Buildings account for almost a third of global energy consumption, and of that, a staggering 70% is spent on heating and cooling. Standard strategies to shrink this carbon footprint usually involve better insulation or meticulously opening and closing windows. While "passive" buildings—those that maintain a comfortable temperature without external energy—currently make up only a tiny fraction of the market, technological breakthroughs are rapidly expanding our options.

The latest breakthrough comes from a team led by Professor Bonghoon Kim at the DGIST (Daegu Gyeongbuk Institute of Science and Technology) in South Korea. They have designed a pioneering 3D device that uses a dynamic architecture to heat or cool surfaces reversibly and efficiently, potentially providing a major boost to sustainable climate control.

You have likely noticed how pine cones open spontaneously to release seeds when they detect environmental moisture. The Korean engineers applied a similar principle to a new device that utilizes an architecture of bimetallic thermal actuators.

Before the technical jargon sends you running, the core idea is relatively straightforward. it is based on the difference in thermal expansion between two bonded metallic materials. As things heat up, one metal expands faster than the other, forcing the structure to curve with nanometre precision. This creates a "smart skin" that reacts to temperature shifts without the need for external sensors, wires, or complex control circuits.

In "Solar Heating (SH) mode", the wings remain folded and flat. In this configuration, the surface boasts a solar absorption rate of over 90%. This turns the facade into a "heat trap" that captures solar radiation even on overcast days to warm the building's internal structure. However, once the ambient temperature hits a critical threshold, the mechanical properties kick in. The 3D wings curl upwards, exposing an inner face specifically designed for passive radiative cooling.

The real innovation of the paper published in Advanced Materials lies in how the device manages thermal emissivity. In “Radiative Cooling (RC) mode”, the 3D structure exposes a surface with low solar absorption and extremely high emissivity within the “atmospheric transparency window.”

This allows the heat built up inside the building to be expelled as infrared radiation. This radiation passes through the Earth's atmosphere and travels directly into the cold void of deep space. In physical terms, it turns a building’s roof or facade into a “cosmic radiator” that cools the structure below the ambient temperature without using a single watt of electricity.

The research, led by Professor Hoe Joon Kim, has delivered unprecedented results. Theoretical simulations by the DGIST team confirm that this technology can slash cooling power requirements by 6.8% during the summer. In the colder months, the solar heating mode achieves a 5.6% reduction in the energy needed for heating.

At I'MNOVATION #Hub, we are keeping a close eye on how new designs and materials are supplementing traditional HVAC systems:

• High-reflectance pigments and radiative vooling: Smart coatings based on titanium dioxide or porous polymers that reflect almost the entire solar infrared spectrum. These can keep roofs up to 10°C cooler than conventional asphalt. One recent application even allows for the harvesting of atmospheric water.
• Phase Change Materials (PCMs): Substances integrated into concrete or plaster that act as a thermal battery, absorbing heat as they melt during the day and releasing it as they solidify at night. This reduces heating requirements.
• Biomimetic passive ventilation: Porous structures that, much like termite mounds, optimise natural airflow and cooling.

Implementing these technologies on a large scale could significantly mitigate the “urban heat island” effect. By preventing buildings from soaking up heat—and instead reflecting it or beaming it into space—we can lower street temperatures and improve urban quality of life.

Buildings aren't the only ones to benefit. We recently explored a prototype bus shelter that uses solar power to circulate water from an underground tank, cooling the immediate area to make the wait more bearable for passengers.

The conclusion is simple: if we want to tackle extreme temperatures and heatwaves while keeping electricity bills down, passive climate control systems are a highly effective, future-proof alternative in an era of climate change.

Source:

• TechXplore