Ashrae Journal - October 2008 - 90

Liquid Desiccant Air Conditioners By John Dieckmann, Member ASHRAE; Kurt Roth, Ph.D., Associate Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE L iquid 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. 90 ASHRAE Journal 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 desicashrae.org October 2008

Ashrae Journal - October 2008

Table of Contents for the Digital Edition of Ashrae Journal - October 2008

Ashrae Journal - October 2008

Contents

Commentary
Industry News
Letters
ASHRAE HQ Dedication Booklet
ASHRAE Rebuilds HQ to Reflect Mission
Feature Articles
ASHRAE's Living Lab
Meetings and Shows
Sizing Pipe Using Life-Cycle Costs
Feature Articles
Energy-Efficient Data Centers: A Close-Coupled Row Solution
The Future of 90.1
Ventilation Standard for Health Care Facilities
Grosvenor Humidity Chart
ASHRAE Research Report
ASHRAE Honor Roll
Building Sciences
Special Products
Emerging Technologies
Product Showplace
Classified Advertisting
Advertising Index
Ashrae Journal - October 2008 - Ashrae Journal - October 2008
Ashrae Journal - October 2008 - Cover2
Ashrae Journal - October 2008 - 1
Ashrae Journal - October 2008 - 2
Ashrae Journal - October 2008 -

Contents