It’s not the heat, it’s the humidity. That line is often heard in some places to explain the discomfort of a tropic-like summer day. But it also explains the heavy load on an air conditioning system in those same places. Using a desiccant dehumidifier to remove latent heat from the air can reduce the load on an air-conditioning and refrigeration system.

In traditional cooling applications, a single air conditioning unit is used to reduce both the sensible and latent heat below its dew point, which decreases the air’s ability to hold moisture. However, this method has its own problems. Reducing humidity in this method often requires lowering the air temperature below a comfortable level, which can then require the now dry air to be reheated for comfort sake. If the temperature is taken too low, the cooling surfaces of the coils may reach subfreezing temperatures, which can create ice build up on evaporator coils and require more defrost cycles, says Mel Meyers, president of Bry-Air, West Sunbury, Ohio, manufacturers of refrigeration and dehumidification systems.

A desiccant dehumidifier will be more effective at latent cooling because it decouples the latent load from the primary air conditioning system. The desiccant system can be integrated into an HVAC/R system and can account for 30 percent of the total cooling load, and can lower relative humidity (RH) levels to 50 percent or below. For example, a desiccant dehumidifier can be linked to a natural gas-fired absorption chiller. The desiccant removes the latent heat and the absorption chiller provides sensible cooling. This frees the chiller to more efficiently reduce sensible heat, which can result in overall energy savings.

This diagram of a Kathabar liquid desiccant dehumidifier shows the path of hot moist air and dry, conditioned air.

Manufacturers of desiccant dehumidifiers are developing a variety of new systems to dehumidify homes and office building, ice rinks and supermarkets, injection molding facilities, pharmaceutical labs, and just about everything in between. The desiccant systems range in size from around 300 cfm to 84,000 cfm. They can be arranged in vertical or horizontal orientations, and be installed indoors or as a roof top drop-in. Relatively new to the landscape are desiccant systems that capture a building’s exhaust air, transfer the energy to the inlet air stream and conditioning it. These units are referred to as enthalpy recovery systems.

Desiccant materials fall into two main categories: solids and liquids. These materials include silica gel, activated alumina, lithium chloride, chloride salt, and various glycols. The desiccants can be an adsorbent, collecting moisture like a sponge, or absorbent in which the desiccants undergo a chemical or physical change as they collect moisture. Absorbent desiccants are usually liquids, or solids that become liquid as they absorb moisture. Lithium chloride, for example, is a hygroscopic salt that collects water vapor by absorption. This particular desiccant has a 94 percent moisture capture rate and a 100 percent kill rate for microorganisms.

Solid desiccants are used in a desiccant wheel, which features a desiccant such as silica gel that is impregnated into paper sheets and wound onto a rotor. The sheets resemble corrugated cardboard, says Myers. The wheel rotates through a humid air stream and when the section of the wheel that is impregnated with the desiccant is exposed to the air, moisture is captured as the air passes through. The wheel then rotates past a separate, reactivating (regenerating) air stream, which is heated to about 300 DegF.

Enlarge this picture Energy use estimates comparing liquid, dry, and conventional cooling systems.

With a liquid system, the desiccant is sprayed into a conditioning chamber where it contacts the air to be dehumidified. The moisture-laden desiccant is then pumped to a regenerator where a heating coil heats the solution to about 170 DegF to reactivate the desiccant, says Mike Harvey, product manager for Kathabar Dehumidification Systems, Buffalo, N.Y., a manufacturer of dehumidification systems.

Despite the extra steps involved in dehumidifying the air and regenerating the desiccants, a number of studies through the years have identified desiccant systems as energy savers. The U.S. Dept. of Energy and Georgia State University studied a hybrid air conditioner-desiccant system at a Georgia school. The study found that the system maintained a relative humidity near 50 percent, making the school more comfortable and allowing the school administration to raise the average thermostat setting 2.4 DegF to about 73 DegF, while reducing energy consumption by about 10 percent.

While this application saved energy, not every application will result in overall savings. These factors can change by local energy costs, humidity levels, the velocity, and quantity of the inlet air, and the desired latent and sensible cooling levels. In general, these systems are best used when low humidity is required (less than 40 percent RH), the latent load is large compared to the sensible load, and the cost of energy to regenerate the desiccant – removing moisture and preparing the material for reuse – is low when compared to chilling the air below its dewpoint. Regenerating the desiccant is the biggest cost factor.

Enlarge this picture Estimated cost of a project comparing a liquid desiccant system to a dry desiccant system and a conventional cooling system.

The reason it must be regenerated is that a desiccant is a hygroscopic substance that attracts and holds water molecules from the surrounding air. Desiccants have a low surface vapor pressure, says Meyers, and if the desiccant is cool and dry and its surface vapor pressure is low, it can attract moisture from the air, which has a high vapor pressure when it is moist. After the desiccant becomes wet and hot, its surface vapor pressure is high, and the material is saturated and will not absorb additional moisture. This is when it goes through the regeneration process. The material is subjected to heat to remove the moisture. Dry desiccant systems must be heated to about 300 DegF, while temepratures as low as 130 degF can be used on liquid desiccant systems.

This continual process of regeneration is a major source of energy consumption. Heating the desiccant is usually done with steam, hot water, direct and indirect gas heaters and burners, or through electric heaters. In addition, if a liquid desiccant is used, the side stream of desiccant solution must be cooled before reuse, which also uses energy. In all, factoring in the power to run the desiccant dehumidifier and regenerate the material, a typical liquid desiccant system will use 1,800 BTUs per pound of moisture. In comparison, dehumidifying with a vapor compression system takes about 970 BTUs per pound of moisture.

To reduce energy consumption, liquid desiccant suppliers have developed various ways to use alternative energy sources to regenerate the desiccant and cool the desiccant solution after reactivation. Instead of using electricity for the energy to remove the moisture, waste heat can be used, and waste heat is a resource that almost every company has, says Harvey. Many companies, he says, have waste heat streams that reach the required temperature. If the waste heat falls short of the 170 DegF mark, a common temperature to regenerate liquid desiccant, a staged system can be used that will incorporate heaters to elevate the temperature up to the required level, says Brian Demers, an application engineer with Kathabar. To cool the desiccant, many buildings use water from coolant towers and other frequently occurring sources.

Sensible Only vs. Enthalpy Recovery. Source: Kathabar

The most common waste-heat sources are boiler systems. A secondary coil runs from the boiler and through the desiccant solution, which heats it to the reactivation level. Another method, while not commonly used, is the waste heat from a combined heat and power (CHP) system. A DOE analysis that studied CHP and desiccant systems, found that an 84,000 scfm liquid desiccant system could be reactivated with the waste heat from a 2 MW CHP reciprocating engine or a bank of microturbines. Desiccants used in a 5,000 scfm system could be regenerated by a 30 kW to 100 kW CHP engine or microturbine. In a similar scenario, such as the hybrid desiccant dehumidification system and natural-gas fired absorption chiller, the combustion of the natural gas used to power the chiller can be used to regenerate the desiccant.

Solar energy is another power source that has been successfully used to regenerate liquid desiccants. Other applications used solar panels, which Demers says can easily heat water to 200 DegF.

Another form of wasted energy comes in the form of exhaust air, which some suppliers are capturing and using to control both latent cooling and sensible heat. Harvey says that the exhaust air, which is cooler and dry or warm and humid depending on the application, is captured and used to precool or precondition the air coming back into a building. Harvey says that the Kathabar Twin-Cel system can capture exhaust air and transfer up to 65 percent of the total exhaust energy (enthalpy) over to the inlet air stream. By preconditioning the ventilation air entering a building with the available energy in the exhaust air leaving the building, these systems reduce heating/cooling and humidifying/dehumidifying energy use, he says. Generally, this air recovery equipment will save about three times as much energy in summer, and about 20 percent more energy in winter than sensible-only equipment. An advantage to enthalpy recovery systems, which use lithium chloride, is that they do not experience cross-contamination such as the transfer of microbiologicals from the exhaust stream to the supply stream. The lithium chloride scrubs the air before it can contaminate the air in the supply stream.

This liquid desiccant system was used for a historic building in Boston, Mass., in which temperature and humidity control was needed. Using a software program developed at Kathabar called the Economic Value Comparison, which factors in local weather data and local energy costs over a one year period, the company determined that the Twin-Cel liquid desiccant system was a more efficient system to use as compared to a dry desiccant system linked to a cooling system, or a conventional air conditioning system. (See Table 1.)

For this project, no preheat or precooling was required. The return air was 2,800 CFM. The return conditions were 68 DegF, and 45 grains/lb (absolute humidity). The goal was to reach 60 DegF and 40 grains/lb. In this application, the liquid system was projected to annually use 35,040 refrigeration tons to remove the moisture and 2,292 therms of energy for desiccant regeneration. In the same application, a silica gel wheel would use 38,411 refrigeration tons and 5,686 therms to regenerate the desiccant. (This is a higher figure than for the liquid desiccant, as the temperature required to regenerate a dry desiccant is greater than to regenerate a liquid desiccant.) The conventional cooling system would use 62,148 refrigeration tons and 3,709 therms to reheat after the evaporator coil.

Dry systems too can be used for latent and sensible cooling. These come in the form of dual-wheel units that include a desiccant-based wheel and a sensible-only heat wheel along with a conventional chilled water or direct expansion coil. During the cooling mode, the sensible-only wheel is used as a post-cooler to partially cool hot dry air off the desiccant wheel while preheating regeneration air or to reheat the over-cooled dehumidified air.

While desiccant dehumidification systems can dramatically reduce the load on an air-conditoning system, overall energy savings can be offset because of the cost to regenerate desiccant material. However, the manufacturers have found ways to use an energy source other than refrigeration compressors. Instead, low cost, or waste energy can be used. The desiccant system can be tied to heat sources such as solar panels, cooling towers, and other heat streams. They can be linked to HVAC/R systems in an effort to get the most efficient latent and sensible cooling possible.

In addition, if an air conditioning system and desiccant dehumidifier were designed in tandem to work together, this could allow the redesign of the air conditioner side so that it used less energy by reducing the size of a compressor or chiller package while still achieving the same or greater sensible-cooling capability.