Thermally-Activated Technologies

Solid-Desiccant and Liquid-Desiccant Systems for Dehumidification

The DOE/ORNL Program has partnered with industry and NREL to establish desiccant technologies to reduce peak demand, better control building humidity levels, and improve building Indoor Air Quality (IAQ) with increased ventilation rates. Advanced Ventilation Air Conditioning (VAC) designs using recovered energy are the key to efficiently controlling buildings’ indoor humidity/environmental quality/comfort and worker productivity.

More than 90% of all commercial buildings in the U.S. are air conditioned with some type of packaged equipment – mainly rooftop units. The HVAC industry has found this equipment compact, reliable, and cost effective. However, improved humidity-control products with improved latent to sensible load performance are needed by the building industry to satisfy American Society of Heating, Refrigeration and Air-Conditioning Engineers, Inc. (ASHRAE) Standard 62 ventilation requirements with minimal or reduced energy use impact.

The hybrid desiccant/vapor-compression products coming out of this program are the first packaged rooftop units that can maintain both temperature and humidity, independently, while delivering any percentage of outdoor air required. These products can precisely match the sensible and latent loads required by the space, rather than meeting the sensible load and accepting whatever humidity level comes with it.

As with many emerging technologies, first generation VAC desiccant equipment has not been without its performance, design, and manufacturing challenges. The goal of the desiccant work is to develop and expand a new class of desiccant technologies into broader, mainstream markets for indoor comfort, health, and humidity control through cost-shared and collaborative programs with industry. Waste heat from DG systems fits well with the low-grade regeneration heat requirements of solid and liquid desiccant systems.

Laboratory Desiccant Performance Evaluation
ORNL is supporting industry with laboratory benchmark testing and evaluation of desiccant system performance. Through User Agreements, ORNL is able to generate controlled laboratory measurements to map performance and verify control strategies for integrated desiccant/vapor-compression system operation, to provide diagnostic support for field data evaluations, and to verify desiccant CHP integration potential. Data and results from contemporary systems will assure the availability of important performance results of interest to the U.S. HVAC and IES/CHP engineering communities.

In addition to this extensive laboratory testing of hybrid solid-desiccant and liquid desiccant systems, ORNL continues to develop and promote desiccant and thermal system test methods and rating certification standards. ORNL has provided laboratory data to ASHRAE and ARI in support of this effort and will be verifying the draft "method of test" that will provide rating and certification standards of TAT desiccant and IES integrated based equipment.

Technology Development Projects
Three types of systems are under development. The first, a rooftop liquid desiccant air conditioner, offers the best opportunity for increased coefficient of performance (COP) efficiency of the desiccant dehumidification process as compared to a solid-desiccant, wheel system. This work was performed by Kathabar, Incorporated and is at the laboratory-scale development stage.

The other two system types are both based on solid desiccants and are at a more advanced development stage. These products impact the packaged equipment, rooftop market in the United States, which accounts for 90-95% of the commercial building air conditioning marketplace. Significant energy and operating cost savings possible with these products offer the best opportunity to dramatically reduce building energy use and efficiently utilize low grade, localized waste heat for building conditioning.

A unitary, packaged system concept, patented by SEMCO, Incorporated, integrates an active desiccant wheel with conventional vapor-compression air conditioning equipment. SEMCO introduced this product, which uses the desiccant to pre-condition the fresh outside air, in 2004. This product can be adapted for seamless integration with waste heat recovery from distributed generation equipment.

A solid desiccant-based hybrid system for all-electric air conditioning has been developed by the Trane Company working with the Florida Solar Energy Center (FSEC), the University of Central Florida, and AirXchange, Inc. This all-electric, desiccant hybrid product uses waste heat from the system's air conditioning condenser to thermally regenerate the desiccant component. Certified testing shows that it reduces HVAC energy use by 50% compared to systems that use reheat for humidity control and by 10% compared to heat pipe/run-around-loop alternatives.

If you have any questions or comments regarding this section or the CHP Technologies Program in general, please contact us.