Liquid Desiccant Dehumidification For Challenging Environments
The main advantage of liquid desiccant systems is that they remove humidity with far less energy input than either conventional cooling coils or solid desiccant approaches. According to the National Renewable Energy Laboratories (NREL) in Golden, CO, desiccant cooling and dehumidification is a thermally driven air conditioning technology and a “valuable tool in the industry’s arsenal of space conditioning options.”2
The benefits of desiccant systems over vapor-compression units are twofold: a fundamentally more efficient humidity-removal process and the potential to use low-temperature waste heat, solar thermal energy, or natural gas — or a combination of these energy sources — to regenerate the desiccant. These approaches reduce primary electrical consumption and allow endusers to reduce peak electrical demand. They also use less ozone-depleting refrigerant.
Yet there’s an additional advantage in effectiveness of humidity control, which has attracted the eye of HVAC engineers and facility owners dealing with requirements for low humidity, such as ice rinks or supermarkets, or outsized humidity loads caused by high ventilation air needs or very wet occupancies (such as swimming pools and manufacturing processes). Even the food-processing industry, with its energy-intensive steamers and cookers and daily washdowns, has proven quick to adopt liquid desiccant technology. Appreciable operating cost reductions have been calculated as compared to solid desiccants and vapor compression systems.
However, one might ask, why liquid desiccant? It’s a fair question, best addressed by focusing on recent experience and independent testing.
In fact, the moisture removal effectiveness and system efficiencies of packaged liquid desiccant dehumidification systems have been well established. A report by QinetiQ North America, an independent testing lab in McLean, VA, concluded that liquid desiccant systems have operating costs that tend to be about 50% less than those of other, comparable dehumidification systems (Table 1). The QinetiQ study also noted that new packaged dehumidification systems are made from corrosion-resistant materials, so the systems no longer suffer from carryover and other corrosive effects of the lithium chloride solutions used.3
Within these novel packaged systems are new designs of liquid-to-air contactors that contain desiccant liquids without using mist eliminators. According to NREL’s Eric J. Kozubal, a researcher in distributed thermal energy technologies, “This is a critical advance that eliminates the maintenance traditionally associated with liquid systems, making possible their broad application as packaged systems.” In addition, the packaged units allow for independent cooling and dehumidification in a single HVAC component, which can simplify and reduce system costs. Lastly, says Kozubal, many liquid desiccant systems are being integrated with combined-heat-and-power (CHP) systems to use excess waste heat during off-peak periods to heat and regenerate the desiccant solution.4
NEW SCIENCE FOR
“Liquid desiccant has been used in building HVAC systems since the 1930s, mostly in industrial situations where it has always vied with solid desiccants,” said Dr. Andrew Lowenstein, president of the R&D laboratory AIL Research (Princeton, NJ) and inventor of a plate contactor array for liquid desiccant systems.
Liquid desiccant air conditioning removes moisture (latent heat) from airstreams by means of a solution of lithium chloride (LiCl) or halide salts, according to Lowenstein. Concentrated, cooled desiccant flows into an absorber and through a mass transfer surface while counterflowing air passes through the same surface, transferring heat and moisture out of the air into the desiccant liquid. As the desiccant dilutes and passes out of the absorber, the moisture collected is transferred to the regenerator. This can be accomplished either by transferring the entire desiccant liquid volume to the regenerator, or, in some cases, more efficiently by simply allowing the excess moisture to migrate to the regenerator using diffusion-driven process. In the regenerator, the solution is heated and flowed onto another mass transfer surface, losing moisture to another counterflowing airstream.
The use of a liquid allows engineers and operators to further boost system efficiency by incorporating heat exchangers, heat pumps, or cooling coils. These components help balance heat transfer and mass transfer, ensuring input energy is distributed as efficiently as possible. Being able to simultaneously dry and cool the treated airstream may provide an increase in efficiency over solid desiccant rotors, which carry heat from the regeneration process back into the airstream that may need to be cooled to a desired temperature range.
The effectiveness of liquid desiccant A/C systems for deep drying, precise humidity control, and highly saturated conditions have made them particularly appealing for industrial end uses.5
“For inherently wet processes and operations, you need to determine what humidity you’re going to get from their process to calculate those loads,” said Wayne Montgomery, P.E., a senior mechanical engineer at Food Facility Engineering, Yakima, WA. “Often a piece of dehumidification equipment can save money on the cooling and refrigeration equipment required.” Effective dehumidification can also help with downtime needed to control excessive condensation or water vapor.
In the past, process manufacturers typically employed HVAC equipment types for process environmental control, such as direct expansion (DX) systems or chilled water systems. These have effectively dealt with temperature variations within the plants, but for humidity they have proven ineffective or costly due to the low temperatures required to extract moisture — generally at temperatures much lower than those needed for space cooling. A further challenge awaits this scenario according to Montgomery. “This means the engineers have to oversize the refrigeration evaporators to handle the humidity load,” he says. “Often it also requires use of a defrost cycle because frost forms on the coils, so they’re using heaters and shutting down the refrigeration units for defrosting.” Liquid desiccant systems have shown an alternative to this kind of inefficient humidity extraction and defrost cycling.
The systems have also been successfully applied for a range of commercial and institutional projects as well, say engineers, ranging from nursing homes to cleanrooms.
COMMERCIAL AND INDUSTRIAL CASE STUDIES
A few examples demonstrate how liquid desiccant HVAC systems have been employed to boost plant energy efficiency while also benefiting public health, corporate sustainability and occupant safety and comfort. These include both industrial and commercial applications of a variety of types.
• Regents Park Nursing Home. This South Florida facility faced serious condensation problems, odors, and air quality concerns in two large cafeterias. Using conventional air conditioning systems made the interiors too cold for elderly residents and increased the risk of microbiological growth on cooling coils. Packaged liquid-desiccant dehumidification systems were installed in the large common areas, eliminating the condensate and associated problems within hours.
• Huawei cleanroom. Manufacturing telecommunications equipment in a clean room facility requires precise control of humidity levels and IAQ, and Huawei Technologies — located outside of warm, humid Shanghai — sought a cost-effective solution for their plant. The company’s engineers considered a standard HVAC solution that would have used about 45 kW to chill water, 29 kW to reheat air to the design point, and 11 kW for other system needs. By contrast, the pair of packaged liquid desiccant units ultimately selected by Huawei consumes only 53 kW, yielding 62% energy cost savings and a brisk 16-week payback.
• Europlast food processing. Europlast’s meat plant was beleaguered by air quality and humidity issues due to the hot, wet process conditions. The production hall was cooled but relative humidity levels reached 85% during summer months, putting the facility at a high risk of mold and mildew growth. Production was halted to allow for a retrofit to improve ambient conditions. Europlast considered an oversized vapor-compression system as well as solid and liquid desiccant options and settled on a packaged liquid desiccant dehumidifier with a built-in heat pump, which maintains mandatory environmental conditions of 71°F and 50% rh regardless of weather conditions.
• Duty Free Americas Miami warehouse. For a warehouse facility in southern Florida owned by Duty Free Americas, sensitive goods such as cosmetics, tobacco, watches, and chocolate required cool and dry conditions. The existing HVAC system could cool the air but not remove enough humidity to be effective. Two liquid-desiccant units were added and integrated with the existing air-handling system to maintain ideal constant conditions of 76° and 52% rh.
• Confectionery plant. As part of a demonstration project to confirm energy savings and effectiveness, operators of a candy and gum factory in a subtropical climate tested a liquid desiccant dehumidification unit with a heat pipe, retrofitted to an existing AHU, to condition supply OA to a gum production area. The new liquid-desiccant system cost about 28% less to purchase and install than a comparable desiccant wheel system and cut associated cooling power loads from 37,800 kWh per month to about 27,000 kWh per month for the single unit.
• Meat processing facility production areas. To address the humidity, mildew, and mold generated by continual cleaning of production areas — which also caused slip dangers — engineers at this food company demonstrated how waste heat from a process biofuel incinerator could be used to power new liquid desiccant dehumidifiers. For the dehumidification system’s heat sink, the team tapped into an existing cooling tower, actually reducing necessary chiller capacity even though additional cooling was achieved. The six new rooftop packaged dehumidification units slashed HVAC energy costs by 40%, and airborne particulate levels (5 microns or greater) dropped by about 75%, while almost 95% of microorganisms were eliminated. This helped address concerns about listeria, salmonella, and E. coli bacteria.
HEALTH CHALLENGES IN INDUSTRY
The risks of microbiological contamination for food processors are vast. The recent E. coli outbreak in Germany cost the nation’s farmers at least $600 million in the first weeks, and Spanish exporters lost $200 million per week. According to FDA, a food manufacturer’s risks of microbiological contamination — and worker health dangers such as Legionella — are directly proportional to the levels of wetness and humidity in production areas. Listeria, for example, can exist on persistently damp surfaces even in refrigerated areas of plants. “Humidity control is becoming more and more important for these reasons,” says Montgomery.
In fact, the prospect of an outbreak is so financially disastrous that food manufacturers are adamant about keeping floors dry and conducting powerful daily deep cleaning at least once per day, with high-temperature, high-pressure washdowns on every surface in production areas, says Gunawardena.
After cleaning, processing spaces resemble massive steamrooms, which must be vented and dried before resuming production. In addition, Kozubal points out that today’s industrial and commercial buildings have tighter enclosures that improve efficiency but also tend to cause humidity and condensation challenges. “This reduces the thermal loads and so the standard air conditioning systems often don’t operate sufficiently to control humidity, which can be the most energy-intensive load in the building,” he explains.
Liquid desiccant systems can improve humidity management, removing moisture without cooling air to saturation, keeping the relative humidity of supply air below 70%. In this way, supply-air ducts remain dry, helping to reduce mold and bacterial growth.
Dehumidifiers also reduce condensation on cooling coils as well as water accumulation in drain pans, both of which can be breeding grounds for bacteria. In addition, the scavenging action of liquid desiccant systems can improve IAQ by removing airborne contaminants.
APPLYING LIQUID DESICCANT
With increased safety requirements, many HVAC challenges can be addressed with liquid desiccant dehumidification, says AIL Research’s Lowenstein. Common situations ideal for liquid desiccants include:
• Where the facility has to overcool and reheat the air to maintain comfort conditions — a common scenario in all but the most arid climates.
• Where dewpoints lower than 40° make it very difficult for a vapor compression system to dehumidify without risk of freezing.
• Where recoverable heat is available at between 140° and 220°.
• Where waste heat or thermal energy is available, the already considerable energy savings from liquid dehumidification units can be even greater.
“Liquid desiccants are cheap cooling storage devices, requiring three to five times less volume than comparable ice energy storage systems,” says Kozubal. Confirming the benefits, in a recent article John Dieckmann described how “a desiccant system integrated with a CHP system could generate strong desiccant during off-peak times when excess waste heat is available and store strong desiccant to provide cooling capacity during periods of peak electric demand.”6
In humid environments, the energy savings are most pronounced, says Dieckmann. The liquid desiccant handles the primary latent load, avoiding the need to overcool the OA supply, decreasing reheat needs and allowing interior cooling to handle ambient interior moisture. In this way, the A/C chiller system operates at optimal desiccant.7With dryer interior conditions, another benefit is that indoor temperature setpoints can be higher by as much as 5°, without any perceived change in occupant comfort. Dieckmann concludes that new-generation liquid desiccant systems used as a DOAS in warm, humid climates can achieve significant energy savings.
Liquid desiccants have similar moisture-absorbing properties as solid desiccants, but they absorb the humidity portion of a typical air conditioning load without adding the heat, says Hugh Williams, a mechanical engineer and industrial department head for KGS Group, Winnipeg. “Getting around heating requirements for using solid desiccants is a huge challenge,” Williams adds, referring to the heat needed to regenerate solid desiccant wheels.
One major drawback of early-generation liquid desiccant systems was an effect known as carryover, in which lithium chloride desiccant would become entrained in cross air flows, causing corrosion in downstream ductwork.
According to NREL’s Kozubal, newer liquid-desiccant dehumidifier and A/C designs available on the U.S. market employ advanced liquid-to-air contactors that contain the desiccant without mist eliminators. The mass-manufactured contactors require a minimal pressure drop as compared to desiccant wheels, and represent “a critical advance that eliminates the maintenance traditionally associated with liquid systems,” allowing broader application of the packaged systems. Liquid desiccant systems also allow a number of application benefits that are unachievable with solid desiccant technology, including integration with CHP systems:
Making effective use of low-temperature waste-heat sources, as well as thermal energy from fuels cells and solar collectors.
The ability to physically separate the A/C and regeneration functions. This allows the liquid desiccant to be piped to a useful humid location and then piped back to a heat source for regeneration.
The option to store regenerated desiccant solution, which allows flexibility in the timing of the regeneration function — for example, it can occur when waste heat is available. Desiccant storage — essentially accumulated chemical energy with A/C potential — can be more efficient than thermal energy storage.
Lab testing supports assumptions made in the field. QinetiQ conducted a economic analysis of packaged liquid desiccant units and two alternate dehumidification approaches: a solid desiccant wheel for dehumidification followed by a vapor compression unit use for the removal of sensible heat from the process airstream; and a vapor compression unit used for dehumidification followed by sensible heating (reheat) of the process air to the final desired delivery temperature.8
As shown in the Table 1, the liquid desiccant systems perform competitively when compared to other conventional dehumidification equipment types — on operating costs and maintenance costs as well as on a first-cost basis, QinetiQ concluded. (For details on the study, see the sidebar, “A Cost Comparison of Dehumidification Approaches.”) For unconditioned process air treated to acceptable supply air at several test conditions, operating cost savings for the liquid desiccant system ranged from 27% to as high as 55% per unit. These implied significant reductions in operating costs, in part due to the elimination of process heat inputs.
CONCLUSION: BENEFITS FOR INDUSTRY
A growing number of facilities with challenging humidity conditions are employing liquid desiccant dehumidification systems, some saving up to half of its outlays for operating HVAC dehumidification. Experience at food plants shows that liquid desiccant systems can maintain tight tolerances for demanding process environments, such as preventing excessive condensation. The most significant benefit of using liquid desiccant systems is the potential for reduction in maintenance and total operating costs — in some cases cutting energy requirements by half. Liquid desiccant systems have also been shown to improve IAQ by reducing humidity loads as well as airborne microbiologicals. The liquid-based systems also require little maintenance.
Just as important, liquid desiccant systems can be implemented using creative approaches to heat-recovery, CHP, and even renewable energy solutions such as solar thermal collection. With the energy-efficiency gains of this inherently economical approach, facilities can further reduce greenhouse gas emissions and carbon footprint, making operations not only leaner but also greener. One reason for its effective performance is that liquid desiccant systems cool and dehumidify simultaneously — yet with independent control of each function.
A growing number of industrial and commercial HVAC installations are taking advantage of the resulting efficiencies, increasingly specifying liquid desiccants for hospitals, fitness centers, museums, and storage facilities. ES
Dieckmann, Roth & Brodrick. “Liquid Desiccant Air Conditioners.” ASHRAE Journal. October 2008, p. 90.
NREL website, www.nrel.gov/dtet/thermal_ac.html
QinetiQ North America/Technology Solutions Group. ”DuCool Dehumidification Equipment Technical Evaluation,” Waltham, MA. December 2009: 9-16.
TIAX, LLC. “Energy Consumption Characteristics of Commercial Building HVAC Systems—Volume III: Energy Savings Potential,” Report to DOE, BTP. 2002. http://doas-radiant.psu.edu/DOE_report.pdf.