New membrane enables us to harvest 'osmotic' energy from water

You've likely heard of solar energy, but what is osmotic energy?

River
  • Osmotic power plants harvest energy from the difference in pressure or salinity between salt and freshwater using a semi-permeable membrane.
  • One of the major challenges for this kind of renewable energy, however, has been developing effective and durable membranes.
  • Now, new research demonstrates a durable and effective membrane that could significantly improve osmotic energy collection.


By now, everybody has heard of solar and wind energy. You're probably familiar with hydroelectric energy as well, and maybe even geothermal energy. But few are familiar with osmotic energy.

Osmotic energy plants are fairly rare as one of the key components in their use — a semi-permeable membrane — tends to break down, requiring frequent replacement and driving up operational costs. Now, new research has uncovered a better, more durable membrane that may lead to significantly better returns for this kind of renewable energy.

What exactly is osmotic energy?

Tofte Osmotic Power Plant

An image taken inside of the world's first osmotic power plant at Tofte, Norway, 2009. The project has since been shelved due to its high operating costs, highlighting the need for better, more efficient technology.

POPPE, CORNELIUS/AFP via Getty Images

Osmotic energy takes advantage of the differences in pressure and salinity between fresh and seawater to generate electricity. Its only waste product is brackish water, which is simply water that is saltier than freshwater but less so than seawater. While it doesn't generate large amounts of energy compared to other renewable energy sources, it is remarkably consistent. The energy derived from wind turbines and solar panels fluctuates tremendously with the weather, time, and local climate, but osmotic energy works more or less the same year-round wherever fresh and saltwater meet.

Osmosis, in general, is the process by which liquid moves from a dilute to a concentrated solution through a semi-permeable membrane. It occurs in your body all the time, as its critical for fundamental biological processes.

Osmotic power plants typically use one of two major techniques. In pressure-retarded osmosis (PRO), freshwater is gathered in one tank while saltwater is kept in another. In between, a membrane separates the two. This membrane has special properties that only permit freshwater to pass through, but not saltwater. As a result, the freshwater is drawn through the membrane, diluting the saltwater in the corresponding tank but also raising the pressure. From this pressure, we can derive energy.

The other technique, reverse electrodialysis osmosis (RED), takes advantage of the fact that saltwater contains more positive and negative ions than fresh water. Normally, these ions would travel into the freshwater, balancing out the solution. But when harvesting osmotic energy, a membrane can selectively allow only the positive or negative ions to pass through, turning tanks of salt and fresh water into a kind of battery that passively generates electricity.

Inspired by bone and cartilage

But the reason why we don't see more of either of these plants is because of the membrane. Osmotic membranes are delicate and must retain specific characteristics in order to remain semi-permeable. Exposed to the elements, they tend to degrade over time.

Recent research described in the journal Joule presents a new, durable membrane inspired by bone and cartilage that lasts. This membrane would be used in RED applications.

Bone is a very strong material, but it doesn't permit the transportation of ions, while flimsier material like cartilage permits ions to pass through easily. A membrane for osmotic energy would require both strength and the ability to transport ions.

Using this as inspiration, the researchers developed a membrane consisting of layers of boron nitride and aramid nanofibers. Boron nitride had shown promise in previous membranes but tended to develop cracks over time. To address this, the researchers investigated the use of a class of synthetic fibers frequently used in Kevlar: Aramid nanofibers. By layering boron nitride and the aramid nanofibers, the researchers had developed a material that was sturdy enough to last while remaining flexible and efficient in transporting ions.

The researchers found that not only does this generate power to a similar degree as commercial RED osmotic power plants, but it also performs for a remarkably long time. They cycled the membrane 20 times, observing its efficiency over the course of 200 hours, and found no drop in performance whatsoever.

Moreover, the membrane can function well in a wide range of pH and temperatures. Other membranes only perform well under specific conditions and need to be regularly replaced, increasing the amount of energy they require to be maintained. Implementing a more durable, longer-lasting membrane in a power plant would mean in effect that the plant could generate more power, as it would require less energy to maintain.

While the study only served as a proof of concept, it does show that we're getting better and better at addressing the problems with renewable energy. Not only that, but it highlights how much energy available to us is out there — so long as we're willing to think creatively and look in the right places. With any luck, we might start to see more osmotic energy plants operating at the mouths of the world's rivers.


This is what aliens would 'hear' if they flew by Earth

A Mercury-bound spacecraft's noisy flyby of our home planet.

Image source: sdecoret on Shutterstock/ESA/Big Think
Surprising Science
  • There is no sound in space, but if there was, this is what it might sound like passing by Earth.
  • A spacecraft bound for Mercury recorded data while swinging around our planet, and that data was converted into sound.
  • Yes, in space no one can hear you scream, but this is still some chill stuff.

First off, let's be clear what we mean by "hear" here. (Here, here!)

Sound, as we know it, requires air. What our ears capture is actually oscillating waves of fluctuating air pressure. Cilia, fibers in our ears, respond to these fluctuations by firing off corresponding clusters of tones at different pitches to our brains. This is what we perceive as sound.

All of which is to say, sound requires air, and space is notoriously void of that. So, in terms of human-perceivable sound, it's silent out there. Nonetheless, there can be cyclical events in space — such as oscillating values in streams of captured data — that can be mapped to pitches, and thus made audible.

BepiColombo

Image source: European Space Agency

The European Space Agency's BepiColombo spacecraft took off from Kourou, French Guyana on October 20, 2019, on its way to Mercury. To reduce its speed for the proper trajectory to Mercury, BepiColombo executed a "gravity-assist flyby," slinging itself around the Earth before leaving home. Over the course of its 34-minute flyby, its two data recorders captured five data sets that Italy's National Institute for Astrophysics (INAF) enhanced and converted into sound waves.

Into and out of Earth's shadow

In April, BepiColombo began its closest approach to Earth, ranging from 256,393 kilometers (159,315 miles) to 129,488 kilometers (80,460 miles) away. The audio above starts as BepiColombo begins to sneak into the Earth's shadow facing away from the sun.

The data was captured by BepiColombo's Italian Spring Accelerometer (ISA) instrument. Says Carmelo Magnafico of the ISA team, "When the spacecraft enters the shadow and the force of the Sun disappears, we can hear a slight vibration. The solar panels, previously flexed by the Sun, then find a new balance. Upon exiting the shadow, we can hear the effect again."

In addition to making for some cool sounds, the phenomenon allowed the ISA team to confirm just how sensitive their instrument is. "This is an extraordinary situation," says Carmelo. "Since we started the cruise, we have only been in direct sunshine, so we did not have the possibility to check effectively whether our instrument is measuring the variations of the force of the sunlight."

When the craft arrives at Mercury, the ISA will be tasked with studying the planets gravity.

Magentosphere melody

The second clip is derived from data captured by BepiColombo's MPO-MAG magnetometer, AKA MERMAG, as the craft traveled through Earth's magnetosphere, the area surrounding the planet that's determined by the its magnetic field.

BepiColombo eventually entered the hellish mangentosheath, the region battered by cosmic plasma from the sun before the craft passed into the relatively peaceful magentopause that marks the transition between the magnetosphere and Earth's own magnetic field.

MERMAG will map Mercury's magnetosphere, as well as the magnetic state of the planet's interior. As a secondary objective, it will assess the interaction of the solar wind, Mercury's magnetic field, and the planet, analyzing the dynamics of the magnetosphere and its interaction with Mercury.

Recording session over, BepiColombo is now slipping through space silently with its arrival at Mercury planned for 2025.

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