Air conditioners are used to regulate the temperature and relative humidity in enclosed space. The effectiveness and efficiency of an air conditioner are triangulated among three main parameters viz., thermal comfort, energy consumption and indoor air quality. Among these, the most important factor is the Indoor Air Quality (IAQ), as it has a direct impact on the health of the occupant. IAQ inside air-conditioned space depends on the admission of fresh air. In the case of split air conditioners, there is no provision for fresh air admission which causes poor IAQ, while in centralized air conditioning; the quantity of fresh air is often inadequate, resulting in poor dilution of indoor air pollutant. Generation of CO2 is mainly from human respiration and it is a direct indicator of occupancy. IAQ in terms of CO2 concentration deteriorates with an increase in the number of occupants inside the built space.
The biggest challenge for the existing air conditioners is to maintain good IAQ inside the built space in terms of CO2 concentration. This is due to the continuous recirculation of conditioned air inside the work environment. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) standard 62.1–2016, stipulates a permissible limit of up to 1000 ppm of CO2 concentration inside air-conditioned space. However, enclosed air-conditioned buildings with higher occupancy are found to be exceeding the permissible CO2 limits. Higher CO2 concentration would mean that occupants spend time in an inadequate oxygen environment that could lead to hypoxia, a condition of inadequate oxygen in tissues or normal cells. Hence, there is a need to monitor indoor air quality periodically in air-conditioned spaces.
IAQ is assessed either by human perception or by measuring the pollutant concentration with necessary scale values. Ventilation is perhaps the area in which the most changes have been proposed in response to the emergency caused by this virus. However, other strategies are possible, such as source control and the extraction of pollutants. The latter incorporates clean technologies, an emergent area with respect to IAQ. Method: Various air treatment technologies can be used to control contaminants, includes mainly physicochemical technologies (e.g., filtration, adsorption, UV-photocatalytic oxidation, ultraviolet disinfection and ionization) and biological technologies as shown in Fig. 1 (e.g., plant purification methods and microalgae-based methods).
In adsorption, gas molecules adhere to the surface of a solid sorbent. The sorbents are designed to have very large surface areas for the capture a large quantity of gaseous contaminants. One of the most common adsorbent materials is activated carbon. Activated carbon is made by the destructive distillation of the non-carbon materials in wood, coconut shells, etc. This leaves a carbon material with very small pores and large surface area available for adsorption.
Other adsorption materials include activated alumina, zeolite, clay, and silica gel. Adsorption is the most widely used process for the removal of gaseous contaminants from indoor air in commercial buildings. However, adsorbent materials do not adsorb all contaminants equally. Furthermore, some vapours may not be retained on activated carbon by physical adsorption because of their high volatility. This approach enables a tailored and more accurate process design and additionally, it can also assist in the physical location of the removal unit and sensors to control its operation. Two different examples of application of this methodology are provided: control of CO2 in tightly closed environments.
Desiccant dehumidification-based adsorption cooling technologies
Adsorption consists of capturing air pollutants on the surface of an adsorbent material. It has been successfully applied for retaining both volatile organic compounds and inorganic pollutants on adsorbents, such as activated carbon, zeolites, silica gel, activated alumina, mineral clay and some polymers. The most commonly used are activated carbon and hydrophobic zeolites, due to their high surface area and adsorption capacity. Activated carbon has high porosity and is a non-polar adsorbent. It can be produced from agricultural wastes such as sugarcane bagasse, apple pomace or coconut shell for more cost-effective pollutants removal.
Due to its microporous structure and large surface area, activated carbon is able to remove up to 100 mg m−3 of Volatile Organic Compounds (VOC)s, although medium and high molecular weight volatiles are better adsorbed on activated carbon than low molecular weight volatiles. Adsorbents can be easily incorporated into building materials and/or integrated into interior surfaces to remove air pollutants with no additional energy input and minimal byproduct formation; for this reason, they are classified as Passive Removal Materials (PRMs).
Passive removal materials enable ozone control, for example, in susceptible populations with health benefits, creating healthy indoor environments. Hybrid technologies of adsorption combined with other methods have been proposed for pollutants removal from indoor air. It is investigated that the technical feasibility of a hybrid system composed of activated carbon and photocatalytic oxidation for controlling indoor air levels of BTEX (benzene, toluene, ethylbenzene, and xylenes) at low concentrations (of 0.1–1 ppmv). It is seen that this hybrid system can enhance control efficiency of BTEX in indoor air levels with higher removal efficiencies (close to 100%) compared to using activated carbon alone (with removal efficiencies close to or higher than 90%), with a negligible addition to indoor CO levels from the photocatalytic oxidation process.
However, some drawbacks of this technology include the decrease in removal efficiency with increasing relative humidity due to the capillary condensation of water vapour inside the activated carbon that blocks the adsorption sites. Additionally, the adsorption capacity of activated carbon decreases with increasing inlet concentrations. Furthermore, it is reported that the temperature of 300 °C is necessary to obtain significant desorption yields (75–95%) and the adsorbents need to be regularly replaced in order to avoid re-emission of already adsorbed compounds.
Adsorption materials can not only act as a sink for airborne pollutants, but also for excess moisture through adsorption. For example, a medium density fiberboard modified with walnut shell was investigated to regulate relative humidity, toluene, limonene, dodecane and formaldehyde. A negative feature of the adsorption technology is the possibility of the deposition and development of airborne bacteria on the adsorbent surface due to the high biocompatibility of these materials. Additionally, adsorption technology does not treat or destroy contaminants, but they are simply transferred from one phase to another, producing a hazardous solid waste that must be further treated and/or disposed of correctly.
The rotating honeycomb wheel is a finely divided desiccant impregnated into a semi-ceramic structure, maximizing the surface area of the desiccant material. The appearance of the honeycomb wheel resembles corrugated cardboard that has been rolled up into the shape of a wheel. The air passes through the flutes formed by the corrugations, and the wheel rotates through the process and reactivation airstreams. The flutes served as individual desiccant-lined air ducts, which maximizes the surface area of the desiccant presented to the air stream. The rotating honeycomb wheel design has several advantages.
The structure is lightweight and very porous. Different types of desiccants can be arranged into a honeycomb wheel configuration for different applications. The design allowed for laminar flow within the individual flutes, reducing air pressure resistance compared to packed beds. This allowed the honeycomb wheel to operate efficiently for low dew point and high-capacity applications.
The honeycomb wheels are very light, and their rotating mass is very low compared to their high moisture removal capacity. The result is an energy efficient unit. The design is simple, reliable, and easy to maintain. The design is the most widely installed of all desiccant dehumidifiers in ambient pressure applications. Additionally, the honeycomb design is the most appropriate dehumidifier configuration for air-conditioning applications. A working desiccant honeycomb unit with environmental chambers at the outlets of both the regeneration side and process side of the unit was utilized to determine removal efficiencies. See Fig. 2.
The desiccant unit was modified with an additional heater and temperature controller to allow the operating temperature of the regeneration cycle (cycle that removes moisture from the wheel) to be varied between 38ºC and 182ºC. This allowed testing of the wheel at various increments of temperature. The Collison nebulizer was operated at 40 psig, which generated a liquid generation rate of 16.5 ml/hr, allowing to introduce a known concentration of fungi per volume of water as an aerosol into the test mechanism to generate a known and constant airborne fungal concentration. Pressure was applied to the nebulizer with an air compressor as shown in Fig. 3.
IoT based sorptive cooling systems
Considering the life expectancy and reliable single-hop communication abilities, IoT monitoring systems are believed to be the most reliable solutions for IAQ measurement and efficient control. With lower latencies and lesser power consumption, these systems also demand lesser efforts for maintenance. IoT based real-time monitoring systems are known as smart systems; consequently, most of the researchers and industrial manufacturers are more attracted to this architecture. Experts reveal that the IoT system can monitor a large number of parameters, even without compromising system performance. However, very few researchers in the past few years have worked on prediction systems in the field of IAQ monitoring. Studies reveal that it is much easier to combine IoT monitoring systems to machine learning and deep learning networks to initiate reliable prediction decisions. It is a significant area of work for new age researchers.
Conclusions
Desiccant-based dehumidification and adsorption cooling capable of capturing both volatile organic compounds and inorganic pollutants. Adsorbents can be easily incorporated into building materials and/or integrated into interior surfaces to remove air pollutants with no additional energy input and minimal byproduct formation. A negative aspect of the adsorption technology is the possibility of the development of airborne bacteria on the adsorbent surface due to the high biocompatibility of these materials. Additionally, adsorption technology produces a hazardous solid waste that must be further treated and/or disposed of correctly.
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.
