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Water vapor adsorption isotherms for: (a) silica gel at 25 1C; (b) silica gelLiCl at 25 1C; (c) sepiolite at 23 1C; (d) sepiolite-carbon by physical activation with steam at 23 1C; (e) sepiolite-carbon by chemical activation with KOH at 23 1C; (f) CaCl 2 -SiO 2 sol-gel at 25 1C; (g) CaCl 2 -MCM-41 at 20 1C; (h) silica gel-LiBr at 20 1C [5].
Contexts in source publication
Context 1
... kinds of composite desiccants have been developed and studied which gave better results as compared to conventional silica gel [16,35,36]. Keeping in view the available conditions of temperature and humidity, a certain composite material may be required to perform efficiently within the range of vapor pressure. As shown in Fig. 2 [5] the composite materials enable water sorption variability at different relative pressures which encourages many humidity based AC ...
Context 2
... Principle and features of desiccant air-conditioning Fig. 6 shows the basic principle of DAC in which the air is dehumidified by natural sorption process between the desiccant and humid air. The driving force is water vapor pressure difference between the desiccant and process air. For detail insight of adsorption isotherm please refer to Fig. 2. Once the equilibrium condition between the desiccant and air arises the water sorption process will be stopped unless external forces are applied i.e. increasing air pressure or decreasing the ...
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Citations
... Although various artificial zeolites are available, they have some disadvantages, such as low adsorption capacity and high energy requirements for desorption, which limit their use in atmospheric water harvesting systems. [32,33]. ...
This study investigates the thermodynamic performance of various adsorbent materials used in adsorption-based atmospheric water harvesting (AWH), with a focus on their application in arid regions, such as Iraq. While significant advancements have been made in developing adsorbents capable of capturing water at low humidity levels, the lack of detailed thermodynamic analysis has hindered optimal operational conditions for these materials. The study evaluates the thermodynamic efficiencies of MOF-808, MCM-41, and COF-432, comparing their performance with other materials like MOF-801, MOF-803, and Ni2Cl2BTDD. Key findings indicate that source temperature and outlet relative humidity significantly influences system efficiency, with higher source temperatures generally improving thermal efficiency. However, lower outlet relative humidity, especially at higher temperatures, can further enhance system efficiency, emphasizing the importance of effective humidity control. Additionally, the study highlights that Iraq’s elevated ambient temperatures may exacerbate efficiency losses due to higher thermal sink temperatures, underscoring the need for adaptive design strategies. The performance of various adsorbents was found to be highly material-dependent, with certain adsorbents offering superior thermal and second-law efficiencies under specific conditions. This study offers crucial insights into optimizing adsorbent selection and system design for AWH systems, particularly in challenging climates.
... Besides, the desiccant materials are often environmentally friendly compared with traditional refrigerants. Some desiccant materials can be easily regenerated at a low temperature, which could utilize the waste heat sources from industry and thermal solar energy [9,10]. ...
Air-conditioning (A/C) systems in tropical regions are characterized by significant energy consumption for latent load handling. Decoupling of the latent load from the A/C units can be achieved using a dedicated dehumidification system while the A/C systems handle only the sensible heat at high efficiencies. Desiccants are widely used in industry, and adsorbent materials that exhibit a unique isotherm shape, i.e. "S shape", have been developed extensively. Recently, activated carbons (ACs) have been discussed as effective adsorbents for dehumidification applications. Although pristine ACs are considered to be hydrophobic materials, certain surface treatments initiate surface phenomena that promote water vapour uptake at relative pressures above 0.4 due to microdroplet aggregation. This work reviews and reports the latest developments of sustainable activated carbons for dehumidification using a multiscale approach spanning from the sustainable precursor selection, “green” activation processes and surface functionalization, adsorption thermodynamics, and system-level developments. With the focus on sustainability, we demonstrate that water adsorption and viable adsorption range are gradually improving with the progressing research, and they are reaching operational values required for practical use. The unique adsorption process of water onto ACs is further explained in detail using solvation theory on the microdomains created by the hydrophilic functional groups while providing clarification of thermodynamic properties adopting the specificities of water/activated carbon adsorption pair. The predicted performance of a desiccant dehumidification system utilizing activated carbon is evaluated using the local weather conditions of numerous major cities worldwide. The highest dehumidification performances of activated carbon, as indicated by the unified SDP (specific dehumidification power) value, are reached particularly in cities that suffer from high humidity and temperature the most proving the viability of this cheap and sustainable material.
Graphical abstract
... To improve energy efficiency, free cold sources such as tap water, solar cooling, radiant sky cooling, nocturnal cooling, ice storage, underground water, and evaporative cooling can be used. Sultan et al. (2015) reviewed the DWS with evaporative coolers as the cold source and pointed out that the use of evaporative cooling technology could improve system performance and obtain a payback period of less than five years. Table 4 shows a summary of the literature on the use of cold sources. ...
Humidity control is very important in production and life, especially in low dew-point industries where the dew-point temperature is less than 0 °C. A low dew-point desiccant wheel system (LDDWS) can meet the ambient humidity ratio required for production well. The mechanisms, configuration, and optimization of the LDDWS are comprehensively reviewed. The air heat-moisture handling processes of the LDDWS are initially introduced. The influence of the cold and heat source configurations on the dehumidification capacity and energy consumption is illustrated. Free cold sources, renewable heat sources, and heat pump technology can effectively increase energy utilization, thereby reducing refrigeration consumption and promoting energy savings. The optimization measures of system performance, including efficient desiccant materials, desiccant wheel partitioning, heat recovery technology, and low dew-point return air recycling, are presented. These optimization measures can effectively improve the dehumidification capacity and reduce the consumption of primary energy. In addition, to enhance the further development of LDDWS, this paper presents prospects for future developments in terms of industrial demand, dehumidification capacity, thermodynamic cycle and energy utilization.
... At the same time, the evaporator in vapor compression systems is suitable for meeting sensible cooling demands and efficiently decreasing the air temperature. The benefits of using a desiccant wheel heat-recovery unit include [114]: ...
This work evaluates the impact of air renewal on energy consumption for indoor environments. For this purpose, an analysis of the problem of air renewal at a Brazilian level was carried out, as well as research into the energy impact of air renewal without energy recovery and the different existing technologies for recovering energy from renewed air. On the other hand, the influence of heat-recovery systems was analyzed in three Brazilian cities (Manaus, São Paulo, and Brasília) for different environments, where a classroom in Manaus has an approximately 50% external air factor and a 42% sensible heat factor. However, classrooms in São Paulo and Brasília have a lower external air factor (27% and 8%, respectively) and a higher sensible heat factor (61% and 78%, respectively). Considering a system with heat recovery, the external air factor decreases to 23%, 10%, and 3% for Manaus, São Paulo, and Brasília, respectively. This allows us to understand the influence of heat-recovery systems, which reduce the external air factor and increase the sensible heat factor.
... Zeolites typically provide a higher water uptake than other inorganic materials. However, their standalone application in AWH is constrained by the high energy requirement for the desorption step [92,93]. ...
... They reviewed desiccant air conditioning systems and concluded that the main attractive feature of desiccant systems is that they can be regenerated by low-temperature energy sources or waste heat. Again, a review of the desiccant wheel-based cooling systems was written by Ge et al. (2014) and Sultan et al. (2015), but the focus of this review was on the distinct solar-powered system and hybrid solar-powered single and double-stage systems. Further, Mujahid Rafique et al. (2015) have done a detailed review of the desiccant-based evaporative air conditioning system, which is available in the literature, and reported that an evaporative cooler integrated with a desiccant wheel system can provide thermal comfort in hot and humid weather. ...
This review provides a comprehensive summary of research pertaining to the purge section within desiccant wheels featuring multi-sector configurations. Additionally, it encompasses
discussions on innovative wheel designs such as non-adiabatic desiccant wheels and the achievement of two-stage dehumidification from a single wheel employing multi-sector
approaches. The review begins by providing a concise historical overview of the desiccant wheel, followed by a systematic classification of the research conducted in this area.
Subsequently, various categorizations are presented in a logical sequence, offering a structured understanding of the subject matter. Central to the critical findings of this review is the identification of an optimal purge wheel sector angle, which not only decreases the energy consumption of the desiccant wheel but also significantly reduces the exit temperature of process air. Moreover, the review highlights the potential of achieving isothermal dehumidification through the utilization of non-adiabatic rotary desiccant wheels. Furthermore, the introduction of a multi-sector desiccant wheel is one of the key successes in
obtaining two-stage dehumidification and getting multi-output like cooling, heating with dehumidification, and heating with humidification. These are all efficiently derived from a single wheel.
... Consequently, in hot and humid regions and by employing a vapor compression air conditioning system, the process of removing moisture from the outdoor air consumes a large fraction of the total energy consumed by the air conditioning system. Therefore, solid desiccant cooling sounds to be a true solution for removing the humidity from the outdoor air before supplying it to the conditioned zone, where thermal energy is required to reduce the moisture instead of electric power [1][2][3][4]. ...
... So, the process air goes through the CC at point (3), where pump #3 supplies chilled water from the absorption chiller to the CC. Finally, the supplied air enters the studied office at point (4). ...
Desiccant air-conditioning systems are considered a competitive alternative to conventional vapor compression cycles. However, the literature lacks detailed studies of such systems when coupled with thermally active buildings and powered by solar energy due to the complexity of modeling and controlling the system. In the present study, a novel hybrid desiccant-dehumidification-absorption system, driven by external compound parabolic concentrators (DDA-XCPC), is used to serve a typical office space in a hot climate zone. The entire system is modeled and simulated dynamically in the TRNSYS environment. The current study investigates the proposed system performance in terms of the achieved thermal comfort, energy consumption, solar fraction, life cycle costs, and carbon emissions. The results reveal superior thermal comfort levels with percentages of people dissatisfied below 5.6 %. The daily solar fraction ranged between 41 and 90 %, with an average of 49 %, whereas the coefficient of performance varied between 0.46 and 1.38 while having a season average of 0.86. Compared to a conventional system, driven by a gas boiler, the proposed system increased the life cycle costs by 6,824 USD due to the large capital investment of the solar thermal loop. However, each 1,000 USD of additional expenses was accompanied by a cutdown of 4,619 kg of CO2 emissions. Increments in the solar system’s capacity (i.e., solar collector area and storage tank volume) have adverse and favorable impacts on the economic and environmental performances, respectively. Therefore, the system can be a viable option whenever green energy production is prioritized and considerably incentivized.
... Air humidity is a crucial environmental indicator that significantly influences human production and daily activities. Currently, the prevailing method for regulating air humidity involves mechanical approaches such as humidifiers, which not only consume excessive energy but also have detrimental effects on the ecological environment [1,2]. In contrast, humidity-controlling materials offer an innovative solution by automatically adjusting the air's relative humidity based on the material's moisture absorption and desorption characteristics, responsive to the environmental humidity levels. ...
This paper focuses on the preparation and evaluation of a novel humidity-control material, vermiculite/(sodium polyacrylate(AA)–acrylamide(AM)), using inverse suspension polymerization. Acrylic acid and acrylamide were introduced into the interlayer of modified vermiculite during the polymerization process, leading to the formation of a strong association with the modified vermiculite. The addition of vermiculite increased the specific surface area and pore volume of the composites. To investigate the moisture absorption and desorption properties of the composites, an orthogonal experiment and single-factor experiment were conducted to analyze the impacts of vermiculite content, neutralization degree, and the mass ratio of AA to AM. According to the control experiment, the addition of vermiculite was found to enhance the pore structure and surface morphology of the composite material, surpassing both vermiculite and PAA-AM copolymer in terms of humidity control capacity and rate. The optimal preparation conditions were identified as follows: vermiculite mass fraction of 4 wt%, a neutralization degree of 90%, and mAA:mAM = 4:1. The moisture absorption rate and moisture release rate of the composite material prepared under these conditions are 1.285 g/g and 1.172 g/g. The humidity control process of the composite material is governed by pseudo second-order kinetics, which encompasses the complete adsorption process. These results indicate that the vermiculite/PAA-AM composite humidity control material has excellent humidity control performance and is a simple and efficient humidity control method.
... 1. Systems that only use vapor compression cycles; for example, a sequential system with only one vapor compression cycle that can switch between low-SHR and sensible only modes using a variable speed compressor [12] and a system that uses multiple vapor compression cycles to handle sensible and latent loads separately [13]. 2. Systems that consist of or are enhanced by non-vapor compression devices, such as liquid desiccant [14][15][16][17], solid desiccants [18][19][20][21], and/or membranes [22][23][24][25][26][27]. ...
... Desiccant materials are hygroscopic substances that have a high affinity to water. Commonly used solid desiccant materials are silica gel, LiCl, and molecular sieves [18]. The principle of solid desiccant cooling is the independent control of sensible load and latent load by absorbing water vapor using desiccant materials. ...
Dehumidification plays a significant role in space conditioning energy use. Conventional vapor compression cooling systems employ dewpoint condensation to deal with latent loads. In contrast, separate sensible latent cooling (SSLC) and other advanced alternative dehumidification systems can significantly reduce the electricity usage for dehumidification. The second law efficiency can be used as a benchmark to evaluate thermodynamic performance of alternative dehumidification systems. However, limitations exist in previous studies that define the thermodynamic reversible limits and second law efficiency for cooling and dehumidification systems. This work presents a new physics-based definition for the reversible limit and the second law efficiencies for cooling and dehumidification systems with air recirculation. The new framework is then extended to define a novel performance metric, the seasonal second law efficiency, to form a universal benchmark for assessing various cooling and dehumidification systems. Five cooling and dehumidification systems including magnetocaloric cooling, solid desiccant dehumidification, and membrane dehumidification are evaluated using this benchmark. Steady-state thermodynamic models are constructed for each system. Second law efficiency for each system under various outdoor temperatures and indoor sensible heat ratios (SHR) are calculated. The annual electricity usage of the five systems is used to justify the seasonal second law efficiency definition. The results show that compared to conventional vapor compression systems with mechanical dehumidification, the membrane-based AMX-R cycle can reduce annual electricity use by 12.2%-22.2% and increase the seasonal second law efficiency by up to 36%.
... Evaporative cooling, recognized as an environmentally friendly and energy-efficient technology, has been increasingly employed in cooling equipment [1,2], and HVAC (heat, ventilation, and air conditioning) systems [3]. This passive cooling technology not only offers a significant cost advantage but also avoids the use of ozone-depleting refrigerants, such as chlorofluorocarbons, making it a sustainable choice [4]. Studies have shown that HVAC systems account for about 50% of the total energy consumption in the building sector [5]. ...