Abstract: In 1990 about 3.9 quads of primary energy were used for air conditioning (cooling and ventilation) of buildings. The energy used for
air conditioning buildings is expected to rise in the 1990's and beyond as the population shifts to the warmer southern states and personal computer
use increases in office buildings. In the 1990s, the air conditioning industry is faced with several challenges:
New approaches to space conditioning will be required to resolve these economic, environmental, and regulatory issues. Desiccant cooling and dehumidification,
a technology known for some time, may provide important advantages in solving air conditioning problems.
- increased energy efficiencies,
- improved indoor air quality,
- growing concern for improved comfort and environmental control,
- increased ventilation requirements,
- reduction of chlororfluorocarbons (CFCs), and
- rising peak demand charges.
In a typical desiccant system, the moisture (latent load) in the process air is removed by a desiccant material in a dehumidifier, then the temperature
(sensible load) of the dried process air is reduced to the desired comfort conditions by sensible coolers (e.g. heat exchangers, evaporative coolers,
cooling coils). The latent and sensible loads are handled separately and more efficiently in components designed to remove that load. The desiccant
in the dehumidifier is regenerated (reactivated) by application of heat to release the moisture, which is exhausted to the outdoors. The heat for
regneration can be provided from a number of energy sources such as solar, waste heat, natural gas, and off-peak electricity. The desiccant
can be either solid or liquid. In solid desiccant systems, air is passed through a bed of adsorptive material. Air is dried and moisture is adsorbed
by the desiccant. When the desiccant is saturated, hot air is passed through the bed, releasing the moisture. Typically, the desiccant is loaded into
a disc that rotates between the process and regeneration airstreams. In a liquid desiccant system, a concentrated liquid desiccant is sprayed in a contactor
containing cooling coils or packing materials) to absorb moisture from humid air stream passing through the contactor. The liquid desiccant leaving the
contactor is diluted with removed water. The diluted liquid desiccant is heated or sprayed into the regeneration air stream to remove and release the
moisture and reconcentrate the liquid desiccant.
The best circumstances for use of desiccant cooling and dehumidification are: (1) need for humidity control, (2) economic
benefits from using low humidity, (3) high latent load versus sensible load, (4) low thermal energy cost versus high electric energy cost, and
(5) need for dry cooling coils and duct work to avoid microbial growth.
- Desiccant systems offer significant potential for energy savings (0.1-0.4 quads) and reduced consumption of fossil fuels. The electrical energy
consumption is small, and the source of thermal energy can be diverse (i.e., solar, waste heat, natural gas).
- With desiccant systems the use of CFCs is eliminated (if used in conjunction with evaporative coolers) or reduced (if integrated with vapor
compression units). CFCs contribute to depletion of the earth's ozone layer and will be banned by the end of the century.
- Indoor air quality is improved because of higher ventilation and fresh air rates associated with desiccant systems. Such systems also offer lower
humidity levels and the capability to remove airborne pollutants.
- With desiccant systems, air humidity and temperature are controlled separately, enabling better control of humidity
Desiccant cooling and dehumidification can be applied to many types of buildings: supermarkets, hotels and motels, office buildings, hospitals and nursing
homes, restaurants, health clubs and swimming pools, and residences. The success of desiccant cooling is being realized in supermarketss, which use four
times more energy per unit floor space than most commercial buildings. Use of desiccant technology to provide dry, cool air for hotels and motels in humid climates
(to avoid mold and mildew damage as a result of excess moisture) is expected to be the next major application of the technology.
Status of Technology: More than 15 years of research and development, funded by the Department of Energy, Gas Research Institute, utilities, and
the private sector, have resulted in significant improvements in the performance, cost, and reliability of desiccant dehumidification and cooling systems.
Currently, they are competitive in the market for a few specialized applications such as supermarkets. Investigation of desiccant cooling systems
during the past decades have revealed that such systems have great potential to compete and complement the conventional, electrically driven vapor
compression systems. There are approximately 900 citations in the literature on the subject of desiccant cooling. The following are some of the recent
advances and findings for desiccant cooling technology:
- Type 1M (moderate) isotherm has been identified as the "preferred" shape of isotherm for solid desiccants in cooling applications. The higher performance from a
desiccant with as Type 1M isotherm is usually obtained when staged regeneration is used.
- Several Type 1M desiccants have been identified/synthesized and R&D efforts are underway to incorporate these materials into inexpensive dehumidifier structures.
- Desiccant materials have the potential to remove airborne pollutants, thereby improving indoor air quality.
- Desiccant degradation/contamination is not expected to be a barrier/problem for HVAC applicaiton of solid desiccant cooling technology.
- Rotary dehumidifiers with laminar flow passages (e.g. corrugated, honey comb, or parallel plate) have become the choice for solid desiccant cooling systems.
- Low-pressure drop, relatively inexpensive, laminar-flow, corrugated dehumidifiers using silica gel or molecular sieves have been entered onto the market.
- Low-cost solid desiccant dehumidifiers using Type 1M material are being manufactured by a few organizations.
- A "humidity pump" gas-fired liquid desiccant dehumidifier module has been developed that can be added to the electrically-driven refrigeration
air conditioner for removing latent load.
Future Research and Development Needs: While significant progress has been made in efficiency and cost effectiveness of desiccant cooling
systems for air conditioning, commercial systems and designs are available only for special applications such as supermarkets and other low-humidity
applications. Lowering costs and improvements in the efficiency, size, reliablity, and life-expectancy of components and systems are necessary to advance penetration of the
desiccant cooling technology into broader commercial air conditioning market.
- The concept of "staged regeneration" for solid desiccant dehumidifers has been reintroduced. Combined with Type 1M desiccant and high regeneration
temperatures, staged regeneration is expected to reduce the size of air-to-air heat exchangers, thus reducing system size and cost.
- Advanced desiccant cycles have been proposed that have thermal COPs above 1.7. Higher COPs are achieved through use of a larger number of components.
- A gas-fired prototype closed-cycle desiccant heat pump unit has been developed using Zeolite/water with a cooling COP above 1.2.
- A variety of modeling tools for analyzing the performance of desiccant cooling systems and components have been developed and validated.
- Integrated desiccant cooling systems using desiccants for dehumidification and conventional vapor compression for cooling have been successfully
demonstrated for a few commercial buildings and many supermarkets.
The reulst of the last 15 years of R&D and the goal of mass market penetration have given desiccant cooling technology a direction for the future and
shaped R&D needs. In our opinion, the high priority R&D areas are:
Concluding Remarks: Desiccant cooling technology has the potential to significantly affect the air conditioning market and its energy use.
Desiccant cooling has many benefits including lower energy consumption, using renewable energy or waste heat, lower use of CFCs, and improving
indoor air quality. Advances in the last decade have resulted in successful application of the technolgy in niche markets. However, penetration into
the air conditioning mass market requires further improvement in efficiency and reliability, reduction in size and cost, and improvement in
technology acceptance by the building community. Investments in further research and development in materials, components, and systems are needed
and justified considering the potential of the desiccant cooling technology.
- Development of low-cost solid desiccant materials with optimum properties for high-,medium-, and low-temperature regeneration applications.
- Fundamental research on water/vapor desiccant interaction for improving sorption behavior.
- Development of low-cost, noncorrosive, low-vapor pressure, and safe liquid desiccants with desirable sorption properties.
- Study of desiccants as air purifiers for improving indoor air quality.
- Developing low-cost, high-performance, compact solid and liquid dehumidifiers
- Reducing the cost and energy requirements for desiccant regeneration device
- Development of control strategies and components.
- Demonstration of the state-of-the-art desiccant systems for air conditioning of various buildings
- Conducting system performance and economic analysis studies
- Development of efficient, cost-effective systems utilitizing waste heat for desiccant regeneration
- Development of user-friendly design and analysis tools
- Transfer of technology to consultant engineers, building community, and HVAC designers and manufacturers
Keywords: desiccant, dehumidifier
National Renewable Energy Laboratory
1617 Cole Boulevard
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