Unexpected wall designs that can help lower building temperatures

A wall with a zigzag pattern can stay up to 3°C (5.4°F) cooler than a normal wall, without using any energy. This approach reduces the energy used by cooling systems and helps to curb global warming.

“Such designs can lead to cooler buildings,” says Qilong Cheng of Columbia University in New York, “and therefore reduce the energy consumption for cooling.”

As global temperatures rise and more people can afford air conditioners, their use is skyrocketing. Greenhouse gas emissions from cooling could increase more than threefold by 2050. As a result, many teams are working to develop passive cooling solutions that don't require any energy.

For example, simply painting roofs white can help keep buildings and cities cooler by allowing them to reflect more sunlight.

This method works even better if the roof is coated with a material that reflects most of the sunlight but emits infrared light within the transparency of the atmosphere — a range of wavelengths that aren't absorbed by atmospheric molecules such as carbon dioxide.

“Infrared light in this band can pass through the atmosphere and potentially reach space,” Chen said.

Materials with these properties have a great cooling effect on an upward-facing roof, but are less effective on walls. The problem is that materials that emit infrared light tend to absorb it, and surfaces close to walls, such as concrete pavements, can emit large amounts of infrared light.

The solution proposed by Chen and his team is to create a wall that has a series of projections that run parallel to the ground, forming a zigzag shape when viewed from the side. To visualize this, think of a staircase sloping upward at angles between 45 and 90 degrees.

The key is that the upward-facing zigzag surfaces (the treads in the staircase analogy) are atmospheric transparency windows that radiate large amounts of heat, while the downward-facing, outward-facing zigzag surfaces (the risers) reflect infrared heat rather than absorbing it.

To test the idea, the team built a 1-metre-tall model with both a zigzag and flat surface.When they placed it outside in New Jersey during the summer, the zigzag surface was 2°C cooler than the flat surface over a 24-hour average, and 3°C cooler between 1pm and 2pm.

Chen says there are plenty of inexpensive materials with the necessary properties. Existing buildings could be retrofitted by adding corrugated panels. The cooling effect inside a building varies depending on other factors, such as the size of the building's windows, but simulations suggest it could reduce temperatures by up to 2°C and cut the energy needed for cooling by up to a quarter.

The zigzag cooling wall is only suitable for hot climates, as colder regions would need more heating in winter. But Chen and his colleagues say They proposed a design with hinged “fins.” It can be raised in the winter to increase heat absorption and lowered in the summer to minimize heat absorption.