Passive Day Cooling Tech Promising for Building Temp Regulation

August 09, 2022 by Darshil Patel

Researchers develop an indoor test setup for determining the daytime cooling performance of passive cooling materials.

In recent decades, the use of air conditioning systems for cooling buildings has increased dramatically. With this, energy consumption, especially during the summer, has increased significantly. Moreover, energy consumption is expected to rise further with an improved standard of living and increased world population.


Passive Day Cooling Technology

Passive day cooling is a promising technology that maintains indoor temperature by avoiding the heating of buildings by solar radiation and dissipating accumulated heat by natural heat sinks without any energy consumption. 

The approach works with natural cooling, utilizing available day energy from the environment and building materials to dissipate heat. Therefore, natural cooling depends on the architectural design and how well the building materials can dissipate heat.


Passive Cooling Materials

Incorporating passive cooling materials within the building requires analysis of design variables to improve cooling, efficiency, and integration with building elements. The aim is to integrate and achieve high efficiency and comfort and provide natural lighting, which depends on air temperature, sun rays, wind, and humidity. It is essential to analyze their relationship to the building.


Graphical illustration of elements that affect the building’s indoor temperature. Image used courtesy of Bayreuth University


Usually, two techniques are preferred to evaluate a material's passive cooling performance: optical spectroscopy and field testing. The first technique determines the spectral absorption of material in both solar and mid-infrared regions to derive the passive cooling performance of the material. However, the optical properties of the materials are hard to determine, especially for materials with irregular surface topography, which can result in imprecise comparison.

Field testing is a controlled method to obtain the steady-state temperature of a material and its cooling power at ambient temperature. Here, the outdoor elements are mimicked in a controlled environment to reliably compare the cooling performance of different materials.

University of Bayreuth researchers have created a test system to characterize and compare materials used for passive cooling. The developed test system is the first step toward a standardized, globally accepted test system for comparing and analyzing high-performance passive cooling materials, according to the researchers.


The Test System for Passive Cooling Materials

The test system replicates the crucial elements that influence the cooling performance: sunlight, a thermal radiation absorber, a filter that only allows light rays of specific wavelengths to pass through, and a heatable gas flow to set up the surrounding temperature. The radiation absorber is an aluminum dome cooled with liquid nitrogen to mimic the nighttime environment. The setup, therefore, allows for the simulation of natural day and night environments on a miniature scale. The filter ensures low absorption in wavelengths between 0.3 - 2.5 micrometers and high emission over the mid-infrared regions required for an ideal daytime environment.


Dr. Qimeng Song, first author of the study, with the newly developed measurement setup. Image used courtesy of Bayreuth University


The researchers report that this is the only way to characterize cooling materials with high precision and the setup is robust, cost-effective, and reduces significant technical effort. The researchers tested and determined the cooling performance of three materials with their setup: a silver mirror (Ag), a film of polydimethylsiloxane (PDMS) on silver, and a graphite-coated silicon wafer.


Feature image used courtesy of Pixabay