From Heat to Cool: Elastocaloric Systems in the Defossilisation Journey

Heating and cooling account for nearly half of global final energy consumption in developed countries, surpassing both electricity (20%) and transportation (30%). These sectors are responsible for over 40% of the world’s energy-related CO2 emissions. With the rapid growth of emerging economies and the advancing threat of global warming, cooling demand is set to increase by 45% by 2050 compared to 2016.

Heating and Cooling Needs Across Sectors

• Buildings: Residential and commercial buildings require temperatures below 100°C, with systems ranging from small-scale solutions to large district heating networks. Each sector evolves at its own pace and demands tailored innovation.
• Industry: Industrial uses are split between low-temperature processes (food, chemicals) and high-temperature needs (over 1,000°C) for materials like steel, cement, and glass. These high-temperature processes remain predominantly dependent on fossil fuels (89% in 2020), and electrifying them efficiently, especially above 150°C, remains a major challenge.

Cooling is also crucial for various sectors, including urban buildings, storage, transportation, daily living, and industries such as agri-food, pharmaceuticals, medical, construction, and catering. Temperature requirements range from +14°C to -80°C for refrigeration and freezing, with cryogenic systems sometimes reaching as low as -150°C.

The Evolution of Cooling Technologies

The most widespread cooling technology today is based on the gas compression-expansion cycle, with air conditioning (AC) as its most advanced form. AC systems range from small single-room units to large installations for entire buildings and districts.

Most air-conditioning systems use electricity, but larger ones may also utilize natural gas, waste heat, or solar energy. Currently, ACs account for nearly 20% of global electricity use in buildings, a figure expected to rise with economic and demographic growth in warmer regions.

How Modern Cooling and Heating Systems Work

These technologies are based on the principle that liquids absorb heat when evaporating into gases and release heat when condensing back to liquids (the classical Carnot cycle). Special chemical compounds called refrigerants, which easily change states at low temperatures, are used in a closed-loop circuit.

Advanced Solid-State Materials: A New Era in Heating and Cooling

Traditional heating and cooling technologies, while effective, have significant drawbacks: high energy consumption and the use of refrigerants with high global warming potential. As a result, developing more efficient, environmentally friendly, and cost-effective alternatives is imperative.

One promising solution lies in the unique properties of certain solid materials (magnetocaloric, thermoelectric, and elastocaloric), which heat up or cool down when subjected to external stimuli, mimicking the heat transfer and entropy change of gas-based cycles:

• Magnetocaloric effect: Heat transfer occurs when a magnetic field is applied.
• Electrocaloric effect: Heat transfer results from an applied electric field causing polarization changes.
• Mechanocaloric effect: Heat transfer and temperature change result from mechanical stress.
• Barocaloric effect: Heat transfer is produced by isostatic pressure.
• Elastocaloric effect: Temperature changes occur during deformation (stretching, compressing, twisting).

Elastocaloric materials exhibit marked temperature changes when mechanical stress is applied and then removed, a process known as the elastocaloric effect. This reversible thermal response offers promising alternatives to traditional vapor compression refrigeration, as demonstrated by laboratory and proof-of-concept studies.

Understanding the Elastocaloric Effect

Common elastocaloric materials include shape memory alloys (SMAs) (Nickel based: Nitinol, NiTi) and cross-linked polymers (elastomers). They undergo phase changes activated by specific temperatures and stress levels, depending on their properties with a cycle very similar to a Carnot cycle.

• Stress Application: Applying mechanical stress (from 0 to σmax) increases the temperature of the material due to phase transformations (e.g., martensite in shape memory alloys, strain-induced crystallization in elastomers).
• Latent Heat Release: Once the maximum stress and temperature are reached, the material releases latent heat, which is then dissipated via a heat sink.
• Stress Removal: Releasing the stress causes the material to cool further (e.g., transition to austenite in alloys, amorphous phase in elastomers).
• Heat Absorption: The material absorbs heat from the environment, returning to its initial temperature.

The cycle is reversible: once the stress is removed, the material regains its original structure and the temperature change is reversed. This process can substitute core elements of vapor-compression cycles: using elastomeric materials in place of refrigerants, actuators instead of compressors, and solid-state phase changes instead of evaporation and condensation.

Optimizing Elastocaloric Systems: Balancing Potential and Efficiency

The performance material coefficient (COPmat) is key to assessing the cooling potential of an elastocaloric material. 

Certain elastocaloric materials can achieve values of COP exceeding 70%, outpacing conventional refrigerant-based heat pumps (typical COP between 40% and 60%).  However, the overall system COP is considerably reduced du to the loss in the heat exchanger, actuator, regeneration and auxiliary power use. Efficiency suffers due to mechanical and thermal losses, limited heat/work recovery, and the need for energy-consuming auxiliaries. 

While advances in materials and actuators have improved prototypes, it remains unclear when elastocaloric systems will surpass conventional heat pumps in overall efficiency.

Advantages & Challenges

Advantages

• Higher efficiency and larger temperature swings than other caloric effects.
• Substantial reduction in greenhouse gas emissions and environmental impact.
• Elimination of harmful refrigerants (no volatile fluids or gases involved).
• Reusable and recyclable system components support sustainability.
• Noise reduction compared to traditional systems.

Challenges

• Material fatigue: Repeated mechanical cycling can damage elastocaloric materials, especially shape memory alloys (like NiTi), shortening lifespan.
• Ensuring long-term stability and power density is challenging.
• Scaling up for larger applications and maintaining performance at manageable costs is difficult.
• Current overall system COP is limited by inefficiencies in actuators and heat exchangers.
• Frequent replacement due to fatigue could increase maintenance and environmental costs.

Social Acceptability

Elastocaloric technology could meet critical cooling needs, especially in regions most vulnerable to climate change such as developing countries. 

For this technology to succeed, costs must become affordable, while, at present, they remain high.  Deployment must also factor in urban planning, especially regarding the size of cooling devices. 

Like many emerging technologies, elastocaloric systems rely on critical materials. Studies highlight supply risks in these value chains (especially for nickel) and a potential rise in costs as demand increases.

Contributions and Acknowledgments:

• Enrico Biagini
• Maria Rosanne 
• Benjamin Metayer 
• Camille Riviere
• Elodie du Fornel
• Jan Mertens
• Jean-Pierre Keustermans
• Céline Denis
• Olivier Sala

To Go Further:

More references, information, examples and use cases in ENGIE's 2025 report on Emerging Sustainable Technologies. 

> Download the 2025 report on Sustainable Emerging Technologies <