Indoor air cooling systems are crucial for maintaining good thermal comfort in regions with hot, humid weather. Controlling indoor moisture level resulted to better thermal comfort is a main criterion for air conditioning systems, especially in cases of harsh weather having exceptionally very high moisture content for typical hot and humid weather. Extreme temperatures and humidity levels can occur in nations with extreme moist weather, making proper air cooling system design crucial. Improperly designed air conditioning systems frequently result in poor indoor air quality, health concerns, reduced thermal comfort, and increased energy use.
The poor indoor air quality might cause a 5-8%reduction in office job performance and that poor air quality frequently cannot be sufficient to provide needed ventilation. Additionally, airborne contaminants remain in the indoor circulated air spread over wide area and resulted to poor hygienic indoor conditions. This is why the design professionals, construction professionals, and HVAC engineers generally do not prefer traditional cooling techniques. As the conventional cooling systems are easily available in market at competitive costs and ease of installation make them popular among HVAC systems.
The moisture level as well as temperature required for producing necessary indoor thermal comfort are mainly managed by the same single coil operation in the same unit, resulted to poor control of either parameter in case of conventional cooling. The goal of reducing dry bulb temperature is reached due to this built-in restriction in the system design, but the relative humidity is too high to be comfortable.
Additionally, the inability to manage humidity can result in the formation of mould, and using vapour compression air-conditioning systems to obtain the best indoor air quality and thermal comfort uses more energy. When the latent load is significantly more than the sensible load, desiccant cooling solutions are very beneficial. Desiccants can help clean airstreams of pollutants to enhance the quality of indoor air.
The desiccant is renewed using a thermal energy input. Regeneration energy is equal to the heat required to evaporate the moisture contained in the desiccant – as well as the heat required to elevate the desiccant’s temperature such that its surface vapour pressure is greater than the ambient air. Desorption of the water from the desiccant also requires some energy.
The usage of air conditioning systems has increased significantly during the past several decades, particularly in commercial buildings due to global warming and increased environmental pollution. However, research indicates that it has negative consequences on the increasing power requirement for producing cooling in buildings – and the built environment to maintain required indoor thermal comfort. In order to find option against traditional cooling systems, which consume lot of electrical energy for its operation and running, substantial research has been conducted by many researchers in different parts of world since many years. One such promising method is desiccant cooling, however research into its use is still in its early stages – and has to be looked at more thoroughly.
The increasing levels of energy consumption of cooling applications are one of the primary issues of the current era in building cooling. The majority of the world’s air conditioners now are vapour compression refrigeration-based models, which require enormous quantities of conventional energy to operate. The ozone layer is being destroyed as a result of the usage of this technology, which raised CFC levels. Due of the need for electricity, these applications have contributed to environmental problems including global warming. Scientists are researching alternative AC technologies to address the aforementioned problems as the need for space cooling applications grows.
The traditionally used vapour compression refrigeration system, which is the primary mechanism used by air conditioners, uses a lot of electrical power. Therefore, the recent advances in renewable energy systems and clean, efficient, sustainable technologies are chosen from energy conservation point of view – and substantially reducing major greenhouse gas emissions resulted from the building cooling while ensuring suitable conditions for users’ comfort.
Desiccant cooling is one of these technologies, and it could be able to address the issues raised above. By use of desiccant based innovative dehumidification and cooling techniques moisture of indoor air can be controlled effectively and consume less power as compared to the traditional HVAC devices in building. The desiccant material used in dehumidifier of desiccant-based cooling systems can produce hot and dry air that can be used for the drying process in commercial applications like pharmaceutical, coating, packaging, dying as well as in both commercial and residential buildings for air-conditioning to minimize the moisture content in built environment to produce necessary thermal comfort.
According to the analysis, solid-based desiccant based dehumidification and cooling systems offer a lot of promises for usage in warm, muggy climates. Basic desiccant cooling systems are more effective than conventional cooling kinds in terms of output temperature, but any improvement over the primary configuration, such as the integration of open/closed type, led to better innovation. The air conditioner should correctly manage the building’s sensible and latent loads to offer the circumstances for indoor comfort.
Hybrid solid desiccant cooling systems
The hybrid Solid Desiccant Cooling Systems’ (SDCSs) configuration, the only heat source for the regeneration process is the condenser of the integrated heat pump, as shown in Figure 1. In this configuration, chilled water is generated by the evaporator and stored in a chilled water tank. The chilled water is pumped to pre-cooling and cooling coils in order to cool process air from the environment. In the integrated heat pump sub-system, an additional condenser installed outside the air handling unit can eject excessive condensation heat to the surroundings – and avoid the over-loading of the compressor due to higher condensation temperature in the refrigeration cycle. The return air stream gains heat from the condenser inside the air handling unit, then heats the upper part of the solid desiccant wheel and causes the vapour pressure in the solid desiccant to be higher than the vapour pressure in the air stream. The moisture in the solid desiccant is ejected to the surroundings with the air stream. The moisture removal from the desiccant wheel by additional heat in the return air stream is called the regeneration process.
Opportunities and future scope
Due to the benefits of free energy, solar-assisted desiccant-based dehumidification and cooling systems have been extensively explored. India has a large significant solar radiations availability, which is thought to have better future among the other major renewable available energy sources. In sub-tropical environment (21–31°C DBT, 65–85% RH), performance analysis of the solar-assisted desiccant based dehumidification and cooling systems by simulation revealed boosting major performance of the HVAC. Like other new ideas, solar power technology might have a high initial cost, but over time, it can result insignificant savings.
Traditionally, the regeneration procedure may be carried out using an electric heater. Low-grade energy, on the other hand, is seen as a wise and practical substitute. Examples include freely available renewable enormous sun radiations, industrial process heat in form of waste industrial heat and exhaust heat recovered to produced warm or hot water released from many process industries. For instance, the regenerator and dehumidifier in an innovative cooling ideas of using the liquid desiccant-based dehumidification and cooling system may have the same type of component arrangements and packing materials, but their internal flow processes may be in opposition to one another. Before flowing into the regenerator, the diluted liquid desiccant solution is warmed by passing through the regenerator. It is necessary to use interchanged heat from hot fluid to cold fluid before passing intermediately to the absorber and regenerator. If the primary heat source is unable to provide the necessary heat for regeneration, an ancillary heater is also required.
The solar collector regenerators are divided into many sub categories like four types of convection: forced convection, natural convection, open type, and closed type. The mechanical air flow type solar collector regenerator used currently is the most popular because it is currently proving to be the most efficient. Solar energy has been used in earlier publications different sources of reactivation energy have been mentioned.
Mostly countries in tropical area that have been exposed to greater solar radiations can be harvested year-round primarily using different solar collectors.
A hybrid cooling system using solid desiccant and vapour compression, which uses direct solar radiations in form of reactivation heat, has been critically reviewed. For passing cooling in desiccant dehumidification solar radiations exposed to the building can be controlled by use of effective shading or double glazing techniques. Green rooftop with harvesting solar energy by use of electric photovoltaic panels fitted. Based on concentration ratio, various operating temperatures can be obtained, several solar radiation collection systems are used to harvest reactivation energy.
There is still a dearth of research on the optimization of system parameters for variable environment in terms of outdoor temperature and humidity. CFCs-based refrigerants, which are mostly used in conventional HVAC systems, are responsible for global warming. However, those can be eliminated in desiccant based green air conditioning. With newer passive desiccant cooling approach energy consumption can be minimized drastically by incorporating natural system of ventilation in building and by use of double glazing to minimize solar heat gain by applying good shading devices. Green roofing is also a newer approach to provide effective roof insulations. As per earlier investigations, it is seen that almost 24% of residential building energy can be saved by use of passive cooling approach.
Conclusion
The desiccant integrated hybrid cooling is a novel space cooling techniques in built cooling environment and is a promising alternative to traditionally used vapour compression-based cooling, especially in hot and humid climatic conditions for efficiently controlling indoor moisture – but less familiar as compared to that of that of the traditional cooling system in market.
Desiccant cooling advances and Montreal Protocol developments that phase out the use of some fluorocarbons offer alternatives to traditional air conditioning systems. The future requirements for desiccant cooling and the demand for desiccant material, which regenerated near ambient conditions leading to the development, should be influenced by rising electrical energy costs, environmental concerns, and legislative requirements.
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.
