impact on the membrane’s ability to attract or repel positive or negative ions from the pores. Therefore, the membranes can become more efficient by selectively focusing sunlight on to them. The lead author of the paper, Michael Graf claimed that “Output would double during daylight hours.” 

Because of the above, the efficiency of blue energy is growing by leaps and bounds, as teams of scientists from all over the world are working on harnessing this vast, untapped, renewable resource. When fully utilized, blue energy will be a major source of clean energy, helping us reach our goal of powering the world with 100% renewables.

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