ASHRAE Journal - May 2011 - 90

emerging technologies

cool and Dry comfort
L
By John Dieckmann, Member ASHRAE; Alissa cooperman; and James Brodrick, Ph.D., Member ASHRAE

Permeable Membrane-Based Liquid Desiccant Cooling Systems

iquid desiccant dehumidification systems are capable of achieving high COPs and EERs. Despite this, for decades such systems have been

used primarily only in laboratories and highly specialized industrial humidity control applications where well-controlled, low humidity levels are required. The technology is finally reaching a stage of maturity where liquid desiccant systems can enter the general market and be a viable alternative to “dry” cooling systems that do not offer the added benefit of dehumidification.

The basic configuration of a liquid desiccant dehumidification system is illustrated in Figure 1. The systems consist of a circulating liquid desiccant solution, a dehumidification column, a regeneration column, a recuperative heat exchanger and a heat input coil, along with supporting desiccant solution pumps, piping, air ducts, etc. Various liquid desiccant materials are used in liquid desiccant dehumidification systems. The most common are aqueous solutions of calcium chloride or lithium chloride. In the dehumidification column (Figure 1), concentrated desiccant solution flows downward over a mass transfer media that provides good contact between the process airstream and the liquid desiccant solution. Water vapor in the process airstream is absorbed into the desiccant solution, reducing the humidity of the
90 ASHRAE Journal

process airstream and lowering (weakening) the concentration of the desiccant. In the regeneration column, heated, weak desiccant solution flows downward over a mass transfer media. Water evaporates from the desiccant solution and is carried away by purge air, returning the desiccant concentration to a level where it can absorb moisture again in the dehumidification column. Desiccant solution circulates through the system, absorbing moisture from the process air in the dehumidification column, and rejecting moisture in the regeneration column. The regeneration column operates at a higher temperature than the dehumidification column, typically 212°F (100°C). The temperature difference is the thermodynamic potential driving the process. The desiccant solution leaving the dehumidification column needs to be
ashrae.org

heated to the regeneration temperature and the desiccant solution leaving the regeneration column needs to be cooled to the dehumidification temperature. A counterflow heat exchanger between these two desiccant streams can do a significant portion of the heating and cooling, reducing the amount of external heat input and cooling that must be done. With a reasonably effective counterflow heat exchanger and a reasonably efficient heater, a latent cooling COP (the heat of vaporization of the moisture removed divided by the heating value of the fuel input) of 0.7 can be achieved. There are many variations on the basic configuration shown in Figure 1, including crossflow configurations of the dehumidification and regeneration modules (with gravity-driven downflow of the desiccant solution and horizontal flow of the process and purge airstreams). Heating and cooling coils have been embedded within the mass transfer section of the regeneration and dehumidification modules, respectively. To improve the COP, double-effect regeneration has been proposed. Analogous to double-effect generators in absorption chillers, a double-effect regenerator has a high temperature effect where the desiccant solution is heated to a high enough temperature to boil off water as atmospheric pressure steam, then the heat of condensation from the steam supplies the heat input to the second
May 2011

ASHRAE Journal - May 2011

Table of Contents for the Digital Edition of ASHRAE Journal - May 2011