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.

Radiative cooling, a process by which surfaces lose heat by emitting infrared radiation to the cold outer space, has emerged as a promising strategy to curb energy consumption associated with traditional air conditioning. Materials optimized for this purpose typically require high reflectance in the solar spectrum to avoid heating during daytime, alongside strong thermal emission capabilities in the mid-infrared range. However, incorporating vivid coloration into such materials has proven difficult, as pigments usually absorb significant amounts of sunlight, thereby diminishing the cooling effect. The newly developed bilayer ethyl cellulose coating overcomes this limitation by finely tuning its structural properties to achieve both color vibrancy and solar reflectance exceeding 97%.

The key innovation in this work is the use of controlled drying-induced self-stratification during a single casting step. By carefully adjusting the concentration of the ethyl cellulose precursor solution, researchers manipulate the formation of a hierarchically structured bilayer. The top layer is meticulously engineered to have a thickness that produces specific colors via thin-film interference—an optical phenomenon where different wavelengths of light are constructively or destructively interfered as they reflect through a thin film. This approach eliminates the need for traditional coloring agents that absorb sunlight and reduce reflectance.

Beneath the colorful top layer lies a highly porous and scattering bottom layer that plays a critical role in the cooling performance. This bottom layer ensures exceptionally high reflectance across the solar spectrum, effectively minimizing solar absorption and unwanted heating during the day. Simultaneously, it exhibits strong emissivity in the long-wave infrared region, allowing thermal radiation to pass freely to outer space. This unique bilayer architecture thus harmonizes color customization with the stringent optical requirements of passive radiative cooling.

Experimental evidence demonstrated that the bilayer ethyl cellulose coating can achieve up to 9°C sub-ambient cooling under a solar irradiance of 800 W/m². This performance metric is particularly impressive considering the coating’s colored appearance, which traditionally compromises cooling efficiency. By outperforming commercially available colored paints and fluorescence-based colored coatings during field tests conducted in the humid subtropical climate of Hong Kong, the material proves its robustness and applicability in real-world, challenging environmental conditions.

Beyond exceptional cooling, the simplicity of the one-step phase-separation fabrication facilitates scalable manufacturing, which is crucial for practical deployment. Unlike multi-step or lithography-based methods common in layered thin-film materials, this approach drastically reduces complexity and costs. Moreover, the use of sustainable, biomass-derived ethyl cellulose aligns with growing demands for environmentally friendly materials in building and urban infrastructure applications.

Developing colored passive cooling materials has long been constrained by compromises between aesthetics and functionality. Traditionally, the incorporation of dyes and pigments introduced significant solar absorption, defeating the purpose of passive cooling by causing heat gain during sunlight exposure. The presented bilayer coating circumvents these issues by exploiting intrinsic optical effects rather than pigment-based coloration. This paradigm shift provides designers and architects with an unprecedented level of freedom to apply colored surfaces without sacrificing energy-saving benefits.

The hierarchical porous structure responsible for the high solar reflectance simultaneously contributes to durability and weather resistance. Porosity creates multiple scattering events that reflect incident sunlight efficiently, while the intrinsic nature of ethyl cellulose confers mechanical flexibility and environmental tolerance. These properties are vital for coatings intended for external applications on buildings, vehicles, and infrastructure where prolonged exposure to sunlight, moisture, and mechanical stresses is inevitable.

An additional merit of the approach is its broad spectrum tunability, achieved simply by controlling the top layer’s thickness during the drying process. This control enables the full color gamut via thin-film interference, from subtle pastels to vibrant hues, without any trade-off in radiative cooling performance. The seamless integration of color functionality with passive cooling technology could substantially accelerate the adoption of energy-saving surface coatings in urban areas, potentially mitigating urban heat island effects and reducing electricity demand for air conditioning.

Field testing in the subtropical climate of Hong Kong serves as a rigorous benchmark, given the region’s high humidity and intense solar radiation, which typically deteriorate radiative cooling efficacy. The bilayer ethyl cellulose coating’s superior performance in these conditions attests to its practical viability and reliability. Furthermore, the comparison with conventional colored paints and fluorescence-enhanced coatings underscores the material’s competitive advantages in both energy efficiency and appearance.

Looking forward, the integration of this innovative coating into building envelopes, vehicle exteriors, and outdoor equipment offers a new class of multifunctional materials. These could help reduce reliance on energy-consuming cooling systems, thus lowering carbon emissions and operational costs. Additionally, the bio-based nature of ethyl cellulose aligns with circular economy principles, potentially easing concerns related to material sustainability and end-of-life disposal.

In conclusion, the development of one-step-processed bilayer ethyl cellulose coatings represents a significant stride in the field of passive radiative cooling technology. By combining controlled self-stratification, thin-film interference coloration, and hierarchical porosity, researchers have crafted a material that not only achieves sub-ambient temperature reduction but also meets aesthetic and environmental standards. This discovery could catalyze widespread adoption of radiative cooling surfaces, marking a pivotal advancement in sustainable urban design and climate resilience strategies.

The synergy between advanced optical engineering and biomaterial science in this study highlights the transformative potential of interdisciplinary research. As passive cooling materials evolve, such innovations will be critical in addressing the escalating challenges posed by global warming and energy consumption. This promising work paves the way for future explorations into multifunctional coatings that harmonize energy efficiency, aesthetics, and sustainability in unprecedented ways.

Further research may delve into the long-term durability under diverse climatic conditions, large-scale manufacturability, and integration with existing building materials. Moreover, exploring other biomass-derived polymers for similar applications could widen the gamut of sustainable materials in this arena. Given the urgent demand for accessible cooling technologies worldwide, breakthroughs such as this bring us closer to cost-effective, scalable solutions that mitigate climate change effects while enriching built environments.

The implications of this study resonate beyond cooling surfaces. The principle of exploiting self-stratification and thin-film interference for property tuning can inspire innovations across photonics, energy harvesting, and sensor technologies. Ultimately, the intersection of green chemistry, nanostructured materials, and functional design embodied by this research embodies the forward path toward a sustainable and resilient technological future.

Subject of Research: Biomass-derived bilayer ethyl cellulose coatings for passive radiative cooling with full-color tunability through thin-film interference.

Article Title: One-step-processed bilayer ethyl cellulose for full-colour sub-ambient daytime radiative cooling.

Article References:
Liu, Y., Blagojevic, N., Xuan, Q. et al. One-step-processed bilayer ethyl cellulose for full-colour sub-ambient daytime radiative cooling. Nat Energy (2026). https://doi.org/10.1038/s41560-026-02039-0

DOI: https://doi.org/10.1038/s41560-026-02039-0

To address this gap, researchers have developed comprehensive testing procedures meant to analyze both the optical and thermal properties of radiative cooling materials, the specific methodologies of which can be adapted for a variety of formats. The first step in this systematic evaluation is to collect hemispherical reflectance and transmittance spectra using advanced equipment. Two integrating sphere spectrometers are employed to capture the solar spectrum, ranging from 0.3 to 2.5 μm, and the infrared spectrum, from 2.5 to 20 μm. This dual-spectrum analysis is accomplished within a span of two hours, laying the groundwork for a more nuanced understanding of how various materials interact with different wavelengths of light.

Following the spectral analysis, attention is shifted to evaluating the materials’ real-world performance. An outdoor performance-testing platform is meticulously designed to monitor temperature variations that arise when materials with distinct radiative cooling capabilities are deployed. Throughout this assessment, thermal insulation and radiation shielding become vital components to ensure accurate readings. Moreover, the setup takes into account various environmental variables, such as humidity, sunlight intensity, wind velocity, and external temperature, all of which play critical roles in the effectiveness of the radiative cooling strategies employed.

While outdoor testing presents a comprehensive approach to understanding these materials, challenges exist in fully replicating the myriad of factors found in uncontrolled environments. This prompted the development of a compact, indoor testing platform, which, although more limited, still serves as a crucial reference point in assessing the performance of radiative cooling materials. By simulating conditions that mimic real-world scenarios, the indoor mechanism facilitates controlled experiments to yield consistent and reproducible results.

An additional layer of sophistication comes into play with the incorporation of a Proportional-Integral-Derivative (PID) temperature control system. This advanced technology allows researchers to manipulate thermal environments more intricately, thereby simulating various application scenarios encountered in actual use cases with higher fidelity. The outdoor evaluations typically extend over a full week, while the indoor assessments can be concluded in just one day, giving researchers immediate access to data that can influence future material development.

Given the complexity of radiative cooling systems, rapid theoretical performance evaluations emerge as a necessity. To this end, a simple MATLAB-based code has been proposed that allows users to engage in swift analytical assessments. Within a mere ten minutes, researchers can glean crucial information regarding the potential effectiveness of new materials, thus accelerating the overall development process.

The importance of standardizing the evaluation of radiative cooling materials cannot be overstated. As markets for energy-efficient technologies gain traction globally, having a dependable method to assess materials will not only enhance competition but also stimulate innovation within this sector. By making these procedures accessible, researchers can collaborate more effectively, sharing insights and advancements that could lead to revolutionary improvements in radiative cooling applications.

Moreover, the implications extend beyond just technical evaluations; there is an inherent potential for broad societal benefits. In urban areas laden with heat islands, the implementation of radiative cooling technologies can lead to significant energy savings and lower electric bills, ultimately contributing to reduced greenhouse gas emissions. The widespread adoption of such materials can transform public infrastructure into sustainable entities that work in tandem with natural processes.

As scientists delve deeper into the world of radiative cooling, the anticipation surrounding new discoveries remains palpable. Innovations are expected to emerge that do not only enhance cooling efficiency but also utilize waste heat for other beneficial purposes. By marrying technology and sustainability, the radiative cooling frontier stands poised to redefine modern architecture, energy consumption, and environmental stewardship.

Researchers stand on the brink of unlocking a vast potential with radiative cooling materials. As protocols for testing and evaluating these materials become more refined, the pathway to commercial applications will inevitably become clearer. The bright future for sustainable technological advancements, rooted in rigorous research and collaborative effort, continues to illuminate possibilities for a more energy-efficient and environmentally conscious world.

Fundamentally, the drive towards sustainable solutions including radiative cooling techniques is a crucial stride towards addressing climate change and environmental sustainability. As we invest in and explore the capabilities of these innovative materials, it becomes evident that the journey of harnessing the sun’s power for cooling applications is just beginning. With a combination of strategic research, rigorous testing, and community collaboration, a renaissance in energy efficiency driven by radiative cooling seems not only plausible but probable.

Subject of Research: Radiative cooling materials and their performance evaluation methods.

Article Title: Characterization of radiative cooling materials.

Article References:

Wang, Z., Pian, S. & Ma, Y. Characterization of radiative cooling materials. Nat Protoc (2025). https://doi.org/10.1038/s41596-025-01273-2

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41596-025-01273-2

Keywords: Radiative cooling, thermal properties, optical properties, performance evaluation, sustainable materials.