Passive Radiative Cooling – Sending Heat to Space and Reducing Power Consumption of Active Cooling

Keywords: passive cooling, energy saving, EPBD, radiative cooling, global warming, thermal emissivity, cool roof

Introduction

According to the IEA Global Energy Review 2025, electricity demand in 2024 was almost twice as high as the average of the last decade, while the global temperature rise remained unabated. One of the main drivers of the increase in energy demand was active cooling [1]. One of the central questions in this context is how we can reduce this energy demand efficiently and sustainably. To take this question to the extreme: Do we always need (electrical) energy to realize cooling? In this article, we would like to provide an answer based on a novel material that has amazing properties. This material can not only cool surfaces below ambient temperature even in bright sunlight but also send some of the thermal energy back to where it came from: into space. After an initial introduction to the material and its applicability for passive cooling, we will also show examples of how it can be used in combination with active coolants. We will illustrate the energy saving potential in these cases using real measurements. In the course of our explanations, we will also give indications of further necessary research.

Passive Radiative Cooling (PRC)

The relevant physical phenomenon is that of passive radiative cooling (PRC). It is a natural phenomenon that has been used by humanity for centuries, especially in the Arab world. It can usually be experienced in winter when the surface of cars is frozen, although the outside temperatures have not dropped below 0°C. It was only a decade ago that a detailed description of the physical mechanisms behind this phenomenon was created. In 2014, Prof. Raman of Stanford University published his groundbreaking article on passive radiative cooling (PRC) [2]. This article encouraged many scientists to work on creating mass-producible materials that exhibit this cooling effect. One of these scientists, Dr. Suemitsu, founder and CEO of SPACECOOL, managed to develop a simple and thin PVC film that can be easily applied to any type of surface. The effect of this foil on a metal surface is shown in Figure 1. The diagram in the figure compares the temperatures of a) a bare metal surface, b) a metal surface covered with reflective paint, c) the ambient air temperature, and d) the metal surface covered with the SPACECOOL PRC film. The temperature curves show that the cooling effect of the radiative cooling material cools the surface of the steel plate by more than 40°C at noon. What's more, the temperature of the steel plate remains below the ambient temperature throughout the day. So, we are looking at a material that is not only able to reject the heat introduced by sunlight, but also to compensate the heat convection by the surrounding air. This, of course, raises the question of how this is possible. We have already given a technical term as an answer: PRC – passive radiative cooling. In the following, we will fill this term with some more details. Simply put, PRC converts heat into light of a certain spectrum. This allows the heat to be radiated through the atmosphere back into space. The frequency band in which this is possible is also called the “atmospheric window”. Figure 2 shows the two important areas in the electromagnetic spectrum: First, an effective PRC material must have exceptional reflectivity in the solar spectrum, including visible light; Second, the material must be able to emit heat in the atmospheric window, an infrared band that contains wavelengths between 8 and 13μm. The combination of these two effects – solar reflectance and thermal emissivity (radiation) of heat through the atmospheric window – leads to a reduction of surface temperatures. As shown in Figure 2, this can lower the temperature below ambient temperature. Both effects must achieve an efficiency of over 90%, and in fact, current radiative cooling materials such as SPACECOOL show an efficiency of 95% and more in both areas. As a result, thermal emission in the range of 70–100 W/m² can be achieved. This means that up to 100 W of heat is emitted back into space and thus does not contribute to global warming. Thus, it leads to a direct reduction of the warming of the atmosphere by sunlight.

Figure 1. Steel Plate Measurements.

Figure 2. Spectrum.

Measurements vs. lack of standards

Because the scientific description of PRC is so new, there are no standards and regulations on it. The measurements shown in Figure 3 were done as part of the PaRaMetriC project [3], which is currently being carried out by leading meteorological and physical institutes in Europe. This project represents a very important first step towards standardization and will be an important milestone on the way to effective comparability between different materials. The measurement data show how well the radiative cooling materials achieve the aforementioned two effects: In the range of wavelengths up to 1.7 µm, they are extremely reflective and therefore show minimal emissivity (and therefore maximum reflectivity), while in the atmospheric window, which is referred to as the IR atmospheric window, the emissivity reaches a maximum, well over 0.9 (where 1 is the emissivity of an ideal black body). The yellow line indicates the performance of a typical reflective colour that is significantly lower than that of the PRC materials [4].

Figure 3. Measurement Parametric Emissivity.

The measurements in Figures 1 and 3 clearly illustrate the novelty of the situation. Compared to insulating materials, which only slow down the heating of buildings and other objects on which they are applied, PRC materials completely block the penetration of heat and even reduce the temperature relative to the ambient temperature. The performance is also significantly higher than that of reflective paints. The measurements presented represent important first steps, but we need further improvements in terms of standards that allow to compare the cooling performance of the different PRC materials. The solar reflection index (SRI) [5], which was developed parallel to the “Cool Roof” approach, is not sufficient. In this concept a solid white reflective material has an SRI of 100, a solid black material that absorbs all sunlight gets an SRI of 0. Accordingly, the higher the SRI, the cooler the material. Although it is calculated by multiplication of the solar reflectance with the thermal emissivity, the SRI value does not indicate whether the material cools the surface well or not. A high SRI can come from a material with very high solar reflectivity but lower emissivity. It also tells nothing about the directional emissivity, like the measurements in Figure 4 show [4]. The cooling power of the materials is also not measured or indexed with the SRI. For a comparison of passive radiative cooling materials in real life, other factors such as humidity, transparency of the air or wind must also be considered. There is already research available on which geographic areas are most promising for the application of PRC materials [6], but this again needs to be tested in practice. All the questions discussed offer plenty of room for necessary academic research with the aim of developing new measurement standards.

Figure 4. Directional Emissivity Measurements Parametric.

PRC combined with active cooling

As mentioned at the beginning, there will be cases in which the sole application of PRC materials for cooling purposes makes sense. This would be particularly the case if the necessary (electrical) energy for active cooling is not available or affordable. We plan to cover such cases in detail in a separate article. In this article, we would like to focus on two cases where PRC and active cooling are combined: 1) application to the outdoor units of cooling systems (for cooling and refrigeration) and 2) application to buildings equipped with active cooling systems.

Currently, the application to outdoor units of active cooling systems is usually carried out as a retrofit solution. Application of PRC material to already installed cooling systems shows clear positive effects in terms of reduced energy consumption, minimizing failures and extending the life of the systems. A further improvement of the aforementioned positive effects on the system is to be expected if the PRC material would already be implemented at design stage. Figure 5 shows the case of such a retrofit installation on the outdoor units of a cooling system in Kanagawa, Japan. The reduction in surface and interior temperature led to a reduction in electricity consumption of almost 20%. This is a typical example of the possible performance improvements when applied to outdoor units of active cooling or refrigeration systems. Of course, the effect varies depending on factors such as the surface on which the radiative cooling material is applied, the orientation of the outdoor units, the size, etc. We can't go into more detail in this article, but this is clearly another field for academic activities to develop appropriate modelling and simulation.

Figure 5. Measurements of effect on outdoor unit with applied PRC foil.

Figure 6 now shows case 2) - the effect when applied to unit houses with metal walls and roofs and an active AC system. The house on the right side of the picture is completely covered with an adhesive PRC film from SAPCECOOL. The buildings have an insulation layer of 3 cm on the inside. The reference building on the left side of the picture differed only in that the PRC film was not applied to the surface. The buildings are located in Saudi Arabia. The measurement data taken at summer 2024 show that the surface temperature at midday was reduced by almost 8°C compared to the environment. The average reduction in daily electricity consumption for the air conditioning system was almost 30%. In concrete terms, this meant an average reduction in electricity consumption of 34 kWh per day. This would sum up to a reduction of 21 tons of CO₂ equivalent over 15 years. The findings of this field study motivated the manufacturer to equip all unit houses with the PRC surface from SPACECOOL while the material is now officially recognized by the Saudi government's Oil Sustainability Program (OSP).

Figure 6. a) Unit Houses KSA, b) Measurements.

Environmental and sustainability considerations

Of course, there is also the question of what the CO₂ footprint of a PRC material looks like. While we have discussed the immense energy-saving potential and the resulting reduction in the CO₂ footprint, the impact of the material itself may not be forgotten. This cannot be answered in general, as the material compositions of the available solutions vary. As an example, we can consider the latest version of the adhesive film of SPACECOOL, which has an LCA value of 0.67 (kgCO₂/m²) based on IDEAv3. Such a low value underlines how efficient PRC materials can be for reduction of carbon footprint. Especially in Europe, it is also important to mention that the SPACECOOL material does not contain PFAS at all. This is not the case with all materials on the market.

Conclusion

The practical examples in this article should illustrate the positive impact that can be expected when passive radiative cooling materials are combined with active cooling systems. Furthermore, they also should motivate to improve regulations and directives so that the positive effect not only on the energy performance of buildings but also the unique ability of these materials to actively contribute to a reduction of global warming is properly accounted for. While showing a minimal carbon footprint it is probably the only passive technology that is capable of emitting heat back to space.

Finally, it should also encourage researchers to work on further improvements of the materials as well as on the mentioned open questions.

References

[1]     https://www.iea.org/reports/global-energy-review-2025.

[2]     Raman, A., Anoma, M., Zhu, L. et al. Passive radiative cooling below ambient air temperature under direct sunlight. Nature 515, 540–544 (2014). https://doi.org/10.1038/nature13883.

[3]     https://parametric.inrim.it/home.

[4]     Adibekyan, A., Schumacher, J., Pattelli, L. et al. Emissivity and Reflectivity Measurements for Passive Radiative Cooling Technologies. Int J Thermophys 46, 66 (2025). https://doi.org/10.1007/s10765-025-03532-6.

[5]     https://coolroofs.org/documents/CRRC-SRI-Document_2024-04-17.pdf.

[6]     J. Huang, C. Lin, Y. Li, B. Huang, Effects of humidity, aerosol, and cloud on subambient radiative cooling, Int. J. of Heat and Mass Transfer 186 (2022). https://doi.org/10.1016/j.ijheatmasstransfer.2021.122438.