On sultry summer afternoons, heating, ventilation and air conditioning systems provide the much-needed relief from heat and moisture. These systems, which often come with dehumidifiers, are currently not energy efficient and use around 76% of electricity in commercial and residential buildings.
In a new study, researchers at Texas A&M University described an organic material called polyimide that uses less energy to dry air. In addition, the researchers said polyimide-based dehumidifiers can bring the price of HVAC systems down, which are currently costing thousands of dollars.
“In this study, we used an existing and fairly robust polymer and then improved its dehumidification efficiency,” said Hae-Kwon Jeong, McFerrin Professor in the Artie McFerrin Department of Chemical Engineering. “We believe these polymer-based membranes will help develop the next generation of HVAC and dehumidifying technologies that are not only more efficient than current systems, but also have a lower carbon footprint.”
The results of the study are described in the Journal of Membrane Science.
Dehumidifiers remove moisture from the air to a comfortable level of dryness, which among other things improves air quality and eliminates dust mites. The most common dehumidifiers available use refrigerants. These chemicals dehumidify by cooling the air and reducing its ability to carry water. Despite their popularity, refrigerants are a source of greenhouse gases, a major contributor to global warming.
As an alternative material for dehumidification, naturally occurring materials known as zeolites have been widely considered for their drying properties. In contrast to refrigerants, zeolites are desiccants that can absorb moisture in their water-attractive or hydrophilic pores. Although these inorganic materials are green and have excellent dehumidifying properties, zeolite-based dehumidifiers present their own challenges.
“Scaling is a big problem with zeolite membranes,” said Jeong. “First, zeolites are expensive to synthesize. Another problem is the mechanical properties of zeolites. They are weak and need really good support structures that are quite expensive and add to the total cost.”
Jeong and his team turned to an inexpensive organic material called polyimides, which is known for its high rigidity and tolerance to heat and chemicals. At the molecular level, the basic units of these high-performance polymers are repeating ring-shaped imide groups linked together in long chains. Jeong said that the forces of attraction between the imides give the polymer its characteristic strength and thus an advantage over mechanically weak zeolites. However, the dehumidifying properties of the polyimide material had to be improved.
The researchers first created a film by carefully applying polyimide molecules to aluminum oxide platforms a few nanometers wide. Next, they place this film in a highly concentrated sodium hydroxide solution and trigger a chemical process called hydrolysis. The reaction caused the imide molecule groups to break and become hydrophilic. Under a high-performance microscope, the researchers found that the hydrolysis reactions lead to the formation of water-attractive percolation channels or highways within the polyimide material.
When Jeong’s team tested their improved material for dehumidification, they found that their polyimide membrane was very 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 found that these membranes can operate continuously without regeneration because the trapped water molecules exit the other side from a vacuum pump installed in a standard dehumidifier.
Jeong said his team carefully designed their experiments for partial hydrolysis, in which a controlled number of imide groups become hydrophilic.
“The strength of polyimides comes from their intermolecular forces between their chains,” said Jeong. “If too many imides are hydrolyzed, we are left with weak material. On the other hand, if the hydrolysis is too low, the material will not be effective at dehumidifying.”
Although polyimide membranes showed promise in their possible use in dehumidification, Jeong said their performance still lags behind zeolite membranes.
“This is a new approach to improving the properties of a polymer for dehumidification, and much more optimization needs to be done to further improve the performance of this membrane,” said Jeong. “Another key factor for engineering applications is that they have to be cheap, especially if the technology is to be reasonably affordable for homeowners. We’re not there yet, but we are certainly making progress in that direction.”
Sunghwan Park in 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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