As building occupants and even ASHRAE standards are beginning to call for increased levels of fresh air to be circulated in commercial buildings, engineers are looking for systems that will help them introduce higher levels of outside air in an energy-efficient manner.

In fact, in the 1997 ASHRAE Handbook of Fundamentals (Chapter 26), new data for wet-bulb and dew-point temperatures with mean coincident dry-bulb temperature and humidity ratios were included for the first time in order to help engineers design HVAC systems to better handle “latent” loads – the moisture contained in air. In other words, engineers are becoming more concerned about handling humidity in buildings, not only because it is more energy-efficient to cool dry air, but also because it is becoming more apparent that excess moisture can cause mold and bacteria growth and cause “sick building syndrome.” They are realizing that occupant comfort depends as much on moisture control as temperature control.

Controlling moisture becomes more difficult, however, as ASHRAE Standard 62-89 calls for more outside air to be circulated into buildings. As more outside air is used to flush out stale, polluted indoor air, it becomes even more important to remove incoming moisture. Many air-conditioning systems, which are often based on vapor compression cycles, are not up to the task of removing this extra moisture. One solution that holds promise, however, is a technology called desiccant dehumidification.

What Are Desiccants?

Desiccants are materials that remove moisture from air and then release that moisture when exposed to heat. Materials such as silica gel, activated alumina, or synthetic polymers are used in a “desiccant wheel” to adsorb moisture from incoming ventilation air. Thermally regenerated, or “active,” desiccant systems then release the moisture when exposed to heat from sources such as natural gas, electricity, waste heat from other mechanical systems, or even solar power. Passive desiccant wheels, on the other hand, can simply utilize the heat from exhausted building air to release and vent the captured moisture.

Desiccants are attractive because they can remove moisture before the incoming ventilation air temperature is reduced. Typically, a building uses a vapor compression system to cool incoming air. Although those systems are perfectly capable of cooling air to the required indoor temperature (the “sensible” load), that system must overcool air to a temperature below its dew point to squeeze out unwanted moisture. This can be an inefficient process and can tax the air conditioning system on humid days at peak loads (a problem compounded by peak energy costs). Desiccants essentially decouple latent and sensible loads, allowing the capacity of the air conditioning system to be reduced to a point adequate for handling sensible load only. And because desiccants are controlled with a humidistat, an optimum humidity level of 40-60% is assured.

Current Research

Currently, the U.S. DOE’s research program for desiccants is managed by the National Renewable Energy Lab (NREL) and the Oak Ridge National Laboratories (ORNL). “Desiccants are not a panacea, and we’re remaining objective about their potential,” said Phil Fairchild, a researcher in the Heating and Cooling Equipment Research Program at ORNL. “But desiccants have a lot of promise, a lot of things falling into place to make it a more viable technology.”

ORNL and NREL are helping industry develop cost-effective, marketable desiccant systems that are energy efficient and environmentally safe. Currently, the technology is moving into niche markets where, according to Fairchild, “it can solve various problems that cannot be handled adequately by vapor compression air-conditioning systems alone.” Such markets include: supermarkets, where desiccants are being used to reduce frost build-up on refrigerated cases and frozen food products, thus extending shelf-life and reducing the number of energy-intensive defrost cycles; motels and dormitories, where there is a need for large volumes of outside air to replace bathroom exhaust air; schools, where the use of conventional systems to comply with ASHRAE 62-89 can significantly increase operating and first cost expenses; and shopping malls and retail stores, where customer comfort must be maintained over a wide range of occupancy levels.

The potential application of the technology, however, is not limited to only these areas. For example, according to NREL, desiccant dehumidification could reduce total residential electricity demand in humid areas of the country by as much as 25%, and in the process provide drier, more comfortable air and displace chlorofluorocarbon-based cooling systems. According to NREL, desiccant systems can potentially save 400-trillion Btu of energy each year in U.S. buildings and can prevent the emission of more than 24 million tons of carbon dioxide (CO2) by 2010. Both labs are working to help desiccants become more cost competitive and efficient so that they are attractive in the competitive HVAC market.

Market research indicates that desiccant sales may jump from $40 million to $200 million over the next three to four years, with major manufacturers jumping into the game once sales hit $60 to $75 million. Large manufacturers are actively researching desiccants but are cautious as to the present viability of the systems.

“We have been taking a close look at desiccants for a couple of years now,” said Dilip Vyavaharkar, manager Indoor Air Quality Systems for Carrier Corp., a leading manufacturer of HVAC systems. “When it comes to IAQ, humidity control is extremely important, and desiccants are one of the technologies we’re evaluating to help us in this area.” According to Vyavaharkar, however, there are still some issues that have to be addressed, including the cost of the systems and the reliability of their moving parts. “And desiccants allow you to get humidity low, but many times it’s overkill,” he added. Carrier is instead currently promoting its MoistureMiser system, which uses a special vapor compression system to reduce moisture content.

Although desiccants can cost 15-20% more than custom built air handlers – ranging in price from $800 to $1,000 per ton – manufacturers of desiccants counter that in many niche markets, such as grocery stores, movie theaters, schools, and other facilities with high latent loads, paybacks can be achieved in as few as two years. According to reports by the American Gas Cooling Center, a gas industry sponsored organization, many of these niche market facilities have achieved better IAQ and quick paybacks with desiccant systems: a ShopRite supermarket in Newton, NJ, for example, saved between $30,000 and $40,000 in energy costs per year after installing a desiccant system to handle humidity problems instead of expanding the size of its vapor compression system. The natural gas industry is promoting heat-activated systems that derive their heat directly from burning gas, but some in the green building industry believe that the real promise lies in systems that use waste heat from other mechanical systems or in passive desiccants that do not need a heat source at all.

And as for reliability problems with moving parts: according to Larry Klekar, general manager of the DryCool division of Munters Corp., “The wheel in our system has a life expectancy of over 100,000 hours. Considering that the desiccant portion of the system generally operates less than 4,000 hours per year, we are talking about a 25+ year life of the desiccant system.”

Munters DryCool, based in Selma, TX, recently introduced its DiverseAire desiccant system, which is, according to the company, the first application of a single-packaged air-conditioning/dehumifidication system with an active desiccant wheel. It was recently installed in a HEB Supermarket Store in Monterey, Mexico.

Other manufacturers of desiccant systems include Englehard/ICC of Hatboro, PA; SEMCO Inc. of Columbia, MO; Clearwater, FL-based Advanced Thermal Technologies Inc.; Conley, GA-based Octagon Air Systems; Douglasville, GA-based Seasons 4 Inc.; Frederick, MD-based Air Technology Systems, Inc.; and LaRoche Industries Inc. of Baton Rouge, LA.