Modern lifestyle and industrialization come with the consumption of a huge amount of energy for cooling systems to provide thermal comfort in buildings and workplaces. Recent analysis from HVAC designers claims triple demands on air conditioner system worldwide by the next 30 years, which means that almost three-fold energy usage and global warming will result – if no significant changes are made in modifying or replacing available air conditioner systems. This fact opens many research challenges for scientists and industries to study and develop eco-friendly and sustainable alternatives for conventional vapour compression air conditioning systems.
The desiccant cooling system is among those alternatives with very interesting concepts. This is not only energy efficient but also could effectively control the indoor humidity, which is another important factor for human comfort aside from temperature. The ability of separate control of latent and sensible loads, which removes the overcooling and reheating of process air, is an advantage of desiccant cooling system over conventional air conditioning systems, because of a huge decrease in energy consumption and increase in Coefficient Of Performance (COP).
The utilization of desiccant cooling system in industries such as pharmaceutical, food, laboratories, and chemical industries, which need very dry air to maintain severe hygienic conditions constantly, is highly preferred because of its strong control for adjusting humidity and its cost-effectiveness.
DEC systems can dehumidify greenhouses and prevent overheating of conditioned spaces. DEC systems can keep temperatures below 27°C, even when conventional evaporative systems keep temperatures at 28.8°C. In one study, a biomass regenerated DEC system kept a greenhouse at 25°C in July and 25.8°C in September.
This system could provide a better indoor air quality by avoiding condensed water formation which, decreases the growth of biological contaminants such as fungus and bacteria and improve the human health that spend more than 75%–85% of their time indoor.
These advantages accelerated the development and application of desiccant cooling system in both residential and industrial sectors. Based on the type of desiccant (sorbent), the desiccant cooling system is divided into liquid, solid, and advanced desiccant systems. Despite the different advantages of liquid desiccant materials, their disadvantages such as crystallization, carry-over issues, and corrosive nature provide more interest and opportunities for development of solid and advanced desiccant (sorbent) materials.
Merits of desiccant based evaporative cooling systems
The simple evaporative cooler is not useful in hot and humid climates. Under such conditions, an indirect evaporative cooling system can be used in conjunction with other dehumidification systems, which can extract the water vapour from the air. This deficiency of the evaporative cooling system can be overcome by using it in combination with desiccant dehumidifier to dry the air.
The application of adsorbent based dehumidification will allow the effective use of direct as well as indirect evaporative coolers in hot and humid climates. DEC systems can consume 50% less energy than conventional air conditioning systems. DEC systems can improve air quality in interiors. Some other benefits of DEC systems include:
- Solid-based systems: These systems have high potential for use in hot and humid weather.
- Basic direct systems: These systems are more effective than indirect systems in terms of output temperature.
- Two-stage systems: These systems perform better than one-stage systems, with lower humidity ratios and supply temperatures.
However, DEC systems can have high initial costs and may require experienced professionals for construction and maintenance.
Working of desiccant based evaporative cooling system
As mentioned previously, a well suitable alternative of mechanical vapour compression system is evaporative cooling system, which can be efficiently used for air conditioning applications with less power requirements i.e., one fourth of the mechanical vapour-compression. It is an energy saving, cost effective, simple, and environment friendly air conditioning technique. Many researchers have investigated different types of evaporative coolers such as direct, indirect and modified coolers. But evaporative coolers become ineffective in hot and humid climate.
In general, evaporative cooling systems are applied when the wet bulb temperature does not exceed much beyond 25°C frequently. The evaporative cooling units can operate with a high Coefficient Of Performance (COP) in dry climatic conditions. But because of the air saturation of the surrounding air in humid climates, the effectiveness of these cooling units drops remarkably. That is why evaporative cooler is best suited in conjunction with the desiccant dehumidifier, which removes the which removes the moisture from the process air and thus, these cooling units can function effectively.
The desiccant dehumidifier composed of some desiccant material (silica gel, lithium chloride, lithium bromide etc.), which is used to remove the moisture from the moist air. A desiccant material is one which absorbs or adsorbs and hold water vapour from the humid air by the process of absorption or adsorption. The evaporative desiccant cooling system consists of a desiccant dehumidifier, a regenerator, and a cooling unit. The basic working principle of a desiccant dehumidification assisted evaporative cooling system is illustrated in Fig. 1.
The air is dehumidified using desiccant dehumidifier and its temperature is lowered using evaporative cooler or some other cooling device. For continuous operation of the system the desiccant dehumidifier is regenerated by using heat energy provided by solar collectors or some other source of energy as shown in Fig.2. Some heat recovery units are also utilized to make the system more efficient.
The desiccant dehumidifier composed of a desiccant material that removes moisture from the air by the process of dehumidification. Different desiccant materials attract the moisture from the air at different capacities. The desiccant materials can be solid as well as liquid. Silica gel, calcium chloride, lithium bromide, lithium chloride, activated ammonia and natural zeolite are the most commonly used desiccants. The desiccant dehumidifier is regenerated using the heat.
Some advantages of the desiccant cooling technology in conjunction with the evaporative cooler are:
- It can be used for hot and humid climates because evaporative cooling alone is not feasible for such conditions.
- A lot of energy is saved as compared to vapour compression cycle as no pre-heating is required.
- Environment-friendly system because of no use of refrigerant, which affects the ozone layer.
- Separate and better control of sensible and latent loads. The desiccant wheel controlling the latent part and the evaporative cooler controlling the sensible one.
- The overall system has low maintenance cost because it operates at almost atmospheric conditions.
- Low grade energy such as solar, biomass, etc., can be effectively used to drive the system.
Salient features of desiccant assisted evaporative cooling system
The main observations are summarized as follows:
- The conventional air conditioning systems are not suitable for hot and humid climatic conditions because of its high sensible heat ratio.
- These systems can provide the comfortable conditions only when sensible heat ratio is greater than 0.75.
- The evaporative cooling system is a good alternative to conventional air conditioning systems that not only saves energy but also is environmental-friendly.
- In the indirect evaporative system the process air stream does not directly interact with the cooling fluid stream – and is only cooled sensibly while indirect evaporative system moisture is also added to the cooled air stream because process air streams comes indirect contact with the cooling water. The evaporative cooling system becomes less efficient in hot and humid climatic conditions. Under these conditions, evaporative cooling system can be operated in conjunction with desiccant dehumidifier with high efficiency.
- The heat and mass transfer is less affected by the thermal properties (thermal conductivity and porosity) of the packing material used for heat and mass transfer in an indirect evaporative cooler – while main factors to be concerned are cost, shape formation, holding ability, durability, compatibility with water-proof coating.
- In modified evaporative cooler, the air can be cooled to the temperature lower than the temperature achieved by indirect or direct evaporative cooler without disturbing the humidity of the air.
- Low grade thermal energy such as solar, biomass etc. can be effectively used for the continuous operation of the desiccant-based evaporative cooling system.
The desiccant-based evaporative cooling technology can first be targeted for commercial buildings rather than residential buildings because of high energy and financial payback in hot and humid climatic areas. The combination of recent advances in desiccant materials and the evaporative cooling technology must be used to design cheaper, reliable and compact cooling systems. Finally, the technology should be converted into products which are market attractive.
Conclusion
The desiccant-based evaporative cooling system is relatively a new technology and is a good alternative for conventional mechanical vapour compression air conditioning system, especially in hot and humid climatic conditions but less familiar as compared to conventional cooling system. In order to familiarize the desiccant-based evaporative cooling systems, designed activities such as workshop, seminars, onsite visit, lectures, exhibition, and publicizing the research results in dedicated ways are required. Standardizations, legislations, public awareness, and regulations are the main issues that need to be focused for such cooling systems.
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). Published more than 200 Research Articles in reputed International Conferences and Journals. He has also published 8 reputed books and book chapters 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.
