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

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

  • 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.


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Human brains remember certain words more easily than others

A study of the manner in which memory works turns up a surprising thing.

Image Point Fr / Shutterstock
Mind & Brain
  • Researchers have found that some basic words appear to be more memorable than others.
  • Some faces are also easier to commit to memory.
  • Scientists suggest that these words serve as semantic bridges when the brain is searching for a memory.

Cognitive psychologist Weizhen Xie (Zane) of the NIH's National Institute of Neurological Disorders and Stroke (NINDS) works with people who have intractable epilepsy, a form of the disorder that can't be controlled with medications. During research into the brain activity of patients, he and his colleagues discovered something odd about human memory: It appears that certain basic words are consistently more memorable than other basic words.

The research is published in Nature Human Behaviour.

An odd find

Image source: Tsekhmister/Shutterstock

Xie's team was re-analyzing memory tests of 30 epilepsy patients undertaken by Kareem Zaghloul of NINDS.

"Our goal is to find and eliminate the source of these harmful and debilitating seizures," Zaghloul said. "The monitoring period also provides a rare opportunity to record the neural activity that controls other parts of our lives. With the help of these patient volunteers we have been able to uncover some of the blueprints behind our memories."

Specifically, the participants were shown word pairs, such as "hand" and "apple." To better understand how the brain might remember such pairings, after a brief interval, participants were supplied one of the two words and asked to recall the other. Of the 300 words used in the tests, five of them proved to be five times more likely to be recalled: pig, tank, doll, pond, and door.

The scientists were perplexed that these words were so much more memorable than words like "cat," "street," "stair," "couch," and "cloud."

Intrigued, the researchers looked at a second data source from a word test taken by 2,623 healthy individuals via Amazon's Mechanical Turk and found essentially the same thing.

"We saw that some things — in this case, words — may be inherently easier for our brains to recall than others," Zaghloul said. That the Mechanical Turk results were so similar may "provide the strongest evidence to date that what we discovered about how the brain controls memory in this set of patients may also be true for people outside of the study."

Why understanding memory matters

person holding missing piece from human head puzzle

Image source: Orawan Pattarawimonchai/Shutterstock

"Our memories play a fundamental role in who we are and how our brains work," Xie said. "However, one of the biggest challenges of studying memory is that people often remember the same things in different ways, making it difficult for researchers to compare people's performances on memory tests." He added that the search for some kind of unified theory of memory has been going on for over a century.

If a comprehensive understanding of the way memory works can be developed, the researchers say that "we can predict what people should remember in advance and understand how our brains do this, then we might be able to develop better ways to evaluate someone's overall brain health."

Party chat

Image source: joob_in/Shutterstock

Xie's interest in this was piqued during a conversation with Wilma Bainbridge of University of Chicago at a Christmas party a couple of years ago. Bainbridge was, at the time, wrapping up a study of 1,000 volunteers that suggested certain faces are universally more memorable than others.

Bainbridge recalls, "Our exciting finding is that there are some images of people or places that are inherently memorable for all people, even though we have each seen different things in our lives. And if image memorability is so powerful, this means we can know in advance what people are likely to remember or forget."

spinning 3D model of a brain

Temporal lobes

Image source: Anatomography/Wikimedia

At first, the scientists suspected that the memorable words and faces were simply recalled more frequently and were thus easier to recall. They envisioned them as being akin to "highly trafficked spots connected to smaller spots representing the less memorable words." They developed a modeling program based on word frequencies found in books, new articles, and Wikipedia pages. Unfortunately, the model was unable to predict or duplicate the results they saw in their clinical experiments.

Eventually, the researchers came to suspect that the memorability of certain words was linked to the frequency with which the brain used them as semantic links between other memories, making them often-visited hubs in individuals's memory networks, and therefore places the brain jumped to early and often when retrieving memories. This idea was supported by observed activity in participants' anterior temporal lobe, a language center.

In epilepsy patients, these words were so frequently recalled that subjects often shouted them out even when they were incorrect responses to word-pair inquiries.

Seek, find

Modern search engines no longer simply look for raw words when resolving an inquiry: They also look for semantic — contextual and meaning — connections so that the results they present may better anticipate what it is you're looking for. Xie suggests something similar may be happening in the brain: "You know when you type words into a search engine, and it shows you a list of highly relevant guesses? It feels like the search engine is reading your mind. Well, our results suggest that the brains of the subjects in this study did something similar when they tried to recall a paired word, and we think that this may happen when we remember many of our past experiences."

He also notes that it may one day be possible to leverage individuals' apparently wired-in knowledge of their language as a fixed point against which to assess the health of their memory and brain.

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