The preservation of comfortable temperatures and humidity levels in indoor environment of the building structure is one purpose that makes to lower the amount of non-renewable energy needed to accomplish this is a key national objective. The temperature and humidity of the conditioned room rise in the summer due to infiltration, ventilation, solar gains, building envelope conduction, and internal heat and moisture creation by the inhabitants and equipment. Five of these heat loads can be reduced but not entirely eliminated with better machinery, building design, insulation, sea-level rise and ventilation systems. Building functionality determines how much heat and moisture occupants produce and this cannot be considerably changed. An energy-efficient cooling system is a very alluring alternative for the consumer, utility and HVAC business that manufactures and installs the product, particularly if it can be naturally marketed and retrofitted to existing building stock. After many years of study and improvement, conventional vapour compression systems utilising electricity have reached their limits. Electricity prices rise at a rate determined by supply and demand. When the economics provide a favourable rate of return, novel cooling systems utilising renewable energy sources directly are being developed. The energy of the unsaturated atmosphere cannot be used by closed cooling systems where the refrigerant does not come into contact with the atmosphere. Open systems that use water evaporation as a refrigerant in the atmosphere rely on this energy source and need less direct solar energy; for instance, a straightforward evaporative cooler just needs energy to move the water and air. To benefit from this, closed systems may be integrated with open systems. Without dehumidification, open systems function worse in humid environments. Because there are fewer heat losses, a solar collector will produce more heat at low temperatures. These facts prompt us to select open cycles with low temperature heat and dehumidification as viable solutions for a nationwide renewable energy cooling system. Ideal specifications for a nationally marketed and retrofitted system are compactness, minimal cost and maintenance, better performance and reliability.
Working of system
Desiccant-based dehumidification or drying is based on the physical process by which desiccant materials, which can be either liquids or solids, absorb and release water vapour.
The cooling impact of water comes from its evaporation, whereas the heating effect comes from its condensation. Heat is the driving force behind the desiccant cycle and the desorption process. Desiccant materials release moisture when heated. The management of thermal energy inside the system has a significant impact on the control, capacity, efficiency, and economics of desiccant cooling systems. Both open and closed cycles can be used to operate desiccant cooling systems. Open cycles operate at pressures that are relatively close to atmospheric pressure, whereas closed cycles are often run at pressures that are either higher or lower than atmospheric pressure. An open-cycle desiccant cooling system works with 100% fresh air is seen in Fig. 1 (open cycle).
The most typical desiccant cooling system is this one. Room supply is the ambient air which is the hot, dry air that has passed through a desiccant dehumidifier. This air then passes through a practical heat exchanger to be cooled. The dry, cooler air is then sent to the indoor space, after passing via an evaporative cooler to be cooled approaching its wet bulb temperature. The house’s exhaust air leaves the building, travels through the evaporator, where it is cooled and then travels via the sensible heat exchanger to warm the processed air. The heat exchanger’s exhaust air passes via a heat source to reach by increasing its temperature. The desiccant dehumidifier is regenerated using this hot exhaust air. If the process air inlet of dehumidifier uses indoor air, the system is run in the re-circulating mode (closed cycle).
Technological merits and limitations of innovative desiccant cooling
Open sorption systems also called desiccant cooling systems, supply conditioned air to a building, which have a number of benefits, some of which are related to the environment, others to energy conservation and still others to lower equipment startup and ongoing costs. The benefits of desiccant cooling systems are described below.
- There is no use of CFC refrigerants. Water and air are the sole fluids used because open-cycle systems will make up the majority of desiccant cooling systems. Vapour compression cooling systems have an environmentally friendly substitute in desiccant cooling systems.
- Fig. 1 demonstrates that desiccant cooling systems are heat-actuated cooling apparatuses that run on a negligibly small amount of parasitic electric power. Therefore, desiccant cooling systems will lower the amount of electricity used. This is important as it lowers the peak load as a whole.
- Low-temperature heat can be used to operate desiccant cooling systems. The efficiency of such a system will be extremely high if waste heat is available to replenish desiccant material. In addition to saving energy, desiccant cooling systems lower carbon dioxide emissions into the atmosphere if no prime fuel is used in the regeneration of desiccant material.
- By integrating a desiccant cooling system into a traditional cooling system to eliminate latent load, the traditional system can now only handle sensible load. This reduction in flatent load will result in smaller compressors, fans, ducting, etc., which will lower initial equipment costs. In most cases, this cost reduction will be sufficient to cover the cost of desiccant equipment.
- Desiccant dehumidifiers can also absorb unfavourable odours and suspended inborn particles with the addition of extra material, such as activated carbon and therefore enhance Indoor Air Quality (IAQ).
- Construction and maintenance are made easier by the close proximity of air pressure during desiccant cooling system operation (open cycle).
- Desiccant cooling systems have fewer moving parts, which lower their operating and maintenance costs.
- Desiccant cooling systems are extremely efficient for applications requiring 100% fresh air and for cooling rooms with significant latent loads.
- For closed-cycle systems, using the idea of heat regeneration and maintaining crisp thermal waves can result in high system COP.
- The performance of closed-cycle desiccant cooling systems is essentially unaffected by condenser temperature.
- Low refrigeration temperatures can be reached using closed-cycle systems.
- Since energy is stored as chemical rather than thermal energy in liquid desiccant cooling systems, energy storage is practical.
- The relatively high installed cost and lack of a widespread network of skilled service staff are two major drawbacks of desiccant cooling systems.
Conclusion
The innovative desiccant based air conditioning technology has the potential to make major contributions to energy conservation, improve indoor air quality and to reduce the cost of moisture damage to buildings and residential sector. However, state-of-the-art materials, components and systems are not widely understood or in use in the residential building industry. Well-coordinated research and development efforts of HVAC industry, government and research institutions will be needed to fully realize the benefits of this established, but under-utilized HVAC cooling technology.
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
