Desiccant Coated Heat Exchanger Research Articles

Performance Analysis of an Automotive Heat Pump System with Desiccant Coated Heat Exchangers

Performance Analysis of an Automotive Heat Pump System with Desiccant Coated Heat Exchangers

Facile synthesis of Al-based MOF and its applications in desiccant coated heat exchangers

Desiccant coated heat exchangers (DCHEs) which decouple latent load and sensible load are promising for achieving energy-efficient air-conditioning. The dehumidification performance of the DCHE is highly dependent on the desiccant properties. However, current desiccants such as silica gels, zeolites and salt-supported composite sorbents suffer from the problems of low adsorption capacity, high generation temperature or corrosion. Metal-Organic Frameworks (MOFs) with high adsorption capacity and moderate regeneration temperature are investigated for DCHE in this work. A facile method is proposed to synthesize Al-based MOFs (MIL-96 and MIL-100) with aluminum ions from the dissolution of the aluminum element. Material, coating sample and system levels experiments are conducted to evaluate the application of MOFs in the DCHE. The equilibrium water uptake of the Al-based MOFs can reach 0.35 g/g at 20 °C and 70% RH. The MOFs coating exhibited fast kinetics and excellent cycling stability after 100 adsorption-desorption cycles. An experimental setup is built up to study the dehumidification performance and the parameter influence of the MOF coated heat exchanger (MCHE). Results suggest that low inlet air velocity (<1.4 m/s) and high hot water temperature (>50 °C) help to enhance the dehumidification performance of the MCHE. Compared with the silica gel coated heat exchanger (SCHE), the MCHE displays a larger dehumidification capacity which is 2–3 times that of the SCHE under most test conditions. Moreover, the MCHE has stronger universality to different climates. This study is expected to provide a reference for the design of MOF-based dehumidification systems.

Comparative study on the water uptake kinetics and dehumidification performance of silica gel and aluminophosphate zeolites coatings

Desiccant coated heat exchanger (DCHE) systems have significant potential in improving dehumidification performance. The moisture transfer occurring in one single air channel of DCHE can dominate the overall system performance. In this study, the adsorber with an approximate air channel of DCHE was designed to experimentally investigate the kinetics of silica gel and two aluminophosphate zeolites (FAM Z01 and FAM Z05). Their dehumidification performances also were simulated at regeneration temperature range of 50–80 °C to reveal the applicability in air-cooled cross-flow DCHE. Results showed that linear driving force (LDF) model could well describe the dynamic water uptake behaviors. Adsorption rate coefficient kads increased with increasing inlet air humidity ratio and velocity, but decreased with increasing coating thickness. It is worth noting that increasing regeneration temperature had a little effect on kads, while desorption rate coefficient kdes increased significantly. Simulation results showed that moisture removal capacity (MRC) and dehumidification coefficient of performance (DCOP) of FAM Z05 coated DCHE could reach 0.495 g/kg and 0.57 at regeneration temperature of 50 °C, which were 2.3 and 15.4 times higher than these of silica gel and FAM Z01 coated DCHEs. It demonstrated that FAM Z05 was more promising to utilize lower-grade heat energy.

A novel lung-inspired 3D-printed desiccant-coated heat exchanger for high-performance humidity management in buildings

Offering an independent humidity management method for buildings, desiccant-coated heat exchangers (DCHXs) are deemed a promising approach to improve the overall energy efficiency of air conditioning (AC) systems. State-of-the-art DCHXs, however, are bound with conventional HX topologies either providing limited desiccant-air interfacial areas or introducing excessive pressure drop penalties. Here, a novel 3D-printed DCHX concept inspired by the bronchi arrangement of a human lung is introduced to address the shortcomings inherent in existing DCHX designs. The proposed lung-inspired DCHX utilizes two intertwined bicontinuous flow networks enabling highly efficient heat and mass transfer characteristics for augmented adsorption and regeneration processes at low pressure drop penalties. While the first network evenly distributes an incoming air stream through the entire volume of the lung-inspired DCHX, the second network volumetrically splits a cooling water stream within and through the first network. Effects of various parameters including air flow rate, outdoor air humidity ratio, and regeneration temperature on dehumidification performance and energy efficiency of the proposed lung-inspired DCHX were investigated. Experimental results indicated the proposed lung-inspired 3D-printed DCHX outperforms existing DCHX systems by demonstrating an excellent balance between a high volumetric adsorption rate and a low pressure drop penalty. The volumetric adsorption rate of the proposed lung-inspired DCHX technology is 54.8 g/m3-s, a more than two-fold improvement compared with state-of-the-art DCHX systems. Additionally, the lung-inspired DCHX offers high thermal energy efficiency of 56% at a low regeneration temperature of 40 °C. Therefore, the proposed lung-inspired 3D-printed DCHX offers a new solid-desiccant-based air dehumidification pathway for next-generation high-performance AC systems.

Experimental investigation of a novel metal-organic framework (MOF) based humidity pump under high humidity conditions

Latent heat load accounts for a significant proportion of air-conditioning energy consumption and particularly for specific environment in humid climates. Traditional vapor-compression refrigeration dehumidification faces the problem of refrigerant leakage, overcooling and complicated mechanical systems. Here, we report a novel humidity pump that uses semiconductor refrigeration and metal-organic frameworks (MOFs) as dehumidification method, which can efficiently transport moisture from a relatively ‘low-humidity’ space to a high-humidity one. The working principles of the humidity pump were introduced that the process air flows through the cold desiccant coated heat exchanger and then comes into direct contact with the MOF coatings to transfer heat and mass. The dehumidification performance of humidity pump was investigated in high humidity, and the dehumidification coefficient of performance (DCOP), dehumidification rate and moisture removal efficiency using MIL-100(Fe) coatings were calculated. The results indicated that the MOF humidity pump possesses excellent moisture transfer ability.

Desiccant coated heat exchanger and its applications

Desiccant coated heat exchanger (DCHE) has shown great potential for dehumidification in vapor compression cooling systems and has been widely concerned in the research field of air conditioning, as well as heat and mass transfer in recent years. This paper is aimed at providing fundamental knowledge and research progresses on DCHE for dehumidification and cooling. Initially, DCHE dehumidification is chosen as the research object in this review due to its superiority of the principle by comparing with other two typical dehumidification processes, which are condensation dehumidification and rotary-wheel dehumidification. Then, intermittent dehumidification and continuous dehumidification as two operation modes of DCHE are compared. Finally, the variation characteristics of temperature and humidity during operation are discussed. The current studies of DCHE are reviewed from four aspects, including desiccant coating, cooling source, regeneration heating source and basic heat exchanger. The performance analysis of DCHE is then conducted, according to the different effects of above aspects acting on dehumidification performance, cooling performance and energy efficiency of DCHE. The ongoing researches indicate that the rational construction of DCHE is an effective way to achieve its multi-objective performance optimization. The significant progress of DCHE lays the foundation for applications on other fields. However, further research is required in the field of the selection of desiccant coatings and the structure of heat exchangers to broaden its application market.

Preparation and characterization of thermo-responsive composite for adsorption-based dehumidification and water harvesting

Adsorption technologies for adsorption-based dehumidification and water harvesting from the atmosphere have great potential for energy-saving and environmental protection. The key to successful utilization of these technologies is desired properties of adsorbents. However, most existing adsorbents fail to have the characteristic of both high adsorption capacity and good desorption ability under low regeneration temperature such as at 40 °C. In this work, we grafted a thermo-responsive polymer (Poly(N-isopropylacrylamide) PNIPAM) to silica gel and impregnated with lithium chloride to prepare a composite adsorbent. The adsorption capacity of this composite can reach 1.70 g g−1 at 20 °C&70%RH, and 56% of adsorbed moisture can be released within 60 min at 40 °C&10%RH. This method offers a promising route to develop high-performance thermo-responsive composite adsorbents for desiccant coated heat exchanger (DCHE)-based dehumidification and adsorption-based atmospheric water harvesting (ABAWH).

Investigation of a high-efficient hybrid adsorption refrigeration system using desiccant coated heat exchangers

To extract as much energy as possible from a low-grade heat source in the range of 50–95 °C, a hybrid adsorption refrigeration system using desiccant coated heat exchangers is proposed and investigated in this study. This hybrid system can be operated in three working modes to satisfy various demands for cooling capacity output forms and system performance. Models of the adsorption refrigeration system and desiccant coated heat exchangers are integrated to match the two separate systems and evaluate the system performance. The results indicate that 24 °C is the optimal chilled water temperature to establish circulation between the two systems. Mode 1 exhibits the best cooling capacity output, which is 24% and 32% higher than that of Mode 2 and Mode 3, respectively. Mode 2 ranks first in exergetic efficiency with a heat source of 50 °C and reaches a maximum thermal coefficient of performance (COPth) of 11.09% and 0.998, respectively. Mode 3 provides the highest exergetic efficiency at a heat source of 55–95 °C and a cold-water supply. Finally, the performance of the hybrid system is compared with that of a traditional adsorption system to verify the improved efficiency and wider operable heat source range of the proposed system. The hybrid system can operate effectively with a heat source range of 50–95 °C, while the traditional system cannot work below 65 °C. The COPth of Mode 2 is superior to that of the traditional system, and the heat source of 50–65 °C results in a better COPth of Mode 1, while the traditional mode shows higher COPth than that of Mode 3 at a heat source of 65–95 °C. The three modes outperform than the traditional mode in terms of exergetic efficiency. The proposed hybrid system can thus effectively exploit a low-grade heat source with high efficiency, providing a promising solution for efficient energy utilization.

Energy improvement and performance prediction of desiccant coated dehumidifiers based on dimensional and scaling analysis

The evolution of an in-depth understanding and accurate prediction of the dehumidification performance of a solid desiccant coated heat exchanger dehumidifier (SDHED) is severely limited by the unbalanced and unidentified relationships between the product air's thermal conditions and the combined effect of the geometrical parameters under varying operating conditions. Therefore, this paper employs dimensional and scaling analysis (DASA) to unravel the key underlying relationships, leading to improved prediction and optimization of the SDHED’s performance. Firstly, a theoretical model is developed based on the fundamental principles and experimentally validated by employing a SDHED system prototype. Secondly, a DASA is conducted to identify key dimensionless parameters. Two dimensionless variables, δd/δf and β, are found to significantly impact the product air’s properties. Thirdly, new correlations are established to predict the dehumidifier’s outlet humidity and temperature with R2 = 0.965. The results also indicate that the design parameters, H/L and δd/δf, have significant effects on both latent effectiveness and removed latent energy performance of the SDHED. The removed latent energy can be enhanced from 3.9 kW to 8.5 kW when δd/δf and H/L are reduced to 0.1 and 0.04, respectively. In sum, the developed correlations and model present themselves as invaluable tools to facilitate improved design and engineering of SDHED systems.

Multi-mode integrated system of adsorption refrigeration using desiccant coated heat exchangers for ultra-low grade heat utilization

A traditional adsorption refrigeration system cannot run under ultra-low grade heat source temperatures. In order for ultra-low grade heat utilization, a multi-mode integrated system of adsorption refrigeration using desiccant coated heat exchangers (DCHEs) is proposed. The key point of this integrated system is the rise in evaporation temperature by the use of DCHEs, which contributes to lowing the driving temperature and enhancing the system performance. A mathematical model based on the first and second law of thermodynamics perspective is developed, and the performance investigation is conducted to understand the effect of different modes, driving and medium temperatures. The results reveal that Mode 1 has a wider working range, Mode 2 has better performance and Mode 3 can realize the cogeneration of chilled water and dehumidified air. Compared to a traditional system, the integrated system has obviously lower driving temperature and higher performance. The integrated system with Modes 1 and 2 can operate effectively at driving temperatures below 50 oC and its maximum COP and exergetic efficiency can reach around 0.8 and 0.21, respectively. For ultra-low grade heat utilization, the medium temperature has a greater effect on the COP and equal effect on the exergetic efficiency to driving temperature.

Comparative Study on the Water Uptake Kinetics and Dehumidification Performance of Silica Gel and Fams Coatings

Desiccant coated heat exchanger (DCHE) systems handling sensible and latent heat simultaneously have significant potential in improving dehumidification performance. The DCHEs consist of a series of open air channels coated with porous desiccant, and the moisture transfer occurring in DCHEs usually can be represented by one single channel due to their structural symmetry. In this study, the adsorption kinetics of an adsorber with a parallel air channel coated with three low-temperature regenerative desiccants respectively, i.e. silica gel, FAM Z01 and FAM Z05 was experimentally investigated. Their dehumidification performances in DCHEs also were simulated at regeneration temperature range of 50-80°C. Results showed that the linear driving force (LDF) model can well describe the dynamic behavior of water uptake. Adsorption rate coefficient [[EQUATION]] increased with the increase in inlet air humidity ratio and air velocity, but decreased with increasing coating thickness. It is worth noting that the increasing regeneration temperature had a little effect on the [[EQUATION]] , while desorption rate coefficient [[EQUATION]] increased significantly. Simulation results indicated that the DCOP of Z05 DCHE can reach 0.66 at regeneration temperature of 50°C, which is 2.4 and 8.9 times higher than these of silica gel and Z01 DCHEs, respectively.

Review of Hygroscopic Coating on Aluminum Fin Surface of Air Conditioning Heat Exchanger

Air conditioning energy consumption accounts for most building energy consumption, indoor dehumidification is the main cause of air conditioning energy consumption. Optimize the dehumidification methods of air conditioning systems have great significance to the development of green buildings and people’s pursuit of comfort. Improvement of fins on air conditioning heat exchangers is a hot topic of current research and has achieved considerable results in terms of indoor dehumidification and energy saving compared to traditional air conditioners. This paper reviews two kinds of heat exchangers modified by coating, including desiccant-coated heat exchangers and hydrophobic/hydrophilic coated heat exchangers. For desiccant-coated heat exchangers, the preparation methods of advanced desiccant materials and the possibilities of using this material to achieve excellent energy efficiencies were presented, and the operating parameters that affect thermal performance and dehumidification are determined, including airflow temperature, air velocity, inlet air relative humidity, and regeneration temperature. For hydrophobic/hydrophilic coated heat exchangers, different kinds of hybrid hydrophobic-hydrophilic surfaces are highlighted for they are a high water droplet nucleation rate and surface heat transfer efficiency. In addition, the challenges and future works are explained at last. This paper will provide a valuable reference for the follow-up research, which will be helpful for indoor humidity control and reducing the energy consumption of air conditioning.

Multi-criteria decision making of biomass gasification-based cogeneration systems with heat storage and solid dehumidification of desiccant coated heat exchangers

Biomass gasification-based cogeneration systems integrated with dehumidification of desiccant coated heat exchangers, covering the cogeneration system inclusive of sensible heat storage and internal heat recovery (SC-1), the cogeneration system inclusive of sensible heat storage (SC-2), and the cogeneration system exclusive of heat storage and internal heat recovery (SC-3), are introduced and comparatively investigated on the energetic and economic performances. The developed models of the cogeneration systems are embedded into a multi-criteria decision-making framework to find the optimal system configuration and parameters. The most favorable scheme (SC-1) – that reaches an overall energy efficiency of 0.963, net present value of 1,429,400 USD, levelized total cost of 646,835 USD/year, payback period of 3.7, heat storage tank of 7 m3, heat recovery tank of 2.1 m3, jacket heat rate of 290 kW, and nominal electric power of 629 kWe – is determined. The sensitivity analysis indicates that the net present value is most sensitive to the electricity tariffs, followed by the biomass feedstock price and the selling price of electricity to utility grid. The overall energy efficiency of 0.81–0.995, the net present value of −470,400–6,166,900 USD, and the payback period of 1.5–13.1 were achieved against the price variation range of ±80%.

Study on heat transfer and dehumidification performance of desiccant coated microchannel heat exchanger

The desiccant coated heat exchanger is capable of dealing with sensible load and latent load of hot and humid air. The heat and moisture transfer coefficients of the desiccant coated microchannel heat exchanger (DMHE) are derived theoretically and increase with the airflow velocity in the heat exchanger. The microchannel structure optimization is implemented. It is found that the smaller the fin pitch and flat tube pitch are, the greater the heat and moisture transfer coefficients are. The thicker the desiccant coating, the larger the moisture transfer coefficient, the smaller the heat transfer coefficient. The experimental results show that the dehumidification capacity and the thermal performance coefficient of DMHE are 1.59 kgh−1 and 2.09 respectively with the regeneration hot water at 60 °C. The Taguchi method is employed to evaluate the influence of cooling water temperature, airflow velocity, and water flow rate on the dehumidification performance of DMHE. The airflow velocity is the main factor affecting the dehumidification capacity, where lower cooling water temperatures and faster the airflow velocity increases the dehumidification performance.

Performance analysis of an electric vehicle heat pump system with a desiccant dehumidifier

A battery-electric-vehicle (BEV) relies on the state of charge (SoC) of the built-in battery system because it exclusively uses electric energy stored in rechargeable battery packs. The EV's mileage would be reduced due to the electricity consumption when the mobile heat pump system (MHP) is operated. In the case of the the winter season, the heat pump system operates not only for heating the cabin to achieve thermal comfort but also for defogging to maintain driving safety. To ensure the EV’s driving range, the energy consumption of MHP should be minimized as much as possible by recovering waste heat and applying a dehumidifier. For these purposes, the desiccant coated heat exchanger (DCHE) is introduced into the system, which is able to simultaneously transfer heat and mass (water vapor). In this study, the MHP and cabin thermal load models were made, and the DCHE model was validated by experiments using the unit DCHE. Based on the models, the energy consumption of the EV MHP with the DCHE was conducted, and compared to the conventional system, the proposed system required less energy.

Preparation and Application of UiO-66 in Desiccant-Coated Heat Exchanger-Based Desiccant Cooling Systems

Dehumidification performance of a high-efficient compact desiccant cooling component named desiccant-coated heat exchanger (DCHE) highly depends on coated desiccant materials. Currently, mesoporous...

Field synergy analysis on heat and moisture transfer processes of desiccant coated heat exchanger

The theoretical analysis of the field synergy is tried to apply to desiccant coated heat exchangers (DCHEs) to clarify its coupled heat and moisture transfer characteristics and the way to enhance the characteristics. In this paper, the heat and moisture transfer model of DFHE is established and verified by comparison with experimental data. Three field synergy angles between temperature gradient, velocity vectorand moisture gradient are introduced and calculated for different types of DFHEs with different air velocities. Through the analysis, the field synergy method is proved available for analysing the coupled heat and mass transfer process of DCHE. Different heat and mass transfer performance index of different structure sizes of DFHEs are analysed. By comparing the sensible heating capacity, dehumidification capacity, Re and field synergy angles of them, the optimization design direction of structure size is obtained. The DFHE with higher ratio between total transfer area and total volume is proved to display better performances. So by optimazing the types of DCHE, DMHE is introduced, and the superiority of DMHE is proved. And the reason why DMHE presents higher heat and mass transfer capacity is illustrated.

Experimental study and analysis of solar based winter air conditioning system using desiccant coated heat exchanger

In this paper, a solar driven winter air-conditioning system comprising desiccant coated concentric tube heat exchanger (DCCTHE) is analysed experimentally. The setup consists of concentric tube heat exchanger with saturated desiccant coating to achieve continuous humidification of the dry air. Silica gel, activated alumina and combination of these with activated clay are used as desiccant materials in concentric tube heat exchanger alternately. Parabolic trough type solar air heater is used to supply hot air to regenerate the desiccant. The performance of the system shows the capability to provide continuous heating and humidification in the dry and cold winter weather of North India. The experimental unit performance is measured in the form of humidification factor, maximum outlet temperature and coefficient of performance. The maximum temperature of conditioned air achieved is 43.1⁰C with multiple desiccants whereas the dehumidification rate was minimum. The desiccant material with lowest regeneration temperature i.e. silica gel achieved maximum humidification factor of 6.7 g/kg. The COP of the system is maximum i.e. 0.74 for silica gel as desiccant, while for activated alumina and multiple desiccants, it is 0.62 and 0.6 respectively.

Experimental investigation on a dehumidification unit with heat recovery using desiccant coated heat exchanger in waste to energy system

Nowadays, waste-to-energy systems have attracted more and more attentions. Gasification technology is one of the most important methods of waste-to-energy utilization. For further using the residual heat of the gasification system, a desiccant dehumidification unit with heat recovery using desiccant coated heat exchanger (DCHE) driven by waste heat is developed. Two DCHEs are employed to remove moisture from fresh air. The hot water used in the regeneration process comes from the gasification combined heat and power (CHP) subsystem. Under the typical outdoor condition in Singapore (32 °C, 65% RH), the moisture removal of the system reaches 9.32 g/kgDA, the thermal COP is 0.70, and the waste heat utilization ratio approaches 74.2%. To improve the energy efficiency further, a heat recovery subsystem is adopted to recover the waste heat from the exhausted regeneration air. The recommended pre-regeneration time is 3 min, and the moisture removal is 9.01 g/kg DA, while the thermal COP and waste heat utilization ratio are remarkably increased to 1.34% and 86.5%, respectively. Detailed parametric analyses were carried out, including the influences of the cooling water flow rate, hot water flow rate and hot water temperature. This system combines desiccant coated heat exchangers and the gasification CHP subsystem innovatively to improve the energy utilization of waste-to-energy gasification system, which turns out an effective choice to achieve dehumidification and recover waste heat deeply.

Dehumidification Effect of Polymeric Superabsorbent SAP-LiCl Composite Desiccant-Coated Heat Exchanger with Different Cyclic Switching Time

This study investigated a composite polymer desiccant material’s performance, which is prepared by impregnating solid desiccant such as sodium polyacrylate (SAP) on to hygroscopic salts such as lithium chloride (LiCl). Dehumidification performance of the proposed composite polymer desiccant (SAP-LiCl) was analyzed by coating the suitable weight percentage (wt %) of the desiccant onto a single fin-tube heat exchanger (FTHE) system and testing the desiccant-coated heat exchanger (DCHE) in a testing tunnel under various operating conditions. Net dehumidification efficacy of DCHE in terms of sorption and desorption amount and thermal performance (COPth) were analyzed. For instance, with processed air inflow temperature, relative humidity and regeneration temperature setting of 30 °C, 80% RH and 70 °C, DCHE’s sorption, desorption amount and COPth were recorded as high as 945.1 g, 1115.1 g, and 0.39, respectively. It was further realized that the performance of the DCHE could be enhanced by modulating the cyclic switching time for dehumidification and regeneration processes. For instance, with the aforementioned processed airflow conditions, when the cyclic switching time tuned as 60 min instead of 10 min for a total time period of 120 min, there is a net 58% improvement to the COPth of the system. It was further observed that, under the same time period corresponding to the increase in cyclic switching time, the overall COPth can be enhanced; however, the water vapor sorption and desorption amounts of desiccant were decreased.