As recently explained in the journal Nature Communications, engineers at Duke University have demonstrated a dual-mode heating and cooling device for building climate control which, if widely deployed in the U.S., could cut HVAC energy use by nearly 20 percent.

The invention uses a combination of mechanics and materials science to either harness or expels certain wavelengths of light. Depending on conditions, rollers move a sheet back and forth to expose either heat-trapping materials on one half or cooling materials on the other. Specially designed at the nanoscale, one material absorbs the sun’s energy and traps existing heat, while the other reflects light and allows heat to escape through the Earth’s atmosphere and into space.

This is the first demonstration of a reversible thermal contact, which allows users to switch between the two modes for heating or cooling. This allows the material to be movable while still maintaining good thermal contact with the building to either bring the heat in or let heat out.

About 15 percent of energy consumption in the U.S., and more than 30 percent globally, is for the heating and cooling of buildings. This activity is responsible for about 10 percent of global greenhouse gas emissions. Yet, up to now, most approaches to minimize the carbon footprint have only addressed either heating or cooling. That leaves the world’s temperate climate zones that require both heating and cooling out in the cold.  The new device could keep the user either warm or cool as the weather changes.

The specially designed sheet contains a polymer composite as the base which can be expanded or contracted by running electricity through it. This allows the device to maintain contact with the building for transmitting energy while still being able to disengage so that the rollers can switch between modes.

The cooling portion of the sheet has an ultra-thin silver film covered by an even thinner layer of clear silicone, which together reflect the sun’s rays like a mirror.  The unique properties of these materials also convert energy into mid-range infrared light and emit it. The mid-range infrared light does not interact with the gasses in the Earth’s atmosphere and easily passes into outer space.

When a change in weather brings the need for heating, the electrical charge releases, and the rollers pull the sheet along a track. This swaps the cooling, reflective half of the sheet for the heat-absorbing half.

To heat the building beneath, the engineers used an ultra-thin layer of copper-topped by a layer of zinc-copper nanoparticles. Because the nanoparticles are a specific size and are spaced a certain distance apart, they interact with the copper beneath them in a way that traps light onto their surface, allowing the material to absorb more than 93 percent of the sunlight’s heat.

The Duke team sees the device as something that could enhance existing HVAC systems, rather than fully replace them. Instead of directly heating and cooling the building, engineers could use a water panel to take hot or cold water to a heat pump or boiler system. With additional engineering, this could also be used on walls, forming a sort of switchable building envelope.

Moving forward, the team wants to advance this from a prototype to a scalable system ready for manufacturing. Today, there are concerns about the long-term wear and tear on the moving parts and the costs of the specialized materials. The team will investigate whether lower-cost aluminum can substitute for the silver and they are working on a static version that can switch modes chemically rather than mechanically.

Despite the many obstacles, they believe this technology could be an energy-saving boon in the future. They’re already working with a company to determine the ideal locations for deploying this technology.  And because almost every climate zone in the United States requires both heating and cooling at some point throughout the year, the advantages of a dual-mode device such as this are obvious.

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