Elastocalorics - how shape memory alloys can revolutionize next era of cooling

The world is moving toward sustainable solutions in every sector, including cooling technologies, which play a vital role in modern life. From air conditioners and refrigerators to industrial applications, cooling systems are responsible for significant energy consumption and greenhouse gas emissions. Enter elastocalorics and shape memory alloys (SMAs)—a groundbreaking approach with the potential to redefine the cooling landscape

What Are Elastocalorics?

Elastocalorics refer to the temperature change that occurs in certain materials when they undergo stress or strain. Unlike traditional cooling technologies that rely on refrigerants and vapor-compression cycles, elastocaloric cooling exploits a solid-state phase transformation in materials such as shape memory alloys.

The key lies in the latent heat absorbed or released during the transformation, which can efficiently cool or heat a space without harmful chemicals or significant energy input.

Understanding Shape Memory Alloys (SMAs)

What Are SMAs?

Shape memory alloys are a unique class of materials that "remember" their original shape. They can undergo a phase transformation between two states:

1. Martensite (low-temperature phase): Soft and easily deformable.
2. Austenite (high-temperature phase): Hard and stable.

When stress or heat is applied, SMAs transition between these phases, enabling their unique properties.

Why Are SMAs Perfect for Elastocalorics?

SMAs exhibit excellent elastocaloric effects because they release or absorb large amounts of heat during phase transformations. This makes them ideal for solid-state cooling systems. Common SMAs used for elastocalorics include nickel-titanium (NiTi) alloys, known for their durability and high transformation efficiency.

The Mechanics of Elastocaloric Cooling: A Deeper Dive

Elastocaloric cooling is based on the unique thermo-mechanical properties of shape memory alloys (SMAs), leveraging their ability to undergo solid-state phase transformations under stress. Unlike conventional cooling technologies, which rely on chemical refrigerants and mechanical compressors, elastocaloric systems use the latent heat associated with phase changes in SMAs for efficient and eco-friendly cooling. Below, we break down the process into more detail:

1. Application of Stress: Heat Release

Phase Transformation: When a mechanical force (tensile, compressive, or torsional) is applied to an SMA, it undergoes a solid-to-solid phase transformation from the martensite phase (low-temperature) to the austenite phase (high-temperature). This transition is exothermic, meaning heat is released to the environment.

Mechanism of Heat Release: The transformation involves rearranging the atomic structure of the SMA. In the martensitic phase, the crystal structure is distorted, while the austenitic phase has a more ordered, higher-energy arrangement. The additional energy required for this transition comes from the material's internal energy, which is released as heat.

Material Selection: Materials like nickel-titanium (NiTi) are ideal because they exhibit a large latent heat of transformation (on the order of 20-30 J/g) and can sustain repeated phase changes without significant degradation.

2. Release of Stress: Cooling Effect

Reverting to Martensite: When the mechanical force is removed, the SMA reverts from the austenite phase back to the martensite phase. This transition is endothermic, meaning the SMA absorbs heat from its surroundings, effectively cooling the environment.

Microstructural Mechanism: The reversion process involves the reorganization of the crystal structure into a lower-energy, distorted martensitic phase. This structural reorganization "pulls" heat from the surrounding medium, creating the cooling effect.

3. Heat Exchange System

Heat Absorption and Release Management: To harness the temperature changes effectively, elastocaloric cooling systems are equipped with heat exchangers that transfer the heat released during the austenite phase to the environment and extract heat from the surroundings during the martensite phase.

Design Considerations:

Thermal Coupling: The SMA must be in close thermal contact with the heat exchanger to minimize thermal resistance.

Cycle Optimization: The cyclic application and removal of stress must be finely tuned to maximize the temperature lift and cooling efficiency.

4. Cyclic Stress Application

Continuous Cooling: By repeatedly applying and removing stress in a controlled cyclic manner, the system produces continuous cooling.

Stress-Inducing Mechanisms: Stress can be applied through mechanical actuators, rollers, or bending mechanisms, depending on the system design. The mechanical input energy is typically much lower than the energy required by compressors in traditional systems.

Companies using  Elastocaloric Cooling Systems

Several organizations are at the forefront of developing elastocaloric cooling technologies:

• Fraunhofer Institute for Physical Measurement Techniques (IPM): Through the "Elasto-Cool" project, Fraunhofer IPM is creating heat pumps based on the elastocaloric effect, aiming to eliminate harmful refrigerants and boost energy efficiency by 20-30%.Fraunhofer IPM

• Hong Kong University of Science and Technology (HKUST): Researchers at HKUST have developed a multi-material cascading elastocaloric cooling device using nickel-titanium (NiTi) SMAs, achieving a temperature lift of 75 K, surpassing previous records.

Global Energy Savings Potential

According to the International Energy Agency (IEA):

• Space cooling and refrigeration currently account for about 20% of global electricity consumption.
• The demand for cooling is expected to triple by 2050, becoming one of the largest drivers of global electricity demand.

If elastocaloric cooling systems replace traditional systems, the energy savings could be transformative:

1. Global Energy Reduction: A 20-50% reduction in energy consumption for cooling could save 600-1500 terawatt-hours (TWh) annually by 2050. For context, 1,500 TWh is roughly the annual electricity consumption of the entire African continent in 2020.
2. Household Savings: For residential cooling, elastocaloric systems could cut energy costs by up to 30%, saving households hundreds of dollars annually depending on usage.

Carbon Footprint Reduction

Reducing energy consumption in cooling systems also reduces greenhouse gas emissions:

1. CO2 Emissions Avoided: By 2050, elastocaloric cooling could prevent the release of 1-2 gigatons of CO2 annually, equivalent to the emissions from 200-400 million passenger vehicles.
2. Elimination of Refrigerants: Elastocaloric systems eliminate the need for hydrofluorocarbon (HFC) refrigerants, which have global warming potentials up to 1,000-4,000 times that of CO2.

Conclusion

Widespread adoption of elastocaloric cooling could save hundreds of terawatt-hours of energy annually, reduce household and industrial energy costs, and significantly cut greenhouse gas emissions. This would make elastocaloric technology a cornerstone of sustainable cooling and energy efficiency efforts globally