An Organic Material for Energy-Efficient HVAC Technologies of the Future.

Heating, ventilation, and air conditioning (HVAC) systems offer much-needed relief from the harsh heat and humidity on hot summer afternoons. These devices, which also include dehumidifiers, are currently inefficient, using up to 76 percent of the electricity used in commercial and residential buildings.

Researchers from Texas A&M University have identified an organic material called polyimides that requires less energy to dry air in a new report. Furthermore, polyimide-based dehumidifiers, according to the researchers, will reduce the cost of HVAC systems, which usually cost thousands of dollars. 

“In this research, we improved the dehumidification efficiency of an existing and fairly robust polymer,” said Hae-Kwon Jeong, McFerrin Professor in the Artie McFerrin Department of Chemical Engineering. “We believe that these polymer-based membranes would aid in the development of the next generation of HVAC and dehumidifier technologies that are not only more effective but also have a lower carbon footprint than current systems.”

Dehumidifiers remove moisture from the air to a comfortable level of dryness, thereby improving air quality and eliminating dust mites, among other useful functions. The most commonly available dehumidifiers use refrigerants. These chemicals dehumidify by cooling the air and reducing its ability to carry water. However, despite their popularity, refrigerants are a source of greenhouse gases, a major culprit for global warming.

As an alternative material for dehumidification, naturally occurring materials known as zeolites have been widely considered for their drying action. Unlike refrigerants, zeolites are desiccants that can absorb moisture within their water-attractive or hydrophilic pores.

“With zeolite membranes, scaling up is a major problem,” Jeong said. “For starters, zeolites are costly to make. Another problem stems from zeolites' mechanical properties. They are flimsy and require extremely strong supporting structures, which are very costly, increasing the overall cost.”

Jeong and his colleagues turned to polyimides, a low-cost organic material known for its high rigidity and resistance to heat and chemicals. At the molecular level, the basic unit of these high-performance polymers is a long chain of repeating, ring-shaped imide groups.

According to Jeong, the polymer's characteristic strength comes from the attractive forces between the imides, giving it an advantage over mechanically weak zeolites. However, the polyimide material's dehumidification properties needed to be improved.

The researchers started by carefully applying polyimide molecules to alumina platforms that were a few nanometers high. The film was then immersed in a highly concentrated sodium hydroxide solution, which triggered a chemical reaction known as hydrolysis. The imide molecular groups were broken and became hydrophilic as a result of the reaction.

“This is a novel approach to improving the dehumidification property of a polymer, and a lot more optimizations are needed to improve the efficiency of this membrane,” Jeong said. “However, another important factor for engineering applications is cost, particularly if the technology is to be affordable to homeowners. We're not quite there yet, but we're moving in the right direction.”