Desiccant assisted innovative cooling is a better alternative to vapour compression based conventional cooling systems, which consume lot of energy and also increase the CFC level in the environment. It is seen that desiccant cooling system can be the most promising system in hot and humid climatic condition for thermal comfort. For effective use of evaporative cooling techniques in hot and humid climate a desiccant material based rotary wheel can be utilized as a dehumidifier. In a solid desiccant dehumidification system, the moisture in the process air is removed by a desiccant dehumidifier. The temperature of the dried process air is lowered to the desired comfort conditions by sensible coolers. The latent and sensible loads are handled separately and more effectively in components designed to remove the total load. The Desiccant Cooling System (DCS) has proven its feasibility and cost saving in the field of air conditioning.
A desiccant is a hygroscopic substance that induces or sustains a state of dryness (desiccation) in its local vicinity in a moderately well-sealed container. Desiccants may be either solid or liquid. Commonly encountered pre-packaged desiccants are solids and work through absorption or adsorption of water or a combination of the two. Solid desiccants used in air conditioning include silica gel and molecular sieves. In desiccant cooling, incoming air stream dehumidifies by forcing it through a desiccant material and then dries the air to the desired indoor temperature. To make the system working continually, water vapour adsorbed must be driven out of the desiccant material (regeneration) – so that it can be dried enough to adsorb water vapour in the next cycle. This is done by heating the desiccant material to its temperature of regeneration, which is dependent upon the nature of the desiccant used. A desiccant cooling system therefore, comprises principally of three components namely the regeneration heat source, the dehumidifier (desiccant material) and the cooling unit.
Working of desiccant assisted cooling system
A schematic view of the solid desiccant system is shown in Figure 1. At point 1 the outdoor air enters into the solid desiccant wheel. The moisture present in the air is absorbed by the desiccant and air gets dehumidified. Generally honey-comb structure is preferred for the desiccant wheel. Desiccant wheel rotates at low RPM – generally 1/60 times slower than heat recovery wheel. Some portion of desiccant wheel remains in contact with outdoor air and some with regenerative air. Due to dehumidification of air, the temperature of the process air increases. Process air enters into the Heat Recovery Wheel (HRW) at point 2. Here, heat is transferred from hot dehumidified process air to regenerative air. As a result of this, air gets sensibly cooled. There is no direct contact between process air and the regenerative air inside the heat recovery wheel. After the sensible cooling, it enters into the evaporative cooler at point 3, where it is further cooled by direct evaporative cooling process (DEC) and then comfort air is sent inside the room at point 4. After leaving the room i.e. at point 5, return air enters into another Direct Evaporative Cooler (DEC). Here it is again cooled in a Direct Evaporative Cooler (DEC) and then enter into the heat recovery wheel where it is heated by heat transfer through the process air. Now, this air is further heated by a heater. The heat source in the heater may be either conventional or non-conventional. Non-conventional resources like solar energy, waste energy etc., can be utilized. In the next step the regeneration air again enters into the desiccant wheel where, it is further utilized to dehumidify the outdoor air.
Progress in solid desiccant cooling system configuration
As per the recent progress in solid desiccant cooling system configuration, the conventional solid desiccant cooling cycles operating in different modes – namely ventilation, recirculation – (Fig. 2) and mixed along with the recent advancement to basic cycles, which is commonly known as hybrid cycles – have been described in this section. Hybrid system has been designed to supplement the conventional standalone solid desiccant cooling judiciously with conventional vapour compression cooling – as well as or by solar heating (Fig. 3) to enhance the overall performance of the conventional system operating under varying ambient conditions.
By use of desiccant – vapour compression hybrid cooling cycle powered by renewable solar energy for transient climatic conditions, it has been found that this system can easily handle the high humidity air and provide the required cooling comfort. There are various cooling options available but due to health and environmental concern and also in hot and humid climate – where other systems are not as effective, desiccant cooling system can be a good alternative. The desiccant cooling system can also reduce the operating cost and save the energy by using the solar energy or waste heat. Thus, it leads to an eco-friendly cooling solution.
Dr. (Prof.) D. B. Jani; an Associate Professor at GEC, Dahod under Gujarat Technological University (GTU), Ahmedabad; received his Ph.D. in Thermal Science (Mechanical Engineering) from Indian Institute of Technology (IIT) Roorkee. Currently, he is a recognized Ph.D. Supervisor at GTU. He published more than 150 Research Articles in reputed International Conferences and Journals along with 5 popular books. His areas of
research include Desiccant cooling, ANN, TRNSYS and Exergy.