Nowadays, most of the conventional air-conditioning (AC) systems are based on the traditionally used mechanical vapour-compression cooling technology which is driven by an electrical energy. Since the major part of the global electricity comes from fossil fuels, it causes the rising awareness of environmental and sustainability issues. One of the alternatives to the mechanical vapour-compression systems is the desiccant assisted cooling (DAC) process, based on the dehumidification of moist air in present of solid or liquid desiccant material. This results into substantial energy saving for conditioning-built environment as shown in figure 1.
Research conducted by International Institute of Refrigeration in Paris led to the conclusion that the proportion of energy used by the air-conditioning systems in the household and commercial buildings now accounts for nearly 47 per cent. Air-conditioning systems today account for almost 21 per cent of the total energy consumption of India as shown in figure 2. Rising standards of living, technological advancement and increasing population have led to a significant increase in per capita energy demand and thus, total energy consumption in the last few decades. Even though human beings have made much progress in almost every field, but still, we rely on fossil fuels as the primary source of energy to meet our demands.
Desiccant systems can be categorised based on the type of desiccant used:
- Liquid desiccant systems,
- Solid desiccant systems,
- Advanced desiccants which include polymeric desiccant, composite desiccant, bio-desiccant.
Working Principle of Desiccant Cooling
Solid desiccant-based dehumidification system working on principle of adsorption of moisture. In its operation, moisture laden ventilated or recirculated air is first passed through channels of rotary dehumidifier so that the water vapour presented in it is adsorbed by a rotating desiccant wheel due to the pressure difference between them. During the process of dehumidification, adsorption heat rises the temperature of process air at dehumidifier exit. The temperature of this dried process air is then lowered further to the desired room supply conditions by use of heat recovery wheels and vapor compression cooling coil. To make the system working continually, amount of water vapour adsorbed by the rotating desiccant wheel must be driven out of the desiccant material so that it can be dried enough (reactivated) to adsorb water vapor in the next cycle. This is done by heating the desiccant material to its temperature of reactivation which is dependent upon the type of the desiccant used. Energy required for reactivation of rotary dehumidifier is supplied via reactivation heat source either by electrical heater or solar or waste heat. A desiccant based hybrid space cooling system as shown in figure 3, therefore, comprises principally four components, namely the reactivation heat source, the rotary dehumidifier, heat recovery wheel and the traditional vapor compression cooling unit. The comparison of different desiccant used in the desiccant dehumidifier tabulated in Table 1.
The drying capability of solid desiccants is higher than liquid desiccants. They can be cleaned easily. However, they require relatively higher regeneration temperature. Figure 4 shows a desiccant wheel used for dehumidification of process air and regeneration of solid desiccant. Unlike liquid desiccants, the dehumidification and regeneration process in the solid desiccant is always simultaneous. Desiccant wheel is divided into two parts with the help of a stunt. In one part, the humid air passes through the desiccant wheel for dehumidification. In another part, hot air is passed to remove the added water from the solid desiccant.
Advanced Desiccant Materials
Both solid and liquid desiccant have several disadvantages. For instance, the adsorption capacity of silica gel is low and requires high regeneration temperature. Desiccants containing salts of chloride like lithium chloride and calcium chloride may give rise to corrosion problem. In addition, zeolites have low water capacities and a higher cost of regeneration. An intensive research is going on to develop new desiccants with a higher performance. New and advanced desiccant that have been developed with suitable modification in the properties of conventional desiccant like bio-desiccant, composite desiccant and polymeric desiccant has the capability to overcome the limitations of conventional desiccants.
The compound desiccant was developed to ameliorate the performance of desiccant cooling system and concluded that under practical operation a compound desiccant can improve the overall performance of a desiccant wheel by 20–30 per cent over desiccant wheel with silica gel. Number of experimental tests have been carried out by many researchers on compound desiccant materials and they have found that the coefficient of performance of desiccant cooling system may reach 1.28, which is 35 per cent more than the desiccant wheel with only silica gel. Investigation on a novel polymeric desiccant has been carried out and it is found that sorption capacity of super polymer desiccant is 2–3 times higher than silica gel. A micro-porous coordination polymers (MCP) has been investigated and compared its capacity and efficiency with activated alumina and concluded that polymeric desiccants hold potential of commercial use. However, further research on various parameters of advanced desiccants is yet to be conducted to study behaviour of MCP in extended cycles before coming to an actual conclusion.
Desiccant cooling is relatively an advanced space cooling technology and is a good option for traditional vapour compression air conditioning system especially, in hot and humid climate. The desiccant assisted comfort space cooling hybrid air-conditioning technology can separate the latent heat load from the sensible heat load as compared to conventional vapour compression system, which will substantially ameliorate the evaporator temperature and so lowered the power consumption of the VCR compressor. In hybrid cooling systems, solid desiccant dehumidification system used in conjunction conventional vapour compression system have advantage of high efficiency over solid desiccant based evaporative cooling systems in which moisture is added to the cooled air stream because process air steams comes in direct contact with the cooling water becomes less efficient as air becomes saturated in humid conditions. It is also found that hybrid desiccant space cooling system consumes low energy when coupled to free energy sources like solar energy or industrial waste heat as well as release less carbon dioxide emission than the conventional vapor compression system. Thus, if economical factors taken into account the application of renewable energy for desiccant regeneration in hybrid space cooling technology would be more beneficial. Therefore, desiccant based hybrid space cooling techniques is economically and environmentally more feasible for the climates which have high cooling demand, and humid summer characteristics. By making the direction of future research on space cooling towards solid desiccant – vapour compression hybrid space cooling augmenting the contribution of desiccant based hybrid cooling which can bring to the amelioration of comfort, energy and cost savings.