Heating, ventilation, and air conditioning (HVAC) systems often come with dehumidifiers but are currently not energy efficient, guzzling around 76% of the electricity in commercial and residential buildings. Researchers have determined that organic material called polyimides use less energy to dry air. Poly-imide-based dehumidifiers can bring down the price of HVAC systems, which currently cost thousands of dollars.
The researchers took an existing robust polymer and improved its dehumidification efficiency. These polymer-based membranes could help develop the next generation of HVAC and dehumidifier technologies that are not just more efficient than current systems but also have a smaller carbon footprint.
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. But 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 (hydrophilic) pores. Although these inorganic materials are green and have excellent dehumidification properties, zeolite-based dehumidifiers pose challenges of their own. They are expensive to synthesize and are weak — they need good supporting structures that are quite expensive.
The team turned to polyimides, which are known for their high rigidity and tolerance for heat and chemicals. At the molecular level, the basic unit of these high-performance polymers is repeating, ring-shaped imide groups connected together in long chains. The attractive forces between the imides gives the polymer its characteristic strength and thus an advantage over mechanically weak zeolites. But the dehumidification properties of the polyimide material needed enhancement.
The researchers first created a film by applying polyimide molecules on a few nanometers-wide alumina platforms. Next, they put this film in a highly concentrated sodium hydroxide solution, triggering a chemical process called hydrolysis. The reaction caused the imide molecular groups to break and become hydrophilic. When viewed under a high-powered microscope, the researchers uncovered that the hydrolysis reactions lead to the formation of water-attractive percolation channels, or highways, within the polyimide material.
When the team tested the enhanced material for dehumidification, it found that the polyimide membrane was very permeable to water molecules — the membrane was capable of extracting excess moisture from the air by trapping it in the percolation channels. These membranes could be operated continuously without the need for regeneration since the trapped water molecules leave from the other side by a vacuum pump that is installed within a standard dehumidifier.