Air-conditioning systems powered by desiccant dehumidifiers were mostly used in the applications requiring indoor low humidity. For continuous operation, desiccant dehumidifier needs external heat supply for reaction purpose. Regenerating the solid desiccant dehumidifiers with the help of waste heat available from primary energy sources, like industrial process waste heat or diesel engine exhaust in DG room set, not only reduces the electricity consumption but also achieves the substantial fossil energy saving. The energy conservation through industrial waste heat for regeneration is an interesting option to converge the cooling and dehumidification requirements of the conditioned space.
A vapour compression based conventional air conditioner consumes large amount of electrical energy especially in hot and humid climatic conditions due to high latent load owing to large internal occupancy. Desiccant dehumidifier based hybrid air conditioning system is one of the promising alternatives to handle the high latent load efficiently where sensible and latent heat of air are being removed separately.
A desiccant wheel is very similar to a thermal wheel, but with a coating applied for the sole purpose of dehumidifying or drying the air stream. The desiccant is normally Silica Gel. As the wheel turns, the desiccant passes alternately through the incoming air where the moisture is adsorbed, and through a “regenerating” zone where the desiccant is dried and the moisture expelled. The wheel continues to rotate and the adsorbent process is repeated. Regeneration is normally carried out by the use of a heating coil, such as a water or steam coil, or a direct-fired gas burner. Thermal wheels and desiccant wheels are often used in series configuration to provide the required dehumidification as well as recovering the heat.
There are various types of desiccant available on the market, but all dehumidifiers use what is known as Silica Gel as the desiccant within the drying wheels. Strangely silica gel is not a “gel” as the name implies, but in fact a porous granular form of silica that is made from sodium silicate. The internal structure of each silica granule is made up of a network of interconnecting microscopic pores, which by a process called physical adsorption or capillary condensation, attract and holds moisture within each granule. This trapped moisture can then, with the addition of heat, be released from the desiccant. This desiccant can then be used again and again. As low ambient temperatures do not restrict the material, it makes it a more all season drying system. The desiccant assisted dehumidification system used in HVAC industries to remove moisture from air to give better human comfort.
The rotary desiccant dehumidifier used in solid desiccant cooling system may be regenerated through heat recovery carried out either from the exhaust of large internal combustion engine (as shown in Fig. 1) or from micro CHP unit. This resulted in enhancement of both the cooling performance of desiccant dehumidification system and better fuel utilization with higher thermal efficiency of engine. Moreover, in solid desiccant vapour compression hybrid air conditioning system, the regeneration heat needed by the desiccant wheel is supplied by the condenser dissipated heat as shown in Fig. 2.
On the process air side, return room air at state point 1 passes through a desiccant wheel where its moisture is removed and temperature is increased up to state 2 due to the adsorption heat effect. It is sensibly cooled during process 2-3 in air to air sensible heat exchanger. Then after, the process air is cooled by evaporation to supply air state 4 by passing through direct evaporative cooler. On the regeneration side, ambient air at state point 5 is cooled and humidified in another direct evaporative cooler. This air is then sensibly heat exchanged with process air to pre-cool the process air and pre-heats itself. The warm air stream is then heated up further to the required regeneration temperature of desiccants used in rotary dehumidifier via heater. After regenerating the desiccant wheel, this air is then exhausted to ambient at state point 9. A certain portion of the return air steam at the state point 7 bypasses the heater in order to reduce the reactivation heat consumption.
It is seen that the utilization of waste heat for the regeneration of desiccant wheel not only achieves great energy saving, but also marginal reduction in running cost of the system. Furthermore, the efforts are still needed to develop desiccants, which are regenerated to ambient temperature to take account of the low temperature waste heat. This also offers benefit in system COP as compared to that of the traditional system.
Performance of the desiccant wheel based hybrid air conditioning system is economic up to certain humidity level compared to the window air condition alone. If the regeneration temperature increases, the load gets completely separated there by performance of cooling coil improves a lot, 70% to 80% performance of cooling coil is significantly governed by latent load. Hybrid air conditioning can be good option when the humidity level is high.
Dr. (Prof.) D. B. Jani is 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
