An Organic Material for Next Generation Energy-Efficient HVAC Technologies

Polymide membrane

Dehumidifiers with improved polyimide membranes (white disc) will be energy efficient with a smaller ecological footprint. Credit: Dharmesh Patel / Texas A&M Engineering

Texas A&M researchers have improved the dehumidification efficiency of a polymer that could help develop more energy-efficient systems with a smaller carbon footprint.

On balmy summer afternoons, heating, ventilation and air conditioning (HVAC) systems provide much-needed relief from the harsh heat and humidity. These systems, which often come with dehumidifiers, are currently not energy efficient and consume about 76% of the electricity in commercial and residential buildings.

In a new study, researchers at Texas A&M University have described an organic material called polyimides that uses less energy to dry air. In addition, the researchers said polyimide-based dehumidifiers could lower the price of HVAC systems, which currently cost thousands of dollars.

“In this study, we took an existing and fairly robust polymer and then improved the dehumidification efficiency,” said Hae-Kwon Jeong, McFerrin professor in Artie McFerrin’s Department of Chemical Engineering. “We believe these polymer-based membranes will help develop the next generation of HVAC and dehumidification technologies that are not only more efficient than current systems, but also have a smaller carbon footprint.”

The results of the research are described in the Journal of Membrane Science

Dehumidifiers take moisture from the air to a comfortable dryness, improving air quality and eliminating dust mites, among other useful functions. Most commonly available dehumidifiers use refrigerants. These chemicals dehumidify by cooling the air and reducing its ability to transport water. Despite their popularity, however, 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 coolants, zeolites are drying agents that can absorb moisture in their hydrophilic or hydrophilic pores. While these inorganic materials are green and have excellent dehumidifying properties, zeolite-based dehumidifiers pose their own challenges.

“Scaling up is a big problem with zeolite membranes,” said Jeong. First, zeolites are expensive to synthesize. Another problem arises from the mechanical properties of zeolites. They are weak and need really good support structures, which are quite expensive and drive up the overall cost. “

Jeong and his team turned to a cost-effective organic material called polyimides, known for their high rigidity and tolerance to heat and chemicals. At the molecular level, the basic unit of these high performance polymers consists of repeating, ring-shaped imide groups linked together in long chains. Jeong said the attractive forces between the imides give the polymer its characteristic strength and thus an advantage over mechanically weak zeolites. But the dehumidifying properties of the polyimide material needed to be improved.

The researchers first made a film by carefully applying polyimide molecules to a few nanometer-wide aluminum oxide platforms. They then place this film in a highly concentrated sodium hydroxide solution, activating 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 found that the hydrolysis reactions lead to the formation of water-attractive percolation channels or highways in the polyimide material.

When Jeong’s team tested their improved dehumidification material, they found that their polyimide membrane was highly permeable to water molecules. In other words, the membrane was able to remove excess moisture from the air by trapping it in the percolation channels. The researchers noted that these membranes can be used continuously without regeneration, as the trapped water molecules leave the other side through a vacuum pump installed in a standard dehumidifier.

Jeong said his team carefully designed their experiments for partial hydrolysis, where a controlled number of imide groups become hydrophilic.

“The strength of polyimides comes from their intermolecular forces between their chains,” Jeong said. “If too many imides are hydrolysed, weak material remains. On the other hand, if the hydrolysis is too low, the material will not be effective in dehumidifying. “

While polyimide membranes have shown promise in their potential use in dehumidification, Jeong said their performance still lags behind zeolite membranes.

“This is a new approach to improve the properties of a polymer for dehumidification and many more optimizations are needed to further improve the performance of this membrane,” said Jeong. “But another important factor for technical applications is that it has to be cheap, especially if you want the technology to be reasonably affordable for homeowners. We are not there yet, but are certainly taking steps in that direction. “

Reference: “Improving the dehumidification performance of polyimide membranes by generating hydrophilic Poly (amic acid) domains using partial hydrolysis ”by Sunghwan Park and Hae-Kwon Jeong, December 28, 2020, Journal of Membrane Science
DOI: 10.1016 / j.memsci.2020.119006

Sunghwan Park from the Chemical Engineering Department also contributed to this study.

This research is funded by the National Science Foundation and the Qatar National Research Fund.

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