Shape-Memory Material Could Power More Efficient AC
Shape memory alloys (SMAs) are unique metals that can be deformed and then, when heated, return to their original shape. This “memory” is due to a reversible phase change that is triggered when a certain temperature is reached. SMAs can also absorb or release heat when mechanically or thermally stressed and then relaxed. So-called elastocaloric technology that relies on SMAs is a promising sustainable alternative to conventional heating and cooling systems, which are energy-intensive and often use environmentally harmful refrigerants.
Based on these thermal and mechanical properties of nitinol, an SMA made of nickel and titanium, a research team at the Saarland University in Germany has developed an air conditioning technology that works by manipulating bundles of ultrathin nitinol wires or sheets and then letting them relax. Heat can be generated in one location and then dissipated in another location.
Nitinol has two solid-state phases—martensite, formed at lower temperatures, and austenite, which is found at higher temperatures. For shape-memory applications, the alloy is deformed into a new shape in its martensite phase; when heated into the austenite phase, the metal “remembers” the original shape and tries to return to it.
Elastocaloric technology uses this effect in reverse, stressing or relaxing SMA materials to absorb or release heat. When put under tension, the material releases heat into the air; when relaxed, the material extracts heat from the surroundings. A prototype heating and cooling system built by a team at Saarland University led by Paul Motzki, professor and chair of smart material systems for innovative production, relies on a track that continuously rotates bundles of nitinol wires slightly thicker than human hairs. Depending on their location on the track, the wires are either pulled or relaxed to enable them to move heat from one side of the device to the other.
According to Paul Motzki, “The technology itself enables us to consider completely new ways to think ‘cooling’—for example, maybe flat geometries that can be integrated into furniture or textiles.” Photo: Oliver Dietze
Motzki’s prototype can operate continuously, just like standard HVAC systems do. The team has also tested a variety of mediums that flow over the SMA material, including air, water, or a water-glycol mix, to transfer the heat generated by the nitinol to the immediate surroundings.
“Based on the elastocaloric effect,” Motzki said, “the SMA material needs to be mechanically deformed and heat needs to be efficiently transferred. In systems engineering, there is a need for a parallel development of an efficient drive technology with kinematics, mechanical force transmission, and thermodynamics regarding heat transfer and fluid dynamics. The combination of these different aspects of mechanical and electrical engineering is what makes this work unique.”
The Saarland team displayed a mini-refrigerator based on their prototype elastocaloric cooling system at the Hannover Messe earlier this year.
Since developing the prototype, Motzki’s team has focused on specific application-driven system development in several fields, including automotive, domestic cooling/heating or industrial cooling, switching cabinets, data centers, and stationary battery systems.
More Uses of Shape-Memory Alloys: Metallic Memories for Lego-Like Joinery
“Technology maturity is being pushed increasingly worldwide, so I expect new breakthroughs in system performance in the upcoming years, as well as the first commercialization efforts in selected application fields,” Motzki said.
The research team is now developing systems for use in vehicles, with the goal of achieving commercialization within five years. As part of this effort, the team is collaborating with German automaker Volkswagen AG to use elastocaloric technology to develop a lightweight, low-energy air-conditioning system for electric vehicles and to develop an efficient way to the traction batteries in EVs.
The team is also developing an elastocaloric air conditioning system that can be used to cool and heat individual rooms of a residential buildings via ventilation slots in the outer walls.
Advances in Cooling Technology: New Year, New Refrigerants for HVAC Systems
“The technology itself enables us to consider completely new ways to think ‘cooling’—for example, maybe flat geometries that can be integrated into furniture or textiles,” Motzki said. “Suddenly new medical applications become possible, plus many more things we haven’t even thought of yet.”
The researchers are also looking for additional elastocaloric materials and special alloys in addition to nitinol.
“The amounts of needed materials will increase exponentially, once the technology is market-ready,” Motzki said.
Mark Crawford is a technology writer in Corrales, N.M.
Based on these thermal and mechanical properties of nitinol, an SMA made of nickel and titanium, a research team at the Saarland University in Germany has developed an air conditioning technology that works by manipulating bundles of ultrathin nitinol wires or sheets and then letting them relax. Heat can be generated in one location and then dissipated in another location.
Nitinol has two solid-state phases—martensite, formed at lower temperatures, and austenite, which is found at higher temperatures. For shape-memory applications, the alloy is deformed into a new shape in its martensite phase; when heated into the austenite phase, the metal “remembers” the original shape and tries to return to it.
Elastocaloric technology uses this effect in reverse, stressing or relaxing SMA materials to absorb or release heat. When put under tension, the material releases heat into the air; when relaxed, the material extracts heat from the surroundings. A prototype heating and cooling system built by a team at Saarland University led by Paul Motzki, professor and chair of smart material systems for innovative production, relies on a track that continuously rotates bundles of nitinol wires slightly thicker than human hairs. Depending on their location on the track, the wires are either pulled or relaxed to enable them to move heat from one side of the device to the other.
According to Paul Motzki, “The technology itself enables us to consider completely new ways to think ‘cooling’—for example, maybe flat geometries that can be integrated into furniture or textiles.” Photo: Oliver Dietze
“Based on the elastocaloric effect,” Motzki said, “the SMA material needs to be mechanically deformed and heat needs to be efficiently transferred. In systems engineering, there is a need for a parallel development of an efficient drive technology with kinematics, mechanical force transmission, and thermodynamics regarding heat transfer and fluid dynamics. The combination of these different aspects of mechanical and electrical engineering is what makes this work unique.”
The Saarland team displayed a mini-refrigerator based on their prototype elastocaloric cooling system at the Hannover Messe earlier this year.
New Directions
For Motzki, the biggest surprise was the superior performance of a first prototype that had yet to be optimized, showcasing the technology’s big potential. Whether using the system for cooling or heating, the team was able to achieve temperature differentials of around 20 °C for a single-stage component. Multi-stage systems should be able to achieve much greater temperature differences.Since developing the prototype, Motzki’s team has focused on specific application-driven system development in several fields, including automotive, domestic cooling/heating or industrial cooling, switching cabinets, data centers, and stationary battery systems.
More Uses of Shape-Memory Alloys: Metallic Memories for Lego-Like Joinery
“Technology maturity is being pushed increasingly worldwide, so I expect new breakthroughs in system performance in the upcoming years, as well as the first commercialization efforts in selected application fields,” Motzki said.
The research team is now developing systems for use in vehicles, with the goal of achieving commercialization within five years. As part of this effort, the team is collaborating with German automaker Volkswagen AG to use elastocaloric technology to develop a lightweight, low-energy air-conditioning system for electric vehicles and to develop an efficient way to the traction batteries in EVs.
The team is also developing an elastocaloric air conditioning system that can be used to cool and heat individual rooms of a residential buildings via ventilation slots in the outer walls.
Advances in Cooling Technology: New Year, New Refrigerants for HVAC Systems
“The technology itself enables us to consider completely new ways to think ‘cooling’—for example, maybe flat geometries that can be integrated into furniture or textiles,” Motzki said. “Suddenly new medical applications become possible, plus many more things we haven’t even thought of yet.”
The researchers are also looking for additional elastocaloric materials and special alloys in addition to nitinol.
“The amounts of needed materials will increase exponentially, once the technology is market-ready,” Motzki said.
Mark Crawford is a technology writer in Corrales, N.M.
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