The energy-efficient method for using 100 percent outdoor air in buildings – WBIW

WEST LAFAYETTE — It is now generally known that circulating outdoor air in buildings is safer than recirculating indoor air. That point was brought home by the pandemic. The problem is, it’s just not cost effective.

That may soon change. Engineers at Purdue University have proposed a system that combines new membrane technology with the latest HVAC systems to make 100 percent outdoor air systems more energy efficient and economically viable, especially in hot, humid climates. They say their system can save up to 66 percent in energy costs for large buildings that choose to use the safer outdoor air.

Using 100 percent outdoor air in an HVAC system is safer, but much more expensive and energy-intensive. Purdue University engineers (left to right) David Warsinger, Andrew Fix and James Braun are working on an energy-efficient system that uses membranes to dehumidify the outside air before it enters the HVAC system. (Purdue University photo/Jared Pike)

Previous research at Purdue has shown that HVAC (heating, ventilation and air conditioning) systems are a key factor in the spread of airborne diseases such as COVID in indoor environments such as office buildings, restaurants and airplanes.

James E. Brown

“Most people don’t realize the complexity of a modern HVAC system,” said James E. Brown, the Herrick Professor of Engineering and director of the Center for High-Quality Buildings Building at Purdu. “There’s a specific good spot for humidity in an indoor environment — between 40 and 60 percent. Any drier than that, and people feel uncomfortable; any more humid, and you’re at risk of mold and other problems.”

Simply opening windows is not a solution.

“If you introduce outside air, the humidity in a building can fluctuate enormously. It is an incredible challenge to maintain the right balance between temperature, humidity, human comfort and total cost.”

In a typical HVAC system, Braun says, nearly 40 percent of the energy is used to dehumidify the air. This makes heating or cooling outside air even more energy-intensive and expensive.

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

Their research has been published in Applied energy.

Andrew Fix

“The membrane is key,” said Andrew Fix, a Purdue doctoral student in mechanical engineering and lead author of the article. We use membranes that are vapor selective, meaning they only allow water vapor to pass when a pressure difference is applied, but block the air. By bypassing the air across these membranes, we can extract water vapor from the air, reducing the load on the motors and compressors running the refrigeration cycle.”

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

“The hotter and more humid it gets, the better our system works,” Fix says. “This is an important finding because as the climate continues to warm around the world, locations that want to use 100 percent outdoor air can now afford it economically.”

The researchers are working on building a physical prototype to validate their computer models. But there is more at stake now than just 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, it can even be a matter of life and death. Hopefully this work will help make all of our indoor spaces safer.”

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

Information: Jared Pike

Sources: David Warsinger, crossing@purdue.edu; James Braun, jbraun@purdue.edu

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