Liquid Desiccants and DOAS Systems

While the pandemic will hopefully end and millions of people across the country will slowly return to work and school in the coming months, there will still be many lingering air quality concerns that need to be addressed by facility managers.

According to a popular report published by the EPA on indoor air quality, the average American spends about 90 percent of their time indoors, where there is a much higher concentration of pollutants compared to outdoor environments. These indoor pollutants include combustion sources, cleaning products, degrading and new building materials, pet dander, radon and mold.

A DOAS is a special HVAC system for providing outdoor air ventilation that conditions the outdoor air and regulates indoor humidity.

An efficient solution that has gained momentum in recent years is the installation and use of a special outdoor air system (DOAS). A DOAS is a special HVAC system for providing outdoor air ventilation that conditions the outdoor air and regulates indoor humidity.

From an energy standpoint, the removal of moisture from the air represents a significant portion of the energy load borne by HVAC systems, which in total accounted for an estimated 27 percent of all commercial energy consumption by 2020. Mechanical refrigeration (including that performed by vapor compression systems) relies on cooling the air to remove moisture. Traditional vapor compression systems provide dehumidification by cooling the air below its dew point to remove the moisture and then reheating the air to bring it to a target temperature. These systems must supercool the air to meet all the requirements of today’s indoor air quality, which is a dilemma for today’s facility managers. As it stands, traditional mechanical cooling systems can provide optimal indoor air quality or optimal energy performance, but they have failed to achieve both comfort and sustainability at the same time. As the industry aims to achieve their best (and smallest) carbon footprint by 2021, reducing these types of inefficiencies while meeting necessary consumer health and comfort requirements will be critical for all types of HVAC systems.

DOAS is just one answer to this problem of overcooling and warming. DOAS is trying to shift dehumidification to a machine that is explicitly designed for that. In addition, a DOAS configuration decouples the control of humidity and temperature, which are respectively handled by a DOAS and a parallel, independent system. In theory, this decoupled control should improve the control of each parameter.

With the membrane-based module system, air, water and a liquid desiccant work in harmony to dehumidify and cool the outdoor air in one step.

To manage outdoor air without the inefficiencies of a traditional DOAS, some companies are turning to liquid desiccants. Historically, the use of liquid desiccants in HVAC systems to maximize dehumidification efficiency has been limited because many of the suitable desiccants, such as lithium chloride, are corrosive to metal. A recent innovation that we have seen to overcome this challenge is the development of a membrane-based module system that contains three moving elements: air, water and liquid desiccant. With the membrane-based module system, the three fluids work in harmony to dehumidify and cool the outside air in one step. A membrane placed between the air and the closed water channel allows for the exchange of water vapor but prevents the liquid desiccant from entering the air stream, solving the corrosion problem.

Using this one-step liquid desiccant DOAS to cool and dehumidify the outdoor air, as opposed to the overcooling vapor compression method, can provide significant energy savings, between 40 and 50 percent, according to the National Renewable Energy Laboratory (NREL). Not only will this dramatically reduce a building’s energy costs, but it will also help mitigate the expected rapid growth in greenhouse gas emissions associated with increased ventilation requirements in the coming years.

Benefits for building residents

As for the residents in these buildings, using this system allows for better ventilation. Improved ventilation improves indoor air quality as it dilutes indoor air pollutants and contaminants such as volatile organic compounds (VOCs) and CO2 expelled by humans breathing space. In addition, improved humidity control will in turn improve mould/microbial control in the indoor environment and improve occupant comfort, resulting in better indoor air quality. With the ability to deliver clean air without the significant energy burden found in other best-in-class systems, liquid desiccants can help steer DOAS trends toward a greener, healthier future.

Gary Clark is vice president, air management and business development, Emerson.Gary ClarkGary ClarkEmerson

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