Materials

Refrigerants not required: Flexible metal cooling prototype demonstrates extreme efficiency

Refrigerants not required: Fle...
The prototype heating/cooling system uses the remarkable properties of shape-memory nitinol metals for environmentally friendly cooling and heating
The prototype heating/cooling system uses the remarkable properties of shape-memory nitinol metals for environmentally friendly cooling and heating
View 2 Images
PhD students Felix Welsch and Susanne Marie Kirsch with the first machine that cools air with nickel-titanium muscles
1/2
PhD students Felix Welsch and Susanne Marie Kirsch with the first machine that cools air with nickel-titanium muscles
The prototype heating/cooling system uses the remarkable properties of shape-memory nitinol metals for environmentally friendly cooling and heating
2/2
The prototype heating/cooling system uses the remarkable properties of shape-memory nitinol metals for environmentally friendly cooling and heating

A German research team has prototyped an extraordinary heating/cooling system that stresses and unloads nickel-titanium "muscle wires" to create heated and cooled air at twice the efficiency of a heat pump or three times the efficiency of an air conditioner. Crucially, the device also uses no refrigerant gases, meaning it's a much more environmentally friendly way to heat or cool a space.

The device is based on a peculiar property of certain shape-memory metal alloys that spring back into shape after being deformed. In some cases – particularly with nickel-titanium, also known as nitinol – these metals absorb significant amounts of heat when they're bent out of shape, and then release that heat when they're allowed to revert to their normal shape. The difference between the loaded wire and the released wire can be as much as 20° C (36° F).

The cooling device is thus quite simple in concept. It uses a rotating cylinder covered in nitinol wire bundles. The wires are loaded up as they pass through one side, sucking heat out of the air and storing it up. Then as they rotate past the other side, they're allowed to snap back into shape, dumping the heat on the second side. Air is blown through chambers on each side, giving you one feed of heated air and another feed of cool air.

The Saarland University team has been experimenting with the device to figure out the optimal convergence of wire loading, rotational speed, and how many wires should be in a bundle to create the biggest possible heat differential between the two sides from a given energy input.

And the results seem very exciting. The Saarland University team claims that "the heating or cooling power of the system is up to thirty times greater than the mechanical power required to load and unload the alloy wire bundles" depending on the type of alloy used. They say that this makes their new system more than twice as good as a conventional heat pump and three times as good as a conventional refrigerator.

"Our new technology is also environmentally friendly and does not harm the climate, as the heat transfer mechanism does not use liquids or vapors," says Professor Stefan Seelecke, the university's Chairman of Intelligent Metal Systems. "So the air in an air-conditioning system can be cooled directly without the need for an intermediate heat exchanger, and we don't have to use leak-free, high-pressure piping."

PhD students Felix Welsch and Susanne Marie Kirsch with the first machine that cools air with nickel-titanium muscles
PhD students Felix Welsch and Susanne Marie Kirsch with the first machine that cools air with nickel-titanium muscles

The idea of achieving an air cooling or heating effect simply by bending and unbending little pieces of metal sounds bizarre. What about metal fatigue? How long will those alloy wires last in these variable temperature environments before they get brittle and break off?

Well, that's one area where nitinol is very different from other metals. Indeed, it's most famous for its use in medical devices, particularly implantable ones such as stents, where its remarkable flexibility allows a stent to bend, crush, stretch and twist with the artery as the body moves.

Verdict Medical Devices summed up nitinol's fatigue properties thus: "it is by far the most resistant metal known to high-amplitude strain-controlled fatigue environments. Several applications … employ nitinol solely because of its extraordinary fatigue resistance."

The cooling device described above would appear to fit the description of a high-amplitude, strain-controlled application. But, of course, it'll be up to the researchers, or a future commercial team, to demonstrate how long you can expect your nitinol reverse cycle air conditioner to last.

It certainly looks like an exciting piece of technology, with the capability to reduce energy use in heating/cooling as well as taking refrigerant gases out of the equation.

Source: Saarland University

13 comments
JimFox
1. Sounds too good to be true 2. Can it transfer from lab to commercialisation? 3. Would it overcome resistance from established systems, if so?
MerlinGuy
One of the most interesting articles about new tech I have read in a long, long time. Sadly, they brought up metal fatigue but then never really answered the question. A simple "it can run x% as many cycles as a traditional air conditioner" would have made the technology sound usable.
guzmanchinky
Sounds absolutely revolutionary if they can make the material last! Very cool! (pun intended). This would be great for van dwellers, a silent air con system.
paul314
Not quite perpetual motion, but with that huge ratio of cooling power to mechanical energy needed to operate the thing, can they run a heat engine off the difference between the heated and cooled air streams and use that to turn the wheel?
Rusty Harris
And it only cost 1,000 times more than a conventional system, and probably will use more electricity than a conventional system LOL.
Cryptonoetic
Useless in high humidity environments.
Nik
guzmanchinky; I think you missed something. This system will require TWO fans, or one larger fan ducted in two directions, so its unlikely to be silent, and as it will be blowing across wires, that may make even more noise. However, any system that requires no polluting chemicals to be disposed of when the system becomes redundant must be good. There are thousands of old scrap fridges waiting to be decommissioned and their toxic refrigerants neutralised, but as they deteriorate, the refrigerants tend to escape anyway, and they are a serious threat to the ozone layer. Lets hope it reaches production, in an affordable form.
toyhouse
Not only is there no mention of useful cycles for the material, but does the effect weaken with age, (similar to recharge cycles on batteries),? But the concept is very interesting and most welcome as air-conditioning alone is one the world biggest users of energy. We certainly need alternatives to the current tech. Something worth watching I think.
Mark R Windsor
I don't believe it! Heat pumps have a Coefficient of Performance of greater than 1.0 (100%) because they are not making heat; they simply move existing heat against it's natural gradient. What is proposed here does not involve moving heat but making it by bending the wires. The basic laws of energy conservation are being broken. Where is that heat coming from other than than the work put into bending the wires? Where does all the extra "free" energy come from?
charles000
I'm more than a bit skeptical here, but I will explain why. I've actually worked with NiTiNOL, years ago at LBL (Lawrence Berkeley Lab) on several different variations of motors, and various research platforms characterizing the thermal and crystallographic state phase transition properties of this alloy. As for the fatigue aspects of the alloy, MTBF (meantime between failure) can in fact extend through many millions of state phase transition cycles. But that's not the limitation here . . , The limitation is the efficiency of mechanical work vs. thermal gradient produced by the material as it is mechanically forced into its respective state phase transition cycles. The amount of thermal gradient produced by mechanically cycling the material is no where even close to the amount of mechanical energy required to induce that state phase change. We tried this, many times, with myriad different configurations, annealing thermal set points and variations of the nickel / titanium ratios in the alloy. In other words, the material can be mechanically cycled with external force to produce heat or absorb heat (cooling) as the contortion of the material induces the aforementioned state phase change, or . . . the material can be subject to alternate exposure to hot and cold environments, which causes a state phase change in the alloy (thus performing mechanical work). The success we did have consisted of solar powered mechanical motors configured with NiTiNOL wire loops that cycled between immersion in hot and cold water vessels, forcing the contraction and expansion of the wire loops around geared bobbins. The concept was to create solar powered mechanical motors that operated on the thermal gradient between hot (solar heated) and cold (from wells) water. NiTiNOL is commercially available in a variety of alloy ratios and annealing thermal set points to further customize the state phase transition vs. temperature thresholds, but the actual process of trying to cool air by cycling it past NiTiNOL as it is transitioning into its cold phase would require a phenomenal amount of energy spent, not to mention an extremely large surface area vs. CFM flow rate of air to actually produce effective cooling. I could be convinced otherwise, but I would very much like to see some hard data, engineering notes, or even 3rd party peer reviewed research on this before getting too excited about this general idea.