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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Review of liquid desiccant air dehumidification systems coupled with heat pump: System configurations, component design, and performance

Vapor Compression Systems (VCS) are the most common air conditioning technology. VCS cool the air to its dew point temperature (overcooling) to remove water vapor in the air through condensation and then reheats the air back to the comfort temperature for direct use. The VCS process is inefficient due to overcooling and reheating. Liquid Desiccant Dehumidification (LDD) is a potentially energy-efficient air conditioning. LDD removes water vapor in the process air using liquid desiccant’s high-water affinity. It hybrids with sensible cooling to control temperature and humidity separately. The LD in the LDD becomes weak after dehumidification. The LDD needs additional heating to regenerate the weak Liquid Desiccant (LD) to a high concentration for dehumidification. Earlier versions of the LDD systems use highly concentrated liquid desiccant (large water removal capability) to dehumidify the air by only dealing with latent load. It leads to highly elevated temperatures above 60 °C of heat sources (combustion or electric resistance-based heating) for regeneration. The energy needed for the elevated temperature heat resource significantly reduces or demolishes the benefit of LDD systems. In the recent two decades, researchers have investigated a new configured LDD system that couples an LDD with a heat pump at both dehumidification and regeneration sides for better efficiency. The heat pump provides cooling (from the evaporator) for both dehumidification and sensible cooling and simultaneous heating from the condenser for regeneration. The highly integrated system (HP-LDD) with improved efficiency enables the LDD to operate at lower concentrations and temperatures in dehumidification and regeneration. This paper depicts the working principle behind HP-LDD and its heating and cooling requirements. It reviews the comparison between the HP-LDD systems and the conventional LDD systems regarding system configurations, component design, energy efficiency, and dehumidification performance characteristics. The main findings from the review include the preferred use of packed bed over membrane-based dehumidifiers, the use of internally cooled dehumidifiers enabled by the HP cooling capacity, the high dispersion of HP operation conditions, and the dependence of dehumidification performance on various dehumidifiers. Finally, an outlook for future research on HP-LDD strategies is presented based on the reviewed works and their limitations.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2022

Thermodynamic Model and Initial Experimental Investigation of Air Dehumidification through Electrically Charged Vapor Capturing Electrostatic Droplets

The removal of water vapor from the air to reduce relative humidity is a well known indoor environmental comfort requirement. Common dehumidification approaches require a substantial amount of energy and usually involve the cooling of atmospheric humid air below its dew point or the use of absorbent/adsorbent materials to extract water vapor out of the air. More recently, researchers investigated the effect of electrostatic forces for enhancing water vapor condensation with the goal to reduce the energy consumption associated with dehumidification. However, the studies are limited, and there is a lack of correlations that can predict the dehumidification rate. In this thesis, a broad theoretical investigation to study the electrostatically enhanced condensation processes was carried out. These processes consist of the use of highly charged particles, preferably highly charged water droplets to attract polar water vapor molecules to their surfaces and promote condensation, a phenomenon known as dielectrophoresis. The electrical charge promotes the reduction of the vapor pressure on the droplets' surface with respect to the saturated pressure predicted by the Kelvin equation on curved surfaces and, consequently, the equilibrium between evaporation and condensation is shifted towards condensation. This investigation resulted in the development of a thermodynamics model, which was able to predict an effective size range of the charged droplets for optimal dehumidification, under ideal conditions. The range resulted in about 2 to 4 m in diameter, while the electrical charge was kept to the maximum limit predicted by the Rayleigh model. A sensitivity analysis on the variation of the size and the charge of the charged droplets was also performed. In terms of dehumidification rates, when six electrospray heads, i.e. the maximum number of heads tested in the experimental campaign, were considered, the model predicted very limited rates, way lower than the target of this research, i.e. a dehumidification rate in terms of relative humidity of 5 % with an air flow rate of 5 cfm. While ways to increase this rate existed, their implementation proved difficult for this initial investigation. According to the requirements predicted by the model, the use of electrosprays appeared the most suitable solution for the production of small but highly charged droplets. The electrospray features and operational modes were studied in detail and an electrospray assembly was designed to be tested in this initial experimental investigation. For this initial investigation, an experimental system was designed and built to validate the thermodynamics model and with the goal to achieve a dehumidification rate of 5 % in terms of relative humidity with an air flow rate of 5 cfm. The experimental system consisted of a wind tunnel test apparatus, dew point sensors, thermocouples for temperature measurements, pressure sensors and other devices. The experimental system was equipped with a computerized data acquisition and storage system. The system was suitable to evaluate the air water content differential before and after the test section where the electrospray heads were installed. The air water content differential was evaluated in terms of the ∆ω/ω_1 ratio, to eliminate the dependence on the dry bulb temperature along the test apparatus. This work presented initial experimental data for electrostatically enhanced dehumidification processes. Working parameters, such as air and water flow rates, high voltage potentials and polarity, deionized water types with different electrical conductivity, were varied to find the best combination for an improved dehumidification. With the set up used and the conditions considered, it was never possible to achieve a 5 % dehumidification rate with 5 cfm. In general, the dehumidification rate was always limited and lower than 1 % for all the air flow rates considered and mainly within the uncertainty of the dew point sensors. These results confirmed the already limited predictions of the thermodynamics model, for ideal conditions and the interesting and promising results obtained in some experimental investigations available in the open literature were not achievable in this thesis. The more probable reasons were the higher air flow rates considered and the complete absence of the use of cooling power, which was mainly implemented to facilitate the vapor condensation.

Morcelli, Stefano↗
DOE OSTI2025

Numerical analysis of air dehumidification through electrospray-enhanced vortical flow cyclone separator

The dehumidification of air is a highly energy-intensive process, typically requiring vapor compression systems to cool the air significantly below its dew point temperature. Here, this study numerically investigates air dehumidification achieved by electrostatic and dielectrophoresis forces in a converging duct and centrifugal principles in cyclone separators. An electrospray was employed to energize water droplets, which then captured water vapor via dielectrophoresis. A novelty of the present study is the inclusion of a cyclone separator positioned downstream of the electrospray. This combination increased the dehumidification efficiency and controlled the flow of the condensate. By integrating electrospray technology and applying an electric field, this study examines the impact of particle separation efficiency, pressure drops, and cyclone performance on air dehumidification. The control parameters for air dehumidification were investigated. Three cyclones were modelled for airflow rates of 5 cubic feet per minute [cfm] (0.0024 m 3 /s), 38 cfm (0.0179 m 3 /s), and 200 cfm (0.094 m 3 /s). The results indicated air dehumidification was up to 5.7 % under environmental conditions, similar to those used to validate the CFD model. The degree of air dehumidification was primarily influenced by the electric charge acquired by the water droplets, their flight trajectory, and the vorticity intensity within the cyclone. Air dehumidification could increase by up to 13 % by increasing the number of droplets injected into the air stream, the flight time, and the electrical charge. The breakdown electric field strength threshold near the electrospray was a limiting factor for further enhancing dehumidification performance. This study examines the optimization of an electro-assisted dehumidification system that includes cyclone-based droplet separation through numerical analysis. The results provide insights into the design of energy-efficient air treatment technologies.

42 ENGINEERING↗
DOE OSTI2023

Demonstrating non-isothermal vacuum membrane air dehumidification for efficient next-generation air conditioning

Vacuum membrane air dehumidification has gained significant interest in recent years as a highly efficient means of air dehumidification. Prior theoretical modeling work by the authors introduced the Active Membrane Energy Exchanger (AMX) concept, which combines active heat exchange and vacuum membrane dehumidification into one non-isothermal system, and found that it could outperform other air conditioning technologies under many conditions. However, no experimental literature exists on combining active heat exchangers and vacuum membrane dehumidification. Here, the goal of the present study is to evaluate the dehumidification (mass transfer) performance of the AMX concept relative to isothermal membrane dehumidification through three main methodologies: (1) membrane material-level characterization, (2) experimental prototype development and testing, and (3) computational fluid dynamics (CFD) simulations. The dense membranes used in this work showed up to a 40% increase in water vapor permeance at cooler temperatures, and the prototype system showed up to a 6% increase in humidity removal when the air was simultaneously cooled. Furthermore, the membrane module-integrated heat exchange tubes provided additional mixing and turbulence, leading to a 4–8% increase in humidity removal. The upper limit coefficient of performance was equal to approximately 2.2, necessitating advanced system designs to improve efficiency. This study is the first to highlight that combining the cooling and vacuum dehumidification processes can improve dehumidification performance.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2024

A multiphase flow model of water droplets dielectrophoretic-induced air dehumidification phenomena

Air humidity in indoor spaces plays a critical role in human comfort and health. Dehumidification systems are used for building humidity controls, but they can take significant energy consumption, especially in geographic locations with high outdoor humidity and warm climates. Consequently, there is a growing demand for innovative dehumidification processes that consume minimal energy. Dielectrophoretic air dehumidification represents one such promising approach. However, it has not garnered significant attention due to the absence of engineering models and simulation tools capable of evaluating its performance and limitations at large-scale airflows. A new numerical multiphase CFD model, which is also experimentally validated, is developed in a customized Reacting Foam solver based on OpenFOAM® version 9. The newly developed model seeks to decrease substantial energy consumption and lower costs by leveraging the dielectrophoretic phenomenon to regulate moisture levels in the air. The solver integrates a hybrid Eulerian-Lagrangian framework to track the droplet's trajectory and growth rate while solving the continuum equations for the moist air. An electrospray produces electrically charged droplets, which grow during their in-flight trajectories as water vapor condenses onto their surfaces. The role of electrostatic forces in promoting vapor condensation within a high-gradient electrical field is investigated, and the dielectrophoretic vapor nucleation process on charged water droplets is discussed. The CFD model was validated against results from the literature and from proof-of-concept experiments conducted by the authors, which showed a 2 % air dehumidification with a single electrospray and airflow rate of 5 cubic feet per minute. The simulation results indicated that augmenting the number of electrically charged spray droplets increased the dehumidification of the air to 25 %. The initial mean droplet diameter, the orientation of the injector and relative humidity significantly influence the assessment of dehumidification. As a result, scaling up this approach to larger airflow volumes is identified as a potential future research direction.

42 ENGINEERING↗
DOE OSTI2024

Dual-module humidity pump for efficient air dehumidification: Demonstration and performance limitations

Condensation dehumidification in conventional air conditioning technologies is energy-intensive, accounting for up to 50% of building cooling energy used in some climates. Selective vacuum membrane dehumidification (VMD) is one of the leading alternative dehumidification technologies due to its potential for significant energy savings, and the “dual-module humidity pump” is one of the most promising VMD concepts. Here, this work is the first to provide experimental proof-of-concept for the dual-module humidity pump system and provides the first thermodynamic modeling framework that accounts for realistic steady-state operating limitations, both of which are lacking in the current literature. Additionally, this work is the first to provide a system design solution that overcomes practical challenges associated with air accumulation in the vacuum channels. The experimental results in this work show that the current prototype can remove up to 45% of the water vapor in the air stream, and the vapor pressure difference in the vapor rejection module needs to be approximately 2–4 times greater than that of the dehumidification module in order to maintain balanced mass transfer. The thermodynamic model applied to typical air conditioning conditions shows that the ideal dehumidification (latent) COPs can reach up to 40, but practical COPs are limited to approximately 10. Furthermore, the model shows that the overall energy efficiency increases as the membrane air selectivity increases, though this improvement gradually starts to diminish when the membrane selectivity is increased above 10,000.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2025

On the validity and limitations of 1D model for heat and mass transfer performance evaluation in a multilayer binder-free desiccant dehumidifier: isothermal dehumidification with internal cooling

Efficient humidity control is essential for maintaining indoor thermal comfort, yet conventional vapor-compression-based dehumidifiers are energy-intensive. Employing separate sensible and latent cooling through desiccant-coated heat exchangers (DCHEs) combined with evaporative coolers offers energy savings of up to 80 % compared to conventional systems. However, the dehumidification performance of DCHEs remains limited due to the use of polymer binders for coating desiccant materials onto heat exchange surfaces. In our previous study, we developed a multilayer fixed-bed binder-free desiccant dehumidifier (MFBDD) that demonstrated high dehumidification capacity and low pressure drop compared to rotary desiccant wheels. Nevertheless, its potential for further enhancement through internal cooling and the use of step-shaped adsorption isotherms has not been explored. In this study, a physics-based one-dimensional (1D) transient model is developed and validated to capture the coupled heat and mass transfer processes in the MFBDD and extended to simulate internal cooling using a high-capacity composite metal–organic framework, MIL-101/GO-6 (water uptake ≈1.6 g/g within 35–47 % RH). The model enables detailed analysis of local air and bed temperature dynamics and quantifies how internal cooling affects the dehumidification performance under a wide range of operating conditions. Results show that integrating internal cooling and using MIL-101/GO-6 enhance mass adsorbed, moisture removal capacity, and dehumidification effectiveness by 50 %–99 % compared with the M.S. Gel baseline. The study further reveals that achieving near-isothermal operation requires simultaneous enhancement of the convective heat transfer coefficient and heat exchange surface area. In conclusion, this work provides the first detailed physical insight into the interplay between internal cooling and step-shaped isotherms in a binder-free desiccant device and establishes a validated modeling framework for scaling up and system-level performance evaluation of next-generation energy-efficient dehumidification systems.

Heat and mass transfer↗
DOE OSTI2021

Analysis of Electro-static Assisted Air Dehumidification

The removal of water vapor from the air to reduce relative humidity is a well-known indoor environmental comfort requirement. Common dehumidification approaches require a substantial amount of energy and usually involve the cooling of atmospheric humid air below its dew point or the use of absorbent/adsorbent materials to extract water vapor out of the air. More recently, researchers investigated the effect of electrostatic forces for enhancing water vapor condensation. However, the studies are limited, and there is a lack of correlations that can predict the dehumidification rate. Also, the findings from the literature focused mainly on small flow rates on the order of one to two cfm. Electrically-enhanced condensation consists of the use of highly charged particles, preferably highly charged water droplets, that attract polar water vapor molecules to their surfaces and promote condensation, a phenomenon known as dielectrophoresis. An effect of the electric charge is the reduction of the vapor pressure on the droplets' surface with respect to the saturated pressure predicted by the Kelvin equation. Consequently, the equilibrium between evaporation and condensation is shifted towards condensation. Following the application of the modified Kelvin-Thomson theory, we developed a preliminary physics-based model to predict an effective size range of the charged droplets for optimal dehumidification. The range resulted in about 2 to 4 μm in diameter, under few simplifying assumptions. The effect of the size and the charge of the electrosprayed droplets on the overall dehumidification rate was briefly discussed. The use of electrosprays to produce small but highly charged droplets was broadly discussed in this paper. The cone-jet mode was identified as the most suitable electrospray operational mode, and it generated droplets of small size and high electrical charge. The cone-jet stability was also analyzed in detail. The preliminary data of the present work and the model results indicated that several electrospray heads were required to achieve a 5% dehumidification rate for airflow rates of about 5 cfm.

Morcelli, Stefano↗
DOE OSTI2021

Analysis of New Data of Electro-static Assisted Air Dehumidification Processes

The removal of water vapor from the air to reduce relative humidity is a well-known indoor environmental comfort requirement. Common dehumidification approaches require a substantial amount of energy and usually involve the cooling of atmospheric humid air below its dew point or the use of absorbent/adsorbent materials to extract water vapor out of the air. More recently, researchers investigated the effect of electrostatic forces for enhancing water vapor condensation. However, the studies are limited, and there is a lack of correlations that can predict the dehumidification rate. Also, the findings from the literature focused mainly on small flow rates on the order of one to two cfm. Electrically-enhanced condensation consists of the use of highly charged particles, preferably highly charged water droplets, that attract polar water vapor molecules to their surfaces and promote condensation, a phenomenon known as dielectrophoresis. An effect of the electric charge is the reduction of the vapor pressure on the droplets' surface with respect to the saturated pressure predicted by the Kelvin equation. Consequently, the equilibrium between evaporation and condensation is shifted towards condensation. Following the application of the modified Kelvin-Thomson theory, we developed a preliminary physics-based model to predict an effective size range of the charged droplets for optimal dehumidification. The range resulted in about 2 to 4 m in diameter, under few simplifying assumptions. The effect of the size and the charge of the electrosprayed droplets on the overall dehumidification rate was briefly discussed. The use of electrosprays to produce small but highly charged droplets was broadly discussed in this paper. The cone-jet mode was identified as the most suitable electrospray operational mode, and it generated droplets of small size and high electrical charge. The cone-jet stability was also analyzed in detail. The preliminary data of the present work and the model results indicated that several electrospray heads were required to achieve a 5% dehumidification rate for airflow rates of about 5 cfm.

Morcelli, Stefano↗
DOE OSTI2020

A Performance analysis of the Claridge-Culp-Liu dehumidification process: A novel approach for drying moist air based on membrane separation, vacuum compression and sub-atmospheric condensation

This article covers a basic model for analyzing the performance of the Claridge-Culp-Liu dehumidification process. The fundamental process efficiency limit for dehumidification is close to COP Carnot , but for the eight dehumidification cases examined, the limiting or ideal energy use required is 26% to 56% that of a Carnot condensing system as shown in an earlier paper. The model presented in this paper is used to show the membrane system performance reduction caused by finite membrane area, finite water vapor permeance, non-zero air permeance, non-zero system air pressure drop, non-ideal compressors, vacuum pumps, and condensers. The performance of a “conservative” membrane system based on the use of existing components is computed for eight specific conditions along with that of a “target” system that assumes expected component performance after additional future component development. The “conservative” membrane system would use 36% to 66% as much energy as a system with a COP=7 chiller to produce the same dehumidification for the eight cases examined while the “target” system would use 15% to 40% the energy of a system with a COP=7 chiller. In addition to the significant energy reduction over conventional technology, the membrane system offers the advantages of: 1) no HFC refrigerant use; 2) direct isothermal control over humidity ratio setpoint; 3) maximum capacity occurs at design conditions; and 4) system generates pure water extracted from air as a by-product.

42 ENGINEERING↗
DOE OSTI2021

An experimental study on dehumidification and regeneration performance of a new nonporous membrane-based heat and mass exchanger using an ionic liquid desiccant

As a promising alternative to inefficient vapor-compression-based air conditioning, liquid desiccant dehumidification uses a liquid desiccant in contact with the humid air absorbing the moisture. However, it has not gained much market share due to the issues related to the carryover, corrosion, fouling, and crystallization of liquid desiccant. The research has developed a new membrane-based exchanger, which uses a non-corrosive ionic liquid desiccant and nonporous tubular membranes to address these issues. As the second generation of the prototype, the new exchanger was tested at various operating conditions. According to the experimental data, when it is used in the dehumidification loop, the new membrane-based exchanger achieves a specific vapor transportation rate of 778.6 g/(h–m 2 ) and an average water vapor flux of 0.316 g/(h-m 2 -Pa), which is a significant improvement compared with the previous designs using nonporous membranes. It also achieved a comparable or even better dehumidification performance compared with the dehumidifiers that use porous membrane and conventional liquid desiccants. However, the regeneration performance is not as good as its dehumidification. It is mainly caused by the high operating temperature required in the regeneration loop. The experimental data and findings provide first-hand experimental data and enhance the understanding of advanced membrane-based ionic liquid desiccant systems.

42 ENGINEERING↗
DOE OSTI2022

Effect of geometry and operational parameters on the dehumidification performance of a desiccant coated heat exchanger

Solid desiccant dehumidification systems are an alternative to vapor-compression-based air dehumidification systems. They use solid desiccant materials to adsorb air moisture in space cooling. DCHE can integrate the cooling component in the heat exchanger to achieve higher dehumidification capacity. Comprehensive numeric modeling would be necessary to assist in the design and operation of the DCHE under development to achieve maximum performance. Here, we provide the details of a heat and mass transfer model developed for this purpose. The model aims to cover a gap in existing models by independently modeling heat transfer on the solid desiccant, fin, and tube. The model results were compared with an experimental reference for validation. The air outlet humidity and temperature results for dehumidification and regeneration showed a deviation lower than 15% from the experiment. The validated model was used to perform a parametric study for a series of design and operating conditions. The parametric analysis showed that an increase of 25% in desiccant layer thickness and thermal contact resistance between solid elements reduces moisture removal by 9.1% and 1%, respectively. These results indicate the need to independently model the desiccant layer.

42 ENGINEERING↗
NASA NTRS1976

Application of chemical dehumidification system to a roof fan house at Michoud Assembly Facility at New Orleans, Louisiana

The feasibility of a chemical dehumidification system to reduce the energy consumption associated with dehumidification of the chilled air is assessed. A comparative energy consumption and cost analysis of the chemical dehumidification and existing systems and the savings offered by the proposed chemical dehumidification system over the existing air washer-reheat system are presented.

Source record↗
DOE OSTI2021

Air Dehumidification using Ionic Liquid-Based Fiber Bundle Membrane Contactor

Air dehumidification is essential since excess moisture in the buildings causes discomfort to the occupants, encourages the production of air pathogens such as mold or mildew, and causes corrosion and rotting that degrade building materials. Existing moisture removal processes are mainly focused on condensation and desiccant (liquid or solid) techniques with direct contact between air and desiccant. However, these methods are energy-intensive, or desiccant might be lost or cause corrosion in the process. The main objective of this study is to investigate an ionic liquid-based liquid desiccant absorber based on a membrane fiber bundle. The novel system consists of membrane contactors containing bundles of 10,000 polypropylene fibers. The fiber bundles have 0.3 micron diameter, with ionic liquid flowing inside, and air flowing outside. The fibers provide a high contact area among phases: 1.4 m2 contact surface area in a 0.00015 m3 volume. The ionic liquid as a sorbent stipulates the selectivity for air, prevents the loss of solvent in the operation due to negligible volatility, provides fast diffusion due to low viscosity compared to common ionic liquids, and has high affinity and solubility in water. The dehumidification capacity of the membrane technology was experimentally investigated. The experimental results show that the ionic-liquid based membrane system can effectively remove excess moisture from the air. The novel fiber bundle dehumidification system has a system volume of 0.00798 m3 (7.98 L) and active heat and mass transfer surface area of 8.4 m2. It achieved an average dehumidification of 320 ± 25 W with a volumetric air flowrate of 3.1 m3/min (108 ft3/min).

Turnaoglu, Tugba↗
DOE OSTI2021

The energy saving potential of thermo-responsive desiccants for air dehumidification

The desiccant wheel is a promising technology for energy-efficient humidity control. However, its overall efficiency—hindered by the desiccant materials’ properties—remains low because traditional desiccants (e.g., silica gels) have a single isotherm regardless of their adsorption temperature. Thermo-responsive materials have been proposed to break this fixed affinity to water vapor, with drastically different adsorption isotherms depending on temperature. Its potential for improving dehumidification efficiency, however, has not been addressed. In this paper, we model the potential of a thermo-responsive interpenetrating polymer network (IPN) desiccant with temperature-dependent adsorption isotherms for energy-efficient dehumidification via a validated transient desiccant wheel model for humidity control of buildings. Thermo-responsive desiccants can improve the dehumidification performance due to their thermo-responsive switchable hydrophilicity below/above the lower critical solution temperature. Our analysis shows that thermo-responsive IPN desiccants can potentially reduce energy consumption by up to 30% compared to silica gels. Finally, the savings depend strongly on the critical temperature of the thermo-responsive desiccant and should be higher when the inlet temperatures are expected to be higher.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2022

Sensitivity Analysis of an Electrospray Dehumidification System

Electrically enhanced condensation achieved by an electrospray system can enhance dehumidification in HVAC applications if successfully scaled up. The first step towards scale-up is identifying critical design and operating parameters that significantly impact dehumidification for the electrospray system. Sensitivity analysis is one approach for identifying critical parameters. This paper presents a methodology that can efficiently perform quantitative sensitivity analysis on a computationally expensive high fidelity simulation model, which is a computational fluid dynamics (CFD) model of the electrospray system. The methodology employs surrogate modeling and Sobol sensitivity analysis. We estimate the main and total effect Sobol sensitivity indices of four operating parameters of the electrospray system. The results reveal that the most important parameter is the size of the sprayed droplets, followed by the volumetric flow rate of the bulk air and injected water in the system. The water injection velocity of the spray was found to have little to no impact on dehumidification.

Young, David↗