The stakes are enormous. According to the researchers, cooling and heating now account for almost half of global energy consumption, a share that keeps climbing as heatwaves intensify, cities swell and server farms multiply. Meanwhile, many common refrigerants are themselves potent greenhouse gases that contribute to global warming when they escape into the atmosphere. The technology carrying this burden is the vapor-compression cycle, in which an electricity-driven compressor transfers heat carried by a refrigerant from one location to another — the same basic principle that has served fridges, air conditioners and industrial chillers since the early twentieth century. Elastocaloric solid-state cooling has long been considered one of the most credible alternatives. It relies on a remarkable property of shape-memory alloys: they tend to cool down once a previously applied mechanical load is released. No fluids, no compressor, no fluorinated chemistry — just a metal that changes its crystal structure on command. But until now, even these futuristic systems shared one trait with their ancestors: an electric motor did the heavy lifting.

The physics behind the cooling effect is as striking as it is elegant. When a superelastic shape-memory alloy is mechanically stressed, its crystal lattice abruptly reorganizes from the austenite phase into the martensite phase — a diffusionless reshuffling of atoms that releases latent heat and warms the material. The decisive moment comes when the load is released. The lattice snaps back, the reverse transformation absorbs latent heat from the surroundings, and the material’s temperature plunges by several degrees Celsius within a fraction of a second. Cycled rapidly between loading and unloading, a thin strip of nickel-titanium becomes a heat pump built from solid metal: no refrigerant to leak, no compressor oil, no evaporation and condensation. Among the family of so-called caloric effects — magnetocaloric, electrocaloric and barocaloric — the elastocaloric effect is prized for the magnitude of its temperature change and for relying on nickel-titanium, an alloy that medicine and industry have manufactured at scale for decades.

Yet one stubborn caveat has shadowed elastocaloric cooling from the beginning: something still has to do the squeezing. Every elastocaloric system demonstrated to date has depended on an electrically driven actuator to generate the force that triggers the phase transformation — meaning that even a “solid-state” cooler ultimately needed a motor, and therefore grid electricity, to run. That electrical umbilical cord has left the technology blind to its most tantalizing fuel: abundant low-temperature heat. Industrial waste streams, vehicle exhaust, engine coolant and rooftop solar-thermal collectors all deliver heat in enormous quantities, and cooling demand peaks precisely when the sun blazes — but heat cannot turn an electric motor. The question that has hovered over the field is disarmingly simple: could the actuator itself be driven by heat, so that a cooling system could feed on the very warmth that would otherwise be thrown away? The KIT-led team has now answered it in experiment for the first time.

Their solution is a piece of materials choreography. The prototype couples two ultra-thin nickel-titanium films with complementary functions. The first film works as a heat-responsive actuator built on the classic shape-memory effect: once it is heated up, it starts to shrink, converting thermal energy directly into mechanical work — a genuine pulling force generated without any electric motor in the loop. That motion immediately transfers to the second film, which serves as the elastocaloric refrigerant. Under cyclic loading and unloading, the second film undergoes reversible alterations in its crystal structure that generate cold. In one stroke, heat replaces the electric actuator that previously stood at the heart of every elastocaloric cooling system. “The crucial innovation is that we combine two complementary functions of shape memory alloys, with one film converting heat into mechanical work and the other film converting this work into cold,” said Dr. Jingyuan Xu, who leads the Young Investigator Group of the ZEco Thermal Lab at KIT’s Institute of Microstructure Technology (IMT). “This way, we’re establishing a new approach to drive solid-state cooling, thereby opening up exciting possibilities for the use of waste heat and solar energy.”

The measurements that matter have now been delivered. At an actuator temperature of 86 degrees Celsius — a level comfortably within reach of solar-thermal collectors, engine coolant loops and countless industrial processes — the prototype achieved a temperature difference of 4 degrees Celsius on the component level, while the temperature change inside the elastocaloric refrigerant itself amounted to nearly 13 degrees Celsius. Those figures, the researchers emphasize, represent the first experimental proof that a heat-driven elastocaloric system can genuinely produce cold rather than merely exist on paper. The setup also operated reliably when fed by an external heat source providing 130 degrees Celsius, demonstrating that the concept can digest real-world heat sources rather than laboratory idealizations. “The decisive moment for us was when we were able to measure the cold that had indeed been generated by a heat-driven system,” said Yi-Ting Hsiau, lead author of the study and a doctoral researcher at the IMT. “This showed us that the principle doesn’t just work in theory.”

Conceptually, the device is a heat engine fused with a solid-state heat pump: the actuator film is the engine, converting a temperature difference into mechanical work, while the elastocaloric film is the pump, spending that work to absorb heat from the cold side. Because the working substance is a solid rather than a pressurized gas, there is nothing to leak, no fluorinated chemistry to regulate and no compressor to lubricate, maintain or eventually fail. And because the trigger is thermal, the system can in principle be attached directly to a waste-heat pipe or placed beneath a solar absorber, converting energy that would otherwise simply warm the atmosphere into useful refrigeration. The researchers are careful to frame the current device as a feasibility study rather than an optimized machine — but as a demonstration that heat can replace the electric actuator driving solid-state cooling, it is a first.

The application landscape is correspondingly broad. Processors in computers could, in a sense, cool themselves: the chips that need cooling also radiate the waste heat that would drive the elastocaloric cycle, a self-regulating loop for servers and data centers whose thermal appetite grows with every exabyte. In automobiles, sensitive electronics could be cooled using heat drawn from the drivetrain rather than by taxing the vehicle’s electrical system. Solar-driven air conditioning is an equally natural fit, since the same sunshine that heats a building supplies the energy to cool it, aligning cooling supply with cooling demand hour by hour. In every case the principle is the same: warmth that already exists does the work, and no fresh electricity needs to be spent.

Nobody involved presents the prototype as market-ready. The published device was designed explicitly as a feasibility study and is deliberately not optimized for maximum cooling capacity; the temperature spans it achieved, while scientifically decisive, remain far below what a domestic air conditioner must deliver. The path forward — already under way in the lab — is parallelization: connecting multiple shape-memory film pairs side by side, multiplying cooling power much as individual photovoltaic cells are tiled into solar panels. Scaling up will also mean confronting the fatigue that shape-memory alloys suffer under relentless mechanical cycling and engineering heat exchangers that can feed ultrathin films efficiently. The study was conducted in collaboration with the University of Tsukuba in Japan, which the researchers say paves the way for heat-driven solid-state cooling fit for practical use. Funding came from the Carl Zeiss Foundation through the CZS Nexus project, the Baden-Württemberg Foundation’s elite postdocs program, and the Hector Fellow Academy.

For Xu, the milestone is a starting line rather than a finish line. “We believe that this is only the beginning,” Xu said. “By scaling up this technology we want to develop compact cooling systems that leverage abundantly available heat sources for sustainable cooling.” If that ambition holds, the implications stretch far beyond a laboratory bench in Karlsruhe. Cooling is quietly becoming one of the defining energy problems of a warming century, and a machine that turns discarded heat directly into cold attacks the crisis from both ends — trimming electricity demand while putting neglected thermal energy to work. For a hundred years, cooling has been the story of electricity pushing heat around through chemical refrigerants. Two ribbons of nickel-titanium, driven by nothing more exotic than waste heat or sunlight, have now demonstrated a different ending: heat itself, set against heat, becomes cold.

This ambitious five-year project, underpinned by a significant investment of US$9.4 million from the Grundfos Foundation—the foundation’s largest research grant to date—aims to revolutionize urban cooling by developing intelligent, water-based sustainable systems tailored for megacities. The initiative, titled Sustainable Water-based Cooling in Megacities (SWiM), leverages the complementary strengths of Danish and Singaporean urban infrastructure innovation. Through integrated research spanning engineering, artificial intelligence, and urban planning, SWiM seeks to break the entrenched cycle of high energy consumption and carbon emissions caused by conventional cooling technologies.

District cooling and heating technologies form the foundational expertise upon which this project builds. Denmark, a global pioneer in district heating systems, has long demonstrated the efficiency advantages of centralized thermal energy distribution. Facilities such as the Avedøre Power Station and the Amager Bakke waste-to-energy plant epitomize cutting-edge combined heat and power technologies, providing sustainable, large-scale thermal solutions. Meanwhile, Singapore has adeptly adapted these concepts into district cooling networks optimized for tropical urban conditions. The Marina Bay district’s extensive underground chilled water pipeline system exemplifies this, significantly reducing carbon emissions citywide.

Despite these successes, current district cooling installations in megacities are typically limited in their geographical coverage and scalability. Business districts and housing estates can be served effectively, but extending these benefits to entire cities requires overcoming substantial technical challenges. The SWiM project directly addresses these barriers by focusing on scalable, modular cooling architectures enabled by advanced control systems. These systems are designed to respond dynamically to varying urban environments, demand fluctuations, and operational anomalies.

Central to the SWiM initiative is the development of autonomous control mechanisms that can ensure reliable, fault-tolerant operation without the need for constant expert supervision. This autonomy is critical for deployment in complex urban settings where human error, potential cyber-attacks, and equipment failures could otherwise compromise system integrity. Aarhus University’s expertise in electrical and computer engineering drives this domain, utilizing digital twin technologies that model physical cooling infrastructure and support adaptive control strategies. Such digital replicas provide real-time operational insights, enabling predictive maintenance and optimal system adjustments.

Artificial intelligence plays a transformative role in the SWiM framework. By integrating machine learning algorithms, the system can monitor performance continuously, detect inefficiencies or faults early, and employ predictive analytics to prevent downtime. Notably, the project incorporates smart algorithms that balance the competing demands of cooling load, energy efficiency, and grid stability. This ensures that cooling systems contribute positively to the broader urban energy ecosystem rather than destabilizing it.

A distinctive aspect of this research is its focus on applicability under real-world conditions. SWiM’s approach transcends laboratory testing by constructing physical testbeds at multiple scales — room, floor, and building levels — within Singapore’s urban fabric. These physical environments will be complemented by comprehensive digital twin simulations, enabling scalable replication of system behavior across various city scenarios. Such rigorous validation is essential for transitioning innovations into practical, large-scale solutions that city planners and policymakers can adopt confidently.

The collaborative nature of SWiM, uniting Singaporean and Danish academic and industrial stakeholders, embodies a model for global scientific partnership. With Grundfos Foundation’s funding strategically underpinning the initiative, industry knowledge will be deeply integrated into research outcomes to ensure feasibility and immediate applicability. This collaboration is particularly timely as both Singapore and Denmark pursue ambitious climate objectives—Singapore targeting net-zero emissions by 2050 and Denmark aiming for climate neutrality by 2045.

Professor Madhavi Srinivasan of NTU Singapore highlights the convergence of interdisciplinary expertise in this project, noting how the blend of sustainability science, engineering, and artificial intelligence can yield cutting-edge urban cooling solutions. Similarly, Professor Rafael Wisniewski of Aalborg University underscores the importance of developing systems that are not only theoretically sound but also resilient and user-friendly, capable of deployment without reliance on specialist intervention.

The envisioned integration of digital tools such as Building Information Models (BIM) with real-time monitoring systems promises unprecedented precision in managing energy flow and cooling demands. Professor Peter Gorm Larsen of Aarhus University elaborates on how digital twins will facilitate seamless transitions between operational states, ensuring that cooling resources are allocated efficiently under varying conditions.

SWiM’s innovations aim to disrupt the current paradigm, making cooling systems vital components of sustainable urban infrastructure rather than significant contributors to environmental degradation. By combining low-energy water-based cooling methods with intelligent control architectures and comprehensive urban planning tools, the project charts a visionary pathway for megacities grappling with the twin crises of urban heat and climate change.

As urban populations continue to expand, particularly in tropical megacities, the stakes for sustainable cooling solutions have never been higher. SWiM represents a bold leap forward, promising to reduce city-wide energy consumption for cooling by up to 30 percent—a transformative achievement with profound implications for global carbon emissions and urban liveability.

In the coming years, the success of SWiM will be measured not only by technological milestones but also by its ability to influence policy, shape standards, and catalyze widespread adoption of sustainable cooling infrastructures worldwide. This initiative underscores the critical role of cross-border collaboration and innovation in addressing one of the defining environmental challenges of our time.

Subject of Research: Sustainable urban cooling systems for megacities involving water-based, intelligent district cooling technologies.

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Image Credits: Rasmus Reimer Larsen

Keywords: Applied sciences and engineering, Systems engineering, Mechanical engineering, Electrical engineering, Civil engineering, Computational science