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Adsorption Characteristics of Silica Gel + Water Systems
Abstract
The adsorption characteristics of pure water vapor onto two different types of silica gel at temperatures from (298 to 338) K and at different equilibrium pressures between (500 and 7000) Pa were experimentally studied by a volumetric technique. The thermophysical properties such as the skeletal density, Brunauer−Emmett−Teller surface area, pore size, pore volume, and total porosity of silica gel were determined. The Tóth isotherm model is found to fit all of the experimental data within the experimental errors. The experimental isotherms and the computed enthalpies of adsorption are compared with those of various researchers and found to be consistent with a chiller manufacturer's data.
... Accurate modeling of the isotherm is essential for simulating an adsorption-chiller adsorbent bed, which provides information on the equilibrium uptake of the refrigerant by the adsorbent. The Toth isotherm was employed in this study because the adsorbent was regular density (RD) type silica gel, and water was the adsorbate [56][57][58]. This adsorption model was proposed by Toth in 1971 [59], and several variations have since been developed [21,40,56,58,60,61]. ...
... The Toth isotherm was employed in this study because the adsorbent was regular density (RD) type silica gel, and water was the adsorbate [56][57][58]. This adsorption model was proposed by Toth in 1971 [59], and several variations have since been developed [21,40,56,58,60,61]. The form of the Toth isotherm required to evaluate the adsorbate quantity is given by Eq. (5), and the corresponding parameters A and C are defined by Eqs. ...
... The form of the Toth isotherm required to evaluate the adsorbate quantity is given by Eq. (5), and the corresponding parameters A and C are defined by Eqs. (6) and (7), respectively, using the temperature-dependent Arrhenius equation [21,56]. Parameter B is the Toth constant, which has been proposed to be 12 [21,56], 8 [62], and 7 [46]. ...
Energy usage for space cooling and air conditioning has been rapidly growing in recent years and is expected to increase further owing to global warming and climate change. Adsorption chillers can provide an eco-friendly solution to space cooling because they utilize low-grade thermal energy such as waste hot water produced by the primary power-generation unit. In this study, dynamic analysis of a fuel-cell-system-driven adsorption refrigeration system was performed to expedite the optimal multibed configuration and its operating conditions. The scope of the study was extended to investigate maximizing the waste heat utilization achieved via exergy analysis of multibed configurations. The adsorption-to-desorption time ratio was integrated into the physical configuration of the adsorption chiller, and a new variable was introduced as a "time constant" that controls the cycle time and, under ideal conditions, equates the heating requirements of different bed configurations. Dynamic simulations for different bed configurations were conducted to evaluate the performance, specific cooling capacity, and unutilized exergy in the outlet waste stream. Under the operating conditions of 25 • C and 80 • C cold water and hot water temperatures, respectively, the five-bed configuration with a time ratio of 0.25 proved to be optimal, resulting in increases of 45.8%, 10.2%, and 15.1% in the specific cooling capacity, performance, and exergy efficiency, respectively, compared with those of the conventional two-bed configuration. The four-bed configuration yields very similar results, but with a slightly lower specific cooling capacity.
... Figure 1 illustrates the temperature profile along a bed of RD silica gel. The physical properties and the parameters in and the form of Tóth's isotherm are given by Chua et al. (2002). The thermodynamic states of the silica gel and that of the air used to ventilate the bed have been chosen to ensure that two heat and mass transfer waves propagate through the bed. ...
... The principal aim of this work is to investigate the effect of the form of Tóth's isotherm on the performance of ventilated beds of hygroscopic porous media. This study makes use of the physical properties of RD silica gel reported by Chua et al. (2002) who give the following form of Tóth's isotherm ...
... R v is the gas constant specific to water vapour, T abs is the absolute temperature, and W m is the saturated moisture content of the solid that occurs when the relative humidity of the surrounding air is unity, and t is the Tóth constant. T abs is the absolute temperature, K. Chua et al (2002) provide the following values for the empirical constants for RD silica gel: K 0 = 7.3 × 10 −13 kg.kg −1 .Pa −1 , Q st = 2.693 × 10 6 J.kg −1 , t = 12, W m = 0.45 and R v = 461.5 J.kg −1 .K −1 . Thorpe (2001) points out that the differential heat of sorption is functionally related to the temperature, T , and the fractional relative humidity, r, of the interstitial air, and the moisture content, W , of the solids through in which p s is the saturation vapour pressure of water, Pa. ...
The thermal performance of ventilated beds of hygroscopic porous media depends strongly on the sorption properties of the solid phase. Furthermore, the published literature shows that the predicted performance of ventilated beds of hygroscopic porous media is extremely sensitive to the form and accuracy of the sorption isotherm. This is particularly apparent under idealized conditions when thermodynamic equilibrium is deemed to exist between the solid and fluid phases. When Tóth’s isotherm is invoked in an equilibrium model used to analyse the performance of a bed of initially warm and dry silica gel ventilated with cool, humid air, it is predicted that a shock wave develops downstream of air entering the bed. In contrast, it is shown that non-equilibrium solutions result in a transfer wave, the width of which decreases as the velocity of the interstitial air decreases. The Tóth isotherm gives rise to erroneous values of the integral heat of wetting of silica gel. Previously published studies of simultaneous heat and mass transfer in beds of hygroscopic materials refer to conceptual difficulties in interpreting the results, and some published mathematical analyses are somewhat terse. A secondary aim of this work is to assuage these difficulties by detailing the analyses in Supplementary Information.
... Therefore, handling of these salts is very difficult which ultimately restricts their use as desiccants in most of the industrial applications. On the other hand, solid desiccants comprise different porous materials including silica gel (Chua et al., 2002), activated carbon (Zheng et al., 2016), zeolites (Dzhigit et al., 1971) and molecular sieves (AbdulKareem et al., 2018). Solid desiccants have lesser adsorption capacity than deliquescent salts or liquid desiccants but at the same time they have some other desired properties such as structural stability, easy handling and dryness, therefore, they are preferred over liquid desiccants in most of the industrial applications (Chen et al., 2016). ...
... Although solid desiccants have various structural and handling advantages over liquid desiccants but most of them still have disadvantages like low desiccant performance and high regeneration temperature. Conventional solid desiccants like activated carbon, molecular sieves and silica gel adsorb water vapors mainly by physical adsorption and require high temperature (> 160 • C) for regeneration, therefore, reuse of these conventional desiccants is a high energy consuming process (Chen et al., 2016;Chua et al., 2002). Among different next generation sorbents investigated so far to capture atmospheric water vapors MOFs and covalent organic frameworks (COFs) have a special position (Henninger et al., 2009;Pérez-Carvajal et al., 2019;Seo et al., 2012). ...
... Further, highest adsorption capacity of SPHC-Laponite® was also supported by the highest values of q m and r (Table 4). Desiccant performance of P(SA-co-AM)-SPH, SPHC-Laponite®, SPHC-bentonite and SPHC-palygorskite was compared with other solid desiccants without having any hygroscopic salt and they had comparatively higher adsorption capacity than most of the previously reported adsorbents (Table 5) (Chua et al., 2002;Dzhigit et al., 1971;Gorbach et al., 2004;Kim et al., 2003;Li et al., 2018;Llewellyn et al., 1995; al., 2020b;Oh et al., 2003). Further, regeneration temperature of all these synthesized polymer desiccants was much lower than the other conventional desiccants. ...
Current research work reports the synthesis and performance evaluation of solid polymer desiccant materials based on the superporous hydrogel composites (SPHCs) comprising superporous hydrogel (SPHs) of sodium acrylate (SA) and acrylamide (AM) with different clays such as Laponite®, bentonite and palygorskite. Synthesized polymer desiccants were characterized using SEM, XRD, TGA and FTIR techniques. All the polymer desiccants followed type-III isotherm and the experimental data correlated well with Guggenheim, Anderson and Boer (GAB) model and suggested that these desiccants belong to macroporous adsorbents and adsorb water vapors largely via capillary condensation process. Parental SPH (i.e. P(SA-co-AM)-SPH) and different SPHCs i.e. SPHC-Laponite®, SPHC-bentonite and SPHC-palygorskite exhibited maximum adsorption capacities of 0.12, 0.17, 0.15 and 0.14 gw/gads, respectively at 50% relative humidity and 25 °C, however, at 90% relative humidity adsorption capacities increased upto 0.77, 1.05, 0.93 and 0.87 gw/gads, respectively, which suggested that the hydrophilicity of synthesized desiccants increased with increasing relative humidity. Exceptionally high adsorption capacities of these desiccants were because of the combined effects of highly porous structure of SPHs and hydrophilic nature of clays. Furthermore, experimental kinetics data correlated well with linear driving force model and suggested that the diffusion of water within polymer structure was via type-II diffusion mechanism. During multiple cycle adsorption, SPHCs exhibited much better re-use efficiency than SPHs which suggested that the incorporation of clays within the polymer matrix not only improved desiccant performance but also improved re-use efficiency of polymer desiccants.
... Silika gel is often application as a moisture absorber in packaging products such as medicine and food [1]. Silika gel has attracted a lot of attention because of its superiority, so intensive study of silica gel continues to grow rapidly so far [2][3][4][5][6][7][8]. Various Indonesian Physical Review. ...
... Based on the description mechanisms above, either in acidic or alkaline conditions, bacterial cellulose does have functional groups -OH including also in NDC. It is also evident from the various FTIR testing by other researchers shows that pure cellulose, bacterial cellulose and the NDC have characteristics containing -OH groups [8,32]. NDC is made from coconut water+acetic acid+sugar+bacterial cellulose, coconut water has a high content of OH, acetic acid has a chemical formula CH3COOH also has a content of OH and sugar and bacteria-containing proteins that contain CHO group so it is possible to also contain OH bonds. ...
Nata de Coco (NDC) is a network of cellulose fibers that traps abundant of water. If a freshly made NDC is dried to remove nearly all trapped water, we will get a very hygroscopic material. This material is potential for making water adsorber that might compete the well known silica gel. NDC was prepared using standard methods and its water absorption was investigated. Dried NDC was used in this study. For comparison, we also investigated the water absorption of several commercial NDC. To determine its business prospects, the water absorption of silica gel was also investigated. The results showed that dried NDC could absorb water vapor comparable to the absorption of commercial silica gel. This suggests that dry NDC has the potential as an alternative water vapor absorber in food packaging. The advantages of the NDC compared to silica gel are safer and environmentally benign, and easily decomposed.
... giving a repeatability score of 0.005 and 0.01 for GO and SG materials, respectively. The specific heat for GO and SG of 0.734 and 0.921 were utilised to determine their thermal conductivity [32,33]. The rise in temperature at the rear face of the test sample is measured as a function of time and is used to mathematically determine the thermal diffusivity (α) [34]. ...
... The convergence type was evaluated using Eq. (33) ε course ε fine ...
... Such systems could save energy up to 45 % [9] because of using low-grade heat sources to drive it, including solar heat [10][11][12][13][14], co-generators [15][16][17], and waste heat [18][19][20]. Moreover, gas emissions could be diminished by approximately 38.6% [16]. ...
... Several research studies focus on silica gel-water-based adsorption cooling systems. Chua et al. [38], and Wang et al. [39] investigated the thermophysical and adsorption characteristics of silica gel/water working pairings. Miyazaki et al. [40] found that utilizing based two-bed type adsorption chillers with either silica gel and refrigerant water or CaCl2-in-silica gel and refrigerant water as the adsorbent results in a 6% increase in cooling capacity. ...
... In this study, two climate zones are selected to conduct representative studies. Hourly cooling load, ambient temperature, and relative humidity profiles from Malaysia and Saudi Arabia are adopted from other literature [41][42][43][44][45][46]. The case study for Malaysia is presented in Appendix A. The climate of Saudi Arabia is classified into tropical and subtropical desert climates. ...
... R p and E a represent the average radius of silica gel particles and the activation energy, respectively. the equilibrium uptake q* is expressed by Tóth's isotherm equation [45,46]. ...
An adsorption chiller system is one of the most promising technologies that utilize waste thermal energy to simultaneously produce cooling and potable water. However, the energy utilization optimization and detection of desiccant’s sorption capacity degradation are two unresolved issues that have severely impeded the development and commercial applications of adsorption chiller technologies. This study is pioneered to develop a digital twin platform specifically designed for an experimental four-bed two-evaporator adsorption chiller system prototype. Leveraging this platform, system monitoring, performance prediction, and optimization functions are achieved. Relying on the monitoring function, the digital twin can detect the capacity degradation of desiccant-coated heat exchangers. By employing the prediction and optimization functions, the application performance of the adsorption chiller system under varying ambient and load conditions can be simulated and optimized under real-time operating conditions. Additionally, this work projects a first-time experimental parametric study analysis for a four-bed two-evaporator adsorption chiller system prototype under a heat recovery scheme that considers fourteen operating parameters. Key results revealed that COPth reaches 0.68 when the cycle time is 2240 s. Case studies also showed that the adsorption chiller system can yield significant energy-saving performance for climatic conditions in Malaysia and Saudi Arabia. The proposed digital twin optimization method demonstrates that COPth is enhanced by 8.5 %, 9.5 %, and 8.5 %, respectively. In contrast to the conventional method, optimizing the adsorption chiller’s performance through the digital twin platform enables a reduction of the annual electricity consumption by up to 10.3 %.
... Efficiency of different sorption techniques depends upon the selection of adsorbent materials. Different materials such as silica gel, metal organic frameworks (MoFs), nanomaterials and polymer adsorbents including hydrogels [31,32] have been used as adsorbents in different atmospheric water harvesting applications but most of these materials need high temperature to desorb the captured water vapors. However, hydrogels-based adsorbents desorb the captured water vapors at much lower temperatures therefore, they do not require any additional energy supply [30,[33][34][35]. ...
... Different conventional solid desiccants such as activated carbon, silica gel, zeolites, clays and MoFs provide a wide range of desiccants which can capture large amount of water vapors with extremely high adsorption capacity but most of these desiccants involve the mechanism of physical adsorption and release captured water vapors at higher temperatures (generally higher than 160 • C), therefore, a large amount of energy is needed to release the captured water vapors which is out of the reach of solar photothermal based heating devices [32]. On the other hand, liquid desiccants such as hygroscopic salts (e.g. ...
Freshwater scarcity is one of the world's foremost environmental stress concerns. In the last few years, with sustainable industrial growth and rapidly growing population, the problem of freshwater shortage has encouraged researchers to conduct comprehensive research for the development of advanced water harvesting and wastewater treatment techniques. Natural gums-based hydrogels have been widely used in different water purification and harvesting applications because of their environment friendly nature, high water absorption, adsorption and retention capacities. In this article, we presented an entirely conceptual and critical review of literature mainly focused on the potential of different natural gums-based hydrogel in water harvesting and wastewater treatment applications. First, different categories of natural gums-based hydrogels including stimuli responsive hydrogels, physically and chemically crosslinked hydrogels, were introduced. Then, the emphasis was given on the role of natural gums-based hydrogels in different wastewater treatment applications like adsorption, photocatalysis and flocculation. After that, the latest research progress on the use of natural gums-based hydrogels in atmospheric water harvesting and seawater desalination was discussed. Finally, different challenges and main limitations associated with the use of natural gums-hydrogels in water purification and harvesting applications were discussed to understand the research gaps and drawbacks which need improvements.
... The equilibrium adsorbate concentration in the adsorbent, X eq , depends on the adsorbent-adsorbate working pair. For the silica gel-water working pair it can be obtained as [44]: ...
... The set of parameters, thermophysical properties and working conditions that were adopted for the simulations are summarized in Table 1. Some of these parameters and thermophysical properties were gathered from several studies on the silica gel-water working pair available in literature [44,[50][51][52]. ...
Adsorption heat pumps (AHPs) can play a significant role in the future energy transition policies. However, the technology still needs to be matured and further research is still necessary. In this paper, the detailed model of a whole AHP system for domestic water heating is presented aiming to fulfil the literature gap for models that can simulate the dynamics of these complete heating systems, while maintaining a high level of modeling detail for the adsorbent bed. The model integrates all the main components of the AHP system, namely the evaporator, the condenser, the heater, the water reservoir and the adsorber. The adsorber is modeled by a 2D distributed parameter model with dynamic boundary conditions since the evaporator and condenser’s temperatures vary in a cycle as well as from cycle to cycle. The novel model obtains the detailed temperature, pressure, and uptake fields in the adsorbent bed when integrated in a complete AHP system. Real scale AHP systems should not be accurately modelled by lumped-parameter models due to the heterogeneities on the temperature, pressure, and uptake in the adsorbent bed. The time evolution of the system’s variables over five simulated cycles is obtained, as well as the coefficient of performance (COP) and specific heating power (SHP) of the whole system. For working conditions suitable for domestic water heating the system’s COP is 1.35 and the SHP is 79.3 W.kg−1.s−1.
... Tashiro reported that the pore volume with a pore size of less than 2 nm could be related to the amount of adsorption at lower relative humidity (Tashiro et al., 2004). Chua found that the water vapor isotherm in silica gel with an average pore diameter of 2.2 nm could be explained by a Henry-type equation and the Toth isotherm model (Chua et al., 2002). Chung reported that the amount adsorbed per unit volume of silica gel increased with an increasing micropore zone (Chung and Chung, 1998). ...
... The prepared test material can be compared after the drying process is carried out using a modified bed dryer with a double-condenser compression refrigeration system. The silica gel used for this experiment was Fuji Davison type A. The thermophysical properties of this silica gel, as provided by Fuji Silysia Chemical Ltd., Japan, and from Chua et al. (Chua et al., 2002) (Li et al., 2007a,b), are listed in Table 1. The test sample (silica gel) is shown in Fig. 1. ...
The drying method requires an effort to store food for a longer time. Some drying processes experience technical and economic weaknesses, mainly related to low efficiency, high energy costs, and decreased product quality. Various drying models have been studied to determine the suitability of heat and mass transfer analysis at drying rates in an air dehumidification scheme using different materials, one of which is silica gel. In this case, the researchers examined the effects of humidity, temperature, and airflow rate on the constant drying rate and activation energy of water desorption in silica gel using a packed bed dryer that was modified with a refrigeration system. This modified system aims to reduce specific energy consumption (SEC). The results demonstrate that the constant rate of water desorption in silica gel and the increase in air humidity cause a decrease in the constant value of the water desorption rate in silica gel. However, increases in the temperature and airflow cause an increase in the value of the constant drying rate for water desorption in silica gel, as they cause capillary evaporation. Meanwhile, the activation energy of water desorption in silica gel increases with decreasing air flow rate and increasing inlet air humidity. The attractive force acting on the water molecules from the surface force field on the surrounding walls becomes stronger if the air flow rate decreases or the air humidity increases. From the results and analysis, it is shown that the activation energy of water desorption in silica gel with significant air humidity and low flow rate, of 0.013 kg/kg d.a. (450 lpm), is the highest at 35.16 kJ/mol, whereas in silica gel with air humidity of 0.007 kg/kg d.a. (750 lpm), it is the lowest at 22.92 kJ/mol. Meanwhile, the dryer air flow rate, higher heater temperature, and lower air humidity improve the performance of the bed dryer against its evaporation rate and decrease the Specific Energy Consumption (SEC) value. SEC is also greatly influenced by the use condenser 1, which provides heater power savings of up to 79.1%. Thus, the system is expected to be applied to commercial drying systems that can work at low drying temperatures to maintain drying products and obtain low energy consumption.
... The thermophysical properties of the adsorbent medium utilized in this simulation were acquired based on the characteristics of the Fuji Davison RD 2560 silica gel, which is a commonly employed adsorbent by commercial chiller manufacturers. Experimental studies validating the adsorption kinetics characteristics between the silica gel and the water employed in this simulation are reported in Chua et al. (2002b) and Thu et al. (2013b). Figures 4 and 5 depict the isotherms corresponding to the adsorption and desorption processes, respectively, as a result of the model simulation The initial evaluation of the adsorption kinetics between silica gel and water characterizes the material's behavior under various operating temperatures and enables assistance in determining optimal operating cycle times to maximize the difference between the amounts of adsorbed and desorbed vapor in each cycle. ...
In this study, the effect of using a hybrid solar thermal-activated adsorption desalination system for brackish water is evaluated under the climatic conditions of the Brazilian semiarid region. The proposed theoretical model utilizes climatic data from the meteorological station in Campina Grande, PB, and adsorptive kinetics data of Fuji Davison RD 260 silica gel to predict the performance indices of the specific daily water production (SDWP), specific cooling power (SCP), and coefficient of performance (COP) performance coefficients over a characteristic day. The SDWP value of 6.26 m3/ton, SCP ranging from 50 to 300 W/kg, and an average COP of 0.5 were obtained, considering variations in global horizontal irradiance in the ACDS system and transient ambient temperature. It was observed that both the production of desalinated water and the refrigeration effect increase with the rise in daily solar irradiance. The variation in the number of solar collectors used in the system and their optimality, as well as the variation in the salinity index of the feed source, impacted the evaluated performance coefficients.
... For the present study, Toth's adsorption isotherm [30]is used for the calculation of equilibrium uptake which is given below: The Darcy's law for conservation of momentum in flow through porous media is given as: ...
The objective of this study is to develop a computational model of two-stage silica gel/water adsorption cooling system. Thermal compression is achieved by adsorption, with silica gel (RD type) as the adsorbent and water as the refrigerant. Three dimensional transient heat and mass transfer analysis of adsorption heat exchangers are carried out to derive salient design and performance features. Numerical studies are carried out to evaluate the performance of the beds with respect to key operating parameters. Two different geometric models are considered for the transient study, one having vapor flow only in axial direction through the cylinder, and the other having a thin mesh inserted between the shell and heat exchanger tubes, resulting in a much shorter vapor flow path through the packed bed. Bed temperature distribution, pressure dynamics and the effect of critical depth on uptake of the adsorber heat exchanger are studied numerically. In addition, a parametric study is carried out to determine the significance of silica gel particle diameter on the uptake of the bed. The optimum particle diameter in terms of uptake was found to be 0.8mm. The effect of ambient temperature on the performance of single-stage and two-stage systems is also studied.
... The mechanism of adsorption in porous adsorbents, especially in SPHs, is a combination of different steps in sequence: (i) initial binding to the outer surface; (ii) transportation from the outer surface to inner pores, and (iii) finally diffusion into internal structure mainly via interconnected capillary channels [35]. Desiccant performance was much lower when the RH was less than 50% because, at comparatively lower pressure, the water molecules were This type of isotherm is mostly observed in porous desiccants, which use a capillary condensation process to capture the water vapors [34]. Furthermore, interconnected macroporous structures observed in the SEM images of PAA-SPHs and PAA/NaCl-5 also supported type-III adsorption isotherm. ...
Deliquescent salts are well known for their high-water vapor adsorption capacity, but they form crystalline hydrates and dissolve in adsorbed water; therefore, they cannot be used in most water vapor adsorption applications. To counter this issue, we fabricated solid polymer desiccants comprising polyacrylic acid (PAA) and sodium chloride (NaCl), which were able to capture water vapors from humidity in large quantities and avoid the dissolution of NaCl by keeping it intact inside the polymer matrix. Polymer desiccants, i.e., super-porous hydrogels (SPHs), were synthesized using gas-blowing and foaming techniques to create a porous structure. Due to dense capillary channels, the polymer matrix alone (i.e., without NaCl salt) could capture a high amount of water vapors (0.82 gw/gads). Introducing NaCl salt in the polymer matrix drastically improved desiccant performance (3.1 gw/gads). Further, the polymer matrix avoided salt dissolution in the adsorbed water and kept it intact within the polymer matrix. Adsorption isotherm was found to be type-III isotherm and best explained using GAB and FHH isotherm models, suggesting that the high desiccant performance of synthesized solid polymeric adsorbents was due to the presence of dense capillary channels in the polymer structure and the presence of NaCl salt within the polymer matrix. The adsorption kinetics followed the linear driving force (LDF) model and the case-II type diffusion mechanism. The desorption performance and kinetics of water release from fully hydrated desiccant samples after capturing water vapors were studied at different temperatures, suggesting that the water release rate depends highly on desorption temperature. Furthermore, the synthesized desiccants exhibited good cyclic performance for six adsorption cycles with a little loss in the desiccant performance.
... Over the last 200 years, many sorbent materials have been explored and implemented, 2 including silica gels, 17,18 zeolites, 7,15 ammonia-water mixtures, 3,19 and hygroscopic salts. 20 This interest was initially motivated by the development of sorption refrigeration systems, 2,21 while sorbents have recently attracted significant interest for emerging applications such as atmospheric water harvesting and sustainable cooling. ...
... On the other hand, Chua et al (2002) offered a differentiation of 1-and 2-dimensional systems for predicting the condition of skin burns. The impact of microwave heating on biological tissue thermodynamic conditions has even been investigated (El-Dabe et al 2003). ...
The public's concern over potential health risks related to the absorption of electromagnetic radiation (EMR) has been developing due to the prolonged use of cell phones close to the human body, particularly the human heart. To address these issues, this study aims to present a numerical simulation of EMR in the spectral range (900MHz, 1800MHz, 2400MHz) on the human heart tissue by Matlab program and Finite-Difference Time-Domain (FDTD) method in One Dimension (1D). A mathematical analysis of electromagnetic radiation heating equations in a one-dimensional one-layer model has also been discussed by numerically calculating the transient bioheat transfer equation and Maxwell's equations by using the FDTD to predict the effects of thermal physics properties on the transient temperature of human heart tissue. The results revealed that FDTD is an efficient method to evaluate the thermal effect of EMR due to its perfect boundary condition. It was found that the heart tissue reacts more at 900 MHz compared to 1800 MHz and 2400 MHz, and the tissue absorption is higher at the lower frequency. The effects of various parameters on the temperature increase in the human heart tissue were considered such as electric field, magnetic field, thicknesses, and thermal conductivity of heart tissue. It was found that the frequency most affecting the tissue was 900MHz. Our study find the temperature distribution decreases with the increase in tissue thickness. We note that the results are similar for the different frequencies. It turns out that as the thermal conductivity of the heart tissue increases, the temperature distribution decreases slightly, while no change occurs here the relationship thermal conductivity and the temperature distribution of tissue with the change in the frequency of a wave.
... Lower cooling power was responsible for a long time cycle. The adsorption characteristics of water vapor on two different types of silica gel were studied experimentally by Chua et al. [2]. A volumetric method was used to study temperatures ranging from 25 • C to 65 • C and equilibrium pressures ranging from 0.5 kPa to 7 kPa. ...
The design and thermo-economic studies for a silica gel/water solar adsorption cooling system are presented in this paper using Simulink to perform a numerical analysis of refrigeration systems employing a pair of silica gel and water. A case study for modelling the adsorption system with several cooling load capacities ranging from 10 to 100 Ton of Refrigeration (TR) was also presented. The simulation output results include the desorber temperature , the temperatures of hot water and chilled cooling, the temperatures of two adsorption/desorption beds, condenser and evaporator, the refrigerant and silica gel mass, area, and cost of all parts in the system, mass flow rate and thermal power. Results showed that the cost of the system and the mass of the silica gel were 20,708.4$ and 2266 kg, respectively for 10 TR cooling load, while for 100 TR cooling capacity, it was 189,169.6$ and 22,667 kg, respectively. Results also showed that when the cooling load increased from 10 TR to 100 TR, the area of the condenser, evaporator, bed, and collector increased, and the mass flow rate of the collector, condenser cooling water, chilled water, and the cooling circuit increased with a percentage increase of 9 times.
... Thus, the geographical location of this process is an important consideration because it will affect the humidity of the incoming iScience Article air, and therefore energy required to desiccate the air. The heat energy required to regenerate the waterbed (E W ) for Na-X as a function of temperature (20 to À58 C) and humidity (25%-100% RH) 39 was calculated using the heat of adsorption from the desiccant (silica gel), 40 and the previously calculated parameters from the breakthrough experiments ( Figure 7). For À19 C, À37 C, and À58 C, the sensible energy in the zeolite bed is sufficient to regenerate the desiccant. ...
Global warming threatens the entire planet, and solutions such as direct air capture (DAC) can be used to meet net-zero goals and go beyond. This study investigates using DAC in a 5-step temperature vacuum swing adsorption (TVSA) cycle with adsorbents’ Li-X and Na-X, a readily available industrial zeolite, to capture and concentrate CO2 from air in cold climates. From this study, we report that Na-X in cold conditions having the highest known CO2 adsorption capacity in air of 2.54 mmol/g. This combined with Na-X having a low CO2 heat of adsorption, and fast uptake-rate in comparison to other benchmark materials, allowed for Na-X operating in cold conditions to have the lowest reported DAC operating energy of 1.1 MWh/tonCO2. These findings from this study shows the promise of this process in cold climates of Canada, Alaska, Greenland, and Antarctica to be part of the solution to global warming.
... The sorption mass transport in the porous sorbent solid phase is widely studied using the linear driving force (LDF) kinetics model [53][54][55] and the Toth adsorption isotherm model [56,57], given by Eqs. ...
Sorption thermal energy storage (STES) systems offer great potential for meeting the growing energy needs and
increasing the use of renewable energy resources. However, the complexity and large-scale of current STES
technologies lead to relatively low-efficiency systems, severely limiting their widespread implementation. To
address this challenge, this study presents a novel and compact smart-polymer sorbent (SPS) thermal battery
with passive material-enabled mechanisms for controlled low-temperature thermal energy storage applications.
A numerical model is established to investigate the coupled heat and mass transport processes underlying the
working behavior of the SPS battery. It is also used to reveal the interdependence of the two processes and the
limiting factors for the battery’s thermal performance. The heating performance of several SPS battery designs
was studied as an example, focusing on their application for cold environment thermal protection. Moreover, the
influence of various design parameters was examined, including sorbent selection, void fraction, relative hu-
midity, gate thickness, and gate-to-sorbent area, along with their implications to the coupled mass-heat transport
processes. The results revealed that diffusion-based mass transport is the limiting process affecting the SPS
battery thermal performance. The numerical simulation results also indicated that the sorbent selection should
consider their low recharge temperature (40 ◦C–100 ◦C) and less pronounced temperature-dependent adsorption
behavior to maximize the heating performance of the SPS battery. In addition, the SPS battery heating efficiency
could be enhanced by increasing the sorbent void fraction to 0.35, relative humidity (RH) to 40 %, and gate-to-
sorbent area to 60 % while decreasing the gate thickness to 0.1 μm effectively, which facilitate the diffusion-
controlled mass transport process. Lastly, a single layer SPS battery is beneficial for thermal regulation at low
temperatures; however, using distributed battery designs had a significant thermal performance enhancement
benefit of 90 % compared to the single layer design. Hence, our study reveals the complex coupling effects of the
heat and mass transport processes within the proposed SPS battery and provides insights and guidance (e.g.,
materials selections and SPS design) for engineering emergent small-scale passive STES applications.
... The maximum adsorption capacities are required for efficient water harvesting, and a steep uptake in between 10 and 30% relative humidity (i.e., Type IV or Type V isotherm) is desirable because it allows water adsorption at low relative humidity and allows regeneration of the materials under mild conditions [37,40,41]. However, conventional desiccant materials require high temperatures (>160 • C) to harvest the maximum amount of water [42,43]. The suitable selection criteria of adsorbents for AWH are related to several factors including working capacity, sorption kinetics, cycling stability, temperature and/or pressure swing, and thermal conductivity [37]. ...
Atmospheric water harvesting (AWH) can provide clean and safe drinking water in remote areas. The present study provides a comprehensive review of adsorption-based AWH by using the scientometric approach. The publication types are mainly composed of articles and reviews, accounting for 75.37% and 11.19% of the total, respectively. Among these publications, ~95.1% were published in English and came from 154 different journals which demonstrates that researchers have shown a great interest in this field. However, much less contribution has been received thus far on this topic from Pakistan. Therefore, this study aims to explore a solar-driven adsorption-based AWH system in terms of varying relative humidity (RH), solar irradiance, and various types of adsorbent materials. Geospatial mapping and Monte Carlo simulations are carried out to integrate the operational parameters of the system and materials with Pakistan’s climatic conditions to forecast the AWH potential (L/m2/d). Probability distribution of 100,000 trials is performed by providing lower, mode, and upper values of the independent parameters. The possible outcomes of the adsorbed volume of water are determined by generating random values for the independent parameters within their specified distribution. It was found that MIL-101 (Cr) achieved the highest water-harvesting rate (WHR) of 0.64 to 3.14 (L/m2/d) across Pakistan, whereas the WHR was lowered to 0.58 to 1.59, 0.83 to 0.94, and 0.45 to 1.26 (L/m2/d) for COF-432, zeolite, and silica gel, respectively. Furthermore, parameter optimization and sensitivity analysis are performed to finalize the boundary conditions of the adsorption-based AWH system by ensuring the maximum volume values within the desired specification limits (1–4 L/m2/d).
... Adsorption isotherms of all the desiccants were studied in the humidity range of 20-90% and 25 • C (Fig. 9). All the desiccants exhibited type-III adsorption isotherms as per the IUPAC nomenclature [38,39], which suggested that all these solid polymer desiccants belong to the category of microporous desiccants having highly dense interconnected channels of pores or cavities which are mainly responsible for the adsorption of water vapors via capillary condensation process [40]. Further, P(AA-co-MAM)-SPH and all the SPHCs followed the same adsorption isotherm mechanism, which suggested that the adsorption behavior and porous structure of SPHs were not affected after incorporating zeolites. ...
This work reports the synthesis and use of zeolites based super-porous hydrogel composites (SPHCs) as solid polymer desiccants to capture water vapors from moist air. The SPHCs were composed of super-porous hydrogels (SPHs) of acrylic acid and methacrylamide (i.e. P(AA-co-MAM)-SPHs) as polymer matrix and AQSOA type zeolites, i.e. AQSOA-Z01, AQSOA-Z02 and AQSOA-Z05 zeolites as reinforcing materials. Different structural and morphological properties of SPHCs were characterized using techniques like XRD, SEM, FTIR and TGA-DTG. Desiccant properties of P(AA-co-MAM)-SPH and SPHCs were explored in terms of adsorption isotherm and kinetics. All the desiccants exhibited type-III adsorption isotherm, suggesting water vapors’ adsorption via capillary condensation mechanism and correlated well with GAB and FHH isotherm models. P(AA-co-MAM)-SPH exhibited maximum adsorption capacity of 0.72 gw/gads. However, after reinforcing different zeolites, adsorption capacity increased to 0.80, 0.84 and 0.91 gw/gads in SPHC/AQSOA-Z01, SPHC/AQSOA-Z02 and SPHC/AQSOA-Z05, respectively at 90% relative humidity and 25 °C. Further, the adsorption kinetics data correlated well with linear driving force and intraparticle diffusion models and the adsorption equilibrium was achieved much quicker at 70% relative humidity as compared to 90% relative humidity. SPHCs showed much better re-use efficiency as compared to parental SPH, therefore, reinforcement of zeolites improved the re-use efficiency of P(AA-co-MAM)-SPH.
... The results of other authors also reported poor adsorption of vapor [22][23][24]. Water vapor adsorption studies at low pressures of 500-7000 Pa of A and RD types of silica gel showed the results at the level of 0.4 and 0.45 wt.%, respectively [25]. In the literature, there are also attempts to increase the adsorption of vapor by modifying the adsorbent, e.g., modifying the CaCl 2 silica gel increases the contact surface of the sorbent with water vapor, thus allowing the maximum sorption capacity to be reached at the level of 0.75 wt.%. ...
This paper presents a study of the application of the properties of water vapor as a gas with high potential energy, strongly dependent on temperature and pressure. Analyses of water vapor sorption on two types of silica gels (SG) (90 wt.%) enriched with carbon nanotubes (CNTs) (10 wt.%), in the context of their application in the design of adsorption beds in adsorption cooling and desalination systems were conducted. The sorption experiments were performed by gravimetric method at a relative humidity of 0% < RH < 100% and temperatures of 298 K, 313 K, and 333 K. The addition of CNTs to SG caused a decrease in the sorption capacity and depended on the temperature. As the process temperature increased, a lower SG/CNT mixtures sorption capacity to vapor was obtained. The highest influence of CNTs was observed at the highest temperature, and the average decrease of sorption capacity was several percent. The ratio of SG/CNT sorption capacity to pure SG values was below 1 in most measurements.
Graphene nanoplatelets with high thermal diffusivity are being researched for their ability to improve the thermal characteristics of adsorbents. Similarly, ionic liquids with hydrophilic properties have shown promising sorption and thermal attributes. In this study, novel composite adsorbents are developed, comprising few-layered graphene nanoplatelets and specific ionic liquids (ethyl-methylimidazolium methanesulfonate (EMIMCH3SO3) and ethyl-methylimidazolium chloride (EMIMCl)), along with polyvinyl alcohol binder. The composites, known as GP-CL-30-CP and GP-CH3SO3-30-CP, each contain 30% ionic liquid content. The aim is to capitalise on the superior thermal properties of graphene nanoplatelets and the stability and solvation characteristics of ionic liquids to enhance water and cooling production in adsorption-based cooling and desalination processes, addressing challenges in the water–energy nexus. The findings revealed an improvement in the thermal diffusivity of the composites by 167%, which is 76 times higher than the baseline silica gel. There was an increase in water uptake from 0.3534 kg/kg for silica gel to 0.9648 kg/kg for the composites, representing a 174% enhancement in water sorption, and hence more freshwater water production.
An essential resource for life on the planet is water. Our biological system is severely impacted by contamination and freshwater scarcity due to the massive population increase and industrialization. Adsorption desalination (AD) is a typical solution to the global problem of freshwater scarcity. The layered structure of two-dimensional (2D) materials gives unique interactions with water vapor that can be used as an adsorbent material in AD systems. This paper presents an overview of 2D materials-based-water vapor adsorption and its application in AD systems. It also illustrates the effect of salt hydrate with the 2D vermiculite adsorbent on water vapor adsorption capacity. The effect of adding the 2D materials to metal–organic frameworks adsorbent for water vapor adsorption has been provided. A comparison of various 2D absorbent materials for desalination applications has been presented. It is found that 2D materials such as vermiculite (VMT) and mesoporous silica nanomaterial (MSN) have a high adsorption capacity of water vapor, while graphene oxide showed a substantial improvement in the adsorption properties of MOF materials. The maximum adsorption capacity reached1.94 kgH2O/kgVMT for VMT/LiBr at 23 °C and 0.94 relative pressure, while the MSN achieved 1.12 kgH2O/kgMSN at 25 °C and a relative pressure of 0.95. The results indicated the effectiveness of 2D materials and are highly recommended for improving the water productivity of AD systems.
In the current era, national and international energy strategies are increasingly focused on promoting the adoption of clean and sustainable energy sources. In this perspective, thermal energy storage (TES) is essential in developing sustainable energy systems. Researchers examined thermochemical heat storage because of its benefits over sensible and latent heat storage systems, such as higher energy density and decreased heat loss. Solar energy is a promising alternative among the numerous renewable energy sources. As a result, this study provides an overview of thermochemical heat storage materials, focusing on materials utilized by solar energy systems in buildings. The research examines the storage materials used in relevant studies and the models used to predict and enhance system performance.
As an alternative form of vapor compression air conditioning devices, solid desiccant cooling (SDC) techniques have increasingly been explored recently. The overall performances of SDC primarily rely on the capability of dehumidification and regeneration of desiccant. A desiccant with a great uptake capability and excellent regeneration potential is preferred in an SDC system. Although traditional desiccants like silica gels and zeolites are able to absorb moisture at moderate levels, hygroscopic polymers show a superior ability in moisture sorption and desorption. Significant research has been conducted to investigate the hygroscopic polymers in SDC for household and industrial applications. Here, first, an introduction to SDC systems is presented, and then hygroscopic polymers from natural and synthetic origins are discussed. Synthetic polymers discussed are metal–organic frameworks (MOFs), covalent organic frameworks (COFs), covalent triazine frameworks (CTFs), amorphous porous organic polymers (POPs), polyelectrolytes, and polymer‐based composites. Their dehumidification behaviors in SDC systems, primarily desiccant‐coated heat exchanger (DCHE) systems, are compared and summarized. Binders employed in SDC systems are also summarized, as a proper binder enhances the overall performance of the desiccant system. It can be anticipated that hygroscopic polymers and binder materials would witness extensive applications in the future.
The adverse effects of climate change, the steady depletion of fossil fuels, and the industrialization of developing countries have resulted in an increased supply and demand of renewable thermal energy. Renewable thermal energy sources like solar thermal energy produce fewer local emissions but have a temporally inconsistent power output. The consumer space heating and domestic hot water demands also vary as a function of time. This creates a mismatch between thermal energy supply and demand. Energy storage is one method of solving this problem. However, conventional methods, like hot water storage, are voluminous and can only store heat for short periods of time. Therefore, compact long-term energy storage technologies, like sorption-based energy storage systems, require research and development. The current work aims to identify and develop suitable materials for sorption-based energy storage systems and to determine the effects of operating conditions on the performance of thermal energy storage systems.
A material screening study was performed, which identified MCM-41, SAPO-34, and silica gel, which are all silica-based materials, as suitable materials for sorption-based energy storage. The effects of key operating variables for a silica gel/water-vapour adsorption-based energy storage system were quantified and optimized. The optimized system energy storage density value was nearly double that of unoptimized systems. The effects of salt impregnation were investigated by impregnating different hosts with MgSO4 salt and varying the concentration of the salt in the host material. All composites were stable after three hydration/dehydration cycle. A silica gel/MgSO4 hybrid containing 33 wt% MgSO4 was found to have the highest energy storage density of all of the MgSO4-based composites. Finally, CaCl2, a promising hygroscopic for thermal energy storage was stabilized via impregnation into silica gel and encapsulation in methylcellulose. A novel synthesis technique involving the simultaneous impregnation of silica gel with CaCl2 and encapsulation in methylcellulose produced a stable encapsulated salt-in-matrix composite with a high energy storage performance.
Hygroscopic hydrogels are emerging as scalable and low-cost sorbents for atmospheric water harvesting, dehumidification, passive cooling, and thermal energy storage. However, devices using these materials still exhibit insufficient performance, partly due to limited water vapor uptake of the hydrogels. Here we characterize the swelling dynamics of hydrogels in aqueous lithium-chloride solutions, its implications on hydrogel salt loading, as well as the resulting vapor uptake of the synthesized hydrogel-salt composites. By tuning the salt concentration of the swelling solutions and the crosslinking properties of the gels, we synthesize hygroscopic hydrogels with extremely high salt loadings which enables unprecedented water uptakes of 1.79 g/g and 3.86 g/g at a relative humidity (RH) of 30% and 70%, respectively. At 30% RH, this exceeds previously reported water uptakes of metal-organic frameworks by over 100% and of hydrogels by 15%, bringing the uptake within 93% of the fundamental limit of hygroscopic salts, while avoiding leakage problems common in salt solutions. By modeling the salt-vapor equilibria, we elucidate the maximum leakage-free RH as a function of hydrogel uptake and swelling ratio. These insights guide the design of hydrogels with exceptional hygroscopicity which enable sorption-based devices to tackle water scarcity and the global energy crisis. This article is protected by copyright. All rights reserved.
The adsorption desalination and cooling system (ADCS), a promising technology for alleviating water scarcity, requires adsorbents with high performance, low cost, and low energy consumption. Enhancing the operating efficiency and minimizing the energy consumption of the system entails improving the adsorption performance and thermal conductivity of adsorbents. In this study, nanoparticles (NP) embedded silica gel (SG) composite adsorbents with CaCl2 were synthesized for use in ADCS. The effects of nano-Fe, nano-Cu and carbon nanotubes (CNT) as SG thermal conductivity enhancers were thoroughly compared through microscopic properties, thermal properties and adsorption properties. The experimental results show that SG/CNT/CaCl2 composite performs the largest specific surface area and pore volume of 180.72 m² g⁻¹ and 0.13 cm³ g⁻¹. Scanning electron microscopy (SEM), accelerated surface area and porosimetry (ASAP) and constant thermal analyzer (TCA) were used to characterize the various performance of the enhanced adsorbents comprehensively. SG/CNT/CaCl2 sample exhibits high thermal conductivity and thermal diffusion coefficient, measuring as high as 0.664 W m⁻¹ K⁻¹ and 0.701 × 10⁻⁶ m² s⁻¹, respectively. Based on dynamic water adsorption performance tests, the SG/CNT/CaCl2 composite had an adsorption capacity of 0.665 g g⁻¹, five times higher than SG. The composite’s adsorption capacity and thermal conductivity did not decline significantly during cycling, indicating a high degree of sustainability. Therefore, these results imply that the nanoparticles-embedded SG/CaCl2 adsorbent with excellent integrated properties has promising prospects in adsorption desalination systems.
Graphical abstract
It was investigated how different embedded nanoparticles affected the thermal and adsorption performance of SG. The results showed that the composite SG/CNT/CaCl2 exhibited the best performance, with the thermal conductivity and adsorption capacity being enhanced to 0.664 W m⁻¹ K⁻¹ and 0.665 g g⁻¹, respectively.
Adsorption desalination is prescribed as a promising and eco-friendly solution for mitigating water scarcity, owing to its utilization of low-grade thermal waste and zero liquid brine discharge. The keystones that regulate the performance of the adsorption desalination system (ADS) include nature of adsorbents, system design, and operating conditions. The present study aims to provide a state of the art review on the keystones of ADS. Metal-organic frameworks (MOFs) hold remarkable adsorption capacity and tunable structure. However, hydrothermal instability, high cost, and complex synthesizing procedures are the potential challenges that need to be addressed. The technological advancements in ADS have been classified into: (i) Conventional Approach, (ii) Heat and Mass Recovery Approaches, (iii) Hybridization Approaches, (iv) and Adsorbent Substituting Approach. The study provides critical insight and compares the performance of each approach based on specific daily water production (SDWP), specific cooling power (SCP), and coefficient of performance (COP). The conventional ADS produce SDWP of 4.7 m3/ton/d, however producing zero and/or minimal SCP while using payable energy of 1.50 kWh/m3. In heat/mass recovery approaches, pressure equalization-valve delay schemes and master–slave configuration provide ∼ 5 % additional water adsorption/desorption on/from silica-gel and reduce ∼ 50 % thermal heating load, respectively. Evaporator-condenser amalgamation emphasizes the evaporator temperature of 30–42 °C leading towards ∼ 69 % higher SDWP with zero SCP. Dual stage, multi evaporators/condensers scheme is found supportive in cogenerating feature of ADS thereby improvising COP to ∼ 0.87. In hybridization approach, ejector integrated ADS produces SDWP of 80 m3/ton and COP of 2.22 using payable energy of 0.92 kWh/m3, however, needs experimental validation. In the adsorbent substituting approach, CPO-27(Ni), Emim-Ac/Syloid 72FP, and composite adsorbent manifest the SDWP to higher levels. The operating conditions are sensitive and need to optimize depending on the configuration of ADS. Possible future research directions may include efficient designing/ sizing of evaporators/ condensers, minimizing the heat and mass transfer resistances in adsorber/desorber reactor, optimize the thickness of the adsorbent layer in heat exchangers, and investigating wide range of adsorbent classes that can be driven with very low regeneration temperature.
Recently, the concern towards dye contamination in the aquatic environment has been identified to be crucial in environmental pollution. Elimination of dye contaminants has gained significant attention among researchers due to their potential deleterious risk to human health and the ecosystem. Among the treatment technologies for dye removal from the water system, the utilization of modified graphene oxide as an adsorbent has garnered increasing research interest due to its superior dye adsorption capacity. Hence, this review aims to comprehensively present the classifications and hazards of dyes, types of preparation methods and recent developments in the employment of modified graphene oxide to adsorb dyes from water. Additionally, the primary objective of this review is to emphasize on adsorption performances of modified graphene oxide for dye removal in an aqueous medium, specifically focusing on the adsorption kinetics, adsorption isotherms and the effect of experimental parameters. Further, the pertinent challenges, tremendous opportunities and the future outlook of modified graphene oxide to be employed as a potential aqueous dye adsorbent were also discussed.
Hitherto air-conditioners have severe shortcomings because of the inherent need to maintain the cooling coil temperature significantly below the air's dew point. Their energy efficiency can be improved by employing desiccant wheel dehumidifiers that offer independent humidity control. Although numerous experimental and theoretical studies have evaluated desiccant wheels’ performance, no existing investigation yields a detailed account of its localized temperature and humidity distributions. Therefore, a new three-dimensional mathematical model was developed to study the time-dependent and steady-state temperature, humidity, and reaction distributions. The model predicted the performance with the maximum discrepancy of ±10% in both temperature and humidity ratio. It also revealed that the countercurrent airflow configuration has a more uniform reaction distribution than the concurrent configuration. The parametric analysis using the model revealed that higher regeneration temperatures require smaller regeneration angles. With the regeneration temperatures ranging between 110-50°C, the optimal regeneration angles span 120-180o. Additionally, the dehumidification performance could be easily improved by increasing the wheel's rotation speed at low desiccant mass since coating more desiccant on the wheel is costly and inconvenient.
Atmospheric moisture exploitation (AME) technology has emerged as a promising alternative solution to improve the unbalanced supply-and-demand relationship between reducing essential resources (particularly energy and freshwater) and growing population. In this regard, emerging hygroscopic polymer gels (HPGs) are regarded as desirable materials for AME owing to their considerable hygroscopicity, highly tuneable structures, and easy integration with functional components. This review covers an in-depth and all-around overview of the up-to-date progress in HPGs used for AME. Firstly, the hygroscopic mechanisms of HPGs are revealed, followed by the presentation of state-of-the-art construction strategies, and the relationships between structures and properties are illustrated in detail. Furthermore, diverse cutting-edge applications based on HPGs for energy management and freshwater generation are also introduced, including fuel production, thermal dissipation, electric generation, hygrochromism, freshwater collection, and agricultural irrigation. Finally, we outline current challenges and their development trends of HPGs in AME applications in the coming future.
The CO2 sorption behavior of commercially available zeolites such as 3A, 4A, 5A, and 13X is considered at low temperatures for CO2 removal from ambient air or direct air capture (DAC). Low silica zeolites are typically not effective CO2 sorbents in the presence of water, as they preferentially competitively adsorb water from humid gas streams, resulting in high sorbent regeneration costs. We hypothesize that low silica zeolites may function as efficient physisorbents for DAC if deployed at cold temperatures where the absolute humidity of air is low. Two modes of deployment of low silica zeolites for DAC at cold temperatures are explored here. Based on the CO2 isotherms of the zeolites at -20 °C with different H2O surface loadings, zeolite 5A was selected for evaluation in a competitive H2O and CO2 coadsorption process as the first mode of deployment. Despite the low absolute humidity at -20 °C compared to that at 25 °C, H2O adsorption and accumulation result in a 39% decrease in the CO2 adsorption capacity of 5A, rendering the process energetically expensive. In the second mode of deployment, focusing on estimates of the thermal energy requirements, zeolite 13X with silica gel as a desiccant in a two-stage, two-bed process is found to provide a potentially energetically feasible process (4359 MJ/tCO2) for cold-temperature DAC. Cyclic adsorption and desorption cycles swinging between -20 and 200 °C with 0.04% and 99.9% CO2, respectively, are conducted to experimentally support the thermal energy calculations using a temperature swing adsorption (TSA) process. Water production using available cooling energy from cold ambient air offers the potential to reduce the cost of DAC, as do additional process design modes such as vacuum swing adsorption and advanced heat management systems.
Aeration is one of the most powerful tools available to the managers and handlers of stored grains. Irrespective of the initial temperature and moisture content of the grains, the biological and chemical phenomena that occur in the stored grains are significantly influenced by the wet bulb temperature of the air entering the grain. Several authors have developed mathematical expressions to estimate the rate at which aerated stored grains and other hygroscopic materials cool. They refer to ‘difficulties in comprehension’, remark that ‘results are difficult to interpret’ and ‘necessarily complicated’. The principal objective of this work is to clarify and quantify the factors that determine the rate of cooling aerated food grains. This is achieved by deriving an equation that arises as a direct consequence of the conservation of energy and moisture in aerated grains. It is found that the rate of cooling of aerated grains depends strongly on the sorption isotherm, a mathematical function that relates grain moisture content to the equilibrium relative humidity and temperature of their surrounding atmosphere. The integral heat of wetting, a function of the sorption isotherm, contributes to the effective specific heat of grains. However, its effects are typically neglected in analyses of the rate at which aerated grain cools because published sorption isotherms are insufficiently accurate for them to be included. It is proposed that a multidisciplinary approach be taken to measuring the psychrometric properties of food grains.
Abstract
A 3-bed-7-step vacuum pressure swing adsorption (VPSA) process was developed for carbon capture from dry flue gas with a silica gel adsorbent. The VPSA process is simulated based on a series of detailed models. This paper aims to solve the multi-objective optimization problem of the VPSA process. A trained artificial neural network (ANN) model with double hidden layers was established to optimize the process performance through the metaheuristic algorithm (NSGA-II, MOPSO, MOEA/D, and NSGA-III). Subsequently, the diversity and convergence performance of the metaheuristic algorithm were analyzed and compared. The optimization results show that on the one hand, the purity of CO2 could reach 80.94% with recovery of 90.61%; and on the other hand, the productivity could reach 0.5233 mol/h/kg with energy consumption of 1004.14 kJ/kgCO2 with the constraint of 70% purity and 90% recovery. The results indicate that, the ANN model can predict the performance metrics and dynamic performance of the VPSA process with very high accuracy. Meanwhile, the NSGA-II can obtain a comprehensive set of trade-off alternatives from different optimization problems, which can be used as a powerful reference for the operation of the carbon capture process.
Sorption and desorption with hygroscopic hydrogels hold significant promise for thermal management, passive cooling, thermal energy storage, and atmospheric water harvesting. However, a comprehensive understanding of the energy and mass transport mechanisms in hygroscopic hydrogels remains missing, impeding accurate modeling and optimization. In this work, we develop a model for the simultaneous vapor, water, and heat transfer in hygroscopic hydrogels during sorption and desorption processes. We show that by considering vapor diffusion in the hydrogel micropores, water diffusion in the polymer mesh, and heat transfer in the porous hydrogel, we can accurately capture experimentally observed thermally-driven desorption rates in these hydrogels. Furthermore, we consider three typical operating configurations of hydrogels and elucidate the differences in the transport mechanisms depending on the configuration. Finally, for each of these configurations, we identify key design parameters, including hydrogel thickness, hydrogel shear modulus, heat transfer coefficient, and thermal conductivity, and we parametrically show that by varying these parameters, a hygroscopic hydrogel can desorb up to 128.5%, 14.9%, 69.7%, and 9.6% more water, respectively, relative to the initial water content. This work provides a generic framework to model sorption and desorption processes in hygroscopic hydrogels which can guide the design and optimization in applications of thermal management, passive cooling, thermal energy storage, and atmospheric water harvesting with hydrogels.
This study explores enhancing adsorption system’s performance utilizing sodium polyacrylate (SP) as an adsorbent for the first time. Four innovated SP samples are explored: raw SP, SP/HCl, SP/(NH4)2CO3), and SP/CaCl2 composite for adsorption desalination and cooling applications. Different characterization methods, including X-ray diffraction, nitrogen adsorption isotherm, and water adsorption (isotherms and kinetics) of SP samples, are investigated. Water adsorption experimental results onto SP samples and their numerical fitting with the Dubinin-Astakhov equilibrium model for isotherms and linear driving force model for kinetics have been expressed. The composite SP/CaCl2 had the highest experimental adsorption uptake of 1.26 kgH2O/kg among the studied samples. At 85 °C regeneration temperature, water desalination production per day (SDWP) achieves 15 m³/ton, with a cooling power of 425 W/kg. SDWP could reach 41 m³/ton of SP/CaCl2 per day with heat recovery. The system can obtain an SDWP of 45 m³/ton per day at a regeneration temperature of 95 °C. The findings show that the system can run efficiently using renewable energy, waste heat, or geothermal energy as heat sources.
Even though an electric vehicle has the advantage of high efficiency, low emission, and more space in the engine room as the combustion engine is not used, it has the disadvantage that consumes the stored energy of the battery a lot for heating and defogging due to the absence of the waste heat from the internal combustion engine. The electric vehicle must operate a heat pump system to heat up the cabin air and to remove the moisture, unlike traditional vehicles. Thereby, the energy consumption of the automotive heat pump should be minimized as much as possible for enhancing the driving mileage to relieve range anxiety. Therefore, the desiccant coated heat exchanger is introduced for dehumidification and waste heat recovery by simultaneous heat and mass transfer. To analyze the energy consumption of the automotive heat pump system, the numerical model, which consists of the cabin thermal load model, the desiccant coated heat exchanger model, and the automotive heat pump model, is made with validation using experimental data. The simulation was conducted to investigate the effect of the desiccant coated heat exchanger on the energy usage of an electric vehicle under various operation conditions. Consequently, the results show that the proposed system requires less energy as compared with the conventional cases.
This experimental study demonstrates a thermodynamic cycle based on isothermal regeneration to enhance the exploitation of sorbents and low temperature energy sources, such as solar energy, for atmospheric water harvesting in dry climates. An experimental setup based on silica gel it has been designed to produced liquid water with low regeneration temperature for dry climates with dew point in the range of 2–8 °C and ambient temperature between 20 and 35 °C. Experimental results demonstrated a daily production of 1.5–3.3 L day⁻¹ per square meter of solar field, with a maximum regeneration temperature of 57 °C, ambient temperature up to 35 °C. The thermal energy required to activate the cycle was between 1 and 3 kWh per liter of condensed water.
Air drying experiments by pressure swing adsorption (PSA) were carried out, using two columns packed with silica gel as adsorbent. Measurements were also made of batch adsorption of water vapor on silica gel to obtain input data for the authors' PSA simulation program. Experimental PSA results were compared with this computer simulation. Good agreement was obtained and the trend of experimental results was well explained by simulation for both isothermal and non-isothermal cases. In the simulation, however, mass transfer coefficients two or three times larger than those estimated conventionally from batch measurement were used to obtain good coincidence. These larger mass transfer coefficients were determined by a recently developed method for short adsorption and desorption cycle. The simulation method is useful in predicting the performance of air drying PSA.
Mise au point comportant des definitions generales et la terminologie, la methodologie utilisee, les procedes experimentaux, les interpretations des donnees d'adsorption, les determinations de l'aire superficielle, et les donnees sur la mesoporosite et la microporosite
Theoretical and experimental studies were performed on the recovery of a low-grade waste heat using a silica gel/water adsorption-cooling system composed of four components: two adsorbers, a condenser, and an evaporator. Its cold generation capacity was 1.2 RT to produce chilled water at 4–7 °C. A numerical model was developed which can predict thermal performance of the system. The model prediction showed good agreement with experimental data. Parametric studies were performed using the model to determine the effect of the heat-transfer rate of individual component on the cold generation capacity; the heat-transfer rate of the condenser was found to be the most sensitive variable. By modifying the heat-transfer rates of the condenser and adsorber, the thermal performance could be improved by about three times. The present model can be utilized to investigate and optimize the adsorption-cooling system.
Fundamental experiments on the solar-powered adsorption cooling system were carried out with small-scale apparatus simulating ideally a practical unit by employing a combination of silica-gel and water vapor as an example of the adsorbent-adsorbate combinations. A simple model which takes into account both adsorption properties and apparatus characteristics is proposed to interpret experimental results quantitatively. Then the transitional behavior of heat and mass transfer in continuous adsorption-regeneration experiments was successfully interpreted by the model. The model proposed is not a complete one and is to considered as a first-step model for estimating operation with practical equipment.
Equilibrium adsorption and desorption isotherms of water vapor on a silica gel were measured at 288, 298, and 308 K. Isotherms were of type I according to Brunauer's classification and provided a single characteristic curve when correlated according to Polanyl's potential theory. Literature data on different samples of silica gel exhibited differences in uptake capacity from each other and from the present data. The uptake capacity at 298 K did not change significantly when regeneration temperatures were varied in the range of 373-442 K, nor when the regeneration time was increased from 4 to 16 h. The heats of adsorption suggest that silica gel has an energetically heterogeneous surface for water vapor adsorption.
A multi-bed regenerative adsorption chiller design is proposed. The concept aims to extract the most enthalpy from the low-grade waste heat before it is purged into the drain. It is also able to minimise the chilled water temperature fluctuation so that downstream temperature smoothing device may be downsized or even eliminated in applications where tighter temperature control may be required. The design also avoids a master-and-slave configuration so that materials invested are not under-utilised. Because of the nature of low-grade waste heat utilization, the performance of adsorption chillers is measured in terms of the recovery efficiency, eta instead of the conventional COP. For the same waste heat source flowrate and inlet temperature, a four-bed chiller generates 70% more cooling capacity than a typical two-bed chiller. A six-bed chiller in turn generates 40% more than that of a four-bed chiller. Since the beds can be triggered into operation sequentially during start-up, the risk of ice formation in the evaporator during start-up is greatly reduced compared with that of a two-bed chiller.
Universal thermodynamic modeling of chillers: special application to adsorption chillers
- H T Chua
Chua, H. T. Universal thermodynamic modeling of chillers: special application to adsorption chillers. Ph.D. Thesis, Depart-ment of Mechanical and Production Engineering, The National University of Singapore, Singapore, 1998.
Adsorption equilibrium data handbook Cremer, H. W.; Davis, T. Chemical Engineering Practice; Butter-worths
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Valenzuela, D. P.; Myers, D. P. Adsorption equilibrium data handbook; Prentice Hall: Englewood Cliffs, NJ, 1989. (12) Cremer, H. W.; Davis, T. Chemical Engineering Practice; Butter-worths: London, 1958; Vol. 6, pp 286-287.
Adsorption Engineering; Elsevier: Amsterdam, The Netherlands, 1990. (10) Tó, J. State equations of the solid-gas interface layers
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PTX data for silica gel/water pair, Manufacturer's Pro-prietary Data
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NACC. PTX data for silica gel/water pair, Manufacturer's Pro-prietary Data, Nishiyodo Air Conditioning Co Ltd., Tokyo, Japan, 1992.