The Heating, Ventilation, and Air Conditioning (HVAC) industry has been facing several challenges for more or less the last one decade, including a decrease of energy resources, an increase in energy demand due to population growth, and new regulatory policies. To respond to these challenges, more energy-efficient heating, cooling, ventilation, and dehumidification technologies are needed. However, there are a number of constraints for deployment of energy-efficient HVAC technologies; among them are the imminent phase-out of ChloroFluoroCarbons (CFCs), eventual phase-out of HydrochloroFluoroCarbons (HCFCs), and the increase in ventilation rates for buildings because of concerns regarding Indoor Air Quality (IAQ) and occupant health.
The higher ventilation rates translate into greater cooling loads-in particular, greater latent loads-during cooling seasons when the relative humidity within a building must be kept sufficiently low to inhibit the growth of micro-organisms that cause health problems and also may damage building materials. As a result, air dehumidification has become a very important part of the HVAC function. Desiccant dehumidification and cooling technology can provide energy-efficient solutions for the industry. Desiccant dehumidification technology has a successful track record over more than 60 years for industrial applications – such as product drying and corrosion prevention.
It has also been used for many years in clean rooms, hospitals, museums, and other special cases requiring highly controlled humidity levels. The present study discussed various benefits of the desiccant technology, its potential applications, and factors that drive its future growth.
In recent years, the use of desiccants for dehumidification in air-conditioning applications has been on the rise, and their capital cost has been on the decline. The benefits of desiccant dehumidification are better humidity control, more efficient latent load removal, and reduction of peak electric demands. In regions of the country where the electric utilities are having trouble servicing their peak air-conditioning loads, this energy-efficient technology can assist in meeting that demand.
Working of Solid Desiccant Cooling System (SDCS)
Figure 1 shows a Solid Desiccant Cooling System (SDCS) with desiccant wheel, which is used to dehumidify the air. The wheel consists of desiccant material that rotates in a slow speed, a heater and drive motor. Ambient air (humid) flows through the process air section during which the moisture content of the air will be adsorbed by the desiccant material. In this process, the temperature of air rises slightly since the adsorption process on the wheel releases heat. As its result, this causes the process air leave the wheel with lower humidity. The hot reactivation air from the heater flows through the air channel in the wheel in opposite direction of the ambient air.
The purpose of regeneration of air in the system is to regenerate the desiccant material overtime for continuous adsorption process to occur. In conjunction with this scenario, the water vapour that exists on the desiccant surface is absorbed by the hot air stream, which involves heat transfer. Therefore, the regenerated air exists in the wheel with higher humidity and slightly lower temperature. This will allow the desiccant material to be dry and ready for new process of adsorption. The above sequence is repeated since the wheel is rotating slowly.
There is a small compressor in the system to compress the water vapour from the ambient air into the condenser. The condenser acts as a heat exchanger to convert the ambient air into liquid by condensation before transferring it to the desiccant wheel for dehumidification process.
There were significant progresses that have been made along the investigation on the efficiency and cost effectiveness of SDCS, but still there are some limitations in this research that need to be addressed. It was noticeable that the thermal sensation of solid desiccant cooling system users was not investigated deeply. Thermal sensation is the condition of mind that expresses personal satisfaction of users with thermal environment.
Desiccant dehumidification applications
Desiccant systems are especially useful when the latent load is high (i.e., when the latent-to-sensible heat ratio is high), because they remove moisture more economically than they remove sensible heat. Another desirable situation is when the cost of dehumidification with a desiccant is lower than the cost of dehumidification with a refrigeration system.
This is where thermal energy comes into the picture: there are instances where desiccant regeneration done by waste heat, natural gas, or off-peak electricity is more economical compared to regular electric refrigeration. Because there is no need for reheating with desiccant dehumidification systems, another appropriate use is when conditioned air must be reheated after coming out of a coil to reach a comfortable dry-bulb temperature.
Finally, the use of a desiccant is well-suited to the case where dehumidification is required at levels below freezing dew-point temperatures. For example, an ice arena has a great deal of humidity, but the cooling coil has to cool below the freezing point. In such an environment, dehumidification with desiccants can play a major role.
Conclusion
Lowering the cost of desiccant dehumidification systems and improving their performance will clearly provide more opportunities for desiccant dehumidification technology. Currently, a number of cost-effective applications in the market will result in increased sales during the next several years; but as in other technologies, further R&D and demonstration programs will enhance broader applications of the technology.
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.
