One takeaway from the pandemic is that circulating outdoor air inside buildings is better for health and safety than recirculating indoor air. However, that practice often comes with obstacles: Systems that circulate 100% outdoor air often are less cost-effective and energy-efficient. 

Much of that stems from outdoor air usually being more humid. Traditional HVAC systems use nearly 40% of their energy to remove humidity from the air. That makes heating or cooling outdoor air even more expensive and energy-intensive. 

Purdue University engineers are working on an innovation that would make outdoor air circulation systems more economically viable and energy-efficient. It would especially benefit buildings in warm, humid climates. 

“There’s a specific sweet spot for humidity in an indoor environment — between 40% and 60%. Any drier than that, and people aren’t comfortable; any more humid, and you’re at risk for mold and other problems,” James E. Braun, a Purdue engineering professor, said in a news release. “If you introduce outdoor air, the humidity levels of a building can fluctuate wildly. It’s an incredible challenge to maintain the right balance between temperature, humidity, human comfort, and overall cost.”

Air circulation innovation

Braun teamed up with David Warsinger, a Purdue assistant professor of mechanical engineering, who has expertise in using membranes for water filtration and desalination. Together they designed a system called the Active Membrane Energy Exchanger. It incorporates special membranes into an HVAC system to lower the energy needed to dehumidify outdoor air that is brought into a building.

The membranes pull water vapor out of the air to reduce the load on the refrigeration equipment’s motors and compressors. The membranes are called “vapor selective” because only water, not air, can pass through when a certain pressure is applied.  

The research team used computer modeling to test the system’s effectiveness in a variety of building environments and geographic locations. The system could provide up to 66% lower energy costs than traditional outdoor air circulation systems for large buildings like hospitals. The models showed lower energy use in all locations, but more humid areas — Tampa, Houston, and New Orleans — experienced the greatest energy savings.

“The more hot and humid it gets, the better our system works,” said Andrew Fix, a mechanical engineering doctoral student and lead author of the paper highlighting this research. “This is a key finding, because as the climate continues to warm around the world, locations that want to use 100% outdoor air will now be able to economically afford it.”

What’s next

Currently, the engineers’ concept is just that: an idea. Their next step is to build a physical prototype of the membrane-enhanced system to validate their computer models. They have already filed patent applications for the system.