Desiccant based sorption cooling system is an extension of the evaporative cooling concept in which heat is absorbed by either solid or liquid from the substances in contact with it, during evaporation process. There is also a close relation with the latent heat of vaporization of that liquid and heat absorbed during evaporation. The difference between wet bulb and dry bulb temperatures increases the potential of evaporative cooling.
The desiccant materials can be defined as natural or synthetic substances capable of absorbing or adsorbing water vapour due to the difference of water vapour pressure between the surrounding air and the desiccant surface. Thus, desiccants are used to dehumidify the inlet air to obtain dry air which is then cooled and humidified by evaporative cooling. It could be followed by a vapour compression or any other cooling system if a sensible cooling is required.
The vapour compression system’s energy demand decreases as desiccant cooling is based on the consumption of handle latent loads. And also energy savings may reach up to 40-60% for dry climates since the desiccant cooling system’s performance is strongly linked to weather conditions.
Fig. 1 shows a schematic diagram of a desiccant assisted dehumidification and cooling system. The air stream from atmosphere is passed through the desiccant wheel at state 1 and significantly dehumidified at state 2. The temperature of air stream increase since adsorption or absorption of water vapour is an exothermic reaction and should be decreased by heat wheel. Heat wheel can be used in conjunction with vapour compression system if required cooling is not supplied. At the end of this state, the temperature of the air stream is more excessively decreased and evaporative cooling is applied according to thermal comfort conditions to increase the humidity ratio of the air. Between state 7 and 8, regeneration of the desiccant material carried out required high temperatures by using a heater. The exhaust air stream can be heated up with solar energy or waste heat.
Use of renewable solar heat for desiccant regeneration
Desiccant Assisted Dehumidification and Sorptive Cooling Systems (DADSCSs), which are especially suitable for solar thermal application due to the low temperature demands around 60–80⁰C, are a fixed technology for air-conditioning buildings. Unlike thermally driven chillers, open cooling cycle sorption technology does not produce chilled water. This technology produces directly conditioned air, which is based on the air dehumidification by an absorbent as lithium chloride or silica gel.
Today, the standard open cycle uses rotating sorption wheels, where the outside air humidity captured in the absorbent and then carried out to the exit air heated by waste heat. The energy needed for reactivating the solid desiccant materials used in dehumidifier mainly supplied by renewable solar energy for green cooling and saving significant heat energy by use of solar heat. Solar powered desiccant dehumidification and cooling system is shown on Fig. 2.
So, the desiccant cooling is a solar mechanical system that is fast growing as a passive cooling technology for sustainable HVAC in buildings since the primarily required form of energy is low-temperature heat that can either be supplied through waste heat or by optimal utilization of solar thermal energy.
Microwave regeneration
Microwave regeneration selectively heats the wet desiccant material without increasing the air temperature. Microwaves have been used to regenerate silica gel, activated alumina, activated carbon, zeolite, and molecular sieves. This regeneration method is found to retain the pore structure and original active sites of the desiccants even after many regeneration cycles.
While packed-bed systems have been utilized for microwave regeneration, fluidized beds demonstrated better regeneration performance under microwaves. It is because fluidization leads to a more favourable temperature distribution in the desiccant material and facilitates uniform heating.
Further, microwave regeneration of a zeolite-based desiccant wheel showed that combining hot air and microwave heating processes improved the regeneration efficiency compared to the standalone hot air regeneration method. A schematic representation of the microwave regeneration in desiccant wheels is shown in Fig. 3.
Ultrasound regeneration
Ultrasonic waves are characterized by their frequency and intensity. Different acoustic frequencies are combined with power levels to study the regeneration effect of ultrasound. It is found that a low-frequency, high-intensity ultrasound (20–40 kHz) when transmitted through a silica gel-packed bed, due to the oscillation of molecules, an alternating compression and expansion effect is generated.
The analogy is like squeezing a sponge repeatedly which promotes the desorption of water molecules. Additionally, the desiccant absorbs a part of the ultrasonic energy, and its temperature rises, resulting in higher mass diffusivity. Thus, the driving force for moisture transfer is enhanced. Fig. 4 shows the schematic of the ultrasonic regeneration.
Conclusions
Rapid urbanization, climate change, and thermal comfort requirements have increased building energy demand sharply. Conventional mechanical vapour compression air conditioners are energy-intensive. They utilize an inefficient simultaneous cooling and dehumidification methodology and significantly raise the global warming potential.
Energy-efficient, clean energy-powered air-conditioners are thus necessary to build a sustainable future. Although numerous next-generation cooling technologies have been developed, thermally driven desiccant dehumidifiers show promising potential in improving the air-conditioning process efficiency.
Dr. (Prof.) D.B. Jani received Ph.D. in Thermal Science (Mechanical Engineering) from Indian Institute of Technology (IIT) Roorkee. Currently he is recognized Ph.D. Supervisor at Gujarat Technological University (GTU). He has published more than 245 Research Articles in reputed International Conferences and Journals. He has also published 10 reputed books and book chapters in area of thermal engineering. Working as Academic Editor for the Journal of Materials Science Research and Reviews. 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.
