This is a self-archived – parallel published version of this article in the publication archive of the University of Vaasa. It might differ from the original. Application of deep dehumidification technology in low-humidity industry: A review Author(s): Zhang, Qunli; Li, Yanxin; Zhang, Qiuyue; Ma, Fengge; Lü, Xiaoshu Title: Application of deep dehumidification technology in low-humidity industry: A review Year: 2024 Version: Accepted Manuscript Copyright ©2024 Elsevier. This manuscript version is made available under the Creative Commons Attribution–NonCommercial–NoDerivatives 4.0 International (CC BY–NC–ND 4.0) license, https://creativecommons.org/licenses/by-nc-nd/4.0/ Please cite the original version: Zhang, Q., Li, Y., Zhang, Q., Ma, F. & Lü, X. (2024). Application of deep dehumidification technology in low-humidity industry: A review. Renewable and Sustainable Energy Reviews, 193, 114278. https://doi.org/10.1016/j.rser.2024.114278 Review article Application of deep dehumidification technology in low-humidity industry: A state-of- the-art review Qunli Zhang a,b,*, Yanxin Li a, Qiuyue Zhang a, Fengge Ma a, Xiaoshu Lü b,c,d a Beijing Municipality Key Lab of Heating, Gas Supply, Ventilation and Air Conditioning Engineering, Beijing University of Civil Engineering and Architecture, Beijing 100044, China b Collaborative Innovation Center of Energy Conservation & Emission Reduction and Sustainable Urban- Rural Development in Beijing, Beijing 100044, China c Department of Electrical Engineering and Energy Technology, University of Vaasa, P.O.Box 700, FIN- 65101, Vaasa, Finland d Department of Civil Engineering, Aalto University, P.O.Box 12100, FIN-02130, Espoo, Finland Abstract Humidity regulation plays a pivotal role in both residential and industrial environments, significantly impacting comfort, health, and process efficiency. The integration of dehumidification systems with air conditioning systems allows for the control of temperature and humidity, resulting in a decrease in carbon dioxide emissions. In order to address the demands of industries with low humidity levels, this study offers a comprehensive review of advanced deep dehumidification systems. The study initially delineates the specific ranges for deep dehumidification as outlined in academic research, as well as the humidity levels in low- humidity industries. Evaluation models are proposed for the analysis of the dehumidification performance, energy efficiency, economic feasibility, and environmental impact of the system. The review focuses on the deep dehumidification technology, which encompasses air compression dehumidification, liquid desiccant dehumidification, solid desiccant dehumidification, membrane dehumidification, and coupled dehumidification, with an emphasis on materials, components, and systems flow. The research provides a comprehensive overview of the various potential applications of dehumidification systems, including air humidification, water collection, air purification, intelligent control, and optimization. Moreover, a comprehensive comparative analysis of different dehumidification technologies is conducted with regard to industrial application humidity requirements, energy performance, economic factors, and environmental considerations. Drawing on advanced studies and findings, this research examines the primary areas for future development in * Corresponding author. Qunli Zhang, doctor, professor. Tel: +861068322517. E-mail address: zhangqunli@bucea.edu.cn https://www.sciencedirect.com/topics/engineering/pivotal-role https://www.sciencedirect.com/topics/engineering/dehumidification https://www.sciencedirect.com/topics/engineering/economic-feasibility https://www.sciencedirect.com/topics/engineering/dehumidification-technology https://www.sciencedirect.com/topics/engineering/liquid-desiccant https://www.sciencedirect.com/topics/engineering/humidification https://www.sciencedirect.com/topics/engineering/air-purification Review article advancing deep dehumidification systems. The objective of this study is to propose optimization techniques aimed at enhancing dehumidification efficiency and reducing energy consumption in low-humidity industrial settings. Highlights ⚫ Summarise desiccant, component, and system aspects of deep dehumidification technology ⚫ Provide recommendations for optimizing methods of the dehumidification efficiency and energy efficiency ⚫ Suggest dehumidification systems applications in industries with different humidity ranges ⚫ Analyse energy, economic, and environmental aspects of dehumidification systems ⚫ Discuss main future works for deep dehumidification systems Keywords: Deep humidification; Desiccant; Air humidity; Dew point temperature; Energy consumption Review Nomenclature Abbreviations Symbols PPMV Parts Per Million Volume dR Dehumidification rate (moisture removal capacity), kg/s PPM Parts per million pM Flow rate of process air, kg/s DEG Diethylene glycol inpd , Process air humidity of inlet, kg/s EG Ethylene glycol outpd , Process air humidity of outlet, kg/s TEG Triethylene glycol inequd , Air absolute humidity in equilibrium with the inlet solution at its temperature and concentration, g/kg LiCl Lithium chloride d Dehumidification efficiency (effectiveness) LiBr Lithium bromide tCOP The coefficient of performance of the total dehumidification system CaCl2 Calcium chloride tQ The total heat exchange of process, kW KCOOH Potassium Formate tW The total system energy consumption by heating, cooling, and dehumidification, kW CH3COOK Potassium acetate i The gas to be tested, consisting process air and regeneration air CH3COONa Sodium acetate pC The specific heat capacity, kJ/(kg·K) HCO2K Potassium formate latentW The energy consumption of latent heat, kW PG Propylene glycol eR The water removal value of per unit electricity, kJ/g https://www.sciencedirect.com/topics/engineering/reducing-energy-consumption https://www.sciencedirect.com/topics/engineering/reducing-energy-consumption Review article [Dmin]OAC 1,3-dimethylimidazolium acetate eC The electricity power consumption, kW [Emin]BF4 1-Ethyl−3-methylimidazolium Tetrafluoroborate d Humidity ratio, g/kg [Emin]OAC 1-Ethyl-3-methylimidazolium ratioA The area ratio of desiccant wheel PVP Polyvinylpyrrolidone pA The area of process air, m2 TiO2 Titanium dioxide rA The area of regeneration air, m2 MOFs Metal organic Framework latentCOP The coefficient of latent heat performance of the dehumidification system PVA Polyvinyl alcohol PP Payback period, year PEI Polyetherimide I initial investment, $ PAN Polyacrylonitrile ep Electricity price, $/kWh PDMS Polyacrydimethylsiloxane x conventional energy PES Poly(ether sulfone) xC Conventional energy consumption SPEEK Sulfonated poly (ether ketone) xp Conventional energy price NTU Number of transfer unit 2COm CO2 emissions, kg RH Relative humidity eEF Carbon emission factor of electricity, kgCO2/kWh CO2 Carbon dioxide xEF Carbon emission factor of conventional energy HVAC Heating, ventilation, and air conditioning dsurface Air humidity ratio of the saturated air layer on the surface of the solution, g/kg COP Coefficient of performance Rd Dehumidification rate (moisture removal capacity), kg/s Mp Flow rate of process air, kg/s Pv Water vapor partial pressure of the saturated air layer on the surface of the solution, kPa P System operation pressure, kPa 1. Introduction Paris Agreement, which aims to restrict global climate warming to 1.5 °C, signifies a crucial commitment. The presence of hot and humid air is a contributing factor to the increased energy demand for operating heating, ventilation, and air conditioning (HVAC) systems, which accounts for approximately 20%–40 % of the total energy consumption in buildings [1]. The greenhouse gas emissions from sensible load amount to 599 Mt CO2, constituting 31 % of total https://www.sciencedirect.com/topics/engineering/humid-air https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib1 Review article air conditioning emissions and 53 % of cooling energy emissions [2]. Meanwhile, humidity serves as a crucial indicator in both building and industrial settings, as it plays a significant role in maintaining thermal comfort, physical well-being, and industrial productivity. According to ASHRAE Standard 55–2017, the maximum allowable air humidity ratio in a HVAC system is 12.0 g/kg (within the dry bulb temperature range of 20 °C–28 °C). Low-humidity requirements in industrial applications, including production, transportation, and storage, are detailed in Table 1. The design and selection of dehumidification systems are of particular significance in light of the rapid expansion of global industrialization and construction, taking into account factors such as dehumidification performance and energy consumption. Table 1. Summary of humidity requirements in low-humidity industry. Industry Specific scenarios Air humidity ratio Air dew point temperature Reference Lithium battery Coating and rolling slitting 0.35–3.47 g/kg −26.13 to −1.01 °C [3] Drying and electroy 15 ppmv −60 to −35 °C [4] Formation 0.01–0.23 g/kg −60 to −30 °C [3] Food Meat processing 2.27–4.56 g/kg −6.02–2.6 °C Brewery production 2.27–3.03 g/kg −6.02–2.63 °C Chocolate storage 0.31–1.38 g/kg −27.24–11.61 °C [5] Chemical Natural gas production and dehydration 143 ppmv −40 °C [6] Medical Effervescent tablets 2.03–3.40 g/kg −7.28 to −1.27 °C [3] Soft capsules 2.74–5.56 g/kg −3.8 to −5.4 °C Medical Device Gas 0.04 g/kg −45 °C [7] Storage Low temperature cold storage 0.63 g/kg −20 °C [8] Film library and audio-visual library 3.47–4.51 g/kg −1.02 to −2.46 °C [3] Seed warehouse 1.34–2.68 g/kg −11.95 to −4.09 °C Conventional air conditioning systems are designed to manage both the latent and sensible loads of chilled water (7°C–12 °C) or refrigerant. The optimal dew point temperature can be reached https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib2 https://www.sciencedirect.com/science/article/pii/S1364032124000017#tbl1 Review article as low as 5 °C using chilled water at 7 °C. When the surface temperature of the cooling coil falls below 0 °C, water vapor condenses and freezes on the coil's surface, leading to a decrease in cooling efficiency and the dehumidification effect. A dehumidification-hybrid air conditioning system is characterized as a control system that is independent of temperature and humidity, and it manages the latent load through the dehumidifier and the sensible load through the cooling unit. By elevating the temperature of chilled water and substituting R134a refrigerants with LiCl solutions, the dehumidification-hybrid air conditioning system achieves a reduction in carbon dioxide emissions. The dehumidification-hybrid air conditioning system is capable of attaining a dew point temperature of 0 °C. Dehumidification has garnered considerable attention in the last five years, as illustrated in Fig. 1. A search on the Web of Science using the keyword "dehumidification" yielded a total of 2335 articles published between 2017 and 2023, with the annual number of publications showing a consistent increase (except for 2023). Two-thirds of the publications are attributed to developing countries, with China and India contributing 41.12 % and 10.36 % of the total, respectively. This discovery indicates that dehumidification systems have significant potential for growth in developing nations. Following a more rigorous screening process that excluded articles related to desalination and those that did not comply with the criteria, only 61 review articles related to dehumidification were identified. Out of all the review articles, only 11.5 % have comprehensively examined the materials, components, and systems. Consequently, the most recent and widely referenced articles on various dehumidification technologies were chosen for analysis. As indicated in Table 2, numerous review studies have undertaken a thorough examination of the contemporary dehumidification systems. In the literature review, all the studies thus far have demonstrated that a dehumidification-hybrid air conditioning system can achieve dehumidification at a lower dew point temperature. However, due to the intricate characteristics of dehumidification systems, a comprehensive summary of the impacts of dehumidification capacity, energy, economy, environment, and dehumidifier design has not yet been achieved. Consequently, it is imperative to address the following areas of knowledge deficiency. https://www.sciencedirect.com/science/article/pii/S1364032124000017#fig1 https://www.sciencedirect.com/science/article/pii/S1364032124000017#fig1 https://www.sciencedirect.com/science/article/pii/S1364032124000017#tbl2 Review article Fig. 1. Summary of dehumidification publications from 2017 to 2023 (Web of Science). Table 2. Critical analysis of dehumidification technologies in recent literature reviews. Dehumidification technology Review content Gap Publish year Reference Solid desiccant dehumidifiers Desiccant materials properties, dehumidifiers, regenerators, dehumidification performance, and energy consumption Absence in comparative analysis of different materials and systems, limited application scenarios to building 2020 [9] Solar powered solid desiccant-vapor compression hybrid cooling system Modeling and effects of system performance parameters Absence in impact of solar energy regeneration on dehumidification performance, without detailed consideration of investment payback period 2018 [10] Review article Dehumidification technology Review content Gap Publish year Reference Desiccant air- conditioning systems System configurations, operating processes, and performance indicators Design and control optimizations of system Absence in optimized design for desiccants and dehumidification components 2021 [11] Solid desiccant-based dehumidification systems Configurations, techniques, and current trends of system Absence in evaluation models for solid desiccant dehumidification system 2022 [12] Liquid desiccant dehumidification system Correlations and enhancement approaches for heat and mass transfer of system Absence in optimization researches on system configurations 2019 [13] Liquid desiccant air dehumidification Materials, components, systems and optimization methods Absence in practical engineering application researches 2020 [14] Membrane-based air dehumidification Membrane characteristics, membrane configurations, membrane-related mass transport mechanism, system design and operation, and the mass transfer modeling Absence in research progress beyond membrane materials 2018 [15] The dehumidification capacities of various technologies differ, and this study introduces the concept of deep dehumidification. Deep dehumidification is characterized by the adherence to strict outlet humidity thresholds. While various disciplines have contributed to the definition of these ranges, the primary emphasis has been on parameters such as humidity ratio, dew point temperature, and relative humidity. A thorough examination of these profound dehumidification definitions, as outlined in Table 3, demonstrates significant variation among the studies. Absolute humidity is a parameter that precisely represents the amount of water vapor present in a specific volume of air. The relative humidity frequently fluctuates in response to changes in environmental temperature, and there is a direct correlation between the logarithm of the air dew point temperature and the humidity ratio in the air [16]. The majority of studies have defined the deep dehumidification range as being below 6 g/kg. This is due to the fact that https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib16 Review article the use of chilled water at 7.0 °C can efficiently control indoor humidity levels to as low as 6 g/kg. It would be difficult to decrease the air humidity ratio below this value, given the necessary humidity difference of 2.0–5.0 g/kg for mass transfer [17]. Given the varied interpretations of deep dehumidification across different disciplines, this study establishes deep dehumidification as the condition in which the supply air possesses a humidity ratio below 6 g/kg or an air dew point temperature of 5 °C. Table 3. Summary of research on different deep dehumidification ranges. Deep dehumidification ranges System description Reference Dehumidify the air from 19.26 g/kg to 7.53 g/kg LiCl–H2O solution [18] Air humidity below 7 g/kg No requirements [19] Dehumidify the air from 19 g/kg to 6.8 g/kg LiCl–H2O solution [20] Air humidity below 6.2 g/kg No requirements [21] Air humidity below 6 g/kg Silica gel desiccant wheel [22] Air humidity below 5 g/kg No requirements [[23], [24], [25]] Dehumidify the air from 19 g/kg–14 g/kg to 4 g/kg Two-stage desiccant wheel [26] Air dew point temperature: −20°C to −65 °C No requirements [27] Air relative humidity below 60 % Cooling and solid desiccant [28] Different from previous review efforts, this study is structured in a manner that systematically addresses the following concerns. (1) The proposal outlines the concept of deep dehumidification, emphasizing experimental and simulation research on materials, components, and systems. (2) This research analyzes the significant advancements made by various methods in enhancing dehumidification performance and decreasing energy consumption in systems in recent years. The objective is to design multifunctional systems for future applications. (3) This study examines the structural dimensions, dehumidification performance, energy performance, economic aspects, and environmental impact of various dehumidification systems in practical applications. Design recommendations for appropriate solutions in various areas with dehumidification needs are also introduced. The substantial body of work serves as the basis for conducting quantitative analysis and making recommendations to further investigate the development of the deep dehumidification https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib17 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib18 Review article system. The technical overview of this review study is depicted in Fig. 2. The evaluation models for dehumidification systems are established in Section 2. The advanced method for reviewing and improving desiccant materials, components, and systems is summarized in Section 3. The multifunctional development of dehumidification systems is primarily elucidated in Section 4. Subsequently, Section 5 examines the dehumidification, energy, economic, and environmental performance of various dehumidification systems. The aim of this study is to provide valuable insights and recommendations for future use by researchers and practitioners working in low-humidity environments. Fig. 2. Schematic representation of review methodology in deep dehumidification research. 2. Evaluation models of dehumidification system The research methodologies for dehumidification-hybrid air conditioning systems mainly consist of experimental approaches and simulation techniques. The dehumidification, regeneration, and cooling processes all utilize principles of energy and mass conservation to develop models for heat and mass transfer. 2.1. Dehumidification performance models https://www.sciencedirect.com/science/article/pii/S1364032124000017#fig2 https://www.sciencedirect.com/science/article/pii/S1364032124000017#sec2 https://www.sciencedirect.com/science/article/pii/S1364032124000017#sec3 https://www.sciencedirect.com/science/article/pii/S1364032124000017#sec4 https://www.sciencedirect.com/science/article/pii/S1364032124000017#sec5 Review article To enable the comparative analysis of dehumidification performance across various systems, three metrics are introduced for assessment: outlet humidity ratio, dehumidification rate, and dehumidification efficiency, which can be calculated using Eqs. (1), (2): )( ,, outpinppd ddMR −= (1) inequinp outpinpd dd dd ,, ,, − − = (2) Where, dR is the dehumidification rate (moisture removal capacity), kg/s; pM is the flow rate of process air, kg/s; inpd , and outpd , are the humidity ratio of the process air at the inlet and outlet, respectively, g/kg; inequd , is the air absolute humidity ratio in equilibrium with the inlet solution at its temperature and concentration, g/kg; d is the dehumidification efficiency (effectiveness). 2.2.1. Energy models The energy consumption of a dehumidification system primarily originates from components such as compressors, fans, and regeneration heaters. To account for the energy consumption and COP (coefficient of performance) of the dehumidification system, the energy models are provided below: t tt W QCOP = (3) ipii t= CMQ (4) latent outpinpp W ddCMCOP )( ,, − = p latent (5) Where, tCOP is the coefficient of performance of the total dehumidification system; tQ is the total energy consumption of the heat exchange, kW; tW is the energy consumption by heating, cooling, and dehumidification, kW; i is the gas to be tested, consisting of process air https://www.sciencedirect.com/science/article/pii/S1364032124000017#fd1 https://www.sciencedirect.com/science/article/pii/S1364032124000017#fd2 Review article and regeneration air; pC is the specific heat capacity, kJ/(kg·K); latentW is the energy consumption of latent heat, kW; latentCOP is the coefficient of latent heat performance for the dehumidification system. 2.1.2. Economic models The economic viability of a dehumidification system is primarily assessed by comparing the amount of moisture removed per unit of electrical power consumption and the payback period for different systems, as presented below: d e e R CR = (6)  −− = xxeep pCpCS IP (7) Where, eR is the water removal value - per unit electricity, kJ/g; eC is the electricity power consumption, kWh; PP is the payback period, year; I is the initial investment, $; ep is the electricity price, $/kWh; x is conventional energy, which includes coal, natural gas, and etc; xC is the conventional energy consumption; xp is the conventional energy price. 2.1.3. Environmental models The environmental performance of a dehumidification system is primarily assessed by comparing the levels of CO2 emissions from different systems, as presented below:  += xxeeCO EFCEFCm 2 (8) Where, 2COm is the CO2 emissions, kg; eEF is the carbon emission factor of electricity, kgCO2/kWh; xEF is the carbon emission factor of conventional energy. 3. Deep dehumidification technology 3.1 Air compression dehumidification The principle underlying air compression dehumidification is that, during the compression process in the air compressor, moist air condenses into liquid water. When subjected to wet air Review article pressure, the value of poutd decreases accordingly. The pattern of alteration in air humidity ratio persists even as the air pressure rises from 0.2 MPa to 0.6 MPa, as depicted in Fig. 3. The inlet air humidity ratio decreases from 3.2 g/kg to 1.2 g/kg in the LiBr solution dehumidification system and from 3 g/kg to 1.1 g/kg in the LiCl solution dehumidification system. In high- temperature and high-humidity environments, it is essential to cool and pre-dehumidify the compressor inlet in order to ensure the safe and energy-efficient operation of air compressors. In order to achieve more profound dehumidification, researchers have utilized techniques including refrigeration dehumidification and the combination of dehumidifiers with air compression technology. Fig. 3. Effect of air pressure on outlet air humidity ratio. 3.1.1. Compressed air hybrid refrigeration dehumidification When the temperature of compressed air rises, it is often necessary to employ compressed dehumidification in conjunction with refrigeration dehumidification. The schematic diagram of the compressed air refrigeration dehumidification system is depicted in Fig. 4. The ADL-5000W refrigerated compressed air dehumidifier, manufactured by China's KL Company, is capable of handling 510 Nm3/min of air and consumes 90 kW of power [29]. At a pressure of 0.7 MPa, the pressure dew point may vary between 2 °C and 10 °C, while the atmospheric dew point may range from −25 °C to −17 °C. The process of compressing air results in a significant amount of heat generation, leading to an average outlet temperature of 110 °C. Yang et al. [30] introduced a cascade LiBr/H2O absorption refrigeration/transcritical CO2 system designed to utilize 90–150 °C low-grade waste heat. When the temperature of the driving source decreases, there is a sharp decrease in COP. Moreover, exceeding a LiBr solution concentration of 60 % poses a risk of crystallization [31]. The payback period for the LiBr–H2O double evaporation-absorption heat pump is less than 2 years, and the maximum COP could reach 1.95. In order to https://www.sciencedirect.com/science/article/pii/S1364032124000017#fig3 https://www.sciencedirect.com/science/article/pii/S1364032124000017#fig4 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib29 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib30 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib31 Review article improve operational efficiency, it is important to evaluate the impact of H2O–LiBr absorption chillers on waste heat recovery, the need for a heat source in an adsorption refrigeration system, the economic payback period, energy, exergy, and thermoeconomic analysis, as well as the cooling and heating performance. Fig. 4. Conceptual diagram of compressed air hybrid refrigeration dehumidification system. 3.1.2. Compressed air hybrid solid desiccant dehumidification Solid desiccants, due to their high adsorption capacity, have the potential to reach dew point temperatures as low as −50 °C or even lower [32]. To attain lower dew point temperatures, the use of solid desiccant dehumidification technology with compressed air is frequently utilized [33]. High-flow compressed air technology is frequently utilized in industries such as chemical production. In such applications, the presence of dust particles in the air compressor and inlet can lead to the condensation of moisture, causing the particles to become wet. This may lead to the hindrance of the efficient functioning of the desiccant wheel [34]. The presence of dissolved acidic pollutants can lead to the corrosion of the internal components of the impeller. Compressed air hybrid solid desiccant dehumidification systems are frequently utilized in a packed bed configuration, as illustrated in Fig. 5. Chen et al. [35] employed a UIO-66 (MOF) desiccant to dehumidify the moist compressed air, resulting in a dew point temperature ranging from −20 °C to −30 °C. Subsequently, a zeolite desiccant was employed to further reduce the dew point to −40 °C to −70 °C under conditions of low relative humidity. The strong interaction between zeolite and H2O, along with its capacity to absorb water without causing damage to the crystal structure, enables it to maintain a high adsorption capacity even at low humidity and high temperatures. Zhang et al. [36] proposed a compressed air hybrid solid desiccant dehumidification system, which consisted of two adsorption devices filled with activated alumina particles. The system attained an outlet humidity ratio of less than 0.39 ppm (0.38 g/m3) at a dew point temperature of −70 °C. Notably, the payback period for the investment in this system was exceptionally short, at 0.55 years. However, a difficulty emerged when employing adsorbent materials with elevated regeneration temperatures, as there could https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib32 https://www.sciencedirect.com/topics/engineering/solid-desiccant https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib33 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib34 https://www.sciencedirect.com/topics/engineering/impeller https://www.sciencedirect.com/topics/engineering/compressed-air https://www.sciencedirect.com/science/article/pii/S1364032124000017#fig5 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib35 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib36 Review article be a discrepancy between the regeneration heat available and the necessary driving heat. This discrepancy has the potential to impact the efficiency of dehumidification. External factors such as meteorological conditions and load demand exerted a notable impact on the efficiency of the compressed air system. Under the specified working conditions of 0.60 MPa pressure and 39.7 % relative humidity of the inlet air, it was observed that for every 10 °C rise in inlet air temperature, the energy demand of the air compressor increased by 5.3 % and the energy efficiency decreased by approximately 5.3 % [37]. A compressed air energy storage system serves as an efficient approach for mitigating load fluctuations and facilitating peak shaving. Huang et al. [38] introduced a variable-speed compressed air energy storage system based on a double-fed induction machine, which has the potential to improve off-design energy efficiency by 6 %. Guo et al. [39] investigated the transient characteristics and control methodology of a compressed air energy storage system with the objective of reducing load balancing time and minimizing load overshoot. Fig. 5. Conceptual diagram of compressed air hybrid solid adsorption bed system [35]. . 3.1.3. Compressed air hybrid liquid desiccant dehumidification The dew point of the supply air in a liquid desiccant dehumidification system usually falls within the range of −20 °C–0 °C [40]. To attain reduced humidity levels and increased mass transfer, it is essential to utilize the suitable flow rates of compressed air and liquid desiccant. The diagram illustrating the configuration of the compressed air hybrid liquid desiccant dehumidification system can be observed in Fig. 6. The dehumidification performance of the system will rapidly increase when air is compressed, as demonstrated in Eq. (9). Elevated pressure amplifies the interface disturbance between the solution and the air, thereby augmenting the driving force for mass transfer. It is essential to dynamically assess the pressure fluctuations' intensity in compressed air liquid desiccant systems, vapor locks at the interface, https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib37 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib38 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib39 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib35 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib40 https://www.sciencedirect.com/science/article/pii/S1364032124000017#fig6 https://www.sciencedirect.com/science/article/pii/S1364032124000017#fd9 Review article and the restoration of pressure in the trapped pores. Additionally, it is important to consider the effect of water/air ratio and the sensitivity of air/moist non-equilibrium phase transition cooling to transient characteristics. 𝑑𝑠𝑢𝑟𝑓𝑎𝑐𝑒 = 0.622 𝑃𝑣 𝑃−𝑃𝑣 (9) where dsurface is air humidity ratio of the saturated air layer on the surface of the solution, g/kg; Pv is water vapor partial pressure of the saturated air layer on the surface of the solution, kPa; and P is system operation pressure, kPa. Fig. 6. Schematic chart of compressed air hybrid liquid desiccant dehumidification system [41]. The size of the equipment and the efficiency of dehumidification are significantly influenced by the ratio of liquid desiccant-to-air flow rate. A significantly large installation space would be necessary for the regeneration unit if the liquid desiccant-to-air flow rate ratio is set at a high value. In terms of investment and maintenance expenses, this may not be deemed acceptable. The performance of the compressed air hybrid liquid desiccant dehumidification system, including its COP and dehumidification capacity, deteriorates rapidly as the ratio of liquid desiccant to air decreases. A lower liquid desiccant-to-air ratio accelerates the regeneration rate, whereas a higher ratio promotes dehumidification [42]. Fang et al. [43] introduced a multi-stage internally cooled liquid desiccant dehumidifier with a liquid desiccant-to-air flow rate ratio ranging from 0.05 to 0.18, which is lower than the typical range of 0.25–2.5. The maximum experimental latent effectiveness reached 1.02, representing a two-fold increase compared to that of existing dehumidifiers. Naik and Muthukmar [44] introduced a mixed logistic regression model to forecast the variation in specific humidity within the dehumidifier and regenerator systems, based on operational parameters such as inlet desiccant concentrations and temperature, inlet air temperature, humidity ratio, and liquid desiccant-to-air flow ratio. The careful selection of the airflow configuration has a significant impact on the efficiency of https://www.sciencedirect.com/topics/engineering/partial-pressure https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib41 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib42 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib43 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib44 Review article heat and mass exchange. Flow types encompass parallel flow, counter-flow, and cross-flow. Significantly, the efficiency of the counter-flow module exceeded that of the cross-flow type by almost 10 % [13]. The cross-flow configuration led to increased mixing and turbulence, which in turn improved dehumidification performance by 4–8% compared to the parallel flow configuration [162]. Accurate correlations for mass transfer between liquid desiccant and air are crucial for the operation of the system. Qi et al. [45] introduced a new mass transfer correlation with improved predictive accuracy (20%–30 % overall prediction errors), taking into account flow dynamics, the Marangoni effect, and the conditions of liquid/air contact that influence interface characteristics and wetting factors. This approach aims to address the limitations of existing mass transfer correlations based on experimental data. The contact area between the liquid and air, as well as the instability of the film, were observed to increase with higher liquid Reynolds numbers. By reducing the contact angle from 90° to 10°, the wetting factor almost doubles, resulting in a significant enhancement in mass transfer efficiency. This occurs despite the modest increase in the Sherwood number due to the suppression of film instability. Table 4 provides a comprehensive summary of compressed air liquid desiccant dehumidification systems. Utilizing waste heat from an air compressor can facilitate the regeneration of the liquid desiccant at a temperature of 70 °C [46]. The heat required for regeneration constituted approximately 30 %–35 % of the waste heat that was available [41]. The values for Wc and RE in this system were 10.1 % lower and 0.52 kJ/g lower, respectively, compared to those of the compressed air hybrid refrigeration dehumidification system operating at a pressure of 0.3 MPa [47]. Table 4. Summary of research on the compressed air hybrid liquid desiccant dehumidification system. Solution type Humidity ratio Atmospheric dew point Pressure dew point Application Energy consumption Reference LiCl (0.8 MPa, 46 %) 2.2 g/kg −29.1 °C −7.7 °C Industrial gas Wlatent: 0.46 kW [41] LiBr (0.8 MPa, 58– 63 %) 1.05 g/kg −28.34 °C −6 °C Low dew point industrial scenarios COPt: 2.254 [48] LiCl (0.5 MPa, 34– 40 %) 0.9 g/kg −19 °C −1 °C Pneumatic and measuring instruments Ce: 1.35 kW Re: 9.74 kJ/g [47] 3.2 Liquid desiccant dehumidification https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib13 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib162 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib45 https://www.sciencedirect.com/science/article/pii/S1364032124000017#tbl4 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib46 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib41 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib47 Review article 3.2.1. Liquid desiccant materials Liquid desiccant systems mitigate irreversible losses and demonstrate favorable thermodynamic effectiveness across a broad spectrum of air treatment conditions. The principle of liquid desiccant dehumidification is grounded in the disparity between the saturation vapor pressure of water vapor at the surface of the desiccant solution and the saturation vapor pressure of water vapor in the air at the equivalent temperature. The oxygen-containing functional group of water in the adsorption layer is oriented in the "oxygen-down" configuration, and as additional water molecules are adsorbed, clusters of hydrogen bonds are formed [49]. The disparity in vapor pressure facilitates the movement of water molecules from the atmosphere to the desiccant solution, leading to the dehumidification of the air. The surface vapor pressure of a solution is influenced by both its temperature and concentration, as illustrated in Fig. 7. An increase in the temperature of the solution or a decrease in its concentration results in a corresponding increase in the surface vapor pressure. Wang et al. [50] conducted a study on the small-scale liquid desiccant dehumidification process, highlighting the importance of considering the interconnected processes of absorbent heating and volume expansion in liquid dropout. The study reveals that the droplet growth rate and final expansion ratio are influenced by the relative humidity and ambient temperature. Fig. 7. Relationship between temperature, mass fraction, and equilibrium vapor pressure in dehumidification [51]. Table 5 presents a comprehensive overview of the parameters related to liquid desiccants. During the initial years, dehumidification systems utilized a solution of triethylene glycol (TEG). However, the system's stability was compromised as a result of the elevated viscosity of TEG [53]. Consequently, it is being gradually substituted by metal halide solutions such as LiCl, LiBr, and CaCl2. At identical temperatures, the dehumidification efficiency can be ordered from highest to lowest as follows: LiCl > LiBr > CaCl2 [54,55]. The addition of phase-change materials to an aqueous LiCl desiccant is a significant approach for enhancing dehumidification efficiency. The vapor pressure of the mixture is determined to be 36.4 % lower than that of the https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib49 https://www.sciencedirect.com/science/article/pii/S1364032124000017#fig7 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib50 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib51 https://www.sciencedirect.com/science/article/pii/S1364032124000017#tbl5 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib53 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib54 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib55 Review article pure LiCl desiccant [56]. CaCl2 demonstrates a relatively high equilibrium vapor pressure when exposed to high-temperature conditions. However, its extensive use is constrained by the unstable conditions that are contingent upon the inlet conditions of air and solution [57]. Owing to the propensity of metal salts to undergo crystallization, corrosion, and related issues, the use of weak acid salts like KCOOH, CH3COOK, and CH3COONa has been adopted for liquid dehumidification. At temperatures ranging from 45 °C to 65 °C, the vapor pressure of a KCOOH solution with a concentration of 64.3 wt% to 74.3 wt% is comparable to that of a LiCl solution with a concentration of 33 wt% to 38 wt% [58]. However, weak acids are limited in their capacity to dehumidify due to their high vapor pressure values, even when compared to solutions of LiCl and LiBr at equivalent concentrations. In a single solution, there are unavoidable disadvantages, including corrosion, high vapor pressure, instability, and expense. The composition of the composite solution offers a novel approach for enhancing the dehumidification efficiency of liquid desiccants. For instance, the dehumidification efficiency of LiCl/CaCl2 solution blend exceeds that of LiCl solution by over 20 % [59].The optimal concentration ratio for the mixed solution was determined through experimental investigation [57]. Zhao et al. [60] conducted a comparison of the dehumidification performance of various composite liquid desiccants and found that LiCl–MgCl2 solution exhibited the highest dehumidification performance at equivalent concentrations. Li [61] carried out experimental research utilizing TEG-LiCl solution, PG-LiCl solution, TEG-LiBr solution, and PG-LiBr solution. The vapor pressure of the composite solutions was determined to be lower than that of the individual organic solutions, providing additional evidence for the potential use of organic-salt mixed solutions as desiccants in liquid dehumidification systems. Table 5. Comparative analysis of parameters across different liquid desiccants [40,46,52]. Solution type Dew point temperature (°C) Concentration (%) Toxicity Corrosiveness Stability Price ($/ton) Appliance CaCl2 -3 to −1 40–50 No Medium Stable 284–721 Urban gas DEG −15 to −10 70–95 No Weak Stable Higher than saline solution General gas https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib56 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib57 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib58 https://www.sciencedirect.com/topics/engineering/high-vapor-pressure https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib59 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib57 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib60 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib61 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib46 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib52 Review article Solution type Dew point temperature (°C) Concentration (%) Toxicity Corrosiveness Stability Price ($/ton) Appliance Gycerol 3 to −15 70–100 No Weak Oxidative decompositio – Industry gas Phosphoric acid −15 to −4 80–95 Yes Strong Stable – Laboratory hygroscopic agent Caustic soda Caustic calcium −10 to −4 – Yes Strong Stable – Industrial compressed gas Sulfuric acid −15 to −4 60–70 Yes Strong Stable – Chemical device LiCl −20 to −5 30–50 Yes Medium Stable 7400–8418 Textile, pharmaceutical LiBr −20–0 25–65 Yes Strong Stable 3316–3660 Pharmaceutical, laboratory Ionic liquids have garnered attention in research due to their exceptionally low vapor pressure, non-crystallizability, and low corrosiveness. Under comparable circumstances, [EMIM]BF4 and [Dmim]OAc demonstrate marginally reduced dehumidification efficacy in comparison to LiBr solutions [62,63]. However, the enhancement of dehumidification performance could be achieved by increasing the mass concentration of the ionic liquid solution. Li et al. [64] enhanced the dehumidification efficiency of LiBr and LiCl solutions through the addition of an ionic liquid. The results indicated that the inclusion of [Dmim]Cl had a more pronounced impact in comparison to [Dmim]BF4. Nevertheless, the practical utilization of ionic liquids is constrained by their elevated viscosity, degradability, and toxicity [65]. The induction of the Marangoni effect through the addition of specific chemical surfactants or coating hydrophilic materials to the liquid desiccant can reduce the contact angle and increase the wetting area, thus enhancing the efficiency of water vapor absorption [66]. However, the majority of current surfactants possess odors, are toxic, and are not suitable for direct use in air dehumidification applications. Wen et al. [67,68] conducted a study on a novel non-volatile, odorless, and non-toxic surfactant (PVP–K30), which demonstrated the ability to enhance both Rd and ηd by 22.7 % and 19.9 %, respectively, with the addition of only 0.4 % of PVP-K30. Dong et al. [69] performed experimental research to investigate the impact of TiO2 coating on the reduction of the contact angle of the solution from 84.6° to 8.8°, as well as the increase in the values of Rd and ηd by 1.60 and 1.63, respectively. Simultaneously, it has the potential to achieve an annual 5.7 % reduction in electricity expenses compared to structures lacking TiO2 coating. The uniform dispersion of nanofluids in liquid dehumidifiers has been shown to significantly enhance heat and mass transfer capabilities [70]. Wen et al. [71,72] utilized https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib62 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib63 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib64 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib65 https://www.sciencedirect.com/topics/engineering/hydrophilicity https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib66 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib67 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib68 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib69 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib70 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib71 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib72 Review article mechanical and chemical techniques to disperse multiwall carbon nanotubes in LiCl solutions, resulting in an average relative enhancement of 25.9 % for nanofluids in dehumidification and 24.7 % in regeneration. Low-temperature liquid desiccants exhibit a reduced surface vapor pressure. Low-temperature conditions enhance the driving force and prolong the duration of interaction between the solution and air [73]. Similar to ambient atmospheric temperatures and high-concentration solutions, low temperatures and low-concentration liquid desiccant solutions have the capability to dehumidify [74]. Zhang et al. [75,76] conducted a study and noted that as the specific heat ratio between air and solution approaches 1, it improves both heat and mass transfer capabilities. The adjustment of solution flow rates in both the dehumidifier and regenerator units could be accomplished by utilizing two pumps with different rated flow capacities to tackle heat and mass transfer concerns [23]. A higher flow rate of the solution contributes to improved dehumidification performance; however, it is important to carefully consider the balance between ηd and pump power consumption. 3.2.2. Components and systems Fig. 8 depicts the flowchart of the liquid desiccant dehumidification system. Various types of liquid regenerators encompass packed beds, falling film, spray water, and membrane-based configurations. In order to enhance the dehumidification effectiveness of liquid desiccants, researchers have utilized a variety of approaches to achieve more thorough dehumidification. Fig. 8. Conceptual diagram of liquid desiccant dehumidification system [77]. Internal cooling dehumidifiers have the potential to significantly lower the regeneration temperature of solutions, as well as reduce air pressure drop and droplet transport [78,79]. Shah et al. [80] performed an experimental investigation to compare the dehumidification effectiveness of adiabatic dehumidifiers and internally cooled dehumidifiers using a CaCl2 liquid dehumidifier concentration of 40 %, with ηd value of 67 % and 95 %, respectively. Lun et al. [81] introduced a self-cooling method by incorporating absolute ethanol into LiCl solution to uphold a consistent temperature. The droplets are dispersed into the regeneration chamber via the intense vibration and atomization of the ultrasonic transducer, thereby substantially increasing the surface area https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib73 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib74 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib75 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib76 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib23 https://www.sciencedirect.com/science/article/pii/S1364032124000017#fig8 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib77 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib78 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib79 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib80 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib81 Review article for heat and mass transfer. Yang et al. [82] employed ultrasonic atomization technology, which resulted in a 4.4 °C reduction in regeneration temperature and a 23.4 % decrease in energy consumption during the regeneration process. Yang et al. [83] proposed the integration of an internal cooling system with an ultrasonic atomization liquid desiccant system and defined a suitable cooling power range to enhance cost-effectiveness. Nevertheless, the limitations resulting from ultrasonic atomization, which transports droplets, have hindered its widespread implementation and require further validation. Multi-stage solution dehumidifiers exhibit higher exergy efficiency and a lower outlet humidity ratio compared to single-stage solution dehumidifiers [84]. Peng and Zhuo [85] introduced a hybrid-connected double-stage dehumidifier system. The system employed a CaCl2 solution for pre-dehumidification, whereas LiCl solution was used for deep dehumidification experiments. The regeneration temperature experienced a decrease of 12.2 °C, leading to a slight improvement in the exergy efficiency of the system [86,87]. 3.3. Solid desiccant dehumidification 3.3.1. Solid desiccant materials Solid desiccants eliminate moisture by adsorbing it into their surface pores, with two primary categories identified: physical adsorption and chemical adsorption. During physical adsorption, the adsorbate molecules predominantly establish hydrogen bonds with the surface of the adsorbent, resulting in the interaction of van der Waals forces between the molecules during the physical adsorption [88]. In contrast, solid desiccants like calcium chloride and lithium chloride remove moisture by transforming into crystalline hydrates and forming adsorption chemical bonds. Table 6 provides a comprehensive summary of solid desiccant parameters. Silica gel has been extensively utilized due to its cost-effectiveness, although it is recognized for its comparatively lower adsorption capacity and higher adsorption heat [89]. Activated carbon demonstrates a specific water vapor adsorption capacity only at elevated relative humidity levels, and it is rarely utilized as a sole material for dehumidification [90]. Activated alumina, due to its high affinity for water, has the capability to reach a low dew point temperature of below −70 °C and is frequently employed as an industrial catalyst [91]. Zeolites and molecular sieves demonstrate strong adsorption capabilities in high-temperature and low-humidity environments, allowing for the purification of air to achieve exceptionally low humidity levels and dew point temperatures ranging from approximately −40 °C to −60 °C [92]. The desorption process presents a comparatively greater challenge, as the regeneration temperature is higher than that required for silica gel and activated alumina [93]. In contrast to physical desiccants, chemical desiccants such as metal salts exhibit strong moisture absorption capabilities. However, upon adsorption, they experience deliquescence, leading to the formation of solutions that have the potential to cause corrosive effects on metals [94]. In order to mitigate corrosion and instability caused by material deliquescence, it is feasible to enhance the adsorption capacity by 10 %–40 % through the dispersion of silica gel in a small amount of metal salt materials [95,96]. Nevertheless, an excessive quantity of metal salt materials could potentially overwhelm the sealing system during the sorption process, resulting in corrosion and leakage, thereby significantly constraining the sorption capacity. https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib82 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib83 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib84 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib85 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib86 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib87 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib88 https://www.sciencedirect.com/science/article/pii/S1364032124000017#tbl6 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib89 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib90 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib91 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib92 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib93 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib94 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib95 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib96 Review article Table 6. Comparative analysis of parameters across different solid desiccants [71,72,89,97,98]. Solid desiccant Test conditions Adsorption capacity (g/g) Regeneration temperature (°C) Specific surface area (m2/g) Silica gel 30 °C, 60 % RH 0.1–0.3 70–150 300–800 Molecular sieve/Zeolite NaX 30–40 °C, 60 % RH 0.34 250–350 400–750 NaY 25 °C, 80 % RH 0.25 NaA 25 °C, 60 % RH 0.23 YZSM-20 25 °C, 80 % RH 0.28 Activated carbon ACY-60 27 °C, 60 % RH 0.18–0.29 70 500–1500 ACs 25 °C, 60 % RH 0.25–0.50 – Composite Silica gel-Activated carbon-CaCl2 27 °C, 9 % RH 0.23 – 1117 Al2O3–SiO2 23 °C, 50 % RH 0.19 – – CaCl2/Silica gel 35 °C, 4 % RH 0.19 60–80 – CaCl2-MCM-41 25 °C, 70 % RH 0.75 90–130 325 MIL-101(Cr)- graphite oxide 25 °C, 90 % RH 1.58 60 – MOFs MIL-101(Cr) 30 °C, 60 % RH 1.5–1.7 70–80 3000–4000 MIL-101(Fe) 30 °C, 60 % RH 0.84 70–80 1549 MIL-100(Al) 30 °C, 60 % RH 0.75 70–80 1814 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib89 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib97 Review article Solid desiccant Test conditions Adsorption capacity (g/g) Regeneration temperature (°C) Specific surface area (m2/g) Silica gel 30 °C, 60 % RH 0.1–0.3 70–150 300–800 Others Dried coconut shell 32 °C, 75 % RH 0.30 – – Dried durian peel 32 °C, 75 % RH 0.17 – – The novel MOF-74 (Ni) with expanded benzene rings was developed by Pacific Northwest National Laboratory [99]. MOF material demonstrated not only a water adsorption capacity of 0.9 g/g and an adsorption capacity of 0.8 g/g for the refrigerant R134a. Zeng et al. [100] introduced a thermally responsive material that demonstrated temperature-dependent adsorption isotherms for water vapor, departing from the conventional fixed affinity. The material exhibited improved dehumidification performance in comparison to silica gel desiccants, resulting in a 30 % decrease in energy consumption for dehumidification. Materials scientists have developed water-absorbing polymers using acrylates and acrylamide, which have demonstrated a water absorption capacity of 2–3 times their initial weight [101,102]. The addition of metal salts resulted in a 2–3 times increase in the adsorption capacity of these polymers compared to the raw materials [103]. Dai et al. [104] developed a nanostructured moisture-absorbing gel dehumidification material that demonstrated an adsorption capacity of up to 1.58 g/g. The dehumidification time required to achieve the same level of dehumidification was shorter when compared to the 4A molecular sieve and silica gel. Vivekh et al. [105] developed a composite polymer desiccant that consists of polymers containing hydrophilic and hygroscopic salts. This desiccant demonstrated a 12-fold higher isothermal water absorption capacity compared to silica gel, zeolite, and MOFs. Additionally, it was capable of undergoing condensation heat regeneration at temperatures ranging from 40 °C to 50 °C. However, the synthesis of such materials presents challenges and high costs, resulting in many of them remaining in the experimental phase. Additionally, there exist environmentally sustainable desiccants, including agricultural waste and other biomass materials, which, following appropriate processing, demonstrate specific dehumidification properties [106]. The desiccants were generally unsuitable for deep dehumidification due to their relatively weak adsorption capacity, despite their cost-effectiveness, biodegradability, and environmental friendliness. The adsorption capacity is influenced by the thickness of the desiccant layer. If the desiccant layer is too thin, the contact time may be insufficient for adequate adsorption to take place. Conversely, a greater thickness of the adsorbent layer typically results in enhanced dehumidification performance. For instance, a rise in thickness from 0.05 mm to 0.2 mm has been shown to enhance dehumidification performance by 113 % [107]. However, the https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib99 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib100 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib101 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib102 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib103 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib104 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib105 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib106 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib107 Review article augmented thickness led to a corresponding increase in resistance, leading to notable pressure drop losses and reduced energy efficiency [108]. The stress applied across the material is a result of thermal expansion and contraction, as well as the binding and releasing of water molecules into and out of the desiccant microstructure. These conditions contribute to the generation of stress within the material. Therefore, it requires long-term material and performance stability, as well as favorable water sorption uptake, isotherms, and kinetics. The diagram illustrating the solid desiccant dehumidification system can be found in Fig. 9. The energy required for regenerating solid dehumidification systems is relatively high. Efforts to minimize energy usage during the process of desiccant regeneration are currently a key area of research. Fig. 9. Conceptual diagram of solid desiccant dehumidification system [32,109]. As indicated in Table 7, the techniques for regenerating solid desiccants primarily encompass waste heat regeneration [110], solar energy regeneration [111], and ultrasonic regeneration [112]. Unstable conditions in low-grade heat sources can potentially diminish the dehumidification performance within the dehumidification system [113,114]. Electrically-driven regeneration, a novel method of regeneration, has faced limitations in its performance development as a result of bubble effects and electrode erosion [115]. The use of microwave and ultrasound has been shown to effectively enhance the regeneration capacity of desiccants https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib108 https://www.sciencedirect.com/science/article/pii/S1364032124000017#fig9 https://www.sciencedirect.com/science/article/pii/S1364032124000017#tbl7 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib110 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib111 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib112 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib113 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib114 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib115 Review article and reduce the duration of the regeneration process [116]. Utilizing free renewable energy and waste heat resources can lead to a significant reduction in operational expenses. Table 7. Summary of studies on the regeneration methods. Reference Method Regeneration description Results [113] Experiment Ultrasonic power and thermal power regeneration silica gel 5 W Ultrasonic power and 20 W thermal power Short regeneration time and strong regeneration ability Regeneration temperature and energy reduced 2.7 °C and 26 %, respectively [117] Experiment Microwave and solar radiation regeneration silica gel Regeneration degree: 77.7 % 3.77 and 1.05 times higher than the solar regeneration and the microwave regeneration Max energy efficiency: 19.4 % [118] Experiment Electric furnace and microwave device regeneration activated carbons Microwave regeneration was more rapid and efficient [119] Simulation Purge gas regeneration Optimum purge angle raised COP 30 % Reduced regeneration energy: 22.7 % Saving energy efficiency: 33 % [120] Experiment Heat pump (engine exhaust gas) regeneration Low regeneration temperature and precooling increased ηd Exergy efficiency raised 31.5 % [121] Experiment Thermal regeneration silica gel When the regeneration heat decreased, the decreased in ηd could be effectively suppressed by controlling the constant regeneration temperature mode of the desiccant wheel speed [122] Simulation Solar radiation and waste heat regeneration Solar radiation 1800 W/m2 ηd : 1–5 3.3.2. Desiccant coating heat exchanger The desiccant coating heat exchanger is primarily utilized for dehumidification in residential buildings due to constraints related to latent heat load and fresh air volume. The latent heat load consists primarily of three components: the human body, fresh air, and equipment. Desiccant-coated heat exchangers encompass fin-tube heat exchangers, wire-tube heat exchangers, https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib116 https://www.sciencedirect.com/topics/engineering/heat-exchanger https://www.sciencedirect.com/topics/engineering/heating-load Review article and microchannel heat exchangers. Fin-tube heat exchangers and microchannel heat exchangers are found to reduce heat transfer coefficients by 15 % and 30 % respectively, while also leading to an increase in pressure loss by 55%–60 % compared to conventional heat exchangers [123,124]. When the depth of the fin in the desiccant coating heat exchanger is increased from 44 mm to 88 mm, there is a 40 % increase in the moisture removal capacity and a 10 % increase in COP. A lower fin pitch results in increased resistance to airflow, necessitating a higher energy input to achieve the same flow rate. The mass transfer coefficient and moisture removal rate of microchannel heat exchangers exceed those of fin-tube heat exchangers by 15 % and 150 %, respectively. Fin and tube heat exchangers exhibit a maximum heat transfer coefficient that is lower than the minimum value of the heat transfer coefficient of metal foam [125]. In comparison to the heat exchanger coated with aluminum fumarate, the mass of adsorbate increased by 66.6 % in the heat exchanger coated with composite silica gel [126]. Zeolite-coated heat exchangers have been shown to achieve regeneration at 90 °C, with a moisture removal capacity 5.4 times that of conventional systems and a COP of 0.5 [127]. The utilization of sensible heat recovery in conjunction with desiccant coating heat exchange has been shown to enhance COP by an additional 13 % [128]. 3.3.3. Adsorption bed The adsorption bed is primarily utilized in industrial dehumidification due to its simple design and ease of manufacturing. The increased irreversibility associated with multiple heat transfer resistances results in a reduced capacity of the adsorption bed compared to that of the desiccant-coated heat exchanger. The significant pressure drop requires the utilization of a high-capacity pump to facilitate flow and create additional space for the installation of equipment. The adsorption bed systems are classified into packed beds and fluidized beds based on the desiccant's movement and circulation status. Optimizing the configuration of the adsorption bed is crucial for addressing the limitations associated with poor heat transfer capability and significant pressure drops. Ramzy et al. [129] introduced the concept of an intercooled packed bed and determined that by optimizing the axial position and the length ratio of the intercooling-adsorption bed to be between 0.45 and 0.65, the total adsorbed mass can be increased by 22 %. Shamim et al. [130] introduced a multilayer fixed-bed desiccant dehumidifier without binders, which resulted in a 98 % reduction in pressure drop and a 36 % increase in dehumidification efficiency. Yeboah and Darkwa [117] concluded that helically coiled oscillating heat pipes have the potential to improve heat and mass transfer. They also found that the annular bed exhibited a higher adsorption rate compared to the full bed. Liang et al. [131] introduced an enhanced circulating inclined fluidized bed system featuring a 20 cm particle channel and a 20° incline angle, which demonstrated superior dehumidification performance and occupied a smaller volume. The energy efficiency of this system exceeded that of the circulating erect fluidized bed systems by 77.9 % and the circulating inclined fluidized bed systems by 40.1 %. 3.3.4. Desiccant wheel Due to the continuous cycle, high desiccant material utilization, high efficiency, and low pressure drop, the adsorption bed is extensively employed for dehumidification in industrial https://www.sciencedirect.com/topics/engineering/microchannel https://www.sciencedirect.com/topics/engineering/heat-transfer-coefficient https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib123 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib124 https://www.sciencedirect.com/topics/engineering/tubes-components https://www.sciencedirect.com/topics/engineering/metal-foam https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib125 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib126 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib127 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib128 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib129 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib130 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib117 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib131 Review article and building applications. Bhabhor and Jani [132] conducted a study on the dehumidification performance of various desiccant wheel channel types. They found that the sinusoidal geometry types resulted in a significant reduction in relative humidity of up to 80 %, with the average temperature of the supply air increasing by 38.98 %. This type of geometry was identified as the most effective in dehumidifying the supply process air. Multi-stage desiccant wheel dehumidification systems have been shown to effectively lower the regeneration temperature compared to single-stage desiccant wheel systems [133]. Tu and Hwang [134] carried out experimental research and discovered that decreasing the outdoor air humidity ratio from 19 g/kg to 4 g/kg resulted in regeneration temperatures of 101.7 °C and 72.7 °C for the single and double desiccant wheel systems, respectively. Asadi and Roshanzadeh [135] successfully reduced the required regeneration temperature of the desiccant wheel by 4.1 °C through the regulation of fresh air and return air volumes entering the first and second stages of the wheel. Moreover, there was a 15.5 % increase in COP and a 21.4 % increase in exergy efficiency. Tu and Hwang [134] conducted optimization by taking into account various area ratios, numbers of wheels, and proposed system configurations for diverse application scenarios, as presented in Table 8. The equation for Aratio is calculated as shown in Eq. (10): r pA AA =ratio (9) Where, ratioA is the area ratio of desiccant wheel; pA is the area of process air, m2 ; rA is the area of regeneration air, m2. Table 8. Optimal design configurations for desiccant wheel [134]. ratioA Number of desiccant wheel System configuration 1 3~4 vapour compression cycle 2 1 Electric heater or natural gas burner 1 1 ( NTU of heat recovery unit ≥ 2) With heat recovery 1 3~4 Low regeneration temperature 2 1 Low latentW or high latentCOP The process of solid desiccant dehumidification frequently entails an exothermic reaction. In order to minimize the production of adsorption heat and convert the adiabatic dehumidification process into an isothermal dehumidification process, researchers have utilized the internal cooling desiccant wheel configuration [81,136]. In comparison to conventional desiccant wheels, the dehumidification efficiency could be enhanced by 48 % [136,137]. However, this necessitated a sacrifice in air flow, as the maximum cross-sectional area for dehumidified air accounted for only 28 % of the total. This required a wheel with a diameter 89 % larger to handle the same air volume as the non-internal cooling configuration. In order to reduce the size of the double-stage desiccant wheel system, Ge et al. [138] suggested a dual-stage https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib132 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib133 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib134 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib135 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib134 https://www.sciencedirect.com/science/article/pii/S1364032124000017#tbl8 https://www.sciencedirect.com/science/article/pii/S1364032124000017#fd10 https://www.sciencedirect.com/topics/engineering/exothermic-reaction https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib81 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib136 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib136 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib137 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib138 Review article dehumidification system with a single desiccant wheel. Dividing the desiccant wheel region into four sections reduced the system length, but it also resulted in increased complexity in the air ducts. 3.4. Membrane dehumidification The membrane dehumidification method is a novel approach that exploits H2O selectivity difference between the two ends of the membrane as air flows over distinct membrane components. The water vapor permeability exceeds that of nitrogen, oxygen, and other gases by at least two times. By harnessing the selectivity of the membrane, it efficiently separates moist air from other gases, thereby achieving the separation of water vapor from dry air without generating heat. Following this, the dehumidified air can be cooled and subsequently circulated into the indoor environment with the use of a cooling apparatus. The categorization of different membrane materials is depicted in Fig. 10. Membranes can be classified based on their physical structure and chemical properties, with pore sizes of 0.1 μm and dense membranes with pore sizes of 0.1 nm, respectively [15,139]. Membranes can be classified based on their membrane material into organic membranes (polymers), inorganic membranes (zeolites, ceramics, molecular sieves), and composite membranes. Membranes can be categorized into flat sheet membranes and hollow fiber membranes based on their membrane configuration. The flat sheet membranes have a thickness of approximately 100 μm, whereas the hollow fiber membranes have a diameter of around 500 μm [140]. Hollow fiber membranes offer several advantages, such as a larger surface area and more attractive aesthetics, enhanced heat transfer efficiency, and reduced pressure drop on the gas side. When compared to flat sheet membrane modules of half the size, hollow fiber membrane modules demonstrate equivalent dehumidification performance [141]. However, the construction of hollow fiber membranes presents greater challenges due to the requirements for sealing at both ends, and they are also more prone to particle fouling [142]. https://www.sciencedirect.com/science/article/pii/S1364032124000017#fig10 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib15 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib139 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib140 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib141 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib142 Review article Fig. 10. Classification of different membranes. As shown in Table 9, the permeability and selectivity of the membrane play a crucial role in determining the dehumidification performance. Membranes possessing greater selectivity could enhance the permeation of water vapor on the permeate side to a significant extent. The membrane exhibited a selectivity range of 178 to 16,300, with a preference for higher selectivity to mitigate solution leakage and carryover [15]. Membranes with high permeability exhibit reduced operating pressures, although they are susceptible to decreased selectivity [52]. Achieving the desired airflow and dehumidification level in the supply air with low energy consumption requires a delicate balance between permeability and selectivity. It is crucial to take into account the chemical reactions involved in certain designs, including heat and moisture transfers, water-vapor permeability complexes, high packing density, and selective permeability. Table 9. Comparative analysis of different membrane material properties [15,141]. https://www.sciencedirect.com/science/article/pii/S1364032124000017#tbl9 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib15 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib52 Review article Membrane description Test conditions Permeability mol/(m2·Pa·s) H2O/N2 selectivity PVA-LiCl with coating PEI hollow fiber membrane 27–31 °C, 14.7– 16.7 g/kg 1.1 × 10−6 4240 PAN-PDMS hollow fiber membrane 28–32 °C,18– 22 g/m3 2.8 × 10−6 –4.3 × 10−6 35–220 PES-SPEEK composite hollow fiber membrane 50–65 °C, RH: 44–90 % 5.0 × 10−7–8.4 × 10−7 190000 PES hollow fiber membrane with coating polyamide 30 °C, 30– 32 g/m3 5.0 × 10−7 500 PES hollow fiber Membrane in Polyarmide Matrix of Carboxylated TiO2 Nanoparticles 30 °C,25–27 g/m3 4.5 × 10−7 486 PES hollow fiber membrane with coating CA/PEG 30 °C, RH: 20 % 1.5 × 10−8 175.5 Porous nickel sheet supported NaA zeolite flat membrane (zeolite/Ni) 32 °C, RH: 90 % 6.8 × 10−6 178 Ionic liquid [emim][BF4] membrane 31.4, 28.2 g/kg 3.5 × 10−7 16300 Polysulfone hollow membrane 32 °C, RH: 100 % 1.8 × 10−7 529 Ionic liquid [emim][DCA] membrane 25 °C, RH: 94 % 1.3 × 10−6 1000 The enhancement of water selectivity alongside the augmentation of permeability has been the subject of investigation by numerous polymer scientists. Various factors, including material composition, type, and structure (such as porosity, pore size, and thickness), have been found to exert influence on these properties. The improvement of water selectivity while increasing permeability has been studied by many polymer scientists, and various factors such as material, type, and structure (porosity, pore size, and thickness) influence these properties. As the membrane porosity and pore size increase and the membrane thickness decreases, the moisture content at the air outlet of the membrane module decreases, leading to an increase in dehumidification capacity and efficiency. However, this enhancement also brings about the potential for liquid permeation, thereby increasing the overall resistance to the mass transfer process [52]. Niu et al. [143] conducted a comprehensive analysis of the mechanical strength of membrane materials and determined that a membrane with a porosity of 0.8, a thickness of 0.1 mm, and a pore size of 180 nm exhibits the most effective dehumidification performance for crossflow parallel-plate membrane dehumidification. The inclusion of LiCl has the potential to enhance the mechanical properties of the membrane and also to augment its hygroscopic rate and capacity. The study shows that an increase in the concentration of LiCl leads to a significant improvement in the surface sealing of PVA/PEO polymer. Additionally, the shape of the polymer-formed film transitions from loose to firmly packed [144]. A greater concentration of polymer contributes to a more robust membrane structure, while a higher concentration of https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib52 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib143 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib144 Review article cross-linker increases the density of spherical particles, both of which enhance the strength of the gel [145]. It also serves to enhance the viscosity and viscoelastic moduli of the gel. As salinity increases or pH decreases, both the viscosity and viscoelastic moduli of the gel decrease. Brunauer-Emmett-Teller (BET) model and the radial-basis function artificial neural network model can be employed to determine the equilibrium adsorption capacity and the diffusion coefficient, two crucial parameters in the diffusion-based kinetic model [146]. In order to enhance system performance, it is more advantageous to focus on increasing the membrane moisture diffusivity rather than reducing the membrane heat conductivity. System performance is enhanced by a thinner membrane, although this may result in a slight increase in conductive heat loss [147]. Various studies have also endeavored to alter the membrane profile in order to enhance ηd [148,149]. Greater curvature values lead to reduced permeability flux. When the curvature radius is reduced from 30 mm to 15 mm, ηd of convex and concave membranes increases by 1.96 % and 1.74 %, respectively [149]. One potential approach to enhance the dehumidification efficiency of membranes involves altering the surface hydrophilicity through the application of additives or coatings [52]. Table 10 presents a comparison of the technical characteristics of various membrane dehumidification technologies, including membrane contactors, separation membranes, and absorption membranes. To eliminate the water vapor on the permeate side of the membrane and uphold the water vapor concentration, as depicted in Fig. 11. Table 10. Comparative analysis of different membrane dehumidification technologies [52,139]. Type Membrane contactors Separation membrane Absorption membrane Working principle Surface absorption Selective barrier Absorption materials Driving force and mechanism Concentration gradient (liquid desiccant) Pressure gradient transmembrane transport Hydrophilic property of membrane materials Key parameters Mass transport resistance Membrane wetting H2O/N2 selectivity permeability Solution diffusion Ratio and capacity of absorption Surface area Physical structure Porous, hydrophobicity, physical stability, high surface roughness, high membrane flux Dense, hydrophilic or hydrophobic, physical stability, good mechanical performance Nanofibers, hydrophily, physical stability, good mechanical performance, high surface roughness Price Low Medium High Advantage High membrane flux and ηd Modular design, high ηd, non- regeneration High membrane flux Disadvantage Regeneration expensive Porous membranes exist High energy consumption Low membrane flux Less material selectivity Significantly high price https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib145 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib146 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib147 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib148 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib149 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib149 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib52 https://www.sciencedirect.com/science/article/pii/S1364032124000017#tbl10 https://www.sciencedirect.com/science/article/pii/S1364032124000017#fig11 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib52 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib139 Review article Type Membrane contactors Separation membrane Absorption membrane membrane wetting problems Liquid transport Application Enthalpy exchanger Accurate humidity control Low energy and humidity Fig. 11. Conceptual diagram of generation in membrane permeation process. Employing the pressure difference method to elevate gas pressure necessitates greater membrane strength and increased pressure resistance in component equipment. When the operational pressure exceeds the pressure-bearing capacity of the membrane module, further increasing the working pressure will not enhance the adsorption capacity, but rather lead to damage of the membrane module. During the membrane dehumidification process, a thicker fouling layer necessitates a higher mechanical pressure. The dirt can only be removed when the pressure is sufficient. Two different methods for sweep gas can be utilized: external sweep gas and self-sweeping, which involves adjusting the membrane porosity. Increasing the flow rate of the sweep gas in a suitable manner has the potential to enhance the dehumidification performance [150]. The vacuum pumping method has extensive potential applications and has been one of the most researched methods [15]. However, vacuum pumps that operate between vacuum and atmospheric pressure require substantial energy consumption. This underscores the need for enhanced vacuum pump performance, careful pump selection, and integrated system design to minimize pump power usage. The influence of humidity on pressure reduction in a vacuum-pump system is less significant compared to the impact of temperature. As the vacuum pressure increases, the pressure decreases, leading to a reduction in the overall pumping power. For example, adjusting the vacuum pressure to a range of 0.66 kPa–2.66 kPa results in a nearly 11 % reduction in pressure drop at an air flow rate of 65 L/min [151]. Therefore, it is https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib150 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib15 https://www.sciencedirect.com/topics/engineering/airflow-rate https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib151 Review article possible to enhance dehumidification efficiency and decrease energy consumption by integrating the sweep gas with the vacuum pump [152,153]. Table 11 provides a comprehensive summary of membrane dehumidification systems. To achieve a more profound level of dehumidification, Petukhov et al. [154] were the first to employ chilled water for pre-dehumidification, followed by the use of a membrane contactor for additional dehumidification, resulting in a reduction of the air dew point temperature to −50 °C. The pre-dehumidification of outdoor air can be achieved by integrating membrane dehumidification technology with solid desiccants, followed by further dehumidification using the solid desiccants. This approach maximizes the membrane's dehumidification capacity in high humidity conditions and the solid desiccants' moisture adsorption capacity in low humidity conditions, rendering it appropriate for industrial environments with exceptionally low humidity levels. Table 11. Summary of research on different membrane dehumidification systems. Membrane type Dew point Humidity ratio Application Results Reference A tetrafluoro-ethylene Teflon AF2400 composite plate membrane contactor (TEG solution) −28 °C 86 ppmv Natural gas dehydration Higher gas flow rate and pressure increase the transmembrane water flux [155] Porous polypropylene membrane contactor heat-exchanger with liquid coolant absorbent −30 °C 2.5 % wt Pipeline transport of natural gas low- pressure pneumatic systems −30 to −50 °C dew point Heat consumption: 2.747 kW Energy consumption: 1.31 kW [154] Polydimethylsiloxane and polyvinyltrimethylsilane nonporous flat membrane (TEG solution) −30 °C 0.24 g/kg Removing carbon dioxide CO2 reduction: 10 times [156] Hollow fiber membranes -8 to −13 °C 0.01 vol% Natural gas dehydration Methane recovery: 98 % Natural gas recovery: 97 % [157] Microporous polysulfone substrate plate membrane −50 °C 3.6 mg/cm3 Air Dehydration Under 90 psig pressure operation [158] https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib152 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib153 https://www.sciencedirect.com/science/article/pii/S1364032124000017#tbl11 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib154 Review article 3.5. Factors influencing deep dehumidification technology As shown in Table 12, elevating the temperature and humidity of the incoming air results in higher humidity levels in the outgoing air and a reduction in dehumidification efficiency. The treatment of air is influenced by variations in environmental meteorological factors, and it is advisable to pre-treat outdoor air, such as through the use of pre-cooling equipment or heat exchangers. When utilizing a cooling apparatus, it is crucial to strike a balance between its cooling capacity and dehumidification effectiveness. Reducing the regeneration air temperature or enhancing the regeneration degree can also notably improve the performance of the dehumidification system [159]. Moderate temperatures for regeneration and air velocity have the potential to enhance COP of the system. An increase in air temperature, humidity, and cooling temperature results in a corresponding increase in COP. An optimal cycle time should be determined based on the experimental conditions and dehumidification capacity in order to achieve a higher COP. Table 12. Operational parameters influencing dehumidification performance [160,161]. Empty Cell tpin ↑ dpin ↑ mp ↑ Cooling water temperature↑ Cooling water flow rate↑ Regeneration air temperature ↑ Regeneration air humidity ↑ Regeneration air flow rate↑ ηd ↓ ↓ ↓ ↓ ↑ ↓ ↓ ↑ dpout ↑ ↑ ↑ ↑ ↓ ↓ ↑ ↓ 4. Multifunctional development of dehumidification systems 4.1. Air humidification Numerous studies have examined the humidification effects of dehumidification systems in arid winter regions with the aim of enhancing the utilization rate of dehumidification systems and transforming them into HVAC systems that can operate throughout the year [162]. By modifying the airflow direction and utilizing additional techniques, the system could potentially be utilized for indoor heating and humidification in the winter months. This not only resulted in a 10%–20 % improvement in thermal comfort [163,164] but also effectively decreased the spread of COVID-19 virus [165]. Cai et al. [166] proposed a heat pump liquid desiccant humidification and air conditioning system that incorporated a four-way valve for reversing the direction of refrigerant circulation. The system utilized a solution to absorb heat from the air and release moisture, thereby achieving both heating and humidification of the air. During the winter season, it was essential https://www.sciencedirect.com/science/article/pii/S1364032124000017#tbl12 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib159 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib160 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib161 https://www.sciencedirect.com/topics/engineering/humidification https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib162 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib163 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib164 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib165 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib166 Review article to uphold the solution concentration within the range of 20 %–30 % to prevent freezing problems when outdoor temperatures fell below −15 °C. Substituting the solution with water was more susceptible to freezing [167]. Su et al. [168] introduced a frost-free air-source heat pump system that integrated membrane-based solutions. The dehumidifier was utilized to extract moisture from the air in order to mitigate the occurrence of frosting. The diluted solution underwent regeneration in a membrane regenerator before being utilized for air humidification, effectively addressing concerns related to droplet carryover and frosting. The solid desiccant wheel system provides benefits such as adiabatic humidification and prevents cross-contamination with fresh air, leading to its widespread application. Kawamoto et al. [169] introduced a desiccant wheel system integrated with a heat pump. The system has the capability to increase the humidity levels of outdoor air within the range of 1.8–2.3 g/kg up to 5.8 g/kg prior to its distribution into the indoor environment. Tu et al. [170] implemented a three-stage solid desiccant wheel humidification system. When employing two control strategies, the system was capable of fulfilling more than 96 % of the humidification needs in the cold and arid climate of Urumqi. 4.2. Water harvesting In light of the global water crisis, there has been an increasing emphasis on the use of dehumidification systems for water collection, as noted in recent studies [171]. The collection of condensed water through condensation is a widely used method, and the quality of the resulting water can satisfy both industrial and household demands [165,172]. Zolfagharkhani et al. [173] employed condensation technology to generate around 26 L/day from ambient air, with an energy intensity of 300 Wh/L. However, the majority of solutions were not directly applicable for water collection. Efficient water harvesting often necessitates integration with other materials, such as zeolite, sand, and fabric, to enhance its effectiveness [174]. Solid desiccants have garnered greater interest in comparison to condensation water collection, owing to their superior water recovery efficiency and reduced energy consumption [175]. The water absorption capacities of various solid desiccant materials are depicted in Fig. 12. Kumar et al. [176] conducted a study on a solar-powered water collection system that utilized orange silica gel desiccant, resulting in the production of 0.98 L/day of drinking water. Gao et al. [177] investigated a new membrane-based water collection system, which, however, necessitated a relatively high air pressure. However, it is important to note that membrane water collection systems have limited applications because of their reduced airflow and consequently lower water collection rates [175]. Heidari et al. [178] introduced an air conditioning system that utilizes a silica gel desiccant wheel and an evaporative cooler to capture water condensation from the return air. The system produced 585 L of water in a week, of which around 296 L were utilized by the evaporative cooler, and the remaining 289 L were allocated for domestic hot water use. Tu and Hwang [179] presented a dehumidification water collection system that integrated a multi-stage desiccant wheel with a vapor compression cycle. Before entering the dehumidification process in the evaporator, the air was humidified by the multi-stage desiccant wheel in order to enhance water extraction. This approach has the potential to elevate the evaporation temperature and increase the rate of water collection. https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib167 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib168 https://www.sciencedirect.com/topics/engineering/solid-desiccant https://www.sciencedirect.com/topics/engineering/solid-desiccant https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib169 https://www.sciencedirect.com/topics/engineering/desiccant-wheel https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib170 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib171 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib165 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib172 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib173 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib174 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib175 https://www.sciencedirect.com/science/article/pii/S1364032124000017#fig12 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib176 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib177 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib175 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib178 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib179 Review article Fig. 12. Water harvesting capacity across different solid desiccant materials [181]. The humidification-dehumidification desalination technology plays a crucial role in water collection processes. It utilizes moving air as the medium for transporting water vapor. The warm air is subsequently channeled to the humidifier, where the evaporated seawater increases the moisture content of the air. Freshwater is acquired through the process of dehumidification using an apparatus. In contrast to a conventional direct-contact humidifier-based desalination facility, Tariq et al. [180] suggested the use of an air saturator for a humidification-dehumidification desalination system, which resulted in a 30 % increase in fresh water productivity, a 46 % improvement in recovery ratio, and an 11 % gain-output ratio. The newly implemented system demonstrated a 14 % reduction in cost per L compared to the conventional humidification-dehumidification desalination system, with a cost of 0.030 USD/L. It also exhibited a 7 % reduction in carbon footprint. The integration of humidification-dehumidification desalination systems with refrigeration, heat recovery, and renewable energy presents an effective approach for enhancing freshwater production and reducing operational expenses. 4.3. Air cleaning The desiccants are responsible for extracting moisture and purifying volatile organic compounds, airborne particles, bacteria, and viruses within the air conditioning system [182]. As indicated in Table 13, the occurrence of droplet carryover in the hybrid air conditioning system using liquid desiccant dehumidification may result in a deterioration of indoor air quality, consequently affecting its viability for extensive residential use. However, research has indicated that the levels of bromide and lithium ions emitted into indoor air were found to be below the established air quality standards, thus not presenting any significant health risks [183]. However, the contentious elements of the dehumidification and purification system restricted its wider implementation. Conversely, membrane dehumidification systems largely https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib181 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib180 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib182 https://www.sciencedirect.com/science/article/pii/S1364032124000017#tbl13 https://www.sciencedirect.com/science/article/pii/S1364032124000017#bib183 Review article circumvent the aforementioned issues. Nevertheless, their manufacturing technologies are predominantly in the experimental phase, which impedes their widespread implementation [184]. Table 13. Air cleaning capacity across different dehumidification methods [185]. Empty Cell Liquid desiccant Soild desiccant Condensation Volatile organic compounds Removal Removal Non effect Bacteria Kill or deactivate Non effect Breed Particulates Capture Release Non effect Transfer carry Yes No No Rong et al. [186] conducted a study on the impact of silica gel desiccant wheel systems on three pollutants: toluene, carbon dioxide, and methane. The study revealed that the adsorption capacity for toluene was notably higher than that for carbon dioxide and methane. However, the adsorption capacity of silica gel desiccant for non-polar substances such as toluene and 1,2-dichloroethane was comparatively low [187]. Pang [188] suggested the utilization of MOF for the adsorption of indoor pollutants. At 25 °C, MIL-101 (Cr) exhibited a saturation