Emerging Technology: Liquid Desiccant Air Conditioners
INTRODUCTIONOngoing economic and indoor environmental concerns require HVAC innovation. New liquid desiccant air conditioning systems offer substantial energy savings and greatly improved humidity control in applications where latent loads (moisture) are very high relative to sensible loads. This includes hot-humid climates and applications such as supermarkets where low indoor humidity is required to avoid condensation on case doors, etc. These systems have been common in industry, and new equipment that decreases maintenance and reduces concerns about desiccant salt carryover makes them attractive for space conditioning.
The liquid desiccant air conditioner (LDAC) provides a cost-effective route to latent cooling needed to control indoor humidity while avoiding the high electrical demand of compressor-based approaches to reducing heat loads. The liquid desiccant is a concentrated salt solution that directly absorbs moisture without first cooling the air below its dew point. The sensible heat released as the desiccant absorbs the moisture is also removed from the air so the LDAC both cools and dries the air. The water absorbed by the desiccant is removed to ambient air by heating the desiccant to between 180° and 200°F (or higher). This heat is the primary energy input to the LDAC. The source can be a boiler fired by biofuel, natural gas, or other fossil fuel; solar thermal collectors; or heat recovered from an engine or industrial process (combined heat and power or CHP). The liquid desiccant air conditioner may be particularly attractive for building-scale CHP in applications like supermarkets where humidity control is important and there are high latent loads.
Thermal air conditioners reduce electricity use and peak electrical demand. Those that use waste heat or solar energy also conserve fossil fuels now used to generate the electricity for compressor-based air conditioners. Compared to other thermal technologies - absorption or adsorption chillers - LDAC should exhibit lower capital cost, easier application, lower cost for energy storage, and lower hot-water temperature requirements.
Liquid desiccants have been used to dehumidify air in industrial applications for over 70 years, but adaptation to comfort conditioning faced two high barriers: carryover of salt-containing droplets and high maintenance costs. Because of the high performance of the systems, these attributes have been acceptable in some industrial applications. The new low-flow LDAC should eliminate droplet carryover by using desiccant flow velocities that are 20 to 50 times lower than those used in industrial systems.
CURRENT STATUSWithin the past few years, two companies were unsuccessful in attempts to introduce a liquid desiccant air conditioner for HVAC applications. The products of both companies used high desiccant flow rates comparable to industrial systems, which may have contributed to their lack of success. A third company is now manufacturing and selling a high-flow LDAC. This unit is expected to have higher pressure drops, higher parasitic power, lower thermal COP, and more demanding maintenance requirements than a low-flow LDAC optimized for commercial applications.
Several companies manufacture and sell high-latent air conditioners that can be used to dry a building’s ventilation air. These high-latent air conditioners all use vapor-compression technology, so they do not significantly reduce the peak electrical demands for cooling.
Other companies supply solid wheel desiccants which are typically silica gel based thermally regenerated dehumidifiers. These systems are larger, less efficient, have higher air-side pressure drops, and typically require considerably higher regeneration temperatures than the LDAC. In addition the heat generated as the moisture is absorbed flows into the conditioned space adding to the building sensible load.
ENERGY SAVINGS AND COSTSThe most attractive applications for the LDAC will be in humid climates where its beneficial impact will include: (1) improved IAQ that leads to improved worker productivity in offices and student attentiveness in schools; (2) improved indoor comfort that increases patronage of restaurants, movies, and retail stores; (3) lowered indoor humidity that avoids remediation costs associated with mold and mildew; and (4) direct savings from the elimination (or reduction) of reheat as a means of humidity control. In humid climates with long cooling seasons, the elimination of reheat can reduce annual HVAC costs by 30 percent or more.
As a gas-fired alternative to high-latent electric air conditioners, the LDAC’s operational savings will be largely determined by local gas and electric utility rates and the length of the cooling season. Capital costs may be lower for LDAC (goal is $5/cfm) than for high-latent air conditioners ($8 to $10 per cfm of ventilation). At $0.10 per kWh and $1.00 per therm, the operating costs for the LDAC and electric alternatives will be comparable.
MARKET BARRIERSThe LDAC is unfamiliar to engineers and designers who specify HVAC equipment, and to the trades that install and maintain it. The little exposure that liquid desiccant equipment has had in the past pertains to industrial-type systems, and this exposure has too frequently uncovered maintenance problems from desiccant carryover. Also, many LDAC applications need a cooling tower, which may also require more maintenance than some building owners will accept.
The high cost of solar thermal collectors combined with the relatively low cost of electricity in many parts of the U.S. are also barriers to the wider use of solar and gas-fired LDACs.
NEXT STEPSDemonstrations are needed to document the benefits of the low-flow LDAC including operating and maintenance costs. These early demonstrations, coupled with strong educational and promotional activities, can move the traditionally cautious HVAC community to adopt the technology. In preparation for commercialization, further market analysis is needed to formulate a practical and effective marketing and sales strategy. In addition, further analysis of the synergies with renewable energy and CHP are required to help position LDAC in the “green” market. The thermally driven LDAC could greatly expand the use of CHP for smaller applications by increasing summer load factors. Finally, an LDAC driven by solar thermal collectors would be the lowest cost alternative to converting the country’s cooling needs to a renewable energy source.
Publication date: 09/20/2010