Printer Friendly

Liquid desiccant air conditioners.

Liquid desiccant air conditioners are an approach to effectively manage humidity under challenging conditions such as buildings with high outdoor air (OA) requirements located in humid regions. They remove moisture and latent heat (and, possibly, sensible heat) from process air via a liquid desiccant material, such as lithium chloride (LiCl) or halide salts. (1,2)

Liquid desiccant AC has two essential components, an absorber and a regenerator. In a basic configuration, strong (i.e., concentrated) and cooled liquid desiccant flows into the absorber and down through a packed bed of granular particles (or other enhanced mass transfer surface or packing). Counterflowing return air passes up through the bed, transferring both moisture and heat to the liquid desiccant. The water absorbed from the air dilutes the liquid desiccant leaving the bottom of the packed bed, and flows into the regenerator. In the regenerator, a heat source (gas- or oil-fired, waste heat, solar, etc.) heats the weak liquid desiccant solution, increasing the vapor pressure of the water. When the weak desiccant is sprayed on another packed bed, the absorbed moisture migrates to a counterflowing scavenger air stream to regenerate a concentrated liquid desiccant solution. Subsequently, the return feed from the regenerator passes through a cooling tower or chiller to remove the heat input from the regenerator. Finally, the cooled liquid desiccant solution returns to the absorber to complete the cycle.

Designs usually include a counterflow heat exchanger between the flow exiting the absorber and that exiting the regenerator to reduce the amount of external heating and cooling required. Alternatively, at least one product has used a heat pump system instead of a heat exchanger to increase the quantity of heat transferred. (2)

Some liquid desiccant AC units include a cooling coil downstream of the absorber to provide (primarily) sensible cooling. (2)

Both integrated and distributed systems exist. Integrated systems house the absorber and regenerator in a single unit. In contrast, a distributed system comprises multiple absorbers (typically integrated with air-handling units) and a single, central regenerator, with piping to transfer strong and weak desiccant between the absorbers and the regenerator. (2) For buildings with multiple OA intakes, this facilitates centralized production and storage of strong desiccant.

A desiccant system integrated with a combined heat and power 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.

Overall, several thousand liquid desiccant units are sold in the U.S. each year. Industrial units for deep drying and applications requiring precise humidity control account for most of the liquid desiccant market. Although they have a small portion of the overall commercial buildings space conditioning market, they are used more frequently in applications with requirements for lower humidity, such as ice rinks and the refrigerated and frozen food aisles of supermarkets. (3)

Energy-Savings Potential

Three issues limit the efficiencies of most units to levels below those of interest for HVAC applications. First, heat (the latent heat of vaporization of the absorbed moisture) accumulates in the absorber, reducing its net sensible and latent cooling capacity. Second, many systems use low liquid desiccant concentration gradients that increase the system mass flow significantly relative to higher concentration systems. This increases parasitic energy consumption, both liquid desiccant pumping power and the fan power to drive the air through the packed bed. Third, a single-effect system only uses the regeneration heat input once, inherently limiting the coefficient of performance (COP) to less than one.

Existing liquid desiccant dehumidification systems have thermal COPs of around 0.5 to 0.6, (3) with systems in development approaching 0.7 to 0.8. (1) Since these values do not include the electric energy the units consume, the actual primary energy efficiency is lower.

Developers and manufacturers have produced several modifications to the basic liquid desiccant system to increase its efficiency, including:

* Multiple effect regenerators;

* High-desiccant concentration gradient designs; and

* Evaporatively cooled absorbers.

Multiple-effect regenerators use each unit of heat input to remove two or more units of latent heat from the desiccant solution in the regenerator, increasing the potential COP to more than one. Over the last decade, researchers have worked to develop advanced liquid desiccant air-conditioning systems that would use multiple-effect boilers to achieve thermal COPs in excess of unity. (1,4)

High-concentration gradient systems can greatly decrease pump and blower parasitic energy losses by reducing the liquid desiccant mass flow required to remove a given quantity of moisture. For example, one group uses extended plastic surfaces for the heat exchanger in both the regenerator and conditioner. Its surface has a thin wick that achieves high mass transfer (of moisture) rates with the air, increasing the change in the desiccant concentration in the cycle. This, in turn, enables a dramatic (20- to 30-fold) reduction in the system's desiccant-to-air mass flow ratio relative to conventional liquid desiccant systems, achieving approximately a ten-fold decrease in pump power. In addition, the array of plastic surfaces has an appreciably lower pressure drop than conventional packed beds, reducing fan power draw. In total, the developers project parasitic electricity consumption of just under 0.3kW/ton. (1)

An evaporatively cooled absorber lowers the humidity below the desired indoor level to a level sufficient to manage internal moisture loads. The dry-bulb temperature of the OA would approach 10[degrees]F (5.5[degrees]C) above the wet-bulb temperature and provides moderate sensible cooling of the air. Although at typical design conditions the system provides no sensible cooling to the building, at lower wet bulb temperatures, the air delivery temperature decreases and also provides some sensible cooling.

Overall, unless they use waste or solar heat or triple-effect regeneration, * current liquid desiccant AC units offer little national primary energy-savings potential as a wholesale replacement for vapor compression systems.

In humid environments, however, they can save energy, notably when used as part of a dedicated outdoor air system (DOAS), primarily to dehumidify the OA. Because the liquid desiccant DOAS handles main latent source/load, this eliminates the need to overcool ventilation air to remove humidity and decreases reheat energy consumption. Furthermore, because the indoor AC system needs only to address indoor moisture sources, it requires very limited latent capacity. This allows it to operate at a higher evaporating temperature, which improves the COP relative to a conventional chiller by about 20%. (4)

When used as part of a DOAS for warm and humid OA conditions (dry bulb temperature=86[degrees]F [30[degrees]C], wet-bulb temperature=78[degrees]F [25.6[degrees]C], an advanced liquid desiccant system with a COP of 1.2 (3,5,6,7) could achieve appreciable energy cost savings relative to conventional systems using both conventional reheat and heat pipes (approximately 30% and 5% respectively). Primary energy savings would tend to be more modest relative to reheat (~15%) and negligible relative to heat pipes. (3)

The superior dehumidification performance of desiccant systems at moderate ambient, high humidity conditions also has the potential to save energy by increasing the indoor temperature setpoint, typically by 2[degrees]F to 5[degrees]F (1.1[degrees]C to 2.8[degrees]C). In some instances, occupants respond to the poor dehumidification performance of conventional systems by decreasing the indoor temperature set point to ensure that the unit runs long enough to dehumidify the space. This, in turn, increases the sensible loads because it increases the temperature difference between the outdoor and indoor air. Furthermore, under this condition, the conventional unit provides negligible sensible cooling and very inefficient latent cooling, e.g., EERs of approximately 3 to 4. (8) As a result, desiccant systems can realize significant savings under these conditions.

Liquid desiccant systems can also use lower-temperature waste heat from distributed generation (microturbines, internal combustion engines, fuel cells, etc.) sources, district heating systems, and solar thermal energy to regenerate the desiccant. If this heat is of sufficient quality, e.g., single-effect systems require temperatures of approximately 160[degrees]F - 180[degrees]F (70[degrees]C - 80[degrees]C) for single-effect and 245[degrees]F - 320[degrees]F (120[degrees]C - 160[degrees]C) for double-effect, (3,6,9) it can dramatically improve the economics and energy savings of liquid desiccant AC.

Conversely, using the waste heat to drive a single- or double-effect absorption chiller usually enables significantly greater use of waste heat than desiccant regeneration alone, because absorption systems can meet both latent and sensible loads. Furthermore, some sources of waste heat, such as microturbines, provide waste heat at temperatures that are too high for direct desiccant regeneration. In this case, the higher-quality heat sources require dilution to drive the desiccant system. In practice, its higher quality heat could often be more productively used to drive an absorption cooling cycle. (3)

Market Factors

Liquid desiccant systems can improve humidity management relative to conventional systems. Because the liquid desiccant removes moisture without cooling the air to saturation, the supply air relative humidity falls below 70%. This keeps supply ducts dry and helps avoid mold and bacterial growth. In addition, the scavenging action of liquid desiccant systems could improve indoor air quality by removing airborne contaminants. (2,4)

To date, liquid desiccant systems have achieved limited use in commercial buildings due to their higher energy costs (discussed earler) and first cost, as well as corrosion and liquid carryover challenges.

The cost premium of liquid desiccant AC units remains significant. For example, one group working on advanced liquid desiccant systems projects that a liquid desiccant-based DOAS will cost approximately 65% more than a DOAS using conventional vapor-compression technology. (1) Although the liquid desiccant system might be more attractive in applications with abundant, low-cost waste heat, such a sizeable first-cost premium would likely severely limit its market penetration.

Many developmental and deployed systems use LiCl, which corrodes most metals and, thus, requires design modifications to avoid corrosion. (2,7) In addition, some of desiccant can carry over, i.e., the process air can entrain liquid-desiccant aerosols as it passes through the packed bed and desiccant spray. This can corrode system components downstream of the absorber such as ducts and coils, and, potentially, cause health concerns. (2,4)

Both challenges have been--and remain--the focus of appreciable development efforts. Developers have worked on and manufacturers have commercialized products that use plastic (and, in at least one instance, cellulose) components that resist LiCl corrosion. One solution is to use microporous membranes that allow the migration of water but prevent the migration of the desiccant into the airflow. (2) Another is to use special surfaces designed to form a thin film of desiccant that directly contacts the supply and regeneration airflows. By ensuring that the system operates in a regime where the desiccant and airflows do not form droplets, the developers claim to eliminate desiccant carryover. (1,2)

Several researchers have investigated using solar thermal energy to regenerate desiccants systems. Liquid desiccants can be regenerated by heat at temperatures achieved by flat-plate solar collectors under peak insolation conditions (i.e., around 1,000 W/[m.sup.2] [317 Btu/h x [ft.sup.2] x [degrees]F]). At this peak condition, the flat-plate collectors have an efficiency of about 50% to 60% and cooling systems have thermal COPs (i.e., not including parasitics) of about 0.8. Both cooling capacity and efficiency will decrease as insolation decreases, however, necessitating a backup heat source to supplement and replace the solar thermal energy source. If solar energy supplies a large fraction of the regeneration energy and the parasitics are not excessive, solar thermal-powered liquid desiccant AC can realize large primary energy savings, particularly in humid climates. (1,9)

The cost of solar thermal collectors is a major barrier to the use of solar thermal-powered desiccants. One group that analyzed the cost of a solar thermal-powered liquid-desiccant system estimated that flat-plate solar collectors would have an installed cost of approximately $25/[ft.sup.2] - 40/[ft.sup.2] ($270/[m.sup.2] - $430/[m.sup.2]). (1) Assuming a peak thermal COP of 0.44 and an installed cost of $32.50/[ft.sup.2] ($350/[m.sup.2]), the solar thermal collectors alone would cost about $2,650 per peak ton ($9,320/kW) of cooling capacity. For comparison, new commercial unitary products in the 10-ton range have an installed cost of about $1,000 per ton ($3,500/kW). (10) Widespread deployment of low-cost, lower temperature solar-thermal collectors, e.g., for solar water heating, would reduce the installed cost of solar thermal collectors and improve the economics of solar thermal-powered liquid desiccant AC.

References

(1.) Lowenstein, A., S. Slayzak, and E. Kozubal. 2006. "A zero carryover liquid-desiccant air conditioner for solar applications." Proceedings of the ASME 2006 International Solar Energy Conference. http://tinyurl. com/5agkzp (or www.nrel.gov/docs/fy06osti/39798.pdf).

(2.) Conde-Petit, M. 2007. "Liquid desiccant-based air-conditioning systems--LDACS," Proceedings of the 1st European Conference on Polygeneration. http://tinyurl.com/6o3ksd (or http://six6.region-stuttgart. de/sixcms/media.php/773/19_Conde_M.pdf).

(3.) TIAX. 2004. "Review of Thermally Activated Technologies." Final Report by TIAX LLC to the U.S. Department of Energy, Distributed Energy Program. http://tinyurl.com/67nfet (or www.docstoc.com/ docs/944227/Review-of-Thermally-Activated-Technologies).

(4.) TIAX. 2002. "Energy Consumption Characteristics of Commercial Building HVAC Systems--Volume III: Energy Savings Potential." Final Report to US Department of Energy, Building Technologies Program. http://doas-radiant.psu.edu/DOE_report.pdf.

(5.) Lowenstein, A. and D. Novosel. 1995. "The seasonal performance of a liquid-desiccant air conditioner." ASHRAE Transactions 101(1):679-686.

(6.) Lowenstein, A. 1998. "Advanced Commercial Liquid-Dessicant Technology Development Study." NREL Report NREL/TP-550-24688. http://tinyurl.com/55rq9m (or www.nrel.gov/docs/fy99osti/24688.pdf).

(7.) Lowenstein, A. 2003. "A solar liquid-desiccant air conditioner." Proceedings of the ASES National Solar Energy Conference. http:// tinyurl.com/5jfpn7 (or www.ailr.com/AILR%20Solar%20AC.pdf).

(8.) Kosar, D. 2006. "Dehumidification system enhancements." ASHRAE Journal 48(2):48-58.

(9.) Hwang, Y., R. Radermacher, A.A. Alili, and I. Kubo. 2008. "Review of solar cooling technologies." HVAC&R Research 14(3):507-528.

(10.) Navigant Consulting. 2007. "EIA Technology Forecast Updates--Residential and Commercial Building Technologies--Reference Case Second Edition (Revised)." Presented to the U.S. Department of Energy, Energy Information Administration.

John Dieckmann and Kurt Roth, Ph.D., are principals with TIAX LLC, Cambridge, Mass. James Brodrick, Ph.D., is a project manager with the Building Technologies Program, U.S. Department of Energy, Washington, D.C.

* Triple-effect regeneration could obtain primary energy savings of 20% to 25%. (4)
COPYRIGHT 2008 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2008 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:Emerging Technologies
Author:Dieckmann, John; Roth, Kurt; Brodrick, James
Publication:ASHRAE Journal
Date:Oct 1, 2008
Words:2415
Previous Article:Energy-efficient data centers: a close-coupled row solution.
Next Article:Grosvenor humidity chart.

Terms of use | Privacy policy | Copyright © 2021 Farlex, Inc. | Feedback | For webmasters |