Materials That Seem Impossible: Cooling Buildings Without Electricity
Discover how passive radiative cooling materials reflect sunlight and release heat to the sky, potentially reducing building temperatures and cooling demand.
Materials That Seem Impossible: Cooling Buildings Without Electricity
What if future buildings could reflect the Sun’s heat and release thermal energy into the sky without relying entirely on air conditioning?
A growing challenge in a warming world
When summer temperatures rise, buildings absorb and accumulate heat. Roofs receive direct sunlight, walls become warmer, and indoor temperatures can increase until fans or air-conditioning systems become necessary.
During extreme heat, millions of buildings may need cooling at the same time. This increases electricity demand and can put considerable pressure on power grids.
Scientists are exploring a different approach: developing materials that use the natural properties of sunlight, infrared radiation and the atmosphere to keep surfaces cooler.
This technology is known as passive radiative cooling. Its main advantage is that the cooling process itself can take place without consuming electricity.
Could this approach help buildings cope with increasingly demanding summer conditions?
How can a surface cool down under the Sun?
To understand this technology, we need to start with two simple physical principles.
First, a surface heats up when it absorbs solar energy.
A dark roof absorbs a substantial proportion of the sunlight that reaches it. As a result, its temperature can rise well above the surrounding air temperature.
Second, warm objects emit energy as thermal radiation.
Much of this radiation is infrared, which is invisible to the human eye but plays an important role in the transfer of heat.
Researchers have designed materials that take advantage of both processes. They aim to create surfaces that do two things particularly well:
Reflect a large proportion of incoming sunlight, reducing the amount of solar energy absorbed by the surface.
Emit thermal infrared radiation at wavelengths that can pass relatively efficiently through parts of the atmosphere and escape into space.
One important wavelength range is known as the atmospheric window, approximately between 8 and 13 micrometres (μm). Under suitable conditions, the atmosphere is relatively transparent to some infrared radiation in this range.
By reflecting sunlight while emitting thermal radiation, a specially designed surface can lose more heat than it gains. Under favourable conditions, its temperature can even fall below the surrounding air temperature.
The principle is not to create cold from nothing. It is to reduce the energy absorbed from the Sun and improve the way a surface releases heat.
What materials are used?
There is no single material that can do everything. Researchers are investigating different coatings, thin films and engineered structures designed to control how surfaces interact with sunlight and infrared radiation.
Special reflective coatings
Some coatings are designed to reflect a large proportion of incoming solar radiation. Applied to roofs, they can reduce the amount of solar energy absorbed and help limit surface heating.
Polymer films
Thin polymer films can incorporate microscopic structures that influence how light and thermal radiation interact with the material. Depending on their composition and design, they can combine high solar reflectivity with strong infrared emission.
Multilayer materials
These materials combine several layers with different optical properties. The layers are designed to control which wavelengths are reflected and which are emitted, helping the surface reject solar energy while releasing heat through infrared radiation.
Nanoparticle-based coatings
Nanoparticles can be incorporated into coatings to modify their optical and thermal properties. Their performance depends on the materials used, the particle size and distribution, and the structure of the coating.
Some of these approaches have been experimentally tested, while others still require further development. Cost, durability, resistance to weathering and large-scale manufacturing remain important challenges.
What happens when outdoor temperatures reach 45°C or even 50°C?
This is one of the most important questions for the future of buildings.
When outdoor temperatures become extreme, a roof exposed to direct sunlight can become even hotter than the surrounding air. A surface that reflects much of the incoming solar radiation can limit this heating and potentially reduce the heat transferred into the building.
However, cooling a roof is not the same as cooling an entire building.
Indoor temperatures also depend on insulation, windows, shading, ventilation, humidity, the building’s thermal mass and the heat generated inside.
Weather conditions matter too. High atmospheric humidity and cloud cover can restrict the escape of infrared radiation into space and reduce the effectiveness of radiative cooling. The material’s design and orientation also influence its performance.
For these reasons, it would be misleading to claim that a special coating could keep every building comfortable when the outdoor temperature reaches 50°C.
The more useful questions are practical ones:
How much cooler can the roof become? How much heat can it prevent from entering the building? How does it perform in different climates? And how much electricity could it save by reducing the need for conventional air conditioning?
These are the results that need to be measured under real-world conditions.
Why are these materials not used on every building?
If the principle is so promising, why has it not become standard practice everywhere?
There are several reasons.
Durability: Materials must withstand sunlight, rain, dust, pollution and years of exposure to changing weather.
Cost: Some advanced coatings and multilayer structures require complex manufacturing processes. Their cost must be competitive with existing roofing materials and cooling solutions.
Performance: The amount of heat a material can release per square metre may not be sufficient to meet the full cooling demand of an entire building.
Environmental conditions: Humidity, clouds and the surrounding environment can reduce the amount of heat that escapes through the atmospheric window.
Maintenance: Dust and surface degradation can alter a material’s reflectivity and thermal performance over time.
These challenges mean that passive radiative cooling is not a universal replacement for air conditioning. Its value will depend on the material, the climate, the building design and the way the technology is installed.
A new way to design buildings
For decades, the main response to rising indoor temperatures has been to use machines that remove heat from inside a building and transfer it outdoors.
Passive radiative cooling offers an additional strategy: instead of relying only on removing heat after it has accumulated, we can also design building surfaces to absorb less solar energy and release heat more effectively.
Combined with good insulation, external shading, suitable ventilation and efficient cooling systems, these materials may help reduce the energy needed to keep buildings comfortable.
The greatest potential may lie in combining several approaches rather than relying on a single innovation.
The challenge is to turn promising laboratory results into affordable, durable and effective solutions for real buildings, particularly in regions facing increasingly severe summer heat.
Conclusion: Can we use the sky to cool our buildings?
Passive radiative cooling shows how natural physical processes can be used to reject solar energy and release heat without directly consuming electricity during the cooling process.
It is not a miracle solution to extreme heat, but it is a promising technology that deserves attention as temperatures rise and the demand for cooling increases.
Its real value will depend on how effectively it can reduce heat entering buildings, lower air-conditioning demand and maintain its performance over many years.
Perhaps future buildings will not rely only on more powerful machines to cope with heat. They may also use intelligently designed materials that absorb less solar energy and release part of their heat towards the sky.
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Scientific references
Materials Today Energy (2024).
Energies (2024).
Note: Passive radiative cooling can reduce surface temperatures and potentially lower cooling demand, but results vary with the material, building design and local weather conditions. Real-world performance should be measured rather than assumed.
Font: Suhendri et al. (2020), “Implementation of Passive Radiative Cooling Technology in Buildings: A Review”, Buildings, 10(12), article 215. MDPI. Llicència CC BY 4.0.
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