Liquid desiccant air conditioners

ASHRAE Journal, Oct, 2008 by John Dieckmann, Kurt Roth, James Brodrick

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)