Evaporation-Powered Engine Can Drive a Tiny Car
by Kiona Smith-Strickland / June 16, 2015
Artificial muscles may move the machines of the future, and the only power they need is evaporating water. Researchers at Columbia University have built artificial muscles that expand and contract with changing humidity, and in the lab, they’re already powering LEDs and propelling miniature cars. “Engineered systems rarely, if ever, use evaporation as a source of energy, despite myriad examples of such adaptations in the biological world,” wrote lead author Xi Chen and his colleagues in a paper published in the journal Nature Communications. Of course, there’s a reason for that: evaporation creates pressure changes at a very small scale, but the force it produces doesn’t generally scale up very well. But Chen and his colleagues found a way to bend the rules. They attached a layer of bacterial spores to a thin plastic tape. Spores of the bacterium Bacillus subtilis expand when they’re exposed to humidity, and they contract when they’re dry. That’s because the spores store water in very small, nanometer-scale spaces, so evaporation causes major pressure changes within those tiny spaces.
When thousands of the spores are layered onto a strip of tape, they all expand and contract together, and their combined force is enough to alternately straighten and curl the surface of the tape. Chen and his colleagues call them Hygroscopy Driven Artificial Muscles, or HYDRAs. That tiny breakthrough drove a number of innovations in the lab. They created a “muscle” by putting strips of spores on alternating sides of a longer strip of tape, so that when the spores contract, they make the tape scrunch up instead of curling. Several of these strips together can contract with enough force to lift small weights of 0.2 lbs to 0.7 lbs – fifty times the weight of the strips themselves. They also created a generator by stretching HYRDA strips horizontally over a small container of water covered with shutters. As humidity builds up, the HYDRAs expand and push a beam to open the shutters. Water vapor can now escape through the open shutters, so humidity decreases and the HYDRAs contract, pulling the beam to close the shutters and starting the cycle over. The engine produced an average power of 1.8 microwatts – enough to power a pair of LED lights.
Finally, the researchers had a bit of fun. They attached short HYDRA strips around two concentric rings, a bit like cars on a Ferris wheel, and then they put four or five of these wheels on an axis. Then they placed the whole contraption halfway inside an enclosure with paper walls. When the paper was wet, half of the wheel would be exposed to humid air, so the HYDRAs would expand; the other half would be dry, so the HYDRAs would contract. The idea was that as the HYDRAs furthest from the paper walls dried out and contracted, their movement would shift the whole wheel’s center of mass, creating torque and causing the wheel to rotate. To increase the shifted mass, the researchers attached small acrylic blocks to the ends of the HYDRAs. It worked, and the higher the humidity inside the enclosure, the faster the wheel turned.
But an evaporation-powered miniature Ferris wheel wasn’t enough for Chen and his team. They mounted the engine on a frame with two pairs of wheels, using a rubber belt to connect the rotating engine to the front wheels. As water evaporated from the paper, the engine turned and drove the little quarter-pound car forward. The engineers say HYDRAs could have many other applications, including artificial muscles for robots. And because sources of evaporation are easy to find in nature, and the materials required to build HYDRAs are relatively inexpensive, they suggest that HYDRA engines could be used to provide power in remote locations. That’s still some time in the future, obviously. But muscles made of spores may be up to the challenge – just add a little water.
An engine with living parts
by William Herkewitz / Jun 16, 2015
It might not look like much, but this plastic box is a fully functioning engine—and one that does something no other engine has ever done before. Pulling energy seemingly out of thin air, it harvests power from the ambient evaporation of room-temperature water. A team of bioengineers led by Ozgur Sahin at Columbia University have just created the world’s first evaporation-driven engine, which they report today in the journal Nature Communications. Using nothing more than a puddle of resting water, the engine, which measures less than four inches on each side, can power LED lights and even drive a miniature car. Better yet, Sahin says, the engine costs less than $5 to build.
“This is a very, very impressive breakthrough,” says Peter Fratzl, a biomaterial researcher at the Max-Planck Institute of Colloids and Interfaces in Potsdam, Germany who was not involved in the research. “The engine is essentially harvesting useful amounts of energy from the infinitely small and naturally occurring gradients [in temperature] near the surface of water. These tiny temperature gradients exist everywhere, even in some of the most remote places on Earth.” To understand how the engine works, it helps to understand unique material behind it. The key to Sahin’s astonishing new invention is a new material that Sahin calls HYDRAs (short for hygroscopy-driven artificial muscles). HYDRAs are essentially thin, muscle-like plastic bands that contract and expand with tiny changes in humidity. A pinky finger-length HYDRA band can cycle through contraction and expansion more than a million times with only a slight, and almost negligible, degradation of the material. “And HYDRAs change shape in really quite a dramatic way: they can almost quadruple in length,” Sahin says.
The idea for the HYDRA material came to Sahin more than half a decade ago, when he came across an unusual find in nature. While studying the physical properties of micro-organisms with advanced imaging techniques, he discovered that the spore of the very common grass bacillus bacteria responds in a strange way to tiny amounts of moisture. Although the dormant spore has almost no metabolic activity and does no physical work, its outer shell can soak up and exude ambient levels of evaporated water—expanding and shrinking while doing so. “The spores stay very rigid as they expand and contract in response to humidity,” Sahin says. “That rigidity means their movements come with a whole lot of energy.”
After many experiments, Sahin found a way he could mimic the spore’s unique response. To make HYDRAs, he actually paints the spores onto plastic strips using a laboratory glue. By painting dormant spores in altering patches on both sides of a single strip, the pulsating spores cause the plastic to flex and release in a single direction in response to moisture—just like a spring expanding and contracting. While a material made of living creatures may sound like it should have a short lifespan, Fratzl says that, in fact, HYDRAs are “likely to last for a very, very long time,” he says. “In nature, it’s absolutely critical that these spores survive from decades to even hundreds of years in dormancy, all while responding to outside humidity in this dramatic way without breaking down.”
How do you go from spores on strips to a working engine? The engine is placed over a puddle of room-temperature water, creating a small enclosure. As the water on the surface naturally evaporates, the inside of the engine becomes slightly more humid. This triggers strips of HYDRAs to expand as they soak up some of the new-found humidity. Collectively, these HYDRAs pull on a cord which is attached to a small electromagnetic generator, transforming the cord’s movement into energy. The HYDRAs also pull open a set of four shutters on top of the engine, releasing the humid air. With the shutters open, humidity inside the engine drops. This causes the HYDRAs to shed their water-vapor and contract, which pulls the shutters back closed. And the process repeats, just like an engine’s cycle.
Sahin has found that the engine works at room temperature (around 70 degrees Fahrenheit) with water that’s at a wide range of temperatures—from 60 to 90 degrees F. Because water naturally evaporates faster at higher temperatures, hotter water works best. With 60-degree water, the engine will open and close its shutters once every 40 seconds. At 70 degrees, it does so every 20 seconds. At 90 degrees, it’s every 10. Sahin also created a second engine with his HYDRAs—this one a turbine-style creation that uses the motion of bending HYDRAs to spin a wheel. Placed on top of a miniature car, the entire device slowly ekes forward—again, powered by nothing but evaporating water.
On average, each pull of the engine creates roughly 50 microwatts. That’s a tiny amount of energy, but it’s enough to generate light with an LED by harvesting the energy of a puddle of water that’s doing nothing but existing at room temperature. Sahin also says that the materials used to make the engine are extremely cheap. Even including the HYRDAs, he says it should cost less than $5 to put together. There is plenty of room for improvement, too. For one thing, he says, each HYDRA band uses just 1 percent of energy potential of the bacteria spores. A HYDRA-like material that could make better use of the spores would radically increase usefulness of the device. In fact, Sahin says he already developed another material that could tap into one-third of the spores’ energy potential, but it proved an absolute nightmare to finagle that material into a long-lasting engine. For now, the evaporation engine is just a proof of concept meant to show that this unique type of energy generation really can be accomplished. Whether future devices will ever be able to compete with other renewable energy sources, such as wind or solar energy collection, may be a question that won’t even be answerable for decades. But the promise is there, he says. Just consider the way the planet works: “The power in wind on a global scale primarily comes from evaporation,” he says, “so there’s more power to be had here than there is in the wind.”
Gravity Power in African Homes
by Andre Gobbo / August 22, 2014
“A considerable amount of resources have been dedicated to finding a sustainable means of providing electricity to those suffering from poverty. In order to create a solution for such a complex problem, a tremendous amount of creativity, innovation and resources have been directed toward finding an answer. Thus far, one idea has managed to utilize the same power that keeps our entire earth in orbit — gravity. GravityLight, a new product created by the London-based design consultancy firm Deciwatt, does just that. The light is entirely powered by a specific weight, usually around 10 kilograms. This weight can be anything, including sand, stones or even water. As the weight descends due to gravitational force, the kinetic energy created by this process is converted into light by a handful of small gears within the self-contained mechanism. The light lasts anywhere from 28 minutes to 12 minutes, depending on what setting the light is on. It requires no batteries, and can be used repeatedly without any running costs. The best part about this technology? The entire apparatus only costs $6.”