Solid Desiccant Research Articles

Capturing water vapors from humid air using microporous activated carbon derived from sunflower seed shells

Water extraction from the atmosphere using solid desiccants is becoming an increasingly viable option for human consumption. Using sustainable raw materials is imperative to reduce the carbon footprint in the mass production of such desiccants. In this paper, we synthesized microporous activated carbon from sunflower seed shells (SSS-AC) to capture water vapors from humid or moist air. Sunflower seed shells (SSS) are an agricultural waste product that makes the final product a sustainable and environment-friendly alternative to traditional solid desiccants. Chemical activation was carried out using KOH as the activating agent, and the ratio of activating agent to carbon was 2:1. Different structural and morphological properties of synthesized microporous activated carbon (SSS-AC) were studied using FTIR, XRD, SEM, EDAX, and TGA characterization techniques. The surface area and pore distribution were characterized using N2 adsorption/desorption studies, where a steep initial uptake with a wide capillary condensation step was observed for SSS-AC. The pore distribution of SSS-AC showed high pore volume in the microporous range. The pHpzc value for the SSS-AC was measured to be 10.24 while the HI value was found to be 6.26, indicating an alkaline and hydrophobic surface. Water adsorption studies were carried out at 25, 35, and 45 °C, and the material was found to exhibit type-V isotherm, which is common for microporous activated carbons and indicates the likely occurrence of pore condensation. The SSS-AC reported a maximum adsorption capacity of 0.454 gwat/gads at 90% relative humidity and 25 °C, which is higher than some complex desiccants reported in the literature. The adsorption capacity of SSS-AC decreased with increasing temperature since, at higher temperatures, the pore-filling mechanism is hindered.

Experimental Investigation on Dehumidification Using a Solid Composite Bio Desiccant Internally Cooled Using Nanofluids for Building Cooling

Indoor comfort has become a major factor with advancements in science and technology. This also leads to an increase in greenhouse gases as well as energy consumption. Desiccant-coated heat exchangers are one of the common solutions to these risks and to lower energy usage. In the present work, the capability of a solid composite desiccant blend prepared from coconut shell-based activated carbon and bio char was studied. Aluminum plates have been coated with the prepared solid desiccants. Desiccant-coated heat exchangers were cooled by the cerium oxide nanofluid passing through the pipes connected along the length of the heat exchanger. Air was blown through the plates where dehumidification occurs due to the vapor pressure difference between the air and the desiccant-coated plate. The experiments were conducted by varying the air velocity, water flow rate, and nanoparticle concentration. The nanoparticle volume fraction varied from 0.05% to 0.3%. Different performance parameters such as the moisture removal rate, dehumidification efficiency, cooling capacity, and coefficient of performance (COP) were calculated. Results showed that the performance parameters were enhanced with an increase in the water flow rate as well as the air flow rate. Furthermore, it was seen that with the addition and increase in nanoparticle concentration, the moisture removal rate and dehumidification efficiency were enhanced. In comparison to no addition of nanoparticles, a 0.3% addition of nanoparticles demonstrated a maximum increase in MRR of 53% and dehumidification efficiency of 57%. A maximum reduction of 6.1% in the dehumidification area was achieved by using 0.3% nanoparticles with water. It is recommended to use nanofluids for dehumidification using solid desiccants, which can enhance the performance without having negative influence on the environment.

Coupled heat and humidity control system of narrow-trough solar collector and solid desiccant in Chinese solar greenhouse: Analysis of optical / thermal characteristics and experimental study

Coupled heat and humidity control system of narrow-trough solar collector and solid desiccant in Chinese solar greenhouse: Analysis of optical / thermal characteristics and experimental study

Innovative solid desiccant dehumidification using distributed microwaves

Dehumidification is one of the key challenges facing the air conditioning (AC) industry in the treatment of moist air. Over many decades, the dual role of heat exchangers of AC chillers for the sensible and latent cooling of space has hindered the thermal-lift reduction in the refrigeration cycle due to the requirements of water vapor removal at dew-point and heat rejection to the ambient air. These practical constraints of AC chillers have resulted in the leveling of energy efficiency of mechanical vapor compressors (MVC) for many decades. One promising approach to energy efficiency improvement is the decoupling of dehumidification from sensible processes so that innovative but separate processes can be applied. In this paper, an advanced microwave dehumidification method is investigated in the laboratory, where the microwave (2.45 GHz) energy can be irradiated onto the dipole structure of water vapor molecules, desorbing rapidly from the pores of adsorbent. Results show a significant improvement in performance for microwave dehumidification, up to fourfold, as compared to data available in the literature.

Thermal Energy Storage Capacity of Composite Solid Desiccant Used In Sun Drying Applications Using Paraffin Wax

Abstract: Systems for storing thermal energy have become more important in solar thermal applications in order to supply the energy during overcast and gloomy hours. The capacity of a solid desiccant made up of 60% bentonite, 10% calcium chloride, 20% vermiculite, and 10% cement on a dry weight basis paired with a latent heat storage medium made of paraffin wax is examined in this study. Solid desiccant and paraffin have volume percentages of 90: 10, 80: 20, and 70: 30, respectively. The hollow cylindrical solid desiccant mould is filled with paraffin wax, which has a latent heat of fusion of 253 kJ/kg and a melting point of approximately 61.6 degrees Celsius. An experiment is conducted at air mass flow rate of 0.01, 0.02, and 0.03 kg/m2s with temperatures of 40, 50 and 60OC. The heat storage and retrieval capacity of 80% solid desiccant with 20% paraffin wax at 50oC and 0.02kg/m2s show the better results which are suitable for solar drying.

A comprehensive review on dehumidification system using solid desiccant for thermal comfort in HVAC applications

In comfort, air-conditioning control over the humidity is essential in applications such as pharmaceutical industries, electronic gadgets manufacturing units, the food sector, paper industries, etc. Relative humidity of nearly 50% can effectively prevent the growth and abidance of biological pollutants. There are several methods using which dehumidification can be achieved. The most common method is subcooling air up to the condensation temperature of the moisture trapped and then cooling again, and the condensate is collected. This method is time-consuming and needs to be more effective. On the other hand, desiccants’ dehumidification process is suitable for maintaining humidity in air-conditioning systems and producing dry air. Desiccants with economical materials, high moisture removal rate, low regeneration temperature, and steadiness after many years make a desiccant system more successful in its performance and cost. The effect of change in process air temperature, process air humidity ratio, and regeneration temperature on the system's performance is studied. This research focuses on the functioning of various desiccant dehumidification systems and their applications. A comparative study of the different dehumidification systems found efficiency, low maintenance cost, and high sustainability.

Performance investigation and exergy analysis of a novel recirculated regenerative solid desiccant dehumidification system

Vapor compression dehumidification (VCD) is limited by the wet-bulb temperature, which results in high-energy consumption, especially in enclosed hot and humid spaces. An improved recirculated regenerative solid desiccant-vapor compression (RRSD-VC) hybrid dehumidification system was proposed to achieve highly efficient dehumidification in enclosed cabins, such as those in marines, spacecraft, and underground shelters. An experimental study of the RRSD-VC system was carried out to analyze the dehumidification characteristics and exergy performance. The results indicate that the regenerative airflow ratio (γ) was the most crucial performance indicator of the proposed system. The system moisture removal capacity (Dtot) increased with decreasing precooling temperature and increasing regeneration air temperature, and the highest Dtot was obtained when the optimal γ was reached. The exergy efficiency of the RRSD-VC system (ηex) was inversely proportional to γ. Therefore, a balance of high dehumidification capacity and low energy consumption could be maintained in the system design. Moreover, ηex could be improved by 55.7% when the electric heating was replaced by heating with waste heat recovered from the engine exhaust gas for air regeneration. The results of further parametric analysis showed that the RRSD-VC system was more adaptable than the VCD system to hot and humid conditions.

Household solid desiccant dehumidifier with multiple operating modes: Concept and prototype device

Solid desiccant dehumidification has proven to be more energy-efficient than traditional condensation dehumidification. However, owing to the complexity and large size of existing solid desiccant dehumidifier systems, their application in residential spaces is limited. In this study, we explore the potential of using small solid desiccant dehumidifiers in residential spaces. The dehumidifier uses a variable frequency fan to replace the two fans of a traditional solid desiccant dehumidifier. Meanwhile, the flow direction of the dehumidified air and regeneration air was adjusted via the air valve such that the air in different working modes flows in the same space, thereby reducing the size of the dehumidifier. Moreover, the dehumidifier used an equalizing orifice to ensure the stability and uniformity of the airflow in a small space and used an electric heating tube for regeneration, as opposed to a heat exchanger, to reduce the size of the system. The experimental results of the prototype device show a dehumidification efficiency of more than 85%. The humidity at the inlet of the dehumidifier, air volume, and adsorption heat affected dehumidification performance. The analysis results of the energy coefficient of dehumidification (ECOD) index indicated that through reasonable design and usage patterns, the ECOD of small-sized solid desiccant dehumidifiers can exceed 1.73 kg/(kW·h). Compared to traditional condensing dehumidifiers for home use in the market, the energy efficiency ratio is improved by 8.1%. This indicates that the dehumidifier exhibits a high energy efficiency ratio.

Analysis of a novel solid desiccant evaporative cooling system integrated with a humidification-dehumidification desalination unit

A new configuration of a solid desiccant evaporative cooling system integrated with a humidification-dehumidification (HDH) desalination unit was proposed, and its performance was numerically evaluated. In the proposed system, the humid exhaust air from the desiccant wheel and M-cycle IEC was recycled and supplied to an HDH unit to improve the water production rate. Two operation modes were proposed depending on climate conditions. The system with two operation modes was evaluated under various operating conditions. A case study was performed under Australian climatic conditions. The results showed that Mode 1 had a better cooling performance than Mode 2, with a maximum cooling capacity of 4.76 kW. However, Mode 2 was more energy efficient, with an average COP of 0.71, as no regeneration heat was needed. With the HDH unit, the freshwater produced by the system could fully cover the water usage of the M-cycle IEC in both modes. When the heat source temperature of the HDH unit increased to 70 °C, the water productivity of the system could reach 148.5 kg/h and 42.0 kg/h for Mode 1 and 2, respectively. The findings indicated that the proposed system had good potential for producing freshwater and cooling simultaneously.

Fabrication and characterization of a TiO2 nanoparticles polypropylene membrane: application in indoor air quality maintenance

The production of innovative materials with improved features applicable in many domains is a key application of nanotechnology with far-reaching implications for modern society. Nanoparticle-based polymer composites are quickly becoming one of the most promising new materials, with potential uses spanning the chemical, physical, and biological sciences as well as engineering. Application of nanoparticle-based polymer composites for indoor air quality maintenance was discussed, as were their production, hybrid functionalization, and feasible synthesis procedures (filter membrane). The batch foaming procedure has been used to create foam from the thermoplastic polymer polypropylene (PP). Foaming is a blown process, where carbon dioxide is utilised as the blowing agent. Nanoparticles of titanium oxide (nano TiO2) are also used for reinforcement. Scanning electron microscopy (SEM) was utilised to investigate the NTPMs' surface morphology, while other physio-chemical characteristics were investigated by means of various analytical methods, including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and thermo-gravimetric analysis (TGA). The adsorption isotherm and kinetics of water vapours were analysed to get insight into the water vapour adsorption characteristics of the NTPMs. The kinetics of adsorption pointed to a combination of intraparticle diffusion and liquid field driving processes for the transport of water vapours. Because of their high dehumidification effectiveness, synthetic NTPMs have the potential to replace many of the currently used traditional solid desiccant materials

Review on Performance Analysis of Desiccant-Assisted Hybrid Cooling Systems

Due to the growing expense of fossil fuels and other environmental issues, it is crucial to reduce the energy cost of built environment cooling systems without sacrificing indoor air quality and comfort levels. One option in this area is solid desiccant dehumidification-assisted cooling systems, which employ alternative energy sources like solar and biomass and are also environmentally beneficial by the minimal requirement of refrigerants. The present review discusses the performance analysis of different solid desiccants readily accessible on the market and their composites. Better moisture absorption and a lower regeneration temperature are qualities of a better desiccant. The review also discusses the various solid desiccant dehumidifier designs, their benefits, and their disadvantages. Solid desiccant dehumidifiers now come in various combinations, considerably enhancing system performance. The exergy efficiency of desiccant-integrated evaporative cooling is up to 21.5%, comparable with another HVAC system. A summary of the performance parameters has also been created to assess system performance further. This study will benefit innovation and advancement in solid desiccant materials in the air conditioning market.

Water adsorption kinetics of silica based porous desiccants by volumetric method

The adsorption kinetics of solid desiccants is critical for the design and optimal operation of air-conditioning systems. However, owing to the complex combination of diffusion processes in the interparticle layer and intraparticle mesopores/micropores, adequately illustrating the adsorption kinetics of solid desiccants is difficult. In this study, the adsorption kinetics of a microsphere silica gel bead was investigated using the volumetric method in a limited batch adsorption system. An analytical solution was adopted for the biporous diffusion model to obtain the effective mesopore diffusivity. A comprehensive diffusion model was developed based on the initial estimation by the analytical solution. The model, i.e., the triple diffusion resistance model that includes three transport resistances, viz., interparticle, mesopore, and micropore resistance was subsequently validated through an experiment. The results show that the rate-limiting step in the entire diffusion and adsorption process changes from intraparticle to interparticle diffusion with increasing sample mass. The proposed model considering the effect of interparticle diffusion successfully determines the effective mesopore diffusivity for each sample mass.

Liquid desiccant-based deep dehumidifier working with a novel ionic liquid: Prediction model and performance comparison

Deep dehumidification system with energy conservation and stable operation is expected to be used in industrial occasions with low-humidity environment. Compared with solid desiccant dehumidification, liquid desiccant-based dehumidification is a promising alternative because of its advantages of continuous operation and low regeneration temperature. However, corrosion and crystallization problems exist with liquid desiccant-based deep dehumidifier (LDDD) using salt solutions. In this work, LDDD using a novel ionic liquid (IL) was introduced, and the heat and moisture transfer model was established based on the finite difference method. Meanwhile, the moisture transfer characteristics were numerically simulated under different operating parameters (i.e., inlet humidity ratio wa,in, air velocity va, solution mass concentration X, solution temperature ts and solution volume flow Vs). Besides, the significance of various factors and their interactions were analyzed based on response surface methodology. The numerical model is validated against the experiment of the LDDD test bench with a relative deviation of 3%-8%. Numerical results show that the LDDD using IL can achieve deep air dehumidification, and outlet humidity ratio of less than 4 g/kgda is possible. In particular, the order of significance for influencing factors of outlet humidity ratio is: wa,in > X > ts > va > Vs. Moreover, it is found that dehumidification rate of 60% LiBr solution (saturated state) is less than that of 90% IL. This study establishes a prediction model of moisture transfer between IL and air under the coupling of multiple physical fields, and provides guidance for the application of LDDD using ILs.

Performance evaluation of a novel multi-tray packed bed solid desiccant dehumidification system.

Configuration of the solid desiccant bed plays a critical role in the development of cost-effective thermally driven desiccant air-conditioning systems. In the present study, a novel multi-tray packed bed solid desiccant dehumidifier (MPBDD) was developed that can be used in conjunction with a traditional vapor-compression system. The proposed configuration aims to achieve high moisture removal from the air and minimize the system power consumption. A lab-scale test rig was designed, fabricated, and tested under different operation conditions. The effect of the inlet humidity, temperature, and flow velocity on the system performance has been experimentally investigated. Experimental results show that the proposed configuration exhibited an adsorption capacity of about 27% higher than that of the conventional systems. Moreover, the pressure drop through the newly designed desiccant bed was found to be 87% lower than that of the conventional wheel design. A dimensionless number called Total Performance Factor (TPF) was defined to be the ratio of heat dissipated for desiccant to pumping energy expended overcoming pressure drop in the device. It was found that in certain experiments it attained an average value of around 250. In this regard, the present study is critical for achieving high dehumidification capacity as well as energy savings in air conditioning applications.

Energy effectiveness analysis of desiccant coated heat exchanger based on numerical method

The conventional air-to-air heat pumps rely heavily on the condensation dehumidification principle to regulate air humidity, thereby severely limiting the system energy efficiency. Significant energy efficiency improvements can be realized by coating high-performing solid desiccants on both evaporator and condenser heat exchangers. Thus far, existing fundamental analysis developed to evaluate energy performance improvement and achieve accurate performance prediction of desiccant coated heat exchangers with two-phase refrigerant (DCHERs) has been impeded by the complex physical phenomena, approximations in the heat/mass transfer paths, and the severe reliance on the use of empirical correlations. Therefore, in this work, a mathematical approach has been judiciously developed based on mass, momentum, energy, and species conservation principles in order to study the flow field distribution within DCHERs and predict their performance. The model is extensively validated with experimental data obtained under various working conditions with discrepancies of ±10 % and ±9 % for outlet air temperature and humidity, respectively. The model provides essential information on local temperature and humidity distributions, and key insights on the transient heat and mass transfer phenomena. Further, a general design guideline for the performance enhancement of DCHERs is established. Key results from the study reveal that the air RH varies within 15 % along the DCHER’s flow channel. Thick desiccant coating, high inlet air relative humidity, and long air-desiccant contact time markedly improve latent energy effectiveness to a maximum of 0.51. Lastly, employing the proposed guideline to design the heat exchanger with optimal tube configurations yields a 23 % enhancement of latent energy effectiveness.

Utilization of clay based super-porous hydrogel composites in atmospheric water harvesting

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.

A Technical Review of Available Desiccant Design Techniques

Desiccant wheel system is a topic of research because of increased requirement in HVAC industry and in dehumidification industry. In HVAC industry desiccant cooling system operating on regenerating heat supply from renewable source as solar are making their impact and desiccant wheel plays important role as main part in dehumidification industry. It’s been approximately two decades since the desiccant wheel has made around in market for large level. Many designs and technologies are already available for desiccant wheels and some are available within the research and experiments only. This article provides a better understanding of available research and designs for desiccant wheel systems. Review for difference of liquid and solid desiccants operated cooling systems, various regenerating technologies used for desiccant different desiccant material and different materials for subtract of desiccant wheel, different construction design in terms of number of sectors and direction of air flowing in the desiccant sections prepared and discussed here. Apart from this various simulation models of desiccant systems studied based on mathematical models are studied here. Based on these reviews an effective conclusion is given for desiccant operated systems. At the end of the review future possibilities for advancement in desiccant industries are mentioned.

Experimental Assessment of Rotary Solid Desiccant Dehumidifier Assisted Hybrid Cooling System

"The prime objective of the present investigation is to assess the performance evaluation of rotary solid desiccant dehumidifier integrated vapor-compression integrated hybrid thermally cooling system by experimental investigation for the transient humid conditions. The experimental test set up is precisely designed and practically fabricated so as to obtain the consistent experimental data during performance testing. The main aim of the experimental investigation is to measure the overall system performance of the rotary solid desiccant assisted hybrid dehumidification and cooling system in terms of COP by measurement at various ambient conditions, air stream flow rates and different regeneration temperatures. The desiccant integrated hybrid thermally cooling system (DAHCS) is primarily the environmental protection technique for indoor comfort cooling purpose of the various residential and commercial building applications. This system also reduces the CFC level in the environment because it minimizes the use of conventional refrigerant. It is found that the rotary solid desiccant dehumidifier assisted hybrid cooling system (DAHCS) is alternate suitable option against vapor compression integrated conventional cooling system in transient humid climates. The desiccant assisted hybrid cooling system (DAHCS) has proven their feasibility and energy saving in the field of built environment. It is concluded that this innovative cooling technology is feasible and demand low energy for comfort cooling. This study is very useful in enhancing opportunities to extend further innovation and core research in different areas of desiccant integrated hybrid thermally cooling system.

A novel air-conditioning system with cascading desiccant wheel and liquid desiccant dehumidifier for low-humidity industrial environments

Numerous industrial workshops require low-humidity environments, including lithium battery production, chip fabrication, some pharmaceutical manufacturing, etc. The desiccant wheel system is the conventional air-conditioning system for maintaining the low-humidity environments. However, only high-humidity outdoor air can be used for desiccant regeneration in some mechanical workshops requiring no mechanical ventilation, which leads to a high regeneration temperature and a poor system energy performance. To enhance the performance, a novel air-conditioning system with cascading desiccant wheel and liquid desiccant dehumidifier for low-humidity industrial workshops is proposed in this study. Solid desiccant and liquid desiccant are innovatively integrated in the proposed system for achieving a supply air dew point temperature below −10 °C. When dehumidifying process air from 5 to 1 g/kg in the basic case, the proposed system shows a competitive COP of 1.6. Under weather conditions in Guangzhou, subtropical China, the proposed system consumes 19.6 % and 58.0 % lower electricity during the summer, compared to the integrated system using outdoor air for regeneration and the desiccant wheel based system, respectively. The proposed system particularly applies to low-humidity industrial workshops in tropical and subtropical regions in summer, where there is a huge air humidity difference between indoors and outdoors. It is anticipated that this study could provide reference for energy saving and carbon emission reduction in low-humidity industrial production process.

Imidazolium-based ionic liquid confined into ordered mesoporous MCM-41 for efficient dehumidification

Composite sorbents with hygroscopic salts (like LiCl and CaCl2) impregnated porous matrixes obtained special attention in the past years, especially for solid desiccant dehumidification. However, the drawback of strong corrosivity limited their practical implementation. Alternatively, ionic liquid (1-ethyl-3-methylimidazole acetate) with less corrosivity is confined into ordered mesoporous MCM-41 to fabricate composite sorbent in this study, overcoming the poor mass transfer or limited adsorption capacity of single sorbent. Comprehensive characterizations including SEM, TEM, XRD, FTIR and N2 adsorption prove that the ionic liquid is encapsulated in ordered channels of MCM-41. Sorption isotherms and kinetics indicate that the composite sorbent has superior water sorption performance (0.5 g/g at 20 °C/70 % RH), fast sorption rate under wide humidity range and potential of low temperature-driven regeneration. Moreover, the leakage test and corrosion experiment reveal that the composite sorbent has long-term stability and is corrosion-free to metal components. Further, the dehumidification performance using desiccant coated heat exchanger with reported composite sorbent is evaluated, showing 5 times higher dehumidification capacity than that of MCM-41 and 2 times higher than traditional silica gel (typical ARI summer, 35 °C, 40 % RH). This study identifies a feasible way of using ionic liquids to develop energy-efficient, safety-ensured and low-cost composite sorbents for water sorption-related applications.