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Article Excerpt
INTRODUCTION
In the 20th century, the HVAC industry brought air conditioning out of the lab and into millions of homes and commercial buildings. What was once a luxury is now a necessity in many countries. However, this success now presents the industry with new challenges.
If the air conditioner is to improve the quality of life for billions more people in a rapidly developing world, we face many potential problems. The source of many of these problems is the air conditioner's heavy reliance on electricity. Tremendous amounts of fossil fuels are converted to carbon dioxide each year to produce the power needed by air conditioners. A rapidly growing demand for air conditioning could accelerate global climate change at a time when the world is struggling to reduce it. Air and water pollution could also increase as more power plants are built to meet the demand for electricity. Areas with limited water resources will find these resources overtaxed by the need to cool the new power plants. The reliability of electric systems could be compromised as air conditioning creates high peak demands for power.
Indoor air quality is another challenge to the HVAC industry, particularly in more humid climates. Sick Building Syndrome is a problem that can be corrected through better ventilation. For many types of buildings, ASHRAE's standards have tripled the ventilation rates over those that were common following the energy crisis of the 1970s. However, in humid climates, the increased ventilation can raise indoor humidity to levels that are both uncomfortable and unhealthy.
The vapor-compression cycle is now the foundation of the HVAC industry and will remain so for many years. The following problems are being addressed through a number of approaches including: (1) more efficient designs for air conditioners, (2) more efficient buildings that require less cooling, (3) the conversion of power generation from fossil fuels to sustainable resources, (4) the development of air conditioners that provide more dehumidification, or latent cooling, more efficiently, and (5) a wider implementation of energy storage technologies. Solutions do exist using only vapor-compression technology, but these solutions will increase the cost for air conditioning. Alternatives to the vapor-compression air conditioner may be better able to meet the growing demand while meeting the new economic, environmental, and performance requirements.
THE BASICS OF LIQUID-DESICCANT AIR CONDITIONERS
Although far less mature than vapor-compression technology, air conditioners based on liquid desiccants are one of the more promising new alternatives. Two characteristics of liquid-desiccant air conditioners (LDAC) will be critical to their wider use:
* The LDAC runs mostly on heat; its electric demand is typically one-fourth that of a vapor-compression air conditioner.
* The LDAC is exceptionally good at dehumidifying air; almost all of the cooling it provides can be latent cooling.
The primary objectives of this article are to present a summary of current R&D on LDACs and identify areas where further development will improve the competitiveness of the technology. However, since liquid-desiccant technology is not yet commonly part of HVAC systems, several basic concepts are first presented.
A desiccant is a material that has a strong attraction for water vapor. It is common to classify desiccants as either solid or liquid depending on their normal physical state (although a material such as lithium chloride can be both, absorbing water vapor both as a solid, hydrated salt, or as an aqueous solution). Both solid and liquid desiccants are commonly used in industrial applications where low dew-point air is needed. Solid desiccants are also increasingly being used in HVAC systems to either increase an air conditioner's latent cooling or recover total energy from the building exhaust.
The strength of a desiccant can be measured by its equilibrium vapor pressure (i.e., pressure of water vapor that is in equilibrium with the desiccant). This equilibrium vapor pressure increases roughly exponentially with the temperature of the desiccant/water system. It also increases as the desiccant absorbs water (a dilute liquid desiccant will have a higher equilibrium vapor pressure than a concentrated liquid desiccant).
When the absolute humidity of air that has come into equilibrium with a liquid desiccant of fixed concentration is plotted on a psychrometric chart, the equilibrium line closely follows a line of constant relative humidity. Figure 1 illustrates this behavior for solutions of lithium chloride. A liquid desiccant that is alternately exposed to two environments that are at different relative humidities will move moisture from high to low relative humidity.
[FIGURE 1 OMITTED]
A liquid desiccant can enhance heat transfer by a mechanism that is the inverse of evaporative cooling. When air flows over a surface wetted with water, evaporation from the film of water will lower the temperature of the water-air interface toward the wet-bulb temperature of the air. This wet-bulb temperature is a function of the air's initial temperature and humidity. A line of constant enthalpy that passes through the air's state point intersects the saturation line on a psychrometric chart at approximately the wet-bulb temperature. As shown in Figure 2, the wet-bulb temperature for air at 80[degrees]F (26.7[degrees]C) and 50% RH is 66.7[degrees]F (19.3[degrees]C).
[FIGURE 2 OMITTED]
When air flows over a surface that is wetted with a desiccant, the desiccant can either absorb or desorb water, depending on whether the desiccant's equilibrium relative humidity is above or below the air's relative humidity. If the desiccant absorbs water from the air, heat will be released and the desiccant's temperature will increase. This heating is the inverse of evaporative cooling. By analogy to evaporative cooling, one can define a brine-bulb temperature as the temperature that the desiccant-air interface approaches.
The brine-bulb temperature is a function of a liquid desiccant's concentration and the air's temperature and humidity. As shown in Figure 2, the brine-bulb temperature will always be slightly higher than the temperature at which a line of constant enthalpy from the air state point intersects the equilibrium relative humidity curve for the desiccant. This is because the heat that is released as the desiccant absorbs the water vapor includes the chemical heat of mixing between the desiccant and water, in addition to the vapor-liquid latent heat for the water vapor.
As shown in Figure 2, the brine-bulb temperature for a 43% solution of lithium chloride and air at 86/78[degrees]F (30.0/25.6[degrees]C) dry-bulb/wet-bulb will be 118[degrees]F (47.8[degrees]C). With an ambient wet-bulb temperature of 78[degrees]F (25.6[degrees]C), a typical cooling tower might supply water at 85[degrees]F (29.4[degrees]C). It's impractical to cool the ambient air using this cooling water in a conventional heat exchanger, because the cooling water is only one degree below the air temperature. However, a strong cooling effect could be achieved by wetting the surfaces of the heat exchanger with the 43% lithium chloride.
Of course, one does not get this enhanced cooling for free. If the cooling process is to be continuous, energy must be expended to regenerate the desiccant back to its original concentration.
If ambient air from the preceding example is brought into equilibrium with 43% lithium chloride at 85[degrees]F (29.4[degrees]C), the air will have a dew point of 33.5[degrees]F (0.8[degrees]C), a wet-bulb of 57.8[degrees]F (14.3[degrees]C), and its enthalpy will be reduced from 41.5 Btu/lb (96.3 kJ/kg) to 24.9 Btu/lb (57.8 kJ/kg). This large cooling effect, both in terms of latent cooling and total cooling, and low dew point--both of which are achieved without a compressor--demonstrate the potential for liquid desiccants to become an important part of HVAC systems.
Liquid desiccants have been successfully used to produce dry air for a surprisingly long time. Dr. Russell Bichowsky, working for the Frigidaire Division of General Motors, first used solutions of lithium chloride to dry air in the 1930s. Frigidaire sold the technology to Surface Combustion Corporation (SCC) in the mid-1930s. A residential liquid-desiccant dehumidifier was field tested by SCC shortly after they acquired the technology, but no product was introduced into the market (Griffiths 2007).
Also in the 1930s, the Niagara Blower Company introduced a liquid desiccant technology that used glycol solutions to prevent frost from forming on low-temperature evaporators. Both lithium chloride and glycol continue to be used today in liquid-desiccant dehumidifiers, but their use is limited primarily to industrial applications.
An important objective of this review is to identify the current technology base and R&D needs for moving liquid-desiccant systems into HVAC applications. The state of the art for industrial applications is a useful starting point, but typically, its cost, maintenance, and performance characteristics are not suitable for HVAC applications.
A typical industrial application of a liquid-desiccant system is shown in Figure 3. The conditioner (also commonly called an absorber) is the component that cools and dries the process air. As shown in this figure, the conditioner is a bed of structured contact media, similar to the corrugated fill that might be used in a cooling tower. Liquid desiccant is first cooled in a heat exchanger and then sprayed onto the contact media. The desiccant flow rate must be sufficiently high to ensure complete wetting of the media, meaning it should be about 5 gpm per square foot of face area. The process air is cooled and dried as it comes in contact with the desiccant-wetted surfaces of the contact media. Heat is released as the desiccant absorbs water from the air, but the high flow rate of the desiccant limits its temperature rise to a few degrees.
[FIGURE 3 OMITTED]
The regenerator removes the water that the desiccant has absorbed in the conditioner. The desiccant is regenerated by first heating it to raise its equilibrium vapor pressure. The hot desiccant, typically between 160[degrees]F (71.1[degrees]C) and 200[degrees]F (93.3[degrees]C),...
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