Every developing country like India, is at present facing many challenges in the path of reaching rapid economic growth. One of them is due to producing electricity from the fossil fuel-based thermal power plants, which leads to remarkable amount of CO2 emission. Since the last decade, the energy consumption in India has grown remarkably to about 129%. In hot and humid environment in most parts of India, air conditioning system that provides thermal comfort in various residential and industrial buildings mostly makes use of high grade electricity that creates many environmental pollution problems while generating the electricity in fossil fuel based thermal power stations. So, emphasis on energy requirement by the air conditioners for producing indoor thermal comfort is becoming great priority in the current energy scenario as exponential rise in cooling demand and associated environmental pollution.
Air conditioners in building consume almost about more than 57% total building energy requirements. The main concern of an air conditioner in building is to handle the cooling load (sensible and latent heat) requirement and to maintain necessary thermal comfort. According to ASHRAE standard 55, the Dry Bulb Temperature (DBT) should be in the range of 23°C–27°C and relative humidity in the range of 50%–55% is considered as the standard comfort condition.
In tropical climate, high ambient humidity coupled with increased ventilation flow requirement of modern building design is a major contributor to poor energy performance by the traditionally used Heating, Ventilation, and Air Conditioning (HVAC) systems. Vapour Compression Refrigeration (VCR) systems are presently used to carry out the thermal comfort of various residential and industrial applications. The reason behind the same is its performance stability, compactness, and ease of availability. However, the VCR systems operating on the use of vapour compressor, which consumes high-grade electrical power that creates environment pollution by releasing many greenhouse gases during the production of electricity in fossil fuel-based thermal power stations.
An option to the traditional air conditioners may be absorption cooling that makes use of low-grade solar thermal energy or industrial process waste heat to make energy efficient system for cooling. Sorption cooling mostly makes use of either ammonia–water or lithium bromide–water system. In both the systems, the latent cooling load cannot meet efficiently as it requires cooling of air below its Dew Point Temperature (DPT) and post-reheating to meet desired room supply conditions. To overcome above problems desiccant-assisted evaporative cooling technology can be a good option. According to the types of desiccant materials used in the dehumidifier, the desiccant cooling can be classified either into the solid desiccant- or liquid desiccant cooling system (Fig.1). Even though the liquid desiccant cooling can be very popular, it has many operating problems, such as carry over, corrosion, and crystallization, while the solid desiccant cooling has many advantages, such as environmental friendliness, durability, and compactness.
Solid desiccant systems may be available in many types, such as fixed bed, rotary wheel, or desiccant-coated heat exchangers. Especially for the hot climate, dedicated evaporative coolers can give better performance, but with an increase in humidity their performance degraded. This limitation of evaporative cooler can be overcome by coupling it with desiccant wheel for the effective handling the air moisture separately.
According to the type of its configuration, it is divided into two types – either direct evaporative cooling or indirect evaporative cooling. In case of direct evaporative cooling, the process air comes in direct contact with cooling sprayed water increases its supply moisture in room on its continuous operation. While in case of indirect evaporative cooler, room moisture content remains unaffected for its entire duration of operation. Even though the evaporative coolers require much lesser electrical power for their operation of components, such as air circulation fan or small water circulation pump, they cannot effectively handle the moisture in humid climates. So, the integration of desiccant dehumidification system with the evaporative coolers increases its climatic applicability. Furthermore, it provides amelioration in saving of cost, energy, and power as compared with the traditionally used HVAC systems.
Use of desiccant powered hybrid cooling in modern HVAC
The desiccant-assisted cooling systems are increasingly developed as an alternative to the conventional vapour compression systems. The conventional vapour compression system consumes large amount of energy and causes environmental problems. Emphasis on desiccant systems is becoming a priority in the light of continuing rise in energy demand and increasing cost and various environmental problems, most notably the climate change.
In hot and humid climates, HVAC device becomes inefficient due to increased humidity level of the supply air. Humidity of the outside air combined with ventilation requirement increases the latent load. Conventional VCR systems are not effective in handling the temperature and humidity separately. The use of desiccant cooling systems can improve the humidity control independent of temperature of supply air.
It also improves overall energy efficiency and reduces energy costs. Besides, the desiccant cooling systems allow higher percentage of fresh air to achieve better air quality at lower energy cost. Energy consumption has increased in recent years with the development of worldwide economy. The energy required for the cooling and air conditioning is estimated between 30% and 40% of total energy use. Because of increased living standards and occupants demands, cooling energy demand will further increase. The peak load on the electricity grid increases in hot summer days because of high cooling requirement.
This could cause blackouts and grid failure. Conventional vapour compression systems increase greenhouse gases in the environment responsible for depleting the ozone layer. Solar-assisted desiccant cooling can help to alleviate this problem. The peak cooling demand in summer is associated with high solar radiation availability giving an excellent opportunity to exploit solar-assisted desiccant cooling technology. The optimal use of solar energy is based on the operational strategy that consumes the least electrical energy and uses thermal energy efficiently while maintaining the indoor comfort level at optimum operating cost. Desiccant cooling can be a perfect supplement to the traditional vapour compression system by controlling temperature and humidity independently Fig. 2.
The operating principle of the desiccant assisted hybrid cooling system is shown in Fig. 3. Warm and humid air passes through the desiccant wheel and is dehumidified by adsorption of water. Its temperature increases and humidity ratio decreases. Desiccant wheel absorbs moisture from air due to pressure difference between pressure of moisture in the air and desiccant materials. Then its temperature is lowered in the rotating heat exchanger (air-to-air heat recovery wheel), resulting in a significant pre-cooling of the supply air stream and in the direct humidifier. Return air is used to cool down the process air in the heat exchanger. Then it is heated to regenerate the desiccant wheel. The regeneration of desiccant material can be made by supplying hot air from any waste heat sources or solar energy and to allow a continuous operation of the dehumidification process.
Compared to conventional Vapour Compression Refrigeration Systems (VCRS), the desiccant-based innovative cooling systems have several advantages: they can be driven by low- grade thermal energy and easily realize efficient energy storage that is very suitable for solar energy. Instead of CFCs or HCFCs, their working fluid is a salt solution (in case of liquid desiccant cooling), which is environment-friendly.
Through the spraying of this solution, the processed air is sterilized and de-dusted. Prediction of annual energy consumption requires the partial load performance evaluation of each component in an air conditioning system. In this condition, use of ways of producing cool with top efficiency and lower energy consumption can help in decreasing of energy consumption in department parts so much. Use of liquid suction cooling systems or solid can decrease energy consumption ratio economize depends to different parameters such as (liquid or solid) environment wet or temperature wet or temperature of ventilation less absorbent revival temperature and operation of systems showing a liquid suction system and procure better condition for energy consumption.
Conclusion
Although a desiccant integrated hybrid cooling systems can be used in any air-conditioning operation as it can provide precise control of temperature and humidity of the space, the high initial costs of such systems typically limit their use. The future research on the investigations and use of novel desiccant materials that can be effectively regenerated near about lower temperature (near-ambient) so that the same can be used successfully by the application of economical solar (green) energy is the key of augmenting even greater the contribution that the desiccant cooling can bring to the amelioration of comfort, energy, and cost savings.
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). He has published more than 180 Research Articles in reputed International Conferences and Journals. He has also published 5 reputed books 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.
