The growing global population has increased the demand for energy in many different applications. A large amount of energy is needed to run more machines for people and industrial applications for everyday use as the growth rate rises. A busy life-style has increased national competitiveness and the expectation that multiple tasks be completed simultaneously.
People get uncomfortable from fatigue and perspiration, and the first thing they think to do is turn on the air conditioner. Consequently, it is essential to analyse the cooling system performance while considering the indoor thermal comfort. Mostly people use vapour compression based traditional cooling system to achieve adequate indoor thermal comfort as an air conditioner.
It is undeniable that VCR based traditional air conditioners have helped people to maintain appropriate indoor comfort in case of moderate climate, but in order for the Heating, Ventilation, and Air Conditioning (HVAC) industries to remain viable, they need to give greater consideration to a few important issues like Indoor Air Quality (IAQ) and large ventilation air flow required for that.
One of the issues is that, as fossil fuel availability declines over the coming decades, air conditioners will no longer be able to rely on them for their power source. This might be the cause of the impending decline in air conditioner sustainable energy performance. 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 could not 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 losses for air conditioner users due to rotting on surrounding furniture especially when surrounding relative humidity exceeds 60%. This happens because the system could not 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 could not 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, which 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 make 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.
For the air-cycle, the outdoor air penetrates through the desiccant wheel and it is dried and ramped up due to dissipating heat from the adsorption of water vapour (1-2). Afterwards, the air is blown to Regenerative Evaporative Cooler (REC) and cooled (2-3) in Sensible Heat Exchanges (SHX). And then, the air is supplied to the evaporative cooler (3-4) and the conditioned air is supplied to the indoor space (at state point 4).
In the regeneration channel, the air taken from the room is heated to the regeneration temperature (6–7) and (7-8) at the air to air heat recovery wheel and heating coil successively to desorbs the moisture from the desiccant wheel to regenerate it (8-9), and is exhausted finally (at state point 9).
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.
The desiccant wheel known as the adsorption rotary regenerator, using silica gel for optimal adsorption capabilities. The rotation mechanism, maintained by a maintenance-friendly chain drive, operates at a constant speed of around 10 to 15 rotations per hour. The seals, both radial and axial, separate counter flows and surroundings, ensuring efficient and controlled dehumidification processes. The housing of our units is constructed with stainless steel, offering durability, and is insulated with mineral wool to enhance efficiency in challenging industrial environments when ambient environment is hot and humid.
Technological innovations
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 that 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), comprises 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.
Conclusion
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 that 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 245 Research Articles in reputed International Conferences and Journals. He has also published 10 reputed books and book chapters in the area of thermal engineering. Working as Academic Editor for the Journal of Materials Science Research and Reviews. 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.
