Andrew Lowenstein

A ZERO CARRYOVER LIQUID-DESICCANT AIR CONDITIONER FOR SOLAR APPLICATIONS

A novel liquid-desiccant air conditioner that dries and cools building supply air has been successfully designed, built, and tested. The new air conditioner will transform the use of directcontact liquid-desiccant systems in HVAC applications, improving comfort and indoor air quality, as well as providing energy-efficient humidity control Liquid-desiccant conditioners and regenerators are traditionally implemented as adiabatic beds of contact media that are highly flooded with desiccant. The possibility of droplet carryover into the supply air has limited the sale of these systems in most HVAC applications. The characteristic of the new conditioner and regenerator that distinguishes them from conventional ones is their very low flows of liquid desiccant. Whereas a conventional conditioner operates typically at between 10 and 15 gpm (630 and 946 ml/s) of desiccant per 1000 cfm (0.47 m 3 /s) of process air, the new conditioner operates at 0.5 gpm (32 ml/s) per 1000 cfm (0.47 m 3 /s). At these low flooding rates, the supply air will not entrain droplets of liquid desiccant. This brings performance and maintenance for the new liquid-desiccant technology in line with HVAC market expectations. Low flooding rates are practical only if the liquid desiccant is continually cooled in the conditioner or continually heated in the regenerator as the mass exchange of water occurs. This simultaneous heat and mass exchange is accomplished by using the walls of a parallel-plate plastic heat exchanger as the air/desiccant contact surface. Compared to existing solid- and liquid-desiccant systems, the low-flow technology is more compact, has significantly lower pressure drops and does not “dump” heat back onto the building’s central air conditioner. Tests confirm the high sensible and latent effectiveness of the conditioner, the high COP of the regenerator, and the operation of both components without carryover.

Review of Liquid Desiccant Technology for HVAC Applications

A thermally driven air conditioner that uses liquid desiccants as the working fluid may be an attractive alternative to the compressor-based technology that is now used in most HVAC applications. The operation of a liquid-desiccant air conditioner is first explained and several basic concepts are reviewed. This review focuses on the development of liquid-desiccant conditioners and regenerators that are better suited to comfort conditioning (as opposed to industrial dehumidification). This includes work on conditioners and regenerators that use low flow rates of desiccants and have internal heat exchange. These conditioners and regenerators will have lower pump and fan power than packed-bed units, and will be much less likely to introduce desiccant droplets into the process air. Work to identify and develop regenerators that have COPs over 1.0 and research on alternative liquid desiccants is also reviewed.