You’re Hot Then You’re Cold

The heating and cooling of buildings accounts for a substantial portion of the world's energy consumption, contributing significantly to both environmental impact and economic expenditures. Statistics reveal some staggering facts, with the International Energy Agency estimating that buildings consume about 40 percent of the total energy used globally, with heating and cooling systems being major contributors to this consumption.

The costs associated with indoor climate control are massive. Energy bills for heating and cooling can be crippling for some homeowners and businesses, especially during extreme weather conditions. Additionally, the environmental impact of this energy consumption cannot be overlooked, as it leads to increased emissions of pollutants and contributes to air quality degradation.

This current trajectory of energy consumption poses sustainability challenges, especially amidst escalating populations and urbanization trends. Addressing this issue will require the adoption of more efficient solutions and technologies. Innovations in building design, renewable energy sources, and smart climate control systems are a few of the ideas that have been proposed that may help us achieve the more sustainable future we dream of.

Radiative cooling presents a promising avenue for mitigating these energy consumption issues. This innovative approach leverages the natural process of heat emission from surfaces to the sky, which can effectively cool structures without relying on energy-intensive mechanisms. But despite the potential of this technology, it is known to be capable of overcooling structures at certain times of the year, which can actually increase heating expenses.

But what if that radiative cooling system could be switched on and off as needed? That is the question that was asked by a team of researchers at the Wuhan University of Technology. In answering that question, they developed a device called the Zero-Energy Switchable Radiative Cooler (ZESRC). As you may have guessed from the name, the ZESRC can switch between heating and cooling modes, and requires no energy for operation.

The device is made up of three layers — a solar heating layer on top, a temperature-sensitive actuator in the center, and a radiative cooling layer at the bottom. The temperature-sensitive actuator is tuned to a specific temperature, and when the ambient temperature falls below that mark, it will spread the solar heating layer out to raise its temperature. Conversely, when the ambient temperature is too high, the solar heating layer moves out of the way, exposing the radiative cooling mechanism. This switching process is fully automatic, being driven only by temperature.

To achieve temperature-specific actuation, the middle layer was constructed from a thermally expansible sensitive layer (polyethylene film) and a thermally inert layer (copper tape). The polyethylene film has a higher thermal expansion rate, so as the actuator heats up, it expands along its length. The copper tape expands much less, due to its unique properties. This mismatch in expansion between the materials causes the layer to bend, which lifts it up and moves the top layer out of the way along with it. When the layer cools, the process reverses, flattening the actuator out and moving the top layer back in place.

A number of field experiments were conducted, and a very impressive heating or cooling of over 12 degrees Fahrenheit was observed when using ZESRC. The researchers note that this has the potential to slash building energy use by 14.3 percent. If that pans out in the real world, that could be a major boon to the economy and the environment.