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Performance study of SAPO-34 and FAPO-34 desiccants for desiccant coated heat exchanger systems
Abstract
A DCHE (desiccant coated heat exchanger) is a novel solid desiccant component with desiccant coated onto the surface of a fin-tube heat exchanger, in which desiccant coated aluminum-fin is an important part. It has an impact on overall performance of the whole desiccant cooling system. In this study, different zeolite-like powders (SAPO-34 and FAPO-34) coated aluminum sheets were fabricated. Their performance was tested and analyzed to investigate the feasibility in a DCHE system. Experimental values of surface area and pore parameters for desiccant coatings had a slight reduction because of the existence of binder solution. Adsorption kinetics indicated that the binder solution would not affect water adsorption capacity or adsorption kinetics of original desiccant powder. Moreover, SAPO-34 and FAPO-34 coated aluminum sheets had higher adsorption capacity than that of silica gel. Sorption isotherms were also measured and fitting equations were developed based on Polanyi principle. Finally, their dehumidification performance in DCHE systems was predicted with a mathematical model. Simulation results indicated that the DCHE prepared from FAPO-34 can have 2–3 times larger dehumidification capacity than those from SAPO-34 and silica gel at low regeneration temperatures.
... These devices follow the assumption that reversible adsorption (desorption) is an exothermic (endothermic) process. 20 In recent years, AHP and ACS made with porous materials such as activated carbons, 14,[21][22][23][24][25][26][27][28][29] zeolites, [29][30][31][32][33][34][35][36][37] and metalorganic frameworks (MOFs) 29,[38][39][40][41][42][43] have shown promising performance and benefits in cost and versatility. Pal et al. 44 studied the production of highly porous carbons from vegetable waste for heat pump applications. ...
The building climate industry and its influence on energy consumption have consequences on the environment due to the emission of greenhouse gasses. Improving the efficiency of this sector is essential...
... These devices follow the assumption that reversible adsorption (desorption) is an exothermic (endothermic) process. [20] In recent years, AHP and ACS made with porous materials such as activated carbons, [14,[21][22][23][24][25][26][27][28][29] zeolites, [29][30][31][32][33][34][35][36][37] and metal-organic frameworks (MOFs) [29,[38][39][40][41][42][43] have shown promising performance and benefits in cost and versatility. Pal et al. [44] studied the production of highly porous carbons from vegetable waste for heat pump applications. ...
The building climate industry and its influence on energy consumption have consequences on the environment due to the emission of greenhouse gasses. Improving the efficiency of this sector is essential to reduce the effect on climate change. In recent years, the interest in porous materials in applications such as heat pumps has increased for their promising potential. To assess the performance of adsorption heat pumps and cooling systems, here we discuss a multistep approach based on the processing of adsorption data combined with a thermodynamic model. The process provides properties of interest, such as the coefficient of performance, the working capacity, the specific heat or cooling effect, or the released heat upon adsorption and desorption cycles, and it also has the advantage of identifying the optimal conditions for each adsorbent-fluid pair. To test this method, we select several metal-organic frameworks that differ in topology, chemical composition, and pore size, which we validate with available experiments. Adsorption equilibrium curves were calculated using molecular simulations to describe the adsorption mechanisms of methanol and ethanol as working fluids in the selected adsorbents. Then, using a thermodynamic model we calculate the energetic properties combined with iterative algorithms that simultaneously vary all the required working conditions. We discuss the strong influence of operating temperatures on the performance of heat pump devices. Our findings point to the highly hydrophobic metal azolate framework MAF-6 as a very good candidate for heating and cooling applications for its high working capacity and excellent energy efficiency.
... Researchers are attentive to solid desiccants such as silica gel (Type 3A & Type RD) (Zhang et al., 2014), silica gel (Venegas et al., 2021;Wu et al., 2018), functional adsorbent material Zeolite01(FAM-Z01), molecular sieve (Al-Alili et al., 2015;A. Zendehboudi et al., 2018) AlPO-18 Zeolite, FAPO-34 Zeolite and SAPO-34 Zeolite (Zheng et al., 2015), Aluminum fumarate (Kummer et al., 2017), and polymer/alumina composite desiccant (Chen et al., 2016) for construction of desiccant wheel due to their merits. Fong & Lee (2018) compared the performances of the desiccant wheels made of silica gel (SG), AQSOA-Z02, and CECA-3A using modeling. ...
This paper investigates the dehumidification performance of desiccant wheel (DW) with different fiber paper (FP) substrates such as wood pulp, glass fiber, ceramic fiber, Nomex fiber, and brown wood pulp FP, coated with nano SiO2 as adsorbent for energy efficient air dehumidification. The FP substrates are coated with SiO2 nano-adsorbent by impregnation-dip coating method to the thickness of 5 mils. The adsorption capacity for all the FP substrates was evaluated at temperature of 28°C and 70% relative humidity. It was found that the adsorption capacity increases with increasing the desiccant coatings. The adsorption capacity of GFP was high with a value of 57.6 g•m⁻² and thermal conductivity was improved from 0.038 W•m⁻¹•K⁻¹ to 0.094 W•m⁻¹•K⁻¹ with desorption temperature of 45°C after coating with SiO2 nano-adsorbent. Among varying regeneration temperature from 50‒90°C, the desiccant wheel GFP-SiO2-DW has high dehumidification capacity and dehumidification coefficient of performance (DCOP) of 2.1 g•kg-1 and 2.2 respectively at 50°C. The results confirm that less energy is necessary for the regeneration heater which can be supplied from a low-grade energy source thus reducing the energy consumption for regeneration of the desiccant wheel.
... On the other hand, in a consolidated bed the adsorbent material is in direct contact with the metal substrate due either to direct synthesis of the adsorbent material on the substrate [5][6][7] or to the presence of a binder [8,9]. The zeolite-binder coating approach is a capable and affordable technology to apply, in an economical way, an adsorbent layer on the heat exchanger module [10][11][12]. This allows a higher heat transfer at the coating/metal interface, which reduces cycle time [8,13], but a lower mass transfer due to the presence of the binder and the low porosity of the coating. ...
In this work, a composite adsorbent coating constituted by high SAPO 34 content and a sulfonated recycled poly (ether ether ketone) was investigated for adsorption heat pump technology. Specifically, the effect of polymer recycling on mechanical and thermal properties, as well as on water vapor adsorption and desorption performance, has been investigated. The degree of sulfonation obtained after 48 h of reaction remained approximately unaltered. The degradation of the polymer due to recycling anticipates the degradation of the C-C bonds of the polymer by about 20 °C without affecting the temperature at which the sulfonic groups degrade. From the mechanical point of view, the coating containing 90% zeolite, due to the use of recycled PEEK, evidenced a worsening of only 11.8% in scratch resistance compared to the virgin one, whereas the adhesive strength exhibited an increase of about 23.2% due to better miscibility of the sulfonated recycled polymer. Adsorption/desorption isobars show an almost similar adsorption capacity of the coating produced with recycled polymer compared to the virgin one, confirming that the water vapor diffusion is not hindered by the polymer matrix during the adsorption/desorption process.
... The framework topology of SAPO-34 is comprised of cylinder-like cages (6.7 × 6.7 × 10.0 Å) with 8-ring pore mouths (3.8 × 3.8 Å), and the small pore mouths only allow linear olefins and small molecules to diffuse through these. SAPO-34 is widely used in catalysis, especially in the catalytic reduction of NOx [3], methanol-to-olefins (MTO) conversion [4,5], and CO 2 hydrogenation for light olefin production [6,7], as well as in adsorption applications, such as sorbent in adsorption heat pumps [8,9]. The last application is an alternative to conventional electrically driven systems, such as air conditioning, solar air conditioning, waste heat recovery, and domestic heat pumping. ...
SAPO-34 nanocrystals with sizes of 50–150 nm were obtained via steam-assisted crystallization (SAC) for 5 h at 200 °C from two types of aluminum precursors—aluminum isopropoxide and boehmite. A reaction mixture composition with a small amount of organic template tetraehylammonium hydroxide (TEAOH) was used with the molar ratio TEAOH/Al2O3 = 1/1. The alumina precursor type and duration of the SAC (5 and 24 h) on the crystal size, texture, and acid properties were investigated. The SAPO-34 nanocrystals that we obtained possess a large micropore volume of 0.22–0.24 cm3/g and a specific surface area of 651–695 m2/g. When the crystallization was prolonged for up to 24 h, a SAPO-18 structure appeared, but the micropore and mesopore volumes changed insignificantly. Using boehmite as the aluminum precursor led to higher mesoporosity of the material but a little bit lower acidity when compared with the samples prepared from aluminum isopropoxide. In addition, the method proposed was used for preparing a SAPO-34-coated aluminum adsorber heat exchanger. Thus, the synthesis method proposed is affordable and effective to prepare SAPO-34 highly crystalline nanoparticles, with no need for post-synthetic procedures as the mother liquor separation from nanocrystals.
... Although AlPO-34, SAPO-34, and FAPO-34 have the same framework structure, the SAPO-34 capacity increases more at the low relative humidity range of up to 8 RH%. FAPO-34 ( Table 2, Entry 10), on the other side, acts more efficiently with respect to water uptake in the range of 20-30 % [112,113]. ...
Freshwater resources are being heavily depleted and not replenished at the same high rate, thus, atmospheric water vapor harvesting has earned growing interest. Development of high-performing desiccant or adsorbent materials offering high sorption capacity and selectivity as well as regeneration capability in atmospheric conditions is crucial to tackle water scarcity. The required properties to generate potent water sorbents include pore accessibility, high specific surface area and porosity to enable high capture capacity and kinetics, and hydrothermal resilience to resist cyclic sorption and desorption. Further, polarity, hydrogen bonding, adhesion ability of water molecules to adsorbent surface, and hydrophilic functional groups can boost water adsorption. A highly promising class of water vapor adsorbents is the aluminophosphate molecular sieves (AlPOs), which are microporous zeotype materials. In this review, AlPO-based adsorbents are discussed for water sorption applications and a link is established between performance of materials and their chemical and morphological properties. Synthesis-properties-performance relationships are elucidated in light of synthesis techniques and adsorption behavior, and prospects to enhance AlPOs' water vapor sorption performance toward large-scale water harvesting applications are highlighted.
... Due to the low thermal conductivity of the desiccant on the surface of the DCHE, the desorption efficiency is affected. Many scholars have studied the influence of factors such as desiccant [6], regeneration temperature [7] and size [8] on the performance of DCHE, for improving DCHE performance. However, due to the high cost and time-consuming experiment of dehumidification heat exchangers, various theoretical models of dehumidification heat exchangers have been proposed. ...
For the novel frost-free air source heat pump system (FASHP), the compressor waste heat is used to assist desorption for the desiccant on the surface of the desiccant coated heat exchange (DCHE). It can promote the desorption efficiency of the DCHE in the FASHP and reduce the energy loss, which means the improvement of the stability and performance. In this paper, a DCHE model is established, and its accuracy is verified through experimental data. The model is then used to analyze the effect of working conditions and structural parameters on the adsorption rate, desorption rate, heat exchange and proportion of latent heat in the adsorption and desorption processes. The results show that the latent heat ratio is higher, the heat exchange of the DCHE is lower and the system performance is better when the temperature of the analytical circulating air increases and the velocity decreases to 25 °C/0.5 m/s. In addition, the performance is greatly affected by the fin length and width. With the increase of parameters, the heat exchange and proportion of latent heat can be increased to 115.9 J and 69.0%.
... Despite the advantages of air-cooled CF-DCHEs, the cooling effect provided by the cooling air in the adsorption process is still little. This effect varies according to the cooling airflow velocity [15], process airflow velocity [22], cycle time [23], and several types of desiccant material [24][25][26][27]. Therefore, in this study, we investigate the effect of cooling air and examine its impact on the dehumidification performance. ...
In this study, a crossflow-type heat exchanger coated with aluminophosphate zeolite was tested for desiccant dehumidification with direct-supplied hot water and evaporative cooling using remaining hot water. This evaporative cooling process could accelerate dehumidification by removing the heat of adsorption. Therefore, the influences of the cooling air velocity, regeneration hot water temperature, and adsorption/regeneration cycle time were experimentally investigated. The result showed that the minimum humidity of the adsorption outlet air decreased with an increase in the cooling air velocity and adsorption/desorption time, indicating that the complete regeneration of coated adsorbents is required to significantly decrease humidity. However, the maximum time-averaged amount of dehumidification appeared at an appropriate cycle time, which avoided adsorption saturation during the adsorption period. Further, in any case, increasing the cooling air velocity had a positive impact on both the momentary observed minimum humidity and time-averaged amount of dehumidification.
At all cooling air velocity, the temperature of the hot water contributed to the increase in the time-averaged amount of dehumidification. However, when the hot water temperature was 55 °C, the cooling effect of evaporative was not observed, even when the cooling air velocity reached 1.5 m/s. Considering that the actual temperature of the coated adsorbent was expected to be much lower than that of the supplied hot water, the hot water temperature should have been ≥65 °C.
... The results indicated that highest reduction of water uptake was up to 43% when selecting silica gel type 3A mixed with 3.3 wt% of hydroxyethyl cellulose, yet the combination of silica gel RD and hydroxyethyl cellulose showed almost no reduction in water uptake. Similar experimental conclusion also was proved in different binder-sorbent pairs [86,87]. A selection criterion of binder was summarized in Ref. [44]. ...
Humidity always accompanies human beings’ life and industry production. A high humidity environment not only induces health hazards easily but also aggravates energy consumption. Desiccant-based internally-cooled dehumidification using porous sorbents represented by desiccant coated heat exchanger (DCHE) is regarded as the high efficient humidity regulation, evoking the overwhelming attention of wider scientific and industrial communities, and booming at different fields in air–water-energy nexus. Although extensive efforts on this manner have been promoted, the depth understanding of heat and mass transfer remains vital. In this review, a comprehensive, clear and full-scale route of DCHE is presented from the thermodynamical viewpoint of modeling. Firstly, the basic principle and heat/mass transfer characteristics of DCHE are restated. Subsequently, model insights of different scales are established including nano-pore, meso-scope coating, macroscopic device and system scale, and the advances in the mathematical methods of DCHE are categorized into thermodynamic, resistance network and discrete element methods. The major derivations, model accuracy and characteristics are detailed, and the diffusivities, adsorption equilibrium equation and heat/mass transfer coefficients are summarized. Furthermore, some attempts for design guidance are reviewed and differences of model on system integration scale are discussed. This review would provide a comprehensive understanding of internally-cooled dehumidification, and pave a pathway for further investigation, design and optimization, as well as DCHE-inspired applications like humidity pump, water harvesting, thermal management, etc.
... The higher the COP and SCP are, the better the adsorbent performance. Most of times, and specially for analogous systems, SAPO-34 shows enhanced performance and that is the ground for its popularity for such systems [206][207][208][209][210][211][212][213][214][215][216]. ...
Silicoaluminophosphate-34 (SAPO-34) is a molecular sieve with a unique shape structure and pore framework, favorable acidic properties, and excellent stability under different operating conditions. These unique properties have made SAPO-34 an important material with widespread applications as a catalyst, membrane, and adsorbent in different processes. As a catalyst, SAPO-34 is utilized in converting methanol to olefins (MTO) with outstanding catalytic performance, excellent selectivity, and stability. The only drawback of this molecular sieve is its rapid deactivation in the reaction environment, which hinders its application at large scales. To overcome this drawback, reducing the particle size and tailoring the hierarchal structure have been proposed as two key strategies. As a membrane, SAPO-34 can be exploited in severe conditions (i.e., high temperature and pressure), for example, to separate hydrogen, methane, and carbon dioxide in a membrane reactor, where polymeric membranes cannot be used. In this case, the membrane fabrication method plays a vital role in determining the final performance. With a focus on these applications, this review discusses the preparation of SAPO-34 and its applications as a catalyst in the MTO reaction and as a membrane in gas separation. Key strategies to improve its properties and performance are elaborated. Lastly, complementary applications of this molecular sieve are briefly discussed. This review may provide helpful insights for the development of high-performance SAPO-34-based materials to be applied in different industries.
Graphical abstract
... Ge et al. [32] conducted an experimental comparison between a silicone-coated finned tube heat exchanger and a polymer-coated finned tube heat exchanger, and the results showed that the effective dehumidification time of silicone-coated DCHE was longer, and the average dehumidification capacity was stronger. However, it was reported that when the partial pressure of water vapor is low, the adsorption capacity of silica gel decreases rapidly with the increase of temperature [33,34]. ...
Air conditioning energy consumption accounts for most building energy consumption, indoor dehumidification is the main cause of air conditioning energy consumption. Optimize the dehumidification methods of air conditioning systems have great significance to the development of green buildings and people’s pursuit of comfort. Improvement of fins on air conditioning heat exchangers is a hot topic of current research and has achieved considerable results in terms of indoor dehumidification and energy saving compared to traditional air conditioners. This paper reviews two kinds of heat exchangers modified by coating, including desiccant-coated heat exchangers and hydrophobic/hydrophilic coated heat exchangers. For desiccant-coated heat exchangers, the preparation methods of advanced desiccant materials and the possibilities of using this material to achieve excellent energy efficiencies were presented, and the operating parameters that affect thermal performance and dehumidification are determined, including airflow temperature, air velocity, inlet air relative humidity, and regeneration temperature. For hydrophobic/hydrophilic coated heat exchangers, different kinds of hybrid hydrophobic-hydrophilic surfaces are highlighted for they are a high water droplet nucleation rate and surface heat transfer efficiency. In addition, the challenges and future works are explained at last. This paper will provide a valuable reference for the follow-up research, which will be helpful for indoor humidity control and reducing the energy consumption of air conditioning.
... Among all the adsorbate-adsorption pairs, water vapor and the silico-alumino-phosphate (SAPO)-34 has been considered to be one of the more attractive options considering the easyaccess and environmentally friendly nature of the adsorbate, and the thermal-stability and the high adsorption capacity of the adsorbent material [24]. Although the synthesis methods, pore architecture modification, water vapor adsorption isotherms, coating methods and the application in thermal heat pump for SAPO-34 material have been well studied in the literature [21,[24][25][26][27][28][29][30], the performance of the adsorbent in the packed bed adsorption TES system has not been evaluated from the breakthrough experiments in response to different operation conditions. In this paper, the SAPO-34 powder-form material was pelletized and filled into cylindrical packed beds constructed with different materials for breakthrough experiments using two different cooling methods in order to evaluate how the inlet relative humidity (RH) and regeneration temperature affect the system's energy densities. ...
The water vapor and the silico-alumino-phosphate (SAPO-34) material has been recognized to be one of the better adsorbate-adsorbent pairs for the packed-bed adsorptive thermal energy storage (TES) systems for space heating applications. In this paper, operating conditions including the system construction materials selection, cooling methods of the system after regeneration, relative humidity (RH) of inlet air, and packed-bed regeneration temperatures have been examined for the TES system performance evaluation and process design optimization purposes.
... Coating with binder is quite convenient, and the binder does not affect the adsorption capacity or adsorption kinetics of original adsorbent [151]. But the surface area and the porous parameter would be slightly reduced [154]. Six different solutions with various contents of binders were used to see the impact of binders on the adsorbent adsorption capacity [155]. ...
Owing to its high porosity and high surface area, zeolite has a good adsorption effect on liquid adsorbates such as water, ammonia, organic liquids and gas adsorbates such as volatile organic compounds, hydrocarbons, etc. A large amount of heat is released during the adsorption period when the zeolite adsorbs these gases and liquids. The energy storage, the heat and mass transfer performance of zeolite adsorption is influenced by the selection of adsorbent and adsorbate as well as the design of zeolite bed. In this paper, the mechanism of zeolite adsorption is discussed, and equations that describe the adsorption isotherm and the heat and mass transfer of adsorbate on zeolite are reviewed. From the view of system aspect, the factors (such as the structural design of zeolite bed, the mass flow rate of adsorbate, the operating temperature, and the cycle time) that will affect the heat storage and transfer in adsorption are concluded. In terms of adsorbent/adsorbate, the properties of modified zeolites and zeolite composites are presented, and the reaction behavior of different zeolites and adsorbates is discussed. As for the application of zeolite adsorption system in the energy storage and heat transfer field, zeolite-based heat exchanger (HX), energy storage system (ESS), dehumidifier, energy absorption system (EAS), volatile organic compounds (VOCs) removal system and hydrocarbon (HC) trap are reported. Research results demonstrate that the zeolite adsorption system is promising no matter used alone or in combination with other equipment in energy-saving and environmentally friendly fields.
... Possible adsorbent classes are zeolites, silica gels, activated carbons, metal organic frameworks (MOFs), and composite adsorbents made of a sorbent matrix embedding a hygroscopic salt [6]. Among them, silico-aluminophosphate zeotype materials (SAPO, ALPO) are very attractive due to the low regeneration temperature (<100 • C) and optimal shape of the water adsorption equilibrium curve [7][8][9]. In closed systems research on components optimization is mainly focused on the integration between heat exchanger and adsorbent material, to create the so called adsorber [10]. ...
In this work, novel silicone-SAPO34 composite materials are proposed for application in adsorption thermal energy storage systems. The innovative composite materials were obtained through a mold foaming process activated by a dehydrogenative coupling reaction between properly selected siloxane compounds. Morphology analysis by optical microscopy and measurement of the mechanical properties of the foamed materials at varying zeolite content demonstrated a quite homogeneous open-cell structure and good structural stability of the foam. Water adsorption isotherms of the adsorbent foams expanded in free space and inside paperboard were measured by a gravimetric adsorption apparatus, demonstrating that the presence of the polymeric fraction does not affect the adsorption capacity of the SAPO34 fraction added in the composite foam. Finally, main adsorption and thermodynamic properties of the proposed foam have been compared with those of other adsorbent materials, confirming the possible use of these new composite foams as adsorbent materials for adsorption thermal energy storage systems.
A compact hybrid atmospheric water harvesting device for all-day water production by synergistically harvesting energy from natural sunlight and the cold universe.
Water adsorption capacities of various adsorbents reported in the literature were investigated to define a hydrophobicity index that was plotted vs. water capacity. In this plot, logarithmic curves were proposed to be used as indicators of performance limits of adsorbents, especially for adsorption heat pumps. In spite of their useful adsorption properties, zeolites generally exhibited quite low hydrophobicity, remaining well below the logarithmic curve. In this study, the use of composites of zeolite NaY was examined both theoretically and experimentally for improvements in the water capacity and hydrophobicity. Salt impregnation and hydrothermal synthesis experiments were performed to prepare composites of zeolite NaY with LiCl/MgCl2 salts and activated carbon, respectively. Water capacity and hydrophobicity of zeolite NaY composites were generally superior to those of pure zeolite. Zeolite composites may be advantageous for enhancing adsorption capacity and hydrophobicity of zeolites while eliminating low stability and slow adsorption kinetics of other adsorbents. Interface between two different phases might indicate another opportunity to provide improved adsorption properties for zeolite composites.
Atmospheric water generators produce liquid water from humidity in the air. Hence, this technology provides a pathway to alleviate water scarcity. In contrast to conventional day-night monocyclic systems, multicyclic atmospheric water generators conduct multiple sorption and desorption cycles per day. The specific water production for multicyclic desiccant based atmospheric water generators primarily depends on the water sorption and desorption rates of the desiccant, as opposed to the uptake capacity. The mechanisms governing the equilibrium uptake capacity of desiccants and the interparticle diffusion rate of water vapour are well known, however, the mechanisms governing the intraparticle diffusion and sorption rate of water vapour within desiccants are not well summarised. In this review, methods for the enhancement of the intraparticle water vapour diffusion and macroscopic sorption rate are identified, including the effects of pore microstructure, surface hydrophilicity, and composites. Additionally, desiccants with the highest potential specific water production and lowest potential specific energy consumption are identified. To date, the polyamide 6-LiCl nanofibrous membrane demonstrates the highest ideal specific water production of 230 L/kg/day. The ideal specific energy consumption is similar between the investigated desiccants and primarily depends on the latent heat of sorption. Furthermore, the suitability of various empirical kinetic models for the investigated desiccants is discussed. The variable order model provides a better fit to sorption and desorption kinetic data than the commonly used linear driving force model.
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 enormous energy consumption of space cooling exacerbates the global energy crisis and the environmental governance burden, in which the latent cooling load for dehumidification wastes a notable percentage (∼ 40%) of the electricity available for cooling. But encouragingly, moisture-absorbing metal-organic frameworks (MOFs) coated heat exchangers in air conditioners can decouple and remove latent loads from sensible loads through adsorptive dehumidification. Given the favorable outlook, the present work performs a detailed analysis of the water adsorption of MOFs and emphasizes their optimization techniques to meet the requirements of dehumidifying coating applications. The promising manufacturing techniques of MOF-coated heat exchangers (MCHEs) were reviewed, and a preliminary framework for the selection of MOFs and coating methods was also constructed. After that, the advantageous dehumidification and latent load handling performance of MCHEs and their performance variations under different factors were summarized to guide the optimized applications. Finally, the energy-efficient dehumidification and cooling effects of MCHE-based air conditioners were demonstrated, and strategies for breaking through the limitations and further development were proposed. This timely review motivates readers to design MOFs for dehumidifying coatings and thus develop MCHE-based air conditioners to tackle the environmental pressures stemming from escalating global energy consumption.
Air-conditioning system applies dehumidification by condensation technique, where the cooling coil handles both latent and sensible load. Energy consumption by the system increases by handling both loads. In order to reduce operating energy and load on cooling coil, latent load needs to be reduced or eliminated. Latent load can be handled separately using desiccant-type dehumidifiers irrespective of cooling coil temperature. This paper investigates the dehumidification performance of a desiccant wheels (DWs) made of fiber paper substrates such as Ceramic Fiber, Glass Fiber, Wood Pulp Fiber, Nomex Fiber, and Brown Wood Pulp Paper (BWPP) coated with synthesized mesoporous nano-titania desiccant for energy-efficient air-conditioning. Moisture adsorption ability test was conducted for different fiber papers at 30 °C and 75% RH, where BWPP has maximum moisture adsorption of 38 g m−2 and 0.42 g g−1 due to its hydrophilic nature, with the highest coating ratio of 3.5 g cm−3. The thermal conductivity, nitrogen adsorption/desorption isotherm, pore structure, and thermal stability of the fiber substrates were also analyzed before and after coating. The thermal conductivity of the fiber papers coated with nano-titania desiccant has enhanced compared to the raw fiber papers, and it reaches 0.116 W m−1 K−1 for BWPP. The dehumidification coefficient of performance of BWPP–DW at a 1:1 airflow area ratio, 60 °C regeneration temperature, and 10 rph is 0.82. The BWPP matrix DWs can be reactivated by low-grade energy sources with 0.82 kg kW−1 h−1 and saves energy up to 2.39 times.
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.
High-performance desiccant-coated heat exchangers (DCHEs) can be employed in air-conditioning systems to improve energy efficiency. While there are significant developments in experimental studies, theoretical approaches for analyzing the DCHE are impeded by the complex and coupled heat and mass transport mechanisms. Therefore, a 3D mathematical model has been developed in this work to study the dehumidification performance of a DCHE. Unlike other works reported in the literature, the developed model is able to simulate the complex coupling between heat and mass transfers in the entire volume of the heat exchanger. The model is calibrated and validated with experimental data, and a maximum discrepancy of ±8 % is recorded. A time-dependent study on the performance of the DCHE is carried out to understand the distribution of temperature, reaction rates, and water uptake in the heat exchanger. Effects of key influencing operational factors are investigated and discussed. It is indicated that the DCHE enters a stable performance state regardless of its initial adsorption state. The regeneration time should be equal to the dehumidification time to optimize the performance. A higher thermal COP is achieved under lower cooling/regeneration temperature, lower air flowrate, and shorter half-cycle time, while higher inlet air humidity ratio. Higher coated desiccant mass also provides a higher thermal COP, but this enhancement levels off once the coating grows. For DCHEs with less coating, reducing cycle time is recommended over increasing the coating amount.
The composite adsorption material ACF-silica sol-LiCl (ASL) is prepared by impregnating activated carbon fiber (ACF) with silica sol and LiCl to improve the water vapor adsorption performance. The silica sol provides support to the soft ACF and solidifies it. The silica sol also increases the adhesion area in the ACF for the LiCl. The LiCl can effectively improve the water vapor adsorption performance of the ACF. The adsorption performance of the ASL is optimum when the LiCl impregnation concentration is 430 mg/L. Scanning electron microscope images show that the silica sol and LiCl fill the fibers pores and spaces between the fibers in the ACF after impregnation. And adsorption kinetics of the ASL is characterized using a linear driving force (LDF) model and experiments. Results show that ASL has a high dynamic water absorption rate and adsorption rate coefficient (k). The k of the ASL reaches 1.58 × 10⁻⁴−2.05 × 10⁻⁴, which is 0.27 × 10⁻⁴−0.59 × 10⁻⁴ higher than that of the ACF. Meanwhile, the results of water vapor isothermal adsorption tests show that the impregnation of silica sol and LiCl increases the hydrophilicity of the ACF, and thus greatly enhances its water vapor adsorption capacity. The adsorption capacity of the ACF is almost 0 at P/P0 < 0.2 and reaches a maximum of 0.5479 g/g at P/P0 = 0.65. The adsorption capacity of the ASL steadily increases with the relative pressure. When the relative pressure is close to 1, the adsorption capacity of the ASL reaches 2.2876 g/g, which is 2.67 times the maximum for the ACF.
This study presents an innovative de-coupling cooling technology where latent and sensible cooling loads are handled separately by a desiccant-coated heat exchanger (DCHE)-based dehumidifier and a dew-point evaporative cooler (DEC). The DCHE first removes the moisture of humid outdoor air by adsorption process. Subsequently, the DEC sensibly cools down the dehumidified air, maintaining the humidity ratio. Their performances are investigated numerically by analyzing the heat and mass transfer. The cyclic average outlet values of DCHE are applied to the inlet conditions for DEC simulation. Comparison between the simulation results and the experimental data was carried out and showed good agreement and a similar trend with a maximum discrepancy of 8.6% for DCHE and 3.2% for DEC, respectively. Key results revealed that moisture removal capacity (MRC) and latent cooling capacity (QL) for DCHE are largely affected by varying air dry-bulb and air wet-bulb temperatures, while the almost constant COPth was observed regardless of the variation of temperatures. For the DEC, the higher dew-point effectiveness and wet-bulb effectiveness were observed at the higher dry-bulb temperature and higher humidity ratio, while the higher sensible cooling capacity was observed at the higher dry-bulb temperature and lower humidity ratio.
A two-stage evaporative cooling system with composite activated carbon fiber dehumidification is proposed. After the first evaporative cooling, the high humidity air is dehumidified in the internally cooled desiccant bed, then sent to the second evaporative cooling unit for further cooling. The evaporative cooling unit adopts the fiber wet curtain as the cooling medium. In the internally cooled desiccant bed, composite activated carbon fiber (ASL) is filled, and copper tubes are arranged for cooling/heating. ASL is prepared of activated carbon fiber (ACF) impregnated with silica sol and LiCl, with an equilibrium dehumidification capacity can increase 3.1 times higher than ACF. The average dehumidification capacity of the desiccant bed reaches 5.59 g/g, and the relative humidity of the outlet air decreased to below 50% in 46 min. In the two-stage evaporative cooling system performance experiments, the supply air temperature is 3.1–4.8 °C lower than the single-stage, and the humidity ratio is 0.42–2.66 g/kg lower. The cooling capacity Qc,e, cooling efficiency ղTwo-stage, and coefficient of performance (COP) increases with increasing inlet air temperature and decreasing inlet air relative humidity, air velocity, and cooling water temperature. Qc,e, ղTwo-stage, and COP of the two-stage evaporative cooling system can reach up to 3757.5 W, 1.78, and 10.8, 808.8 W, 0.64, and 2.33 larger than these in the single-stage.
An activated carbon/silica gel/lithium chloride composite desiccant (AC-SL) for air dehumidification is proposed. Lithium chloride and silica gel are impregnated into activated carbon pores to improve the density, thermal conductivity and water vapor adsorption capacity of AC-SL. Nitrogen isothermal adsorption lines, SEM images and Fourier Transform Infrared showed that impregnation significantly reduced AC-SL is specific surface area and pore parameters and increased the content and types of oxygen-containing functional groups on its surface. The sorption kinetics study shows that the equilibrium adsorption capacity of AC-SL is 6.8 times that of activated carbon. The Dubinine-Astakhov equation based on Polanyi potential theory is used for nonlinear fitting of the water vapor isotherms adsorption, and the correlation coefficients of the fitting equations are all above 0.98. When the relative humidity is 60%, the water vapor adsorption capacity of AC-SL reached 0.81 kg kg⁻¹, 4.5 times that of pure activated carbon. The simulation results show that the AC-SL coated heat exchanger's dehumidification capacity is 3.13 kg h⁻¹, which has a higher cost performance than other materials.
Adsorption and desorption properties of desiccants are of key significances to the dehumidification performance of a compact dehumidification component named desiccant coated heat exchanger. This work developed novel activated carbon (AC) - sodium polyacrylate (PAAS) composite coated aluminum sheets with high adsorption property and good desorption ability. Adsorption and desorption properties of AC-PAAS coated aluminum sheets were investigated under different air conditions. Cyclic adsorption-desorption features with internal-cooling/heating were conducted. Results showed that composite coated aluminum sheets had nearly 2–3 times higher water uptake than an AC coated sample in the range of medium-to high-relative humidity (60%–90%RH). More than 95% of adsorbed water in composite samples was released at 70 °C & 20%RH, and 86% was still desorbed when the air temperature was decreased to 40 °C. Adsorption capacities and rate coefficients of composite samples with internal-cooling could be 2.3 and 1.5 times higher than those without. This paper provides a way to develop composite desiccants with high cyclic water uptake performance in the fields of adsorption cooling, solid desiccant air-conditioning and atmospheric water harvesting.
Desiccant coated heat exchanger (DCHE) systems have significant potential in improving dehumidification performance. The moisture transfer occurring in one single air channel of DCHE can dominate the overall system performance. In this study, the adsorber with an approximate air channel of DCHE was designed to experimentally investigate the kinetics of silica gel and two aluminophosphate zeolites (FAM Z01 and FAM Z05). Their dehumidification performances also were simulated at regeneration temperature range of 50–80 °C to reveal the applicability in air-cooled cross-flow DCHE. Results showed that linear driving force (LDF) model could well describe the dynamic water uptake behaviors. Adsorption rate coefficient kads increased with increasing inlet air humidity ratio and velocity, but decreased with increasing coating thickness. It is worth noting that increasing regeneration temperature had a little effect on kads, while desorption rate coefficient kdes increased significantly. Simulation results showed that moisture removal capacity (MRC) and dehumidification performance of coefficient (DCOP) of FAM Z05 coated DCHE could reach 0.495 g/kg and 0.57 at regeneration temperature of 50 °C, which were 2.3 and 15.4 times higher than these of silica gel and FAM Z01 coated DCHEs. It demonstrated that FAM Z05 was more promising to utilize lower-grade heat energy.
Sorption thermal energy storage is considered as a promising method to reduce energy consumption of building heating. MgCl2⋅6H2O could be a good candidate due to its high energy storage density. This paper first summarizes phase equilibrium lines of four MgCl2 hydration/dehydration processes to evaluate reaction enthalpy and entropy variation. Thermal diffusivities and specific heat of MgCl2, MgCl2·6H2O and MgCl2-ENG composite are determined which are compared with common counterparts of MgSO4·7H2O and MgSO4-ENG. Then the performance of open sorption thermal energy storage system is assessed in terms of various sorbents. Results show that the obtained thermal conductivities of MgCl2·6H2O range from 0.15 W m⁻¹ K⁻¹ to 0.96 W m⁻¹ K⁻¹ at the temperature range of 15–70 °C while thermal conductivities of MgSO4·7H2O range between 0.29 W m⁻¹ K⁻¹ and 0.68 W m⁻¹ K⁻¹ at 20–45 °C. At 20 °C testing temperature, thermal conductivities of composite MgCl2 are improved which range between 0.97 W m⁻¹ K⁻¹ and 2.92 W m⁻¹ K⁻¹ under the condition of 500–900 kg m⁻³ density and 67–80% mass ratio of salt. Besides, MgCl2 composite is preferable for open sorption thermal energy storage which could achieve stable heat output for more than 4 h under the conditions of 15.4 °C inlet temperature, 40% relative humidity and 0.26 m s⁻¹ air velocity.
Coating metal–organic frameworks (MOFs) on metal substrates is an important research orientation in the applications of MOFs. However, the existing binder‐based coating method needs repetitive operations and unavoidably plugs the pores of MOFs, resulting in a reduction of the adsorption capacity. Herein, a binder‐free method is proposed to construct the MOF‐on‐metal structure. The well‐intergrown polycrystalline Al‐MOF layer on aluminum substrate is prepared by in situ synthesizing Al‐based MOFs (MIL‐96 and MIL‐100) with aluminum ions from the dissolution of aluminum substrates. The morphology and chemical compositions of the MOF coating layer are systematically characterized, and a pH‐controlled strategy is proposed to regulate the relative proportion of the hybrid MOFs. Importantly, the MOF‐on‐metal structure displays ultrahigh water adsorption capacity of 192.5 g m−2, which is the highest of all reported desiccant‐coated metal structures, and superior cycling stability. Further, the performance of a desiccant heat pump system utilizing MOF‐on‐metal structure is predicted, demonstrating that the operation period is 80% longer than a system with a binder‐based silica gel coating, and the average dehumidification capacity can reach 8.36 g kg−1 dry air. In conclusion, the new method enables the formation of binder‐free, low‐cost, and high‐performance MOF coating and has a broad prospect in energy‐efficient adsorption‐based applications. Metal–organic framework (MOF)‐on‐metal structure composed of Al‐MOF layer and metal substrate is constructed by in situ synthesizing Al‐based MOFs with aluminum ions from the dissolution of aluminum substrates. The binder‐free Al‐MOF layer has an ultrahigh water adsorption capacity of 192.5 g m−2 and superior cycling stability under rigorous conditions and thus has a broad prospect in energy‐efficient adsorption‐based applications.
Adsorption technologies for adsorption-based dehumidification and water harvesting from the atmosphere have great potential for energy-saving and environmental protection. The key to successful utilization of these technologies is desired properties of adsorbents. However, most existing adsorbents fail to have the characteristic of both high adsorption capacity and good desorption ability under low regeneration temperature such as at 40 °C. In this work, we grafted a thermo-responsive polymer (Poly(N-isopropylacrylamide) PNIPAM) to silica gel and impregnated with lithium chloride to prepare a composite adsorbent. The adsorption capacity of this composite can reach 1.70 g g⁻¹ at 20 °C&70%RH, and 56% of adsorbed moisture can be released within 60 min at 40 °C&10%RH. This method offers a promising route to develop high-performance thermo-responsive composite adsorbents for desiccant coated heat exchanger (DCHE)-based dehumidification and adsorption-based atmospheric water harvesting (ABAWH).
Solid desiccant dehumidification is a promising alternative to vapor compression-based air-dehumidification to reduce energy consumption and improve air quality. Desiccant coated heat exchanger (DCHE), as one type of solid desiccant dehumidification system, can improve system performance and efficiency. The thermal performance and moisture removal capability of the DCHE greatly influence its dehumidification performance.
The present work aims to critically analyze the designs, materials, and manufacturing methods of heat exchangers used for solid desiccant coating and look into recent developments in regular heat exchangers, potentially deployed as a substrate for DCHE applications. A comprehensive literature review of publications regarding solid desiccant materials, heat exchangers, manufacturing and coating methods, binder materials, and the performance of heat exchangers has been developed. Alternative heat exchangers are described as better options than fin and tube heat exchangers that have been considered thus far for DCHE applications. Additionally, the heat exchanger's manufacturing process and materials over their thermal and mass exchange performance have been analyzed. Despite the widespread use of fin and tube heat exchangers for solid desiccant coating, recent developments in heat exchanger design, and related fields such as manufacturing methods and materials, open the possibilities for their application in solid desiccant dehumidification systems.
Finally, the authors provide their outlook on possible developments of DCHE technology, aiming to increase systems' energy and dehumidification performance.
Desiccant coated heat exchangers (DCHEs) yield higher dehumidification and thermal efficiency over other solid desiccant dehumidifiers due to their effective removal of the exothermic heat of sorption and improved heat transfer effectiveness. Accordingly, they offer prospective energy and cost savings to several energy-related applications such as heat pumps, chillers, water harvesters, etc. A comprehensive review of the current state-of-the-art in DCHEs is imperative to understand this technology’s marked impact and its performing strategies and capabilities. This chapter first introduces the different types of isotherm and hysteresis profiles and specifies the adsorption mechanisms. Then it presents a list of conventional pure/composite desiccants employed and highlights their limitations. Next, the detailed steps involved during its binder material selection are presented, and a comparison is made between the different types of coating techniques. Different regeneration techniques are then described, and the relevance of thermal regeneration vis-à-vis microwave and ultrasonic methods is established. Lastly, the ideal characteristics of a desiccant are listed, which would pave the way for performance-enhancing synthesis of advanced desiccant materials.
Desiccant coated heat exchanger (DCHE) has shown great potential for dehumidification in vapor compression cooling systems and has been widely concerned in the research field of air conditioning, as well as heat and mass transfer in recent years. This paper is aimed at providing fundamental knowledge and research progresses on DCHE for dehumidification and cooling. Initially, DCHE dehumidification is chosen as the research object in this review due to its superiority of the principle by comparing with other two typical dehumidification processes, which are condensation dehumidification and rotary-wheel dehumidification. Then, intermittent dehumidification and continuous dehumidification as two operation modes of DCHE are compared. Finally, the variation characteristics of temperature and humidity during operation are discussed. The current studies of DCHE are reviewed from four aspects, including desiccant coating, cooling source, regeneration heating source and basic heat exchanger. The performance analysis of DCHE is then conducted, according to the different effects of above aspects acting on dehumidification performance, cooling performance and energy efficiency of DCHE. The ongoing researches indicate that the rational construction of DCHE is an effective way to achieve its multi-objective performance optimization. The significant progress of DCHE lays the foundation for applications on other fields. However, further research is required in the field of the selection of desiccant coatings and the structure of heat exchangers to broaden its application market.
The desiccant coated heat exchanger is capable of dealing with sensible load and latent load of hot and humid air. The heat and moisture transfer coefficients of the desiccant coated microchannel heat exchanger (DMHE) are derived theoretically and increase with the airflow velocity in the heat exchanger. The microchannel structure optimization is implemented. It is found that the smaller the fin pitch and flat tube pitch are, the greater the heat and moisture transfer coefficients are. The thicker the desiccant coating, the larger the moisture transfer coefficient, the smaller the heat transfer coefficient. The experimental results show that the dehumidification capacity and the thermal performance coefficient of DMHE are 1.59 kgh⁻¹ and 2.09 respectively with the regeneration hot water at 60 ℃. The Taguchi method is employed to evaluate the influence of cooling water temperature, airflow velocity, and water flow rate on the dehumidification performance of DMHE. The airflow velocity is the main factor affecting the dehumidification capacity, where lower cooling water temperatures and faster the airflow velocity increases the dehumidification performance.
Dehumidification performance of a high-efficient compact desiccant cooling component named desiccant-coated heat exchanger (DCHE) highly depends on coated desiccant materials. Currently, mesoporous silica gels and salt-supported silica gels have been extensively studied. Silica gels have limited adsorption quantity in the middle relative pressure range, while salt-supported silica gels have potential threat of corrosion. In this work, a metal–organic framework UiO-66 that can achieve good adsorption quantity without corrosion is reported. The equilibrium adsorption test shows that UiO-66 has promising water uptake, especially in the relative pressure range of 0.35–0.65. The adsorption kinetics test displays that UiO-66 exhibits desirable dynamic water uptakes and adsorption rate constant. Dehumidification capacities of a DCHE using UiO-66 are estimated via a mathematical model and compared with silica gel- (SG) and LiCl-supported silica gel (SG/LiCl). Simulation results show that UiO-66 has better dehumidification performance than both SG- and SG/LiCl-coated samples for semiarid and semihumid climates when outdoor temperature is lower than 36 °C. Especially, when the temperature and relative pressure of inlet air are set as 30 °C and 0.35, respectively, its dehumidification capacity can be 2.7 times the amount of the SG-coated one and 1.3 times of the SG/LiCl-coated one. However, it should be more prudent in choosing UiO-66 under hot and humid climates.
A composite adsorbent coating on Al6061 aluminum substrate, based on SAPO34 zeolite filler and sulfonate poly (ether-ether ketone) (S-PEEK) matrix is proposed for sustainable energy saving technologies. The effect of sulfonation degree on the mechanical and physical chemical properties of the composite coating was assessed by synthesizing four batches with a degree of sulfonation of the matrix ranging from about 40%–50%. The degree of sulfonation was measured by titration and confirmed by thermogravimetry and FTIR analysis. For each batch of S-PEEK four coatings at increasing SAPO-34 filler content (in the range 80−95 wt.%) were realized. The composite coatings were homogenous and almost defect free. The mechanical performances, evaluated by scratch and pull-off tests, highlighted that all batches exhibited a suitable behavior for adsorption heat pumps application in mobile systems. Best results were observed by using a relatively high sulfonation time of S-PEEK. Obtained coating exhibited indeed very good adhesive and cohesive properties as well as high water vapor uptakes.
Solid desiccant heat pump system (SDHP) can handle sensible and latent heat load simultaneously with high coefficient of performance (COP). However, in such system, supply air temperature is relatively high and unstable due to the frequent switch between desiccant coated heat exchanger as evaporator and condenser. In order to overcome these drawbacks, the concept of semi-coupled solid desiccant heat pump (SCSDHP) system is proposed in this paper, then a mathematical model is established to investigate the feasibility of this novel system. Calculation shows that under ARI (Air-conditioning and Refrigeration Institute) summer condition silica gel coating with density of 0.25 kg m⁻² is optimal, and area ratio between desiccant coated evaporator and conventional evaporator of 1:1 is recommended. Furthermore, compared with conventional SDHP system, supply air temperature can be 4.5~7°C lower and the variation range of supply air temperature decreases by 16~21°C. Also, effects of main operating parameters including the temperature and relative humidity of outdoor air on the performance of SCSDHP system are investigated.
In hot and humid climates, desiccant air-conditioning systems have been suggested as suitable for improving Indoor Air Quality (IAQ). Generally as dehumidification devices, various types of silica gel rotor have been used to control humidity. In this study, we evaluated the dehumidification performance of the Functional Adsorbent Material - Zeolite (FAM-Z) rotor, which is expected to offer superior dehumidification at lower regeneration temperatures. In this experiment, in which hot and humid air was passed through a FAM-Z rotor (desiccant rotor), we measured the dry bulb temperature and the relative humidity both before and after passage through the FAM-Z rotor, and then calculated the difference in absolute humidity. We also compared dehumidification performance of a FAM-Z rotor and a silica gel rotor. Some parameters, such as the regeneration temperature and airflow, which may affect the performance of the desiccant rotor, were analyzed. As a result, in the case of a regeneration temperature of 50°C, the dehumidification performance of the FAM-Z rotor was superior to that of the silica gel rotor, however when the regeneration temperature was 60°C and above, the silica gel rotor performed better.
This paper addresses the preparation and characterisation of anticorrosive silane- and polypyrrole-based organic coatings
and combinations of the two on aluminium 2024. Layer adsorption studies of organosilanes such as propyl (C3), octyl (C8) and
octadecyl (C18) trimethoxysilane and polypyrrole deposits on the aluminium electrodes reveal only limited protection. Their
anticorrosive power declines when they are subject to highly corrosive environments, such as salt fog cabinets, for extended
periods. The combination of both deposits yields a more protective structure that affords better protection with time. The
best performance is achieved with polypyrrole deposits on silanes due to the excellent bonding between the silane adsorbed
on the surface of the material and the polypyrrole film. Of the three organosilanes used, the one with the shortest chain
performs best. When long-chain organosilanes are used, the polypyrrole film becomes detached due to the lesser interaction
between the layers. Electrochemical impedance spectroscopy and morphological studies of the layers also show the greater adhesion
and lesser deterioration of polypyrrole deposits on silane layers.
J F Danielli et al (eds) London: Academic Press 1973 pp xv + 333 price £11.30
Desiccant coated heat exchanger provides a promising option for desiccant cooling system, since it can handle sensible load and latent load simultaneously within one component. It is fabricated by coating desiccant material on the surface of conventional fin-tube heat exchanger. In order to enhance the performance of conventional silica gel coated heat exchanger (SGCHE), a novel composite silica gel coated heat exchanger (CCHE) is proposed and fabricated. An experimental setup is built to test and compare the dynamic performance of SGCHE and CCHE. Influences of main operation parameters including water temperatures and inlet air conditions on system performance are analyzed in terms of average dehumidification capacity (Davg) and thermal coefficient of performance (COPth). Optimization of cycle switch modes is also discussed. Experimental results show that CCHE has better dehumidification performance compared with SGCHE. In addition, pre-cooling before dehumidification process is found to be advantageous to both Davg and COPth.
Solid composite desiccants are fabricated by impregnating LiCl into pores of SBA-15 and MCM-41 mesoporous silicates. In the study, characteristics including textural properties and equilibrium and dynamic water sorption were tested and analyzed. Research on nitrogen adsorption suggested that because of the impregnated salt, composite desiccants were different from pure silicates. Sorption isotherms were measured and simulated based on Polanyi potential theory. Composite samples had water uptake much higher than that of pure hosts under low and middle relative pressure owing to mutual contribution of both physical and chemical sorption. Sorption kinetics were also tested and fitted with a linear driving force model. The composite desiccant prepared from SBA-15 exhibited higher dynamic sorption quantity and a reasonable diffusion rate constant. Finally, dehumidification performance of the composite desiccant in a novel solid desiccant cooling component was evaluated with a mathematical model. Simulation results indicated that enhanced dehumidification capacity can be achieved using the composite desiccant.
The desiccant based hybrid air conditioning system can control temperature and humidity separately, which consists of adsorption desiccant system and compression type refrigeration system. The adsorption desiccant system obtains the moisture power, and the compression type refrigeration can obtain sensible heat load. Generally, the regenerated energy consumption of the desiccant wheel is very large in the total energy consumption, using electric power, waste heat or renewable energy (solar etc). Heat recovery from the condenser considered as only regenerated energy in the new type adsorption desiccant based hybrid air conditioning system is proposed and discussed in this paper. This method can reduce much energy consumption, and make the air conditioning system structure simple.
Solid desiccant cooling technology has become a research focus for its features of energy-saving and eco-friendly. However, widely adopted rotary desiccant wheel cooling system can't realize inner-cooling dehumidification process. In this paper, a novel self-cooled solid desiccant cooling system (SCDHE) is developed by integrating desiccant coated heat exchanger and regenerative evaporative cooler. In the system, regenerative evaporative cooler is adopted to produce chilled water, which is again pumped into desiccant coated heat exchanger in dehumidification process to realize self-cooled dehumidification process. Similarly, in regeneration process, hot water heated by low grade thermal energy is adopted to regenerate the coated desiccant material. A mathematical model is established to validate the feasibility and to analyze performance of this novel system. Also, effects of ambient air condition are predicted. It is found that SCDHE system is feasible, it can provide satisfied supply air to conditioned room under simulated ARI summer condition, and the required regeneration temperature is from 50 to 80 °C which is lower than rotary wheel desiccant cooling system. Also, there exists an optimal switch time and suitable control mode for system to obtain enhanced performance in terms of cooling power. Compared with conventional DCHE cooling system without regenerative evaporative cooling, SCDHE system can provide satisfied supply air while conventional system cannot, also it can obtain increased cooling power. Under simulation condition, cooling power of SCDHE system increases by about 30% compared with conventional DCHE cooling system.
Based upon the fast development of energy efficiency, energy safety and use of renewable and sustainable energy, various energy systems related to residential refrigeration, power generation and storage have been developing. Some of them are in large scale application, while others are still under development. Current status of residential refrigeration, power generation and energy storage technologies have been briefly summarized in this paper. Also, future residential refrigeration, power generation and energy storage technologies are highlighted, and some roadmaps are discussed.
In recent years, the boost towards the reduction of electrical loads for air conditioning and the decentralization of energy conversion devices are determining an increasing interest in small scale trigeneration systems fueled by natural gas (“gas cooling”), able to shift energy demand in summer from electricity to gas, at the same time allowing the exploitation of natural gas surplus during the warm season. A technology that meets these requirements is represented by desiccant-based dehumidification systems, in which thermal energy for regeneration can be provided by a small scale cogenerator; the main component of these systems is the desiccant wheel, whose performances, in terms of humidity reduction and process air outlet temperature, depend on several operational parameters. The rotational speed of the desiccant wheel is widely recognized as a crucial parameter: if the wheel rotates too fast, the desiccant material does not have enough time to remove the moisture, while if the wheel rotates too slowly, saturation could occur. As a result, there must exist an optimal rotational speed, depending on the operating conditions, that guarantees the best dehumidification performance. Rotational velocity of the desiccant wheel influences the process air temperature exiting the desiccant wheel too; therefore it should be chosen in order to contemporary obtain a high dehumidification performance and an enough low outlet temperature, to reduce the cooling load on the cooling device, in particular if a conventional vapor compression chiller is used, as often occurs in high humidity climates. In this paper, experimental tests on a silica gel desiccant wheel, in order to highlight the effect of rotational speed on its performance, are shown. The adsorbent material is regenerated by thermal energy up to 65 °C. The experimental results were used to calculate some of the most representative performance parameters for the wheel, that are the dehumidification effectiveness, the dehumidification coefficient of performance (DCOP) and the sensible energy ratio (SER). Finally, the influence of process air inlet temperature and humidity, regeneration temperature and the ratio between the regeneration and process air flow rates on the optimal rotational velocity is discussed. It was found that, for the analyzed desiccant wheel, the velocity that optimizes the dehumidification performances varies in the range 5–10 revolutions per hour, depending on operating conditions, while SER monotonically increases with rotational velocity.
The study investigates the new solid desiccant heat pump system applied in the outdoor air conditions of the Asia-Pacific Region. The new system operates in a cyclic process of air cooling/dehumidification and air heating/regeneration as opposed to the typical solid desiccant heat pump system. The experimental set-up was constructed to test the performance of the system. Several combinations of the outdoor air and return air temperature and humidity ratios were used in the investigation. The outdoor air conditions range from warm and very humid (33 °C and 80%) to mild temperature and humidity (28 °C and 60%). The indoor air conditions ranges from 28 °C and 60% to 26 °C and 40%. The result shows the system was good in the reduction of air latent energy content compared to the reduction of air sensible energy content. The system reduced up to 10 g/kg of air humidity ratio even at outdoor air humidity ratio of above 25 g/kg with high outdoor air temperature of 33 °C. The system higher coefficient of performance reached the maximum value of 5.7. The system lowest coefficient of performance was 2.9. The present system coefficient of performance is higher than the ordinary vapor compression heat pump system.
Desiccant coated heat exchanger (DCHE) system can handle latent and sensible load simultaneously by removing the released adsorption heat in dehumidification process. The system can also be driven by low grade thermal energy such as solar energy. In this paper, a dynamic one-dimensional mathematical model validated by experimental data is established to predict the performance of DCHE system, using conventional silica gel as desiccant material. Cooling performance of DCHE system is calculated under ARI (American Air-conditioning and Refrigeration Institute) summer and humid conditions. Simulated results show that the operation time in dehumidification process is a crucial factor for cooling capacity of DCHE system, which can be enhanced by eliminating the initial period with higher outlet air temperature, the largest cooling power of DCHE system increase from 2.6kW to 3.5kW by eliminating first 50s of operation time under ARI summer condition. The results also prove that the system can provide cooling power to indoor condition with selective operation time when regeneration temperature varies from 50°C to 80°C. Besides, the model is adopted to analyze the effects of some structural parameters on system performance under simulated condition. The system performs well in smaller cobber tube external diameter condition, while both transient heat and mass transfer capacity can be enhanced under the condition of smaller distance between the fins.
A method is shown which permits a test of various empirical diffusion equations giving the rate of the mean internal concentration change dq̄/dt as function of the mean internal concentration q̄ and the surface concentration q. (Formula Presented) It is shown that, under the conditions occurring in chromatographic columns with substances following linear or moderately curved adsorption or exchange isotherms (Kd < 3), where conditions are maintained close to equilibrium, eqn. (A) is almost perfect, (B) is fully adequate, while (C) and (D) are not. For strongly curved isotherms (3 < Kd < 100), the gradient of the fronts of advancing solute is very steep, and during the passage of such a boundary the particles are far from equilibrium with the contacting solution. Under these conditions eqn. (D) offers by far the best representation. For almost irreversible exchange (K d > 100), eqn. (C) is superior to (A), (B) and (D). It can be shown, however, that when applied to the practical chromatographic break-through curves, the difference between the four equations, except for irreversible exchange, is hardly big enough to warrant the use of equations more complex than (B).
In this paper a new family of sorbents, specifically designed for ethanol sorption, is presented. The composites were synthesized by a dry impregnation of matrices with an aqueous solution of various salts. The ethanol sorption capacity of the composites, under conditions typical for adsorptive air conditioning cycle, has been measured by using an express method based on the Polanyi principle of temperature invariance. Results obtained show that the best novel composites have the ethanol sorption ability which is higher than that of known ethanol sorbents. The composite LiBr(30 wt.%)/SiO2 appears to show the highest sorption capacity and an uptake variation Δw=0.56 and 0.40g/g for air conditioning and ice making cycles, respectively. They are much larger than those obtained for conventional adsorbents. The correspondent cooling coefficient of performance (COP) was estimated to be 0.66 and 0.61, which is comparable with the COP of the best water sorbents.
In this paper, two desiccant-coated heat exchangers, which are actually fin-tube heat exchanging devices coated with silica gel and polymer materials respectively, are investigated experimentally. Due to the hygroscopic properties of the desiccant materials, both the sensible heat and the latent heat of the process air can be handled by using this kind of heat exchanger. An experimental setup was designed and built to test the performance of this unit. It is found that this desiccant-coated fin-tube heat exchanger well overcomes the side effect of adsorption heat which occurs in desiccant dehumidification process, and achieves good dehumidification performance under given conditions. The silica gel coated heat exchanger behaves better than the polymer one. The influences of regeneration temperature, inlet air temperature and humidity on the system performance in terms of average moisture removal rate Davg and thermal coefficient of performance COPth were also analyzed.
The paper focuses on development of new adsorbent material of aluminophosphate (FAM-Z05) which could be subsequently used in utilization of waste beat below 373 K. To evaluate this new material, water vapor adsorption isotherms of FAM-Z05 were measured at several temperatures. Furthermore, an attempt was made to improve the heat exchanger by coating FAM materials onto rin tube type heat exchanger. Then, the dehumidification performance of direct cooling and heating desiccant system was evaluated. It was found that the water vapor adsorption isotherms of FAM-Z05 were characterized by S-shape and the adsorption character of FAM-Z05 was dependent on temperature. Further, the new adsorbent of FAM-Z05 was able to adsorb water vapor in lower regeneration temperature than FAMZ02 and FAMZ01, which was attributed to FAM-Z05 ability to adsorb water vapor in high humidity range. However, when FAM-Z05 is used in low humidity range, the dehumidification performance of FAM-Z05 might be lower than those of FAM-Z01 and FAM-Z02.
AFI-type structure ferroaluminophosphate zeolite (FAPO-5) was examined as an AHP adsorbent. FAPO-5 with iron content of 2-8mol% (Functional Adsorbent Material-Zeolite 01; FAM-Z01) was selected for further testing. The water vapor adsorption isotherm of FAM-Z01 was S-shaped and highly dependent on temperature, and almost no hysteresis was observed with adsorption/desorption at 333 K and 348 K. No changes were observed in the properties of FAM-Z01 after 200,000 cycles of water vapor adsorption-desorption, indicating that FAM-Z01 is suitably durable for practical use. When the AHP was operated under conditions of TL/TM/TH= 283 K/303 K/333 K, the adsorption capacity of FAM-Z01 was 4 times that of silica gel.
CHA-type structure silicoaluminophosphate zeolite (SAPO-34) was examined as an AHP adsorbent and the influence of silicon content on its adsorption performance was evaluated. SAPO-34 with silicon content of 7.5 mol% (Functional Adsorbent Material-Zeolitc 02; FAM-ZO(2)) was selected for further testing. The water vapor adsorption isotherm of FAM-ZO2 was S-shaped and highly dependent on temperature, and a small hysteresis was observed with adsorption/desorption at 363K. No changes were observed in the properties of FAMZO(2) after 100000 cycles of water vapor adsorption-desorption, indicating that FAM-ZO(2) is Suitable durable for practical use. When the AHP was operated under conditions of T,(L)/T-M/T-H=283 K/313 K/363 K, the adsorption capacity of FAM-ZO(2) was 4.8 times and 3.8 times those of Y zeolite and silica gel.
Ferroaluminophosphate zeolite with an iron content of 5 mol%, named Functional Adsorbent Material Zeolite 01 (FAM-Z01), and the silicoaluminophosphate zeolite with a silicon content of 7.2mol%, named Functional Adsorbent Material Zeolite 02 (FAM-Z02), were studied as adsorbents in a desiccant cooling system. Adsorption isotherms of water vapor were measured by use of a volumetric adsorption apparatus under vacuum and by thermogravimetric analysis at atmospheric pressure. FAM zeolites were able to adsorb water vapor at lower humidity and desorb water vapor at lower temperature than conventional adsorbents such as silica gel or Y type aluminosilicate zeolite. This behavior indicated that FAM zeolites will be useful to extend the working temperature of desiccant cooling to below 70 degrees C. A desiccant rotor containing FAM zeolites or silica gel as the main component was also evaluated at various regeneration temperatures and air conditions. It was found that the dehumidifying performance of the FAM-Z02 rotor was 11-22% higher than that of the silica gel rotor at the regeneration temperature of 50-70 degrees C.
In this study, a novel self-regenerating electric vapor compression heat pump desiccant (HPD) unit operated in the heating and humidification mode during the winter season is introduced. The HPD unit was installed in an office suite for the field test. The performance of the HPD unit and the provided indoor conditions were measured over a wide range of operating conditions. The target indoor humidity ratio was set to 4.4 g/kg, which is the minimum required indoor humidity ratio for a comfortable indoor environment indicated in the ASHRAE winter thermal comfort zone. The seasonal comparison revealed that even though 77.7% of all outdoor humidity ratio data was lower than 4.4 g/kg, 78.2% and 85.8% of all the indoor humidity ratio data of each room were found to be higher than 4.4 g/kg. In addition, due to the significant sensible capacity of the HPD unit, the indoor temperatures could be maintained within 20–25 °C. These results prove that the HPD unit not only properly humidifies the indoors without using any additional water source, like the conventional humidifier, but also helps to keep the indoor temperature at the desired temperature levels.
Integration of the variable refrigerant flow (VRF) and heat pump desiccant (HPD) systems was investigated in a field performance test for a heating season. The HPD systems use only the moisture in the outdoor air and return air to humidify the indoors during ventilation in the heating season. Three different operating modes: non-ventilated, HPD ventilation assisted and HPD ventilation–humidification assisted VRF systems were investigated. It was found that the VRF systems provided an average of 93.5% of the total heating energy for the HPD ventilation assisted mode. The remainder was the recovered heat by the HPD systems during ventilation. The VRF systems provided an average of 46.8% of the total heating energy for the HPD ventilation–humidification assisted mode. The remainder was covered by the HPD systems which provided additional sensible and latent heating. Overall, among the three operating modes, it is concluded that the HPD ventilation–humidification assisted VRF outdoor units consume less energy than the HPD ventilation assisted ones (about the same energy as the non-ventilated ones), while providing the best indoor thermal comfort and indoor quality conditions. For the total system, the HPD ventilation–humidification assisted VRF systems consume less energy than the HPD ventilation assisted ones.
Rotary desiccant air conditioning system, which combines the technologies of desiccant dehumidification and evaporative cooling, is advantageous in being free from CFCs, using low grade thermal energy and controlling humidity and temperature separately. Compared with conventional vapor compression air conditioning system, it preserves the merits of environment-friendly, energy saving, healthy, comfortable, etc. Ongoing research and development works suggest that new desiccant materials and novel system configurations have significant potential for improving the performance and reliability and reducing the cost and size of rotary desiccant dehumidification and air conditioning system, thereby increasing its market competitiveness and breaking out the current fairly small niche market. For the purpose of providing an overview of recent efforts on these issues and showing how rotary desiccant air conditioning systems can be designed and coupled to available thermal energy, this paper presents and analyzes the status of rotary desiccant dehumidification and air conditioning in the following three aspects: the development of advanced desiccant materials, the optimization of system configuration and the utilization of solar energy and other low grade heat sources, such as solar energy, district heating, waste heat and bioenergy. Some key problems to further push forward the research and development of this technology are also summarized.
A novel class of crystalline, microporous, aluminophosphate phases has been discovered that represents the first family of framework oxide molecular sieves synthesized without silica. The new family of aluminophosphate materials (AlPOâ-n)³ currently includes about 20, three-dimensional framework structures, of which at least 14 are microporous and 6 are two-dimensional layer-type materials. The aluminophosphate materials have interesting properties for potential use in adsorptive and catalytic applications, due to both their unique surface selectivity characteristics and novel structures.
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