Desiccants: Because it’s not the heat, it’s the humidity
Horace Kephart, in his book Our Southern Highlanders, described Appalachian mountaineers at the last turn of the century as preferring open, airy habitats to tightly closed buildings. They became claustrophobic, he wrote, if fresh air wasn’t readily available.
These “highlanders” knew something that we’ve had to learn ourselves. As new buildings are more tightly sealed and exclusively air conditioned—particularly commercial and institutional structures—indoor air quality has become an issue with health officials and heating, ventilation and air conditioning equipment manufacturers alike. In the worst instances, “sick building syndrome” can affect the health and productivity of a building’s occupants.
“Air conditioning designers have learned that more outside ventilation is needed to keep air fresher inside,” says Jim Sand of Energy Division’s Buildings Technology Center. “The American Society of Heating, Refrigeration and Air-Conditioning Engineers has written a standard that essentially states that ventilation systems need more outdoor air.”
Toward that purpose, Sand is coordinating a program with DOE, HVAC equipment makers and some smaller firms to promote the use of desiccant air conditioning systems.
Moist air causes that “clammy” feeling and promotes growth of molds, mildew and other irritants. Desiccant systems, Sand says, dry the air more efficiently than do conventional AC systems. That’s because desiccants absorb water. Those inedible packets of silica gel placed in packaged dry goods “to control moisture” are desiccants. The same principle can also be used to remove moisture from buildings.
Conventional AC systems, Sand explains, have two functions: To cool air, called “sensible cooling” and remove moisture, called “latent cooling.”
“On a hot, humid day, the only way a conventional system can remove moisture is to cool the air below the dewpoint,” Sand says. “This cold, moisture-laden air must be reheated before it feels comfortable to building occupants. This reheating is known in the trade as ‘parasitic heat load.’
“Conversely, on a cold, humid day, like spring mornings in East Tennessee, the air must be reheated—the conventional air conditioner sees no need to run at all because outside air is ‘cool’ enough to be used for ventilation in a building, but it isn’t ‘dry’ enough to be comfortable or to maintain a healthy indoor environment.
“Desiccant systems allow you to dehumidify, when you need it, more efficiently than a conventional air conditioner, thereby moving the workload from latent cooling to sensible cooling. It’s the Arizona effect, where a dry 90-degree day feels great, while in humid East Tennessee a 90-degree day can be miserable.”
Although not yet marketed for smaller residential applications, in a large building such as a department store or hospital, desiccant systems provide a way to make the large building comfortable and healthy.
A desiccant system works by blowing air through a revolving wheel that contains desiccant material, which absorbs the water from the air. That air is then circulated through the HVAC system, where it is cooled or heated depending on the need. There is a catch: To regenerate the desiccant wheel, air must be heated and sent through the other side of the desiccant wheel to drive out the water, and that requires energy.
“In Btu’s of water removed versus Btu’s of energy spent drying out the desiccant, you get an energy coefficient of less than one, but it’s still more efficient than running an air conditioner way down in temperature to dehumidify,” Sand says. “Desiccants pay off in three ways: They get the moisture out of the air more efficiently, the dry air doesn’t have to be as cool, and it relieves your cooling system of the water removal load.
“It’s not so much an energy-saving technology, but you pay a much larger penalty with a conventional AC system. Desiccant systems are a way to avoid the dramatic increases in building HVAC costs that would result if conventional air conditioning were used to handle this increased amount of ventilation air.”
Part of ORNL’s task is to help small desiccant system makers work in partnership with larger HVAC manufacturers, who are beginning to build and market HVAC systems using desiccant systems they supply.
“We’re helping to provide enabling technologies and performance standards and ratings for the industry,” Sand says. “Sensors and controls unique to desiccant systems haven’t yet been developed—some humidity sensors still use horsehairs—and good humidity sensors are expensive.
“Some applications of desiccants are a logical slam-dunk,” Sand says. “Hockey rinks almost always use desiccant dehumidification systems because you must cool the floor to keep the ice, and you have a room full of people emitting heat and moisture. If the air’s not dry, you’ll soon have wet ice and fog. Grocery stores, with their aisles of coolers, also are good candidates.”
ORNL recently published a subcontracted report for DOE through its Desiccant Technology and Applications Development Program titled Causes of Indoor Air Quality Problems in Schools: Summary of Scientific Research. The study links poor air circulation and poor humidity control to frequency of respiratory problems in the classroom. It identifies desiccant systems as a way to provide fresher air to young people, who may be more affected by indoor air problems than adults.
“About 10 Georgia and Florida schools are participating in a follow-up field test. Some have conventional air conditioning; some have humidity controls,” Sand says. “We’ll study mold spores, carbon dioxide levels and the presence of biogens in the schools and then compare absenteeism and the necessity for substitute teachers. Schools, particularly in the Southeast, are more closed up and have real problems with mold and mildew. This study will be used to develop strategies for improving indoor air quality, and consequently the learning process, in schools.”
Energy Division’s desiccants research is being funded through DOE’s Office of Building Technologies.B.C.