Dr. Ray Baughman, Director of the Alan G. MacDiarmid Nano Tech Institute at the University of Texas, Dallas, and his team discovered a new technology for refrigeration that is based on twisting and untwisting fibers.
Tech Briefs: What got you interested in studying twist for refrigeration?
Dr. Ray Baughman: We’ve been using twist and coiling to make powerful artificial muscles. We, for example, took ordinary fishing line — polyethylene or nylon — and discovered that if we insert so much twist that we get coiling, we can make powerful artificial muscles that can generate five times more mechanical power during contraction than your car’s engine, normalized to the weight of the engine. Then we found we could run coiled carbon nanotube artificial muscles in reverse to harvest mechanical energy as electrical energy with very high ratio of output electrical power to weight. We worked on this for many years. As part of this work, we looked at using twist of a rubber fiber to see how it affects muscle actuation. It turned out that twisting a rubber fiber actually degrades muscle contraction, but we did discover that rubber has amazing cooling when undergoing a cycle of twist and untwist.
We first observed this cooling a long time ago. Marcio Lima, one of the key scientists in our laboratory, made a jig that used an artificial muscle that was very narrow and was powerful enough to lift a person. It used water — hot water to contract and cold water to expand. He touched the muscle when it didn’t have any water in it after it was relaxed, and he noticed it was cold. The way the harvesters work, the way the muscles work, and the way the caloric materials work, has a commonality that can be understood by thinking of an ordinary metal spring. If you stretch a metal spring, you can maybe elongate it four times its original length. But that elongation is not because the wire is elongated, it’s because when you stretch it, the wire within the spring is actually twisted. Similarly, we can get cooling by just taking a rubber fiber and inserting twist. When you do that you decrease entropy, which causes the rubber fiber to heat and when you release the twist, the increased entropy allows it to cool.
If you have a rubber fiber or a polymer fiber made from coiled fishing line and you stretch it, what you’re really doing is causing torsional rotation within the fiber. In other words, the twist of coiling is being converted to twist within the fiber, or the reverse, depending on the relative handedness (chirality) of twist within the fiber and twist within the coil. By using a chirality of twist within the fiber, which is different from the chirality of coiling, we can make a material that when stretched, cools down. That aspect, we haven’t practically used, but it’s very interesting scientifically that we can tune whether a material heats or cools when it’s stretched.
Tech Briefs: Could you explain a little more about how by changing directions of twist and coiling you can change whether a material cools or heats?
Dr. Baughman: Whether it cools or heats, depends upon whether the fiber within the coil increases or decreases twist. If it increases twist, you go to a more ordered state — one that has less entropy, so if the fiber is isolated from the environment thermally, it heats up. If you twist a homochiral fiber — one that has the same handedness of twist and coiling — and stretch it, you convert some of the twist of coiling to the twist within the fiber, just like you do when you stretch a coiled metal wire spring.
You can make a heterochiral fiber by inserting twist in the fiber then wrapping it on a mandrel in the opposite direction from your inserted twist. If you stretch that out, what happens is you get the twist of the fiber going into the twist of coiling and you have an opposite direction process.
Tech Briefs: The way I think of a cooling process, I think of an air conditioner, which absorbs heat from a room and then sends it through coils, where the heat gets dissipated in the outside environment.
Dr. Baughman: I can explain. In ordinary air conditioning, you’re using the condensation of a gas to produce entropy change. When you condense the gas into a liquid, it loses some of the disorder that’s present in a gas state, so you’re releasing energy as heat. Then when you evaporate the gas within the coils, you get refrigeration. What we do with our device, instead of using a gas to provide entropy changes, we use a solid. For example, if you stretch a rubber band, its entropy decreases, and you get heating. If you relax the rubber band, you get cooling. That’s been known since about 1805. However, if you take a rubber band and try to make a refrigerator out of it, you’ve got a problem. With a rubber band, in order to get high cooling, you’ve got to stretch it by 600%. But in our case, we don’t have to provide a change in stretch, we can just twist. You need a very small amount of stretch — maybe 100%. So, our “twist fridges” based on rubber can be 2/7ths the length of a refrigerator made out of a rubber band.
When we’re inserting twist, we get entropy changes that result in heating in one part of the cycle and cooling in the other. We demonstrated a twist fridge cooler in which we pass flowing water over a fiber as twist is being released. This makes the fiber colder so it’s able to cool the water by about 6.7° in one cycle. If you’d like a greater amount of cooling, you circulate the water through the twist fridge more than one time. So, fundamentally, you’re doing the same thing as in a refrigerator, but instead of using a gas, you’re using a solid material. Your refrigerator can then be much smaller, less weighty, and less expensive. But even more important: there’s a major problem with conventional refrigerators. Between 17 and 20% of global electrical energy is used for refrigeration. That includes cooling houses. But the efficiency of these refrigerators is only about 60%. Using twist in a rubber fiber, we can get a materials efficiency of 67%. That degree of efficiency could have enormous consequences for humanity, in terms of energy savings and smaller amounts of greenhouse gases. Also, the greenhouse gases released by traditional refrigerators are much more potent than carbon dioxide — that’s a real problem.
Tech Briefs: Don’t you have to use mechanical energy to run the cycle?
Dr. Baughman: Yes, mechanical energy is needed to provide refrigeration. The efficiency of refrigeration is related to the energy of cooling divided by the input mechanical energy.
Tech Briefs: Do you do the twisting with a motor?
Dr. Baughman: We had done that, however, for our next paper, we detail how you could use our powerful artificial muscles to insert the twist. Our artificial muscles made from sewing thread and fishing line can rotate a heavy rotor at about 90,000 revolutions per minute. You don’t need that high a speed for this application, but they could be used to twist the material that’s doing the cooling.
Tech Briefs: What causes the muscle to move?
Dr. Baughman: The artificial muscle could be powered electrically, for example — electrochemically.
Tech Briefs: Okay, I understand the first part of the cycle: you release the twist and water flows over it and the water cools. What about when you increase the twist and it heats — where does the heat go?
Dr. Baughman: What you do is to equilibrate with the environment. If I insert twist, I heat the fiber. Then I can pass room temperature water over it to return it to room temperature. I can get both hot water and cold water. During the cooling part of the cycle, I pass room temperature water and I get it cold and in the other part of the cycle, I heat the water. The water can be separated into two streams: a hot-water stream and a cold-water stream.
Tech Briefs: How do you separate the streams?
Dr. Baughman: We showed a simple one-stage twist cooler in our paper. You can use one inlet and one outlet and valves to separate the flows into hot and cold. The cool water can be used for the coils of a refrigerator. Usually you just throw the hot water away or allow it to cool to ambient by using metal vanes. Just like in a regular refrigerator, you have to dissipate the heat.
Tech Briefs: In your paper, you talk about supercoils and coils — what’s the difference?
Dr. Baughman: A supercoil results when you take a coil and coil it upon itself. For polymer fibers, we can’t get supercoiling unless the polymer is elastic. If it’s elastic, like a rubber band, then we can insert a lot of twist. You can think of a model airplane (I have model airplanes around from the old work on artificial muscles). The rubber bands that drove them have gotten so old that they broke). If you take your finger and rotate the propeller to put energy in, first of all you see that you get twist. Then if you rotate more you get coiling, but then if you rotate even more, you get coiling upon the coiling.
Tech Briefs: Does the speed of twisting affect the amount of cooling?
Dr. Baughman: Yes — you want to go as fast as you can consistent with equilibration between the cooling fiber and the surroundings so you can get to use the cooling energy.
Tech Briefs: Could you give me some examples of large and small applications.
Dr. Baughman: The most exciting large application is to replace conventional gas compression refrigerators.
A small application is on circuit boards. You have to get the heat out from the electronics because they’re making devices smaller and smaller and are packaging more and more per unit area and volume. But they generate heat, which you have to remove from the circuit board. Although the heat is usually conducted away you still have to cool the place it’s going to.
Another opportunity is in microfluidic circuits, where you pump liquids around, sometimes you might want to heat or cool them. Our technique could be used within a microfluidic circuit because our fiber diameters could go down to as low as one micron.
So far, we’ve talked about using them as refrigerators, but we can also coat the fiber with a thermochromic material. Then if you twist it, it will turn color as it heats and when you untwist it, it’ll cool down and turn another color. if you stretch a coiled fiber, and it’s homochiral, it’ll increase twist and heat up and when you release it, it will cool down. You can put these fibers into textiles for fashion purposes. When the fiber stretches and heats, it’s one color, then when you release it and it cools down, it changes to another color.
Another color change application is if you want to know whether or not a remotely viewed structure is being strained. You could use color change as a sensor by having a thermochromic material on the surface or you could use the infrared signature.
Tech Briefs: Do you have a sense of the timeframe for the initial commercialization of any of this?
Dr. Baughman: It could occur as quickly as two years. Being realistic, to replace conventional refrigerators, will take at least five years to start penetrating the market. One advantage we have is that for our artificial muscles, we’ve licensed our technology to a major Japanese company called Lintec. They started a laboratory five miles from our institute in order to commercialize our carbon nanotube yarns and sheets and artificial muscles. They’re making and upscaling the artificial muscles. So, we’re not starting from zero as far as commercial activity. They have about 40 people working on technology we licensed to them, and many more than that in Japan.
An edited version of this interview appeared in the December Issue of Tech Briefs.