Isothermal Dehumidification Research Articles

Experimental study on the vacuum dehumidification performance of PVA/CaCl2 selective permeation membrane under vortex conditions

Vacuum membrane dehumidification is a promising power-saving isothermal dehumidification technology, and the development of low-cost and efficient selective permeation membranes is beneficial to its application. In this study, the dehumidification performance and mechanism of the PVA/CaCl2 water vapor selective permeation membrane are investigated experimentally. The effects of the CaCl2 concentration in the casting membrane solution, the permeation side vacuum level, and the inlet air flow rate and relative humidity on the dehumidification performance are discussed. In addition, the effect of the deflector fins on the dehumidification performance is analyzed. The results show that the vortex coupled dissolution-diffusion theory can explain the experimental results. In addition, from the point of view of dehumidification capacity per unit power consumption, the best concentration of CaCl2 in casting membrane solution is 2.08 %, and the worst permeation side vacuum level is 60 kPa. The higher the inlet air flow rate and relative humidity under the deflector fins, the better the dehumidification performance. The dehumidification performance can be significantly improved by the deflector fins, which increase the COP by 38.31 %∼146 % and the selectivity by 35.85 %∼147 %.

Energy saving of vacuum-based membrane dehumidification with indirect evaporative cooling in a low sensible-heat-ratio building

In hot and humid climates, dehumidification generally consumes more energy than cooling, and conventional mechanical vapor compression system is not efficient for dehumidification. Vacuum-based membrane dehumidification system have been developed as a more environmentally friendly alternative for dehumidification due to non-refrigerant system and isothermal dehumidification process. In this study, a novel hybrid air conditioning system that combines vacuum-based membrane dehumidification with an indirect evaporative cooler for pre-cooling is proposed to achieve efficient air conditioning in building with low sensible heat ratio. The energy performance of the proposed system is investigated by detailed thermodynamic simulations and compared with conventional variable air volume (VAV) system and indirect evaporative pre-coolers combined VAV system. The simulation results show that when the average sensible heat ratio of the space is 0.73 and the COP of the vacuum-based membrane dehumidifier is 1.0, the proposed system can reduce energy consumption by 16.1 % compared to conventional VAV system and by 6.4 % compared to indirect evaporative pre-cooler combined VAV system during the cooling season. The energy performance of the proposed system is significantly affected by the sensible heat ratio of the building and the energy efficiency of the vacuum-based membrane dehumidifier. When the COP of the vacuum-based membrane dehumidifier reaches 2.0, the proposed system can achieve an energy savings of 48.2 % compared to conventional VAV system.

Effect of polyvinylpyrrolidone and lithium chloride composite desiccant-coated heat exchangers on dehumidification studies

Cooling systems are necessities that remain energy intensive, particularly in tropical regions with elevated humidities and temperatures. Conventional vapor-compression air-conditioners are often inefficient because the sensible and latent heat loads are intrinsically coupled. To realize more energy-efficient cooling, solid desiccant-coated devices can be introduced to mitigate the latent heat load. Among such devices, desiccant-coated heat exchangers (DCHEs) are attractive for enabling nearly isothermal dehumidification. Hence, desiccants with high sorption capacity and rapid kinetics are essential. Solid polymeric desiccants are well-positioned to meet these requirements but comprehensive studies on the dehumidification performance of solid polymeric desiccant-coated heat exchangers are currently lacking. In this study, we investigated the potential of polyvinylpyrrolidone (PVP) and lithium chloride (LiCl) composites as desiccant materials at both material and system levels in terms of gravimetric water uptake. Specifically, LiCl is a hygroscopic salt bound by the moisture-sorbing PVP matrix. This work also demonstrates how the Elovich model accurately predicted the PVP-LiCl composite’s sorption kinetics. The results show that the optimum PVP-LiCl composite ratio is 66.7 wt%, with an equilibrium sorption capacity of 280 % at 75 % RH and 25 °C, compared to 29 % for silica gel under the same condition. With much superior sorption kinetics and capacity, the PVP-LiCl-coated prototype heat exchanger demonstrated a moisture removal capacity eight times higher than a silica gel-coated one under dynamic flow towards energy savings in air-conditioning systems.

Experimental investigation of a thermo-responsive composite coated heat exchanger for ultra-low grade heat utilization

There is a growing interest in desiccant coated heat exchanger (DCHE)-based systems due to characteristics of isothermal dehumidification and compact structure. To enhance the dehumidification efficiency and reduce the regeneration temperature of DCHE-based systems, a thermo-responsive composite desiccant coated heat exchanger (PSLCHE) was developed. This study focuses on investigating the influence of operating parameters (such as air temperature, relative humidity, regeneration temperature and adsorption/desorption time ratio) on the dynamic performance, average dehumidification capacity (Davg) and thermal coefficient of performance (COPth) of the system. Under the air condition of 30 °C & 60% RH, the PSLCHE demonstrates remarkable performance with the Davg of 7.3 g·kg−1 and COPth of 0.93 at 40 °C. This study provides a route to the design of highly efficient air conditioning systems using thermo-responsive composite for ultra-low grade thermal energy at 40 °C.

Applicability of a dedicated outdoor air system assisted by isothermal dehumidification and evaporative cooling

A dedicated outdoor air system (DOAS) assisted by an isothermal dehumidifier and an indirect evaporative cooler is proposed, and its energy-saving potential is evaluated based on detailed simulations. In the proposed system, an isothermal dehumidifier accommodates the latent cooling load and an indirect evaporative cooler assumes a sensible cooling load. The energy performance of the proposed system was evaluated by comparing it with two conventional dedicated outdoor air systems: a DOAS with a desiccant wheel and a DOAS with a cooling coil. The results indicate that the proposed DOAS can accommodate sensible and latent cooling loads, but it consumes 10% more operating energy because of the low coefficient of performance (COP) (0.67) of the isothermal dehumidifier, despite the free cooling operation by the indirect evaporative cooler. For comparable energy performance, the proposed system requires the COP of the isothermal dehumidifier to be greater than 0.78, which is achieved through improvements in the membrane selectivity and vacuum pump (compression ratio and isentropic efficiency).

Modeling comparison and theoretical study of mass transfer characteristics for desiccant coated air channel under isothermal dehumidification

Coupled heat and mass transfer phenomena are encountered in desiccant coated dehumidifier, and their numerical modeling is becoming indispensable. In this study, three conjugate heat and mass transfer models were compared to clarify the effects of interparticle mass transfer and local kinetic non-equilibrium on modeling accuracy, and the models were validated by experiment results of silica gel and FAM Z01 coated dehumidifiers with single one air channel. Furthermore, a parametric study on the separate mass transfer characteristics of the dehumidifier under isothermal dehumidification was conducted based on one of the improved models. Results demonstrated that the effect of interparticle mass transfer should be involved, while the local kinetic non-equilibrium tends to be considered for desiccants with steep isotherm shape. Parametric study revealed that as air velocity increases, both air- and solid-side mass transfer change significantly, while desiccant layer thickness, interparticle porosity and isotherm shape mainly affect solid-side mass transfer. The average mass transfer Biot numbers Bim of desiccant layer with relative low thickness of 0.1 mm under various parameters are in the same order of magnitude as 1, indicating that the mass transfer resistance on solid side cannot be ignored generally.

Experimental study on performance measurement of planar vacuum membrane dehumidifier with serpentine flow channel designs

In recent years, membrane dehumidification, as a novel dehumidification technology, has received many attentions for energy efficiency in air conditioning systems. Its dehumidification technology is characterized by isothermal dehumidification, which can avoid excess energy consumption. This experimental study focuses on the performance measurement of the planar vacuum membrane dehumidifier. Nafion membrane is used in combination with a planar membrane dehumidifier. A vacuum membrane dehumidification method is also employed to investigate the dehumidification performance. Four serpentine flow channel designs, including three-snake, four-snake, five-snake, and six-snake flow channel designs, are tested at the certain values of temperature and humidity and different inlet air flow rates. Different performance indexes, including water collection, dehumidification rate, pressure loss, and pumping power, are investigated. The results show that the amount of water collected by the cold trap increases with boosting the inlet flow rate in the range of 35 l/min to 55 l/min and it has the largest value when the inlet air flow rate is 55 l/min. However, the amount of water collected by the cold trap decreases with further increase of the inlet flow rate in the range of 55 l/min to 65 l/min.

Experimental and theoretical study on water vapor isothermal adsorption-desorption characteristics of desiccant coated adsorber

Open air channels coated with porous desiccant are widely applied in solid desiccant cooling systems. In this study, the isothermal adsorption and desorption characteristics of silica gel coated single-channel adsorber were investigated. A modified LDF model considering air- and solid-side resistances was derived to evaluate the kinetics and estimate the total mass transfer rate for numerical modelling. Results show that the modified LDF model can well describe the dynamic behavior of isothermal dehumidification. A new numerical model predicting the mass transfer characteristics of dehumidification process was proposed and strictly validated. Parametric studies reveal that the moisture transfer strongly depends on kinetic constants, isotherm shapes and thermal boundaries. The higher kinetic constants, the larger adsorption/desorption rate and the faster equilibrium. It was found that neither too high nor too low shape factor R are conducive to improve dehumidification capacity. The isotherm with R=0.1 is considered as the optimum isotherm shape. With the transition of thermal boundary from ideal isothermal to adiabatic condition, the mass transfer performance decreases. The peak value of transient mass transfer rate of adsorption process under ideal isothermal boundary is almost twice and 2.6 times higher than these of the third type thermal boundary and adiabatic boundary, respectively.

Vapor-selective active membrane energy exchanger for high efficiency outdoor air treatment

As much as 40% of the total load on air conditioning systems can be attributed to condensation dehumidification. However, new water vapor-selective membranes present a unique opportunity to greatly reduce the power requirements for moisture removal by avoiding phase change and have thus been ranked as a top alternative to traditional HVAC systems. To date, however, all such systems have relied on the assumption of constant temperature, even terming the technology “isothermal dehumidification.” This work proposes a membrane-based air cooling and dehumidification approach, referred to as the Active Membrane Energy Exchanger (AMX), which is the first to provide simultaneous, yet decoupled, air cooling and dehumidification. The suggested AMX configuration uses two vapor-selective membrane modules with a water vapor compressor in between them, using the second membrane module to reject vapor into the exhaust stream. Cooling and heating coils in each membrane module channel move heat between the air streams using a vapor compression cycle. A detailed steady-state, thermodynamic model is presented for the AMX integrated within a 100% outdoor air conditioning system. The AMX’s limiting parameters and design considerations like compressor efficiency are systematically analyzed for a broad range of outdoor air conditions and compared against standard and state-of-the-art dedicated outdoor air systems. This new high efficiency approach is found to outperform all other standard and state-of-the-art systems, achieving 1.2–4.7 times the COP over conventional dedicated outdoor air treatment. Lastly, a building simulation case study predicted cooling energy savings as high as 66% in hospital buildings with 100% outdoor systems in hot, humid climates.

The potential for energy savings in U.S. houses by using isothermal dehumidification

Potential energy savings achievable by using isothermal dehumidification instead of conventional air conditioning systems are modeled on a technology-agnostic basis. First, we use simple thermodynamics to model potential device-level energy savings by dehumidifying isothermally instead of by sub-dew-point cooling. A model for dehumidification demand in a prototypical house is then developed, and the device-level model is applied to the house under relevant climactic norms and weather extremes. Finally, we model all U.S. houses by coupling the household model with a statistical model for air infiltration representing all single-family, detached homes in the United States. The results show potential nationwide electricity savings of 8–25 or 5–14% of electricity usage for space cooling in U.S. single-family homes, depending on weather. The results also display large regional differences, with moist hot climates having the greatest potential absolute energy savings, but moist temperate climates having the greatest potential relative energy savings. Individual houses with high balance-point temperatures in moist climate zones are likely to derive the greatest benefit by using isothermal dehumidification over conventional systems. Ultimately, this technology-agnostic analysis provides fundamental, thermodynamics-based insight into where, when, and how isothermal dehumidification devices can reduce the energy and greenhouse gas footprints of space cooling.

Energy saving potential of a vacuum-based membrane dehumidifier in a dedicated outdoor air system

As a novel energy-conservative ventilation system for buildings, a dedicated outdoor air system (DOAS) assisted by a vacuum-based membrane dehumidifier is proposed, and its energy performance is investigated via detailed energy simulations. The vacuum-based membrane dehumidifier dries the air by an isothermal process, which is difficult to realize in conventional dehumidification systems. The proposed system consists of an enthalpy exchanger, a vacuum-based membrane dehumidifier, and a cooling coil. Its dehumidification performance and operation energy are compared with those of the following conventional DOASs: a DOAS that employs a cooling coil (Reference A) and a DOAS that employs an active desiccant wheel (Reference B). To evaluate the energy performance of the proposed system, the energy consumptions of all the DOASs were estimated by simulation programs (i.e., TRNSYS and EES) with mathematical models. The simulation results indicate that the average coefficient of performance of the DOAS we devised (i.e., 1.34) is lower than that of Reference A (i.e., 2.70) and higher than that of Reference B (i.e., 0.80). However, owing to the isothermal dehumidification process, the latent cooling ratio in the case of the proposed system (i.e., 1.0) is significantly higher than those in the cases of Reference A (i.e., 0.43) and Reference B (i.e., 0.11). Consequently, the annual energy consumption of the proposed system is 17.4% and 56.2% lower than those of Reference A and Reference B, respectively.

Experimental study on a fresh air heat pump desiccant dehumidification system using rejected heat

Desiccant coated heat exchanger (DCHE) was proposed for highly efficient dehumidification process due to the adsorption heat could be eased by inner cooling during the sorption process. Meanwhile, heat pump is probably used with DCHE together in humidity control processes because cooling and heating generated by vapor compression heat pump system match with the energy demand of DCHE well. A fresh air heat pump desiccant dehumidification system using rejected heat with desiccant coated heat exchanger (DCHE) was set up and experimentally tested in this paper. Near isothermal dehumidification process could be realized due to the utilization of DCHE which could convert the sensible cooling provided by vapor compression heat pump subsystem to latent cooling to increase the moisture removal of the system. The optimal average moisture removal and COP are 7.02 g/kgDA and 4.10, respectively, under the near ARI humid condition. Most of the total cooling generated by vapor compression heat pump subsystem is used for dehumidification. The moisture removal per kilowatt hour electric power consumption of the system is 1.60–2.88 times that of conventional condensation dehumidification method.

Performance evaluation of an air conditioning system based on quasi isothermal dehumidifcation

This paper presents a first study of novel air conditioning system based on quasi isothermal dehumidification combined with dew-point evaporative cooling through the Maisotsenko cycle (M-Cycle). The dehumidification process is realized in desiccant coated heat exchanger with heat rejection system. The key idea behind the solution is to maximize the cooling effectiveness by creating favourable conditions for dehumidification and evaporative cooling process. This is achieved by rejecting the heat of sorption in the desiccant system and providing cold and dry air for M-Cycle air cooler, which allows the system to be powered by using low-grade heat energy at 50 °C. The analysis is performed on the basis of experimentally validated numerical models of each component combined in a unified model of the system. The goal of the study is to determine the performance of the solution under different operational conditions. In addition two types of M-Cycle air coolers (i.e. cross-flow and regenerative unit) are compared to establish which unit is more suitable as the source of cooling energy for the system. It was established that proposed system is able to provide comfortable room parameters (supply air temperature equal 17 °C and humidity ratio equal 0.010 kg/kg-da) for outdoor air temperatures ranging from 28 to 36 °C and outdoor air humidity ranging from 0.010 to 0.018 kg/kg-da. This means that the proposed solution is able to provide comfortable conditions in temperate and humid climates. Results also indicate that the system consisting of the regenerative M-Cycle HMX allows better cooling load management, offers higher thermal Coefficient of Performance (COPth) and uses less water compared to the system with the cross-flow HMX. However, the system with cross-flow heat and mass exchanger (HMX) has a relatively greater electrical Coefficient of Performance (COPel) in most cases.

Experimental investigation for a non-adiabatic desiccant wheel with a concentric structure at low regeneration temperatures

The desiccant wheel-based air-conditioning system has been considered to be an effective alternative to conventional air-conditioning systems to reduce energy consumption in buildings. However, the adsorption and carryover heat released during the wheel’s working process both significantly restrict dehumidification and energy performance. Moreover, it is also difficult to apply a low temperature of thermal energy, including solar thermal energy and waste heat, in the desiccant wheel-based air-conditioning system. To improve the dehumidification and energy performance, and use a low temperature of thermal energy, this study designed, constructed and tested a non-adiabatic solid desiccant wheel, which is based on a concentric ring design. The wheel’s structure was fabricated from Nylon, using three-dimensional technology with polymer desiccant materials inserted into the channels between each of two rings. Then, cooling water was brought into the narrow passages inside the rings on the process air side of the wheel, with the aim of transforming the dehumidification process from quasi-adiabatic to quasi-isothermal. A series of experiments were then conducted to investigate this system’s performance under various operating conditions. The measurements showed that, when the inlet air and cooling water temperatures were 25 °C and 24 °C, respectively, the dehumidification performance of the new wheel was approximately the same as that of a conventional desiccant wheel. However, when the inlet temperature of the process air was 35 °C, the dehumidification process of the new wheel was very close to the ideal for a desiccant wheel – an isothermal dehumidification process. In this condition, the enthalpy effectiveness of the tested wheel was also 11% higher than that of a conventional wheel.

Performance investigation of an internally cooled desiccant wheel

Desiccant wheels are commonly used to dehumidify air in air-conditioning system to reduce the energy consumption. The objective of this study was to design, test and analyse the performance of a novel tube-shell, internally water-cooled desiccant wheel. Cooling water was used in the supply section of the new wheel to shift the dehumidification process from nearly adiabatic to nearly isothermal. We conducted a series of experiments to investigate the wheel’s performance and used the experimental results to validate a mathematical model. The model was then applied to investigate the influence of modifications to the base design, including changing the number of desiccant layers inside the heat-exchange tubes, and the size and number of tubes. We also analysed the influence of water temperature and flow rate on both dehumidification and temperature rise across the wheel. Our results show that isothermal dehumidification performance can be achieved, but only with no more than two desiccant layers inside the heat-exchange tubes. More layers reduced heat transfer between the air in the innermost layers and the cooling water. Lower cooling water inlet temperatures led to lower air outlet humidity and temperature. A cooling water temperature of approximately 24 °C was required to achieve isothermal dehumidification for process air with inlet temperature of 30 °C and absolute humidity of 16.3 g/kg and regeneration air with the inlet temperature of 50 °C and absolute humidity of 16.3 g/kg. This corresponds to a 48% improvement in dehumidification performance (the maximum absolute humidity change between inlet and outlet process air) compared with a conventional adiabatic desiccant wheel while using super-adsorbent polymer as the desiccant material.

Mathematical modeling and performance evaluation of a desiccant coated fin-tube heat exchanger

A solid-desiccant system that utilizes low grade heat is potentially a viable add-on to conventional HVAC systems since it can help reduce power consumption significantly, for achieving indoor thermal comfort conditions. In contrast to desiccant wheels which carry out adiabatic dehumidification, isothermal dehumidification process that may be realized by a cross-flow heat exchanger is much more efficient. In this paper, a novel mathematical model is developed to simulate heat and mass exchange phenomena of a desiccant coated fin tube heat exchanger (DCFTHX). This model takes solid side mass transfer resistance as well as fin efficiency into consideration. The model is validated using experimental results in the literature. It is also compared against simplified models to establish its utility. A parametric study is then conducted to investigate the effects of geometrical parameters as well as mass flow rate of water and air velocity on dehumidification and adsorption heat removal performance of the DCFTHX as well as the performance of the augmented air-conditioning system under warm and humid ambient conditions. Under the range of parameters and conditions simulated, if low grade waste heat (50 °C hot water) is available for regeneration, integration of DCFTHX sub-system with a conventional air conditioning system can yield as high as 31% energy savings (even when the additional power consumed by pumps and blower fans is accounted for).

Thermodynamic analysis of desiccant wheel coupled to high-temperature heat pump system

A high-temperature heat pump coupled with a desiccant wheel system possesses superiority among the various hybrid desiccant cooling systems due to its compact configuration and especially its ability to make full use of both heating and cooling from the condenser and evaporator of the high-temperature heat pump to achieve the function of self-regeneration. This study investigates the performance of high-temperature heat pump coupled with a desiccant wheel system using the exergy analysis method; the performance is also compared with the hybrid desiccant cooling system with an electric heater. Two improved configurations for the proposed system are also suggested that are beneficial for irreversibility reduction. The results show that high-temperature heat pump coupled with a desiccant wheel system gets rid of the large exergy destruction of the electric heater and then obtains an exergy efficiency that is 19% higher than the hybrid desiccant cooling system. The improved configuration for the proposed system integrating the isothermal dehumidification of a two-stage desiccant wheel and matched heat exchangers is in favor of reducing the exergy input. It is found that the exergy input is decreased by 30.3%, and the cooling exergy efficiency is increased by 46.9%.

Improvement of Dehumidification Performance of Desiccant Rotors Regenerated at a Low Temperature

This paper presents a highly effective desiccant rotor that can be regenerated at a temperature between 20 and 30°C, corresponding to return air exhausted from conditioned spaces. The desiccant rotor consists of a honeycomb structure, which is coated with organic polymer desiccant materials. For a specific operating condition, the desiccant rotor functions as a rotary total heat exchanger. Desiccant rotors with thickness of 0.2 m and more lead to both higher dehumidification and temperature efficiencies compared to conventional total rotary heat exchangers in different states of the inlet process and regeneration airflows. Both the dehumidification and temperature efficiencies achieve 100% at a thickness of 0.4 m, and at rotational speeds between 100 and 300 rph. Dehumidification, together with cooling, is very effective. For the desiccant rotor with a thickness of 0.4 m, the humidity change of the process air corresponds closely to isothermal dehumidification. In terms of the dehumidification and cooling functions, the performance of the desiccant rotor with thickness of 0.2 m and more is very advantageous compared to conventional desiccant rotors and rotary total heat exchangers.

Isothermal dehumidification with a simply thermoelectric device: concept and feasibility

This paper proposed a simple thermoelectric cooler (TEC) device with a heat recovery unit to deliver isothermal dehumidification in a small space where ambient temperature falls into the comfort zone while relative humidity is high. The concept of the system was presented, followed by the analysis of the cold surface temperature to examine its possibility to reach the target. Five different types of TEC products were chosen to make a comparison by using the market available data sheets and results found that for small applications (up to 30 W), the proposed system can easily deliver the isothermal dehumidification with room temperatures of 20–25°C.

Simulation Analysis of the Four Configurations of Solar Desiccant Cooling System Using Evaporative Cooling in Tropical Weather in Malaysia

A high demand for air conditioning systems exists in hot and humid regions because of the warm climate during the year. The high energy consumption of conventional air conditioning system is the reason for our investigation of the solar desiccant cooling system as an energy-efficient cooling system. Four model configurations were considered to determine the best configuration of a solar desiccant cooling system: one-stage ventilation, one-stage recirculation, two-stage ventilation, and two-stage recirculation. These models were stimulated for 8,760 hr of operation under hot and humid weather in Malaysia. Several parameters (i.e., coefficient of performance or COP, room temperature and humidity ratio, and the solar fraction of each system) were evaluated by detecting the temperature and humidity ratio of the different points of each configuration by TRNSYS simulation. The latent and sensible loads of the test room were 0.875 kW and 2.625 kW, respectively. By investigating the simulation results of the four systems, the ventilation modes were found to be higher than the recirculation modes in the one- and two-stage solar desiccant cooling systems. The isothermal dehumidification COP of the two-stage ventilation was higher than that of the two-stage recirculation. Hence, the two-stage ventilation mode desiccant cooling system in a hot and humid area has higher efficiency than the other configurations.