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    Liquid Desiccants and DOAS Systems

    Aug. 5, 2021
    Extended periods of time spent indoors presents a reason for seeking improved indoor air quality and greater energy efficiency.

    While the pandemic is hopefully winding down and millions of people across the country are slowly returning to work and school in the coming months, there will still be many lingering concerns regarding air quality that will need to be remedied 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 supplies, degrading and new building materials, pet dander, radon and mold. 

    A DOAS is a dedicated HVAC system for providing outdoor air ventilation that handles conditioning the outdoor air as well as controlling indoor air humidity.

    An efficient solution that has picked up steam in recent years is the installation and utilization a dedicated outdoor air system (DOAS). A DOAS is a dedicated HVAC system for providing outdoor air ventilation that handles conditioning the outdoor air as well as controlling indoor air humidity. 

    From an energy standpoint, removing humidity from the air is a significant portion of the energy load carried by HVAC systems, which in total, accounted for an estimated 27 percent of all commercial energy use in 2020. Mechanical cooling (including that performed by vapor compression systems) relies on cooling the air to remove moisture. Traditional vapor compression systems accomplish dehumidification by overcooling the air below its dewpoint to remove the moisture and then re-heating the air to get it to a target temperature. These systems must overcool the air in order to meet all of today’s indoor air quality requirements, which presents 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 not been able to achieve both comfort and sustainability at the same time. As the industry strives to put their best (and smallest) carbon foot forward in 2021, reducing these kinds of inefficiencies while meeting the necessary consumer health and comfort requirements will be crucial for all types of HVAC systems. 

    DOAS is just one response to this overcooling and re-heating issue. DOAS attempts to shift dehumidification to a machine explicitly designed for that purpose. In addition, a DOAS configuration decouples control of humidity and temperature, which are handled by a DOAS and a parallel, independent system, respectively. In theory, this decoupled control should improve control of each parameter. 

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

    For managing outdoor air without the inefficiencies of a traditional DOAS, some companies are turning to liquid desiccants. Historically, use of liquid desiccants in HVAC systems to maximize dehumidification efficiency has been limited, as many of the suitable desiccants, such as lithium chloride, are corrosive to metal. One recent innovation we’ve seen to overcome this challenge is the development of a membrane-based, module system that incorporates 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 a single step. A membrane placed between the air and closed water channel allows for the exchange of water vapor but prevents the liquid desiccant from entering the air stream, thus, solving the corrosion issue. 

    Using this single-step, liquid desiccant DOAS to cool and dehumidify outside air as opposed to the over-cooling vapor compression method has the potential to unlock significant energy savings, between 40 and 50 percent, according to the National Renewable Energy Laboratory (NREL).  Not only does this dramatically lower a building’s energy costs, but it also helps mitigate the expected rapid growth of greenhouse gas emissions associated with greater ventilation requirements in the coming years. 

    Benefits for Building Occupants

    As for the occupants inside these buildings, using this system allows for improved ventilation. Improved ventilation improves indoor air quality as it dilutes pollutants and contaminants in indoor air, such as off-gas volatile organic compounds (VOCs) and CO2 expelled by people breathing in the space. In addition, improved humidity control will in turn improve control of mold/microbes in the indoor environment and improve comfort for occupants, yielding improved indoor air quality. With the ability to deliver clean air without the significant energy load found in other best-in-class systems, liquid desiccants could help steer DOAS trends towards a greener, healthier future. 

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