Desiccants exhibit an affinity so strong for moisture that they attract and hold water vapour directly from the surrounding air. This affinity can be regenerated repeatedly by heating the desiccant material to drive off the collected moisture. Desiccants are placed in dehumidifiers, which have traditionally been used in tandem with conventionally used vapour compressed based air-conditioning systems. The systems have been more commonly applied in typical air-conditioning environments that involve large dehumidification load fractions, such as low humidity levels required for operations in many industries.
Lower humidity levels, below the level necessary for comfort, are generally unattainable, cost-effectively, with traditional cooling and reheat. Numerous moisture-sensitive manufacturing, thus, utilize desiccant dehumidification in industrial air-conditioning.
Interest is now being revived in thermal energy driven desiccant dehumidification in non-industrial air-conditioning applications to offset the rising cost of electricity. Lower cost thermal energy, including natural gas, waste heat, solar energy, and other sources, is substituted for electric energy to meet the dehumidification load on the air-conditioning system. In addition, the cost of desiccant dehumidification equipment has decreased considerably to warrant wide use of this method for application outside the industrial field of use. Desiccant dehumidification provides a cost saving to reduce electric air-conditioning capacity and, thus, to lower electric energy costs and power demand charges in many non-industrial air conditioning situations.
Working of desiccant-based dehumidification and cooling systems
Adsorption refers to a desiccant that does not change phase as it collects airborne moisture. Most adsorbents are solids; familiar examples include activated alumina, silica gel, and zeolites (molecular sieves). In absorption, collecting moisture changes the desiccant physically or chemically. Most absorbents, such as solutions of lithium chloride or tri-ethylene glycol in water, are liquids. There are literally hundreds of desiccants, each designed and manufactured for a specific task. They can be categorized by their ability to attract and hold water vapour at specific temperatures and relative humidity. The curve depicting this trait is a desiccant isotherm. Figure 1 shows typical isotherms for the Type I, Type II, and Type III desiccants that are often used for HVAC applications.
Adsorbents, or ‘solid’ desiccants, are the focus of this article. Their most common application is the desiccant wheel, a cylindrical matrix of channels that are coated with or constructed from a solid desiccant. To maximize moisture collection, the wheel rotates slowly – only 10 to 30 rotations per hour—through two air streams (Figure 2). ‘Process’ air passes through one section of the wheel. Desiccant on that section adsorbs water vapour, making the air drier than when it entered. Wheel rotation then exposes the moisture-laden desiccant to a ‘regenerating’ air stream that strips the captured moisture away from the desiccant (desorption). Moisture transfer is enabled by the difference in vapour pressures at the desiccant surface versus the air passing over it.
The desiccant collects moisture when the surface vapour pressure is lower than that of the passing air, and releases it when the surface vapour pressure is higher. For practical purposes, since Relative Humidity (RH) is a function of vapour pressure, the direction of moisture transfer can be characterized by the difference between the relative humidity of the process and regeneration air streams. The desiccant can retain little moisture when the regeneration-air RH is low, so water vapour will migrate from the desiccant to the regeneration air. When the RH of the process air is high, the desiccant can adsorb more moisture from that air stream. Maintaining an adequate difference between the relative humidity of the process and regeneration air streams is essential to dehumidify effectively using a desiccant wheel.
Due to the costs of regeneration and re-cooling, traditional desiccant wheels typically are used only when the required process-air dew point can’t be achieved with standard mechanical equipment. (These costs become even more prohibitive as the price of natural gas rises.)
Recent progress on desiccant based dehumidification and cooling
Air leaving the process side of a series desiccant wheel is cooler than the space, not neutral or warmer. This makes the wheel suitable for use in the mixed air stream –and allows a single unit to both comfort-cool and dehumidify the space. Figure 3 shows an example of a mixed-air air handler with a series desiccant wheel. The desiccant adsorbs water vapour from the air downstream of the cooling coil, enabling the system to deliver drier supply air (at a lower dew point) without lowering the coil temperature. The regeneration side of the wheel is located in the mixed air, upstream of the cooling coil. Because the RH of the air upstream of the coil is much lower than the RH of the air downstream, the adsorbed water vapour transfers upstream – and the cooling coil gets a second chance to remove the transferred water vapour via condensation.
Many systems are configured with the desiccant wheel downstream of the cooling coil, rather than upstream, to better apply the operating principles of cooling coils and desiccants. In this configuration, the process air (OA) first passes through a DX or chilled water cooling coil, where it’s cooled and dehumidified. Then the cool, Saturated Air (CA) passes through the desiccant wheel, which adsorbs moisture from the high-RH air – lowering the dew point but raising the dry-bulb temperature. The resulting Conditioned Air (CA) is dry and warm – but not as hot as in the ‘wheel upstream’ configuration described earlier. Water vapour transfers from the desiccant to the regeneration air (RG) as the wheel rotates into the regeneration air stream.
Mixed Air (MA) entering the regeneration side of the wheel is less humid, about 40% RH due to the low supply-air dew point in this example. At this RH, the wheel can no longer hold the water vapour it adsorbed downstream of the coil. Water vapour released from the wheel passes into the Mixed Air (MA) and then condenses on the cold coil surface. In practice, this typically involves dehumidifying a portion of the air to 55°F DP, and then mixing that dry air with bypassed Return Air (RA) to raise the supply-air temperature to 65°F DB.
A liquid desiccant dehumidifier would typically be used in series with a sensible cooling device to reduce the air’s dry-bulb temperature. Since the second coil is used for sensible cooling only, it will operate dry – this can be advantageous in that it reduces the opportunity for biological contamination. The coil would also operate at a temperature higher than would be used in the desiccant based dehumidification and cooling system, so the Coefficient of Performance (COP) of any applied refrigeration system would be improved. The careful design of the conditioner and its operating parameters, including the ratio of desiccant solution to air flow rates, inlet concentration of desiccant and packing size can result in high dehumidification effectiveness of 0.9 or more.
The effectiveness is based on the potential dehumidification provided by the potential vapour pressure difference between the inlet air and the incoming desiccant. Liquid-to-Air Membrane Energy Exchangers (LAMEEs) are being developed to provide the interface between the air and the liquid desiccant – this prevents any carry-over into the airstream by using semi-permeable membranes. These allow the transfer of water vapour, but do not permit any liquid droplets to be transferred. Effectiveness for dehumidification may be as high as 94% is reported by many researchers’ work earlier in this area.
A novel design of a hybrid system of Vapour Compression Systems (VCS) and Liquid Desiccant Systems (LDS) was proposed earlier by many investigators. In this system, liquid desiccant cooling system along with an indirect evaporative cooler was used to sub cool the refrigerant of the liquid desiccant cooling cycle. Moreover, the desiccant solution was regenerated using the condensing heat of the VCS. Results obtained from the thermodynamic analysis showed that the proposed system attained higher coefficient of performance than conventional system as well as the reverse Carnot cycle under similar working conditions. About 18.6% and 16.3% higher COP was achieved using hot air and ambient air, respectively.
The evaporator and desiccant helps in simultaneously cooling and dehumidifying the process air while the diluted desiccant solution is collected in the weak solution tank. After that, the diluted solution is pumped to absorb heat from the heat exchanger that uses the waste heat rejected from the condenser of the VCS to pre heat the diluted solution. A heating coil in the regenerator tank provides the additional heat required by the solution to completely. The proposed system can attain yearly energy savings of 53% compared to a VCS with a reheat mechanism.
Conclusion
Cooling and desiccant-based dehumidification systems are most economical when used together since they complement each other. The difference in the cost of electrical power and thermal energy will determine the optimum mix of desiccant to cooling-based dehumidification in a given situation.
Desiccants are especially efficient when drying air to create low relative humidity, and cooling-based dehumidification is efficient when drying air to saturated air conditions. It is found that the desiccant system has considerable advantages over the conventional system for the application considered for commercial air-conditioning.
Dr. (Prof.) D.B. Jani received Ph.D. in Thermal Science (Mechanical Engineering) from Indian Institute of Technology (IIT) Roorkee. Currently he is a recognized Ph.D. Supervisor at Gujarat Technological University (GTU). He has published more than 180 Research Articles in reputed International Conferences and Journals. He has also published 5 reputed books in the area of thermal engineering. Presently, he is an Associate Professor at GEC, Dahod, Gujarat Technological University, GTU, Ahmedabad (Education Department, State of Gujarat, India). His area of research is Desiccant cooling, ANN, TRNSYS, and Exergy.
