HVAC System Uses Only Outside Air

The COVID-19 pandemic brought the risk of spreading disease through indoor air recirculation to the forefront. One way to reduce this risk is by circulating only outdoor air through a building. However, in a typical HVAC system, almost 40 percent of the energy consumed is used to dehumidify the air. This makes the heating or cooling of outdoor air even more energy-intensive and costly. 
 
“Most people don’t realize the complexity of a modern HVAC system,” said James E. Braun, Herrick Professor of Engineering and director of the Center for High-Performance Buildings at Purdue University. “There’s a specific sweet spot for humidity in an indoor environment—between 40 percent and 60 percent. Any drier than that, and people aren’t comfortable; any more humid, and you’re at risk for mold and other problems.” 
 
To tackle this challenge, Braun and a research team have proposed a system that combines specialized membrane technology with existing HVAC systems that removes moisture from outside air efficiently, making 100 percent outdoor air systems economically feasible. 
 

How It Works

 
David Warsinger, assistant professor of mechanical engineering at Purdue and an expert in vapor membrane technology, theorized it might be possible to integrate cooling with vapor-selective membranes to accomplish dehumidification and cooling without moisture condensation in a single device.
 
“Previous research related to membrane cooling used separate devices for vapor removal and sensible cooling,” said Braun. “Through simulation and experimental research, we later discovered very significant energy benefits associated with combining these processes when integrated within an overall system concept.”
 
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The technology is quite innovative because it provides both cooling and dehumidification without requiring that the energy exchanger operate below the dewpoint of the incoming air. This leads to significant energy savings because the evaporator of the vapor compression cooling cycle can operate at a higher temperature. 
 
Called the Active Membrane Energy Exchanger (AMX), the system integrates specialized membranes into the HVAC system to reduce the energy required to dehumidify the outside air. Developers believe it is the first approach that provides 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 heat between the air streams using a vapor compression cycle. 
 
“Key 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,” said Warsinger. “This new high-efficiency approach is found to outperform all other standard and state-of-the-art systems, achieving 1.2 to 4.7 times the coefficient of performance over conventional dedicated outdoor air treatment. 
 
A building simulation case study predicted cooling energy savings as high as 66 percent in hospital buildings using 100 percent outdoor systems in hot, humid climates. 
 
“The membrane is the key,” added Andrew Fix, a Purdue doctoral student in mechanical engineering and lead author of their recently published 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 outside air over these vapor-selective membranes, we can pull water vapor out of the air, reducing the load on the motors and compressors that run the refrigeration cooling cycle.” 

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“The biggest challenges have been associated with getting an experimental apparatus built and working properly,” said Braun. “The AMX is a new and unique technology. It required several iterations to get everything working properly. In particular, the membranes are not commercially available components, so we had to do the materials science, experimental design, fabrication, etc. in the process of developing a prototype.” 
 
A big surprise was the superior energy performance of a system based on the AMX technology as compared to state-of-the-art HVAC systems that we determined through simulation. “More recently, we discovered through experimental testing that the membranes perform better when exposed to cooler air, an advantage entirely unique to this particular design,” Braun continued. “That was a big surprise. Overall, the performance of the first-generation prototype AMX has exceeded our expectations.”
 

Moving Forward

 
AMX is the first system to target the combination of active membrane dehumidification and sensible cooling within the same device, exploiting several thermodynamic and material benefits. It can also be used for other applications requiring controlled humidity levels. As a result of this initial success, the team is applying for a patent. 

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Braun, Warsinger, and Fix are carrying out additional research within the Center for High Performance Buildings at Purdue, which is an industrial consortium. A number of companies within the CHPB are following their research and are potentially interested in commercialization.
 
“Our next step is to develop a full-scale prototype system that integrates the AMX technology and to then more fully validate the energy savings potential,” said Braun. “We are in the process of developing a prototype system design. Overall, the technology is in the early stages of development and several years from being a commercial product.”
 
Mark Crawford is an engineering and technology writer in Corrales, N.M.