News

Working with external project Cool White to test and suggest improvements on the locally available white paints

Background

A major challenge in dealing with climate change is the heat accumulated in buildings - for example in factories or schools and especially in the hot areas of Africa. Heat impairs the ability to concentrate and leads to a lower level of well-being. In addition, air conditioning systems require a lot of electricity to cool. Cooling systems are responsible for almost 20 % of electricity consumption and 10 % of greenhouse gas emissions worldwide - and the trend is rising.

Recently, a new class of materials has emerged, which can generate sub-ambient cooling when exposed to the sky, even under direct solar illumination. To achieve this effect, these coatings reflect all solar radiation while emitting thermal infrared radiation in a wavelength range where the Earth atmosphere is transparent. This allows them to dissipate their heat directly to the cold outer space which acts as a renewable heat sink, without any consumption of energy.

Despite a growing interest in developing these materials, there is still a lack of established techniques to evaluate and compare their cooling performance under real-world conditions - a key factor in fostering their adoption and market uptake.

Supporting public buildings in Africa with accurate temperature measurement

Metrology Partnership project Metrological framework for passive radiative cooling technologies (21GRD03, PaRaMetriC) is supporting a development cooperation project in Rwanda and South Africa called Cool White. This project aims to demonstrate the clear cooling effect of white roofs. By white painting the roofs of public buildings such as schools and factories and by advising on suitable paints, room temperatures should be reduced with positive implications for well-being and productivity.

The expertise of the PaRaMetriC consortium has been exploited to measure the optical properties of different Passive Radiative Cooling (PRC) white coatings available on the local market and to train the local Rwandan metrology institute to optimise the measurement of temperature and humidity inside the buildings.

This included assisting the local contacts to select suitable sensors to log ambient conditions, and to position them within the monitored buildings, as well as to provide guidelines on how to communicate the idea of passive radiative cooling more effectively to a non-specialist audience.

To quantitatively investigate the cooling effect of these white colors on the roofs, scientists from the Metrology Partnership project worked with colleagues from Rwanda and South Africa to record temperature and humidity data over a longer period of time, including different seasons and longer rainy seasons - before and after the coating.

The measurements, which are based on the average values of the individual sensors in the period from January 2023 to today, showed a significant drop in temperature after the roofs were painted white. Daytime temperatures observed in a factory building during the warmest six hours decreased by 9.2 °C under the roof (averaged over 4 months) and by 2.3 °C indoors.

This has a doubly positive effect on working conditions. It's not just the actual room temperature that is crucial, but also the significant reduction in infrared heat radiation from the ceiling. It has a positive effect on the subjective feeling of temperature.

PaRaMetriC project participant Dr. Albert Adibekyan, from PTB, Germany’s National Metrology Institute (NMI), explains the cooling effect of white colors in sunshine:

‘Passive Radiative Cooling (PRC) materials ideally allow for temperatures below the ambient temperature without additional air conditioning, even in direct sunlight. This is due to their optical properties, which reflect solar radiation very effectively and at the same time dissipate heat through the infrared transparency window of the Earth's atmosphere.’

For example, on August 7, 2023, an improvement of 20.4 °C was achieved under the roof: While the temperatures under the unpainted school roof were 50.2 °C, the thermometer under the painted school roof showed a temperature of 29.7 °C.

Dr. Christian Monte, also part of the PaRaMetriC project and from PTB, confirms:

‘By applying white paint to roofs, the conditions for schoolchildren and workers in developing and emerging countries are significantly improved. In addition, the resulting cooling effect creates a high potential for energy savings through not required air conditioning within buildings.’

The monitoring campaign is currently in progress.

Project Coordinator Lorenzo Pattelli from INRIM, Italy’s NMI, said

‘The idea that white materials heat up less than darker ones under the sun is part of our everyday experience. However, visible light is only a small fraction of solar irradiance: a material may look white to our eyes and still heat up due to absorbed radiation at different wavelengths. Part of this heat is inevitably re-emitted in the infrared spectrum, but the final heat balance is not trivial to predict due to the varying infrared transparency of the atmospheric window. For these reasons, understanding which paint may be most advantageous is not a simple task, and I was glad to see that the experience matured during the PaRaMetriC project allowed us to identify a promising solution based on the locally available products. The preliminary results coming from the test sites are extremely encouraging, showing major temperature reductions both inside and on the external surfaces of the buildings without any electricity consumption.’

The vision for Cool White is to scale the project, which will include developing a training programme for specialists from developing and emerging countries in order to paint as many roofs and surfaces as possible white using locally available paint.

As regards PaRaMetriC, an important task going forward will be to evaluate the potential energy savings deriving by the application of radiative cooling paints, as well as their stability against weathering agents. The long-term application scenario provided by our collaboration with Cool White provides an excellent opportunity to see these materials tested in the field over a prolonged time span.

This Metrology Partnership project has received funding from the European Partnership on Metrology, co-financed by the European Union Horizon Europe Research and Innovation Programme and from the Participating States.

Want to hear more about EURAMET?

Sign up for EURAMET newsletters and other information

Follow us on LinkedIn and X/Twitter