The increase in world population has made energies to become more demanding for many applications. As growth rate increases, high volume of energy is required to operate more machines for humans and industrial applications for daily lives’ utilization. Busy lifestyle has made nations to be more competitive and several tasks are expected to be performed at the same time. Tiredness and sweat cause people to be uncomfortable and the only thing that comes into their minds is to turn on air conditioners. Therefore, it is imperative that the cooling framework of air conditioner should be analysed, whilst also taking the thermal comfort into concern. Air conditioner has become one widely used innovation by humans in local and modern location to obtain good thermal comfort. The fact that air conditioners have been serving well for humans to provide proper indoor air quality cannot be denied – but there are some critical problems that must be addressed by the Heating, Ventilation, Air Conditioning (HVAC) industry to make them more viable. One of the problems is that the air conditioners won’t rely on fossil fuels anymore to obtain power source in the next few decades due to the declining trend of fossil fuel availability. This can be a potential reason for the drop of air conditioners production in the upcoming years.
There are some limitations in the conventionally used Vapour Compression Refrigeration (VCR) systems, which urge the researchers to find a better replacement for cooling system. One of the problems in vapour compression cycle is that its system has been consuming high volume of electrical energy to operate air conditioners. This is because VCR cannot reduce the cooling load by dehumidification, instead it only uses up power to cool down surroundings by coolants. Besides that, VCR system has also been a major factor for the discharge of carbon dioxide (CO2), and ozone depletion substances such as Chlorinated Fluorocarbon Compounds (CFCs) and Hydro Chlorofluorocarbons (HCFCs) which are considered as potential ozone draining gases. These ozone depletion coolants can indirectly cause skin diseases and asthma for users over long term. High concentration of these coolants is potential to lower the quality of indoor air. A part of it, VCR has caused some major loss for air conditioner users due to rotting on surrounding furniture especially when surrounding relative humidity exceeds 60%. This happens because the system cannot control the environment’s humidity and temperature separately. High humidity causes high amount of water vapour in the air, which promotes the growth of mold on furniture and cause user’s furniture to rot because VCR cannot control the indoor humidity and temperature separately.
This can be replaced by innovating a cooling system that has the capabilities to reduce the indoor humidity and temperature at the same time. One promising idea came up by researchers to provide better cooling quality is by using Solid Desiccant Cooling System (SDCS). SDCS requires the use of solid desiccant materials to adsorb the indoor moisture to keep indoor humidity under control by dehumidification process. Dehumidified air can provide dry and cool air to indoor surrounding and at the same time, reducing the risk of rotting the indoor furniture. SDCS is also able to reduce the energy cost by air conditioners because this system will be operated only under thermal heat, which is from the processed ambient air. As the dehumidification done by SDCS will remove moisture, it will also be able to cool down the surrounding air to a certain extent. This will reduce the need of cooling because the indoor will be more comfortable than usual condition. This will reduce the workload of the air conditioner to run than the usual. So, the air conditioner will be able to reduce power consumption by cutting the running cost. In addition to that, SDCS does not require refrigerant that is another bright side of this system. Therefore, high energy is not required to compress any refrigerant to the system for cooling purposes.
Working of the system
Desiccant cooling is either an open cycle or closed cycle according to its configurations that makes use of a desiccant wheel and thermal wheel to achieve both cooling and dehumidification. Solar thermal energy or waste heat can be used to re-activate the desiccant. The thermal wheel is a rotary heat exchanger positioned within the supply and exhaust air streams in order to recover the heat energy, and the desiccant wheel works in a similar way, additionally re-activating the desiccant. The principle of the cycle is shown in Fig. 1. There are a number of variations depending on the condition of ambient outdoor air, air change requirements and humidity control requirements. It offers high efficiency but requires large air flows and is limited in operational range, but it can be applied in conjunction with a conventional vapour compression cycle or evaporative cooling. Referring to Fig. 2, the outdoor air moisture content is reduced and its temperature increased as it passes through the desiccant wheel. It is sensibly cooled through the thermal wheel, further cooled to the required supply temperature, with some moisture gain as it passes through an evaporative cooler. The return air from indoors, at, say 25°C is passed through an evaporative cooler so that it enters the thermal wheel at a lower temperature and higher moisture content. As it passes through the thermal wheel, it is sensibly heated and further heated by the heater – so that it can re-activate the desiccant before exhausting. The by-pass can be controlled – so that unnecessary heat is not applied. The thermal wheel is illustrated in Fig. 3.
Technological innovation in desiccant cooling
Open sorption systems also called DEC systems supply conditioned air to a building, which is controlled to a specific temperature (lowest temperature is 16°C) and humidity. The principle of open systems is to use the ambient air or a combination together with re-circulated building air for air conditioning of a building (Fig. 4) instead of chilled water. With such open systems building heat is removed by the air flow through the building, and additional fresh air is continuously supplied into the building. Therefore, air conditioning and building ventilation are provided at the same time. For the sorption part (solid-based sorption wheel or liquid-based salt solutions) of such an open system, solar heat is necessary for the regeneration to ensure a continuous operation. The advantage of a solar heat-driven DEC system is that it fulfills all the essential requirements of air conditioning (i.e., control of fresh air temperature, humidity, and volume flow).
In a developing country like India, high volume of waste heat is generated around the year which could be used to produce heat around the year. Thus, a technology which uses thermal energy to provide cooling could be a solution to our rising energy crisis. In many buildings like hotels, hospitals, and industries, there is a demand for hot water along with air cooling. Such a scenario is well suited for the application of Tri-generation concept. A tri-generation system produces three forms of energy, i.e., electricity, heating, and cooling which could be used to generate power, hot water, and air conditioning with suitable equipment. The principle of tri-generation (Fig. 5) is based on the generation of heat energy. Heat captured through burning waste, production of electricity with generators, or heat generated through solar panels could be used to generate hot water through heat transfer equipment or cold/ chilled water with absorption chillers. Potential for using tri-generation systems has been identified to be nearly 500 to 1,000 MW in India. Tri generation technology, also known as Combined Cooling, Heating, and Power (CCHP), consists of a gas engine or a power system operated by burning waste, bio fuel, or fossil fuel to produce electricity. The connected heat recovery system is used as a heat exchanger to recover heat from the engine or exhaust. This recovered heat can be used for heating applications like hot water or a regeneration process in absorption chillers. The electricity produced within the tri-generation process could be used to meet the building loads or power chillers during peak load period. The thermal energy could be diverted to boilers to heat the water used in hospitals, hotels, and industries for numerous purposes and/ or to absorption chiller to heat the absorbent and refrigerant mixture and regenerate the absorbent.
Desiccant cooling systems are increasingly developed as an alternative to the conventional vapour compression systems since the conventional vapour compression system consumes a large amount of energy and causes environmental problems. So, the desiccant systems become a priority because of the continuing rise in energy demand, increasing cost, and climate change. Also, they do not use any refrigerants, which are harmful to the environment. Thereby desiccant systems are better for the environment. Hybrid desiccant cooling systems can be combined with direct, indirect evaporating or vapour compression based conventional cooling systems. They require a smaller regenerative temperature for desiccant. It means that they need less energy. Additional energy such electricity or gas is required for desiccant wheel regeneration. These systems have a sanitizing effect on the air. Airborne microorganisms are killed by passing through the system. Because of the possibility to regulate the comfort level of the relative humidity in the indoor air. There is no dew, so no mold and mildew. Therefore, these systems can prevent building-related illnesses. In addition, desiccant cooling systems can be applied in different climates.
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.