Desiccants are materials that have a strong attraction for water vapor.  It is common to classify desiccants as either solid or liquid (although a compound such as lithium chloride can be both, absorbing water vapor both as a solid, hydrated salt or as an aqueous solution).  

     The strength of a desiccant is measured by its equilibrium vapor pressure with water.  This equilibrium vapor pressure increases roughly exponentially with the temperature.  It also increases as the desiccant absorbs water, i.e., a dilute liquid desiccant will have a higher equilibrium vapor pressure than a concentrated liquid desiccant.

     When the humidity of air in equilibrium with a liquid desiccant of fixed concentration is plotted on a psychrometric chart, the plotted line closely follows a line of constant relative humidity.  The first psychrometric chart at the right illustrates this behavior for solutions of lithium chloride.  A liquid desiccant that is alternately exposed to two environments that are at different relative humidity will move moisture from high to low relative humidity.

     A common format for characterizing a liquid desiccant’s drying potential is the Duhring chart, where the dewpoint of air in equilibrium with a desiccant of fixed concentration is plotted versus the desiccant’s temperature. As shown in the second chart on the right, the near-constant equilibrium relative humidity of a liquid desiccant at fixed concentration is equivalent to lines of constant slope on a Duhring chart.

     Liquid desiccants can help the world meet its almost insatiable demand for air conditioning.   In many applications, they will enable low-demand evaporative cooling to replace high-demand compressor-based air conditioners.  This potential follows from a liquid desiccant's ability to enhance heat transfer by a mechanism that is the inverse of evaporative cooling. 

     When air flows over a surface wetted with water, the evaporation of water lowers the temperature of the wetted surface towards 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 will intersect the saturation line on a psychrometric chart at approximately the wet-bulb temperature.  As shown in the third chart on the right, the wet-bulb temperature for air at 95°F (35.0°C) and 47.3% rh is 78.0°F (25.6°C).

     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.  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 the third chart on the right, the brine-bulb temperature for a 40% solution of lithium chloride and air at 95/78°F (35.0/25.6°C) dry-bulb/wet-bulb will be 113°F (44.9°C).   With an outdoor wet-bulb temperature of 78°F (25.6°C), a typical cooling tower might supply water at 85°F (29.4°C).  While it would be impractical to cool the outdoor air using this cooling water in a conventional air-to-water heat exchanger (i.e., the cooling water is only one degree below the air temperature), a strong cooling effect could be achieved by wetting the surfaces of heat exchanger’s air passages with 40% lithium chloride to create a brine-bulb desiccant-air boundary that approached 113°F (44.9°C).  Assuming the outdoor air came into equilibrium with 40% lithium chloride at 85°F (29.4°C), the cooling effect (i.e., the difference in the outdoor air’s initial and final enthalpy) would be 15.5 Btu/lb (35.9 kJ/kg).

     Although the preceding cooling effect is an idealized upper limit, it is sufficiently large that even after allowing for realistic heat exchanger performance, liquid desiccants applied in combination with evaporative cooling can be an alternative to compressor-based air conditioners and chillers for treating outdoor air. Furthermore, this potential becomes even greater in applications where latent cooling is needed: in the preceding idealized example, the dew-point of the ambient air when it reaches equilibrium with the desiccant is 39.1°F (3.9°C)—a level of dryness that would be difficult to reach with compressor-based cooling systems.

     Although a liquid-desiccant air conditioner that rejected heat via a conventional cooling tower might effectively condition the high-enthalpy ventilation air for a building, conditioning a mix of return air and outdoor air would be far more challenging.  Under AHRI Standard 340/360, a commercial Roof Top Unit (RTU) is rated when conditioning air at  80°F (26.7°C) and 0.0140 kg/kg-da (i.e., a mix of outdoor air and building return air).  At this rating condition, the mixed air's brine-bulb temperature with 40% lithium chloride would be 102.7°F (39.3°C).  The preceding ideal cooling effect for conditioning outdoor air—15.5 Btu/lb (35.9 kJ/kg)—reduces to 8.7 Btu/lb (20.2 kJ/kg) for mixed air.  With commercial RTUs expected to supply cooling at about 400 cfm/ton—roughly equivalent to 6.8 Btu/lb (15.9 kJ/kg)—a liquid desiccant air conditioner rejecting heat via a cooling tower (without assistance from a compressor-based refrigeration loop) would have to perform close to its ideal limit.

     The vapor-compression refrigeration system demonstrated by Willis Carrier in 1902 at the Sackett-Wilhelms Lithographing & Publishing Company in Brooklyn, NY—with surprising few changes—remains the dominant means for air conditioning in the 21st century.  With the planet getting warmer and more humid, and air conditioning becoming less of a luxury and more of an essential need, continuing our dependence on conventional vapor-compression refrigeration may not be sustainable.  As described in the pages of this website, novel applications of liquid desiccants being explored by AIL Research and others can be the key element in protecting the health and comfort of our indoor environments.