Bring the outdoors in: The energy-efficient method for using 100% outdoor air in buildings

WEST LAFAYETTE, Ind. — By now, it’s well known that circulating outdoor air in buildings is safer than recirculating indoor air. That point was driven home by the pandemic. Problem is, it’s just not cost-effective.

That may soon change. Purdue University engineers have proposed a system that combines new membrane technology with the latest HVAC systems to make 100% outdoor air systems more energy-efficient and economically feasible – especially in warm, humid climates. They say their system could save up to 66% in energy costs for large buildings that choose to use the safer outdoor air.

Previous research at Purdue has shown that HVAC systems (heating, ventilating, and air conditioning) are a key factor in spreading airborne diseases like COVID in indoor environments like office buildings, restaurants and airplanes.

“Most people don’t realize the complexity of a modern HVAC system,” said James E. Braun, the Herrick Professor of Engineering and director of the Center for High Performance Buildings at Purdue. “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.”

So, simply opening windows is not a solution.

“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.”

In a typical HVAC system, Braun says, almost 40% of the energy is used to dehumidify the air. That makes the heating or cooling of outdoor air even more energy-intensive and costly.

To solve this problem, Braun teamed up with David Warsinger, assistant professor of mechanical engineering, who specializes in using membranes for water filtration and desalination. They have proposed a system called the Active Membrane Energy Exchanger, which integrates specialized membranes into the HVAC system to reduce the energy required to dehumidify the outside air. Large buildings like hospitals could reduce their energy costs up to 66% with such a system, compared to current fully-outdoor air systems.

Their research has been in published in Applied Energy.

“The membrane is the key,” said Andrew Fix, a Purdue doctoral student in mechanical engineering and lead author of the paper. We use membranes that are vapor selective, meaning they only allow water vapor to pass through when a pressure difference is applied but block air. By passing the air over these membranes, we can pull water vapor out of the air, reducing the load on the motors and compressors that run the refrigeration cooling cycle.”

To gauge the system’s effectiveness, the team used computer models created by Pacific Northwest National Laboratory of hospitals in different climate conditions. Hospitals are ideal test beds because they are large indoor environments, which often require a higher percentage outdoor air in their HVAC systems for safety purposes. The computer models showed an overall reduction in energy usage for all locations using the Active Membrane system. The more hot and humid locations – Tampa, Houston and New Orleans – showed the greatest energy savings.

“The more hot and humid it gets, the better our system works,” Fix said. “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.”

The researchers are working toward building a physical prototype to validate their computer models. But there’s now more at stake than simply saving energy.

“I think COVID was a wake-up call for all of us,” Fix said. “Heating and cooling our buildings is not just a matter of temperature and humidity, but it can actually be a matter of life and death. Hopefully, this work will help to make all of our indoor spaces safer.”

Patent applications for this system have been filed via the Purdue Research Foundation Office of Technology Commercialization. This research is supported by the Center for High Performance Buildings at Purdue University (project number CHPB-50-2020).

About Purdue University

Purdue University is a top public research institution developing practical solutions to today’s toughest challenges. Ranked the No. 5 Most Innovative University in the United States by U.S. News & World Report, Purdue delivers world-changing research and out-of-this-world discovery. Committed to hands-on and online, real-world learning, Purdue offers a transformative education to all. Committed to affordability and accessibility, Purdue has frozen tuition and most fees at 2012-13 levels, enabling more students than ever to graduate debt-free. See how Purdue never stops in the persistent pursuit of the next giant leap at https://purdue.edu/.

Media contact: Kayla Wiles, 765-494-2432, wiles5@purdue.edu

Writer: Jared Pike

Sources: David Warsinger, 240-205-0440, dwarsing@purdue.edu

James Braun, 765-494-9157, jbraun@purdue.edu

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ABSTRACT

Vapor-selective active membrane energy exchanger for high efficiency outdoor air treatment

Andrew J. Fix, James E. Braun, David M. Warsinger

DOI

As much as 40% of the total load on air conditioning systems can be attributed to condensation dehumidification. However, new water vapor-selective membranes present a unique opportunity to greatly reduce the power requirements for moisture removal by avoiding phase change and have thus been ranked as a top alternative to traditional HVAC systems. To date, however, all such systems have relied on the assumption of constant temperature, even terming the technology “isothermal dehumidification.” This work proposes a membrane-based air cooling and dehumidification approach, referred to as the Active Membrane Energy Exchanger (AMX), which is the first to provide simultaneous, yet decoupled, air cooling and dehumidification. The suggested AMX configuration uses two vapor-selective membrane modules with a water vapor compressor in between them, using the second membrane module to reject vapor into the exhaust stream. Cooling and heating coils in each membrane module channel move heat between the air streams using a vapor compression cycle. A detailed steady-state, thermodynamic model is presented for the AMX integrated within a 100% outdoor air conditioning system. The AMX’s limiting parameters and design considerations like compressor efficiency are systematically analyzed for a broad range of outdoor air conditions and compared against standard and state-of-the-art dedicated outdoor air systems. This new high efficiency approach is found to outperform all other standard and state-of-the-art systems, achieving 1.2–4.7 times the COP over conventional dedicated outdoor air treatment. Lastly, a building simulation case study predicted cooling energy savings as high as 66% in hospital buildings with 100% outdoor systems in hot, humid climates.