A dimensionless analysis of heat and mass transport in an adsorber with thin fins; uniform pressure approach

A numerical study on heat and mass transfer in an annular adsorbent bed assisted with radial fins for an isobaric adsorption process is performed. A uniform pressure approach is employed to determine the changes of temperature and adsorbate concentration profiles in the adsorbent bed. The governing equations which are heat transfer equation for the adsorbent bed, mass balance equation for the adsorbent particle, and conduction heat transfer equation for the thin fin are non-dimensionalized in order to reduce number of governing parameters. The number of governing parameters is reduced to four as Kutateladze number, thermal diffusivity ratio, dimensionless fin coefficient and dimensionless parameter of Γ which compares mass diffusion in the adsorbent particle to heat transfer through the adsorbent bed. Temperature and adsorbate concentration contours are plotted for different values of defined dimensionless parameters to discuss heat and mass transfer rate in the bed. The average dimensionless temperature and average adsorbate concentration throughout the adsorption process are also presented to compare heat and mass transfer rate of different cases. The values of dimensionless fin coefficient, Γ number and thermal diffusivity ratio are changed from 0.01 to 100, 1 to 10^− 5 and 0.01 to 100, respectively; while the values of Kutateladze number are 1 and 100. The obtained results revealed that heat transfer rate in an adsorbent bed can be enhanced by the fin when the values of thermal diffusivity ratio and fin coefficient are low (i.e., α^? = 0.01, Λ = 0.01). Furthermore, the use of fin in an adsorbent bed with low values of Γ number (i.e. Γ = 10^− 5) does not increase heat transfer rate, significantly.     

Study of a new solar adsorption refrigerator powered by a parabolic trough collector

This paper presents the study of solar adsorption cooling machine, where the reactor is heated by a parabolic trough collector (PTC) and is coupled with a heat pipe (HP). This reactor contains a porous medium constituted of activated carbon, reacting by adsorption with ammonia.We have developed a model, based on the equilibrium equations of the refrigerant, adsorption isotherms, heat and mass transfer within the adsorbent bed and energy balance in the hybrid system components. From real climatic data, the model computes the performances of the machine. In comparison with other systems powered by flat plate or evacuated tube collectors, the predicted results, have illustrated the ability of the proposed system to achieve a high performance due to high efficiency of PTC, and high flux density of heat pipe.     

新型吸附式制冷系统吸附床的研究进展

吸附床的传热传质性能是提高吸附式制冷效率的关键,优化吸附床的结构能够有效提高整个吸附床的传热传质效率,减少热量损失,提高系统的制冷效率（coefficient of performance, COP）和单位质量吸附剂制冷量（specific cooling power, SCP）。本文介绍了近年来几种新型吸附床的类型,综述了吸附剂侧的固化吸附剂和涂层吸附剂,以及换热器侧的新型换热器结构。最后阐述新型吸附床的未来发展方向和研究重点。     

Simulation of hydrogen storage tank packed with metal-organic framework

Hydrogen adsorption in high surface metal-organic framework (MOF) has generated significant interest over the past decade. We studied hydrogen storage processes of MOF-5 hydrogen storage systems with adsorbents of both the MOF-5 powder (0.13 g/cm³) and its compacted tablet (0.30 g/cm³). The charge–discharge cycles of the two MOF-5 adsorbents were simulated and compared with activated carbon. The physical model is based on mass, momentum and energy conservation equations of the adsorbent-adsorbate system composed of gaseous and adsorbed hydrogen, adsorbent bed and tank wall. The adsorption process was modeled using a modified Dubinin–Astakov (D–A) adsorption isotherm and its associated variational heat of adsorption. The model was implemented by means of finite element analysis software Comsol Multiphysics™, and the system simulation platform Matlab/Simulink™. The thermal average temperature from Comsol simulation is used to fill the gap between the system model and the multi-dimensional models. The heat and mass transfer feature of the model was validated by the experiments of activated carbon, the simulated pressure and temperatures are in good agreement with the experimental results. The model was further validated by the metal-organic framework of Cu-BTC and is being extended its application to MOF-5 in this study. The maximum pressure in the powder MOF-5 tank is much higher than that in the activated carbon tank due to the lower adsorbent density of MOF-5 and resulting lower hydrogen adsorption. The maximum pressure in the compacted MOF-5 tank is a little bit lower than that in the activated carbon tank due to the higher adsorbent density and resulting higher hydrogen adsorption. The temperature swings during the charge–discharge cycle of both MOF-5 tanks are higher than that of the activated carbon tank. These are caused mainly by pressure work in the powder MOF-5 tank and by adsorption heat in the compacted MOF-5 tank. For both MOF-5 hydrogen storage systems, the lumped parameter models implemented by Simulink agree well with experimental pressures and with pressures and thermal average temperatures from Comsol simulation.     

Experimental investigation and mathematical modelling of a solid adsorption refrigeration system

The objective of this work is to study the thermodynamic mechanism and performance of an engine exhaust-powered adsorption refrigeration system using CaCl₂ as adsorbent and NH₃ as refrigerant. A 6 kW nominal refrigerating capacity adsorption refrigerator was developed. The working performance of the refrigerator is presented. It is concluded that the refrigerating capacity at constant evaporating temperatures varies with the input heat into the generator, and the heat transfer affects strongly the mass transfer in the adsorbent, making it work in different mean generation and adsorption temperatures. A conventional test bed was developed for investigating the properties of CaCl₂–NH₃ adsorption/desorption unit tube. A mathematical model based on non-equilibrium thermodynamics was developed to describe the performances of the adsorption refrigerating system.     

Thermal effect of the adsorption heat on an adsorbed natural gas storage and transportation systems

This paper experimentally studies the thermal effect that results from the adsorption heat on both the charge and discharge performance of adsorbed natural gas (ANG) storage and transportation systems. Two storage tanks built with temperature systems and security control were used during the adsorption and desorption process. Temperature, flow rate and discharge amount were recorded experimentally at 2, 3 and 4 MPa adsorption pressures, using different activated carbon (AC) as an adsorbent bed. Results show that the central region of the adsorbent bed suffers from the severest temperature fluctuation of the charge and discharge process. It was observed that the best discharged amount was 4 MPa using, G1220 Extra AC as an absorbent bed. Conclusions detected that it is possible mitigate the temperature fluctuations with improved AC properties and the amount of NG desorbed is linearly proportional to the respective tank’s hydraulic volumes.     

A two-energy equation model for dynamic heat and mass transfer in an adsorbent bed using silica gel/water pair

In this study, the influence of the adsorbent bed dimensions, convective heat transfer coefficient between the cooling fluid and adsorbent bed and the thermal conductivity of the solid adsorbent material on the transient distributions of the solid and gas phase temperature difference, differences in the adsorbate concentration predicted by the instantaneous equilibrium and linear driving force (LDF) models, solid phase temperature, gas pressure and adsorbate concentration inside the adsorbent bed of a solid sorption cooling system have been investigated numerically for a nearly isobaric adsorption process. Silica gel/water is selected as the working pair. A transient two-dimensional local thermal non-equilibrium model has been developed that takes into account both internal and external mass transfer resistances. The local volume averaging method has been used to derive the macro-scale governing conservation equations from the micro-scale equations. It has been found that generally, the effects of the parameters investigated on the transient distributions of the temperature difference between the phases, difference in adsorbate concentration between the instantaneous equilibrium and LDF models, and gas phase pressure gradients are negligible small. The thickness of the adsorbent bed for the given adsorbent bed length and thermal conductivity of the solid adsorbent material have a large influence on the transient distributions of the solid phase temperature and adsorbate concentration. On the other hand, the transient temperature and adsorbate concentration distributions are only slightly affected by the variation of the adsorbent bed length and convective heat transfer for the conditions studied.     

Effects of porosity on heat and mass transfer in a granular adsorbent bed

In the present study, the mechanism of heat and mass transfer in an annulus adsorbent is handled. The heat and mass transfer equations for the adsorbent bed and the mass balance equation for the adsorbent granules are numerically solved to obtain the distributions of temperature, pressure, adsorptive density and adsorbate concentration in the adsorbent bed. The study is performed for the silica gel–water pair and for three different values of porosity as 0.1, 0.2 and 0.3. The distributions of temperature and adsorbate concentration are considerably influenced from the bed porosity. The adsorption period increases with the increase of the porosity value. The porosity affects the pressure and adsorptive density distributions at the beginning of the process and after a relatively short time, the averages of these dependent variables approach to the final equilibrium state.     

基于平行流铝扁管吸附床传热性能的模拟研究

吸附床是吸附式制冷系统的关键部件。吸附床的换热能力对吸附式制冷系统的各项性能有显著影响。文章针对应用于吸附床的传统换热器和扁管换热器的不足之处,设计出一种新型平行流铝扁管吸附床,并建立了该吸附床的二维传热模型,以温度随时间的变化情况为分析指标,分析翅片的间距、高度、厚度,以及吸附剂体积分数等因素对吸附床传热性能的影响,从而优化调整吸附床的结构,提高其换热性能。分析结果表明:当翅片高度约为70 mm时,吸附床的换热能力达到峰值;当翅片厚度大于1.5 mm时,翅片厚度的增加对吸附床传热性能的影响比较微弱;当吸附剂体积分数由0.25逐渐增大至0.45时,吸附剂的等效传热系数约增加了50%。     

Design and Investigation of Heat Transfer in a New Adsorbent Bed With CPC for Solar Adsorption Refrigeration Systems

This article presents the design and the heat transfer study in a novel adsorbent bed with compound parabolic concentrator (CPC) for solar adsorption chillers. The objectives of the study were to investigate the heat transfer in the adsorbent bed experimentally, and to verify the fins layout through finite-element analysis (FEA) simulation. CPCs with different concentration ratios were experimentally tested and an appropriate design of CPC was selected for a prototype. The prototype was designed with the objective of improving the heat and mass transfer ability of the adsorbent bed. Fins were placed in the transverse direction under the receiver area of each CPC. Spaces were provided from three sides of the adsorbent for easy movement of the refrigerant. FEA software was used to study the effect of the fins layout and fins pitch. The experimental results showed that the heat was efficiently transferring up to the end and extended parts of the bed. Simulation results indicated that the present strategy of placing the fins in a transverse direction gives uniform heat distribution compared to a fins layout with fins placed in a longitudinal direction. The proposed design scheme will be helpful to improve the system performance by increasing the heat and mass transfer ability of an adsorbent bed.     

Study of thermal conductivity,permeability, and adsorption performance of consolidated composite activated carbon adsorbent for refrigeration

Composite adsorbents, comprising activated carbon and expanded natural graphite, have been developed, and their thermal conductivity, permeability and adsorption performance were tested. The thermal conductivity varied with the ratio of activated carbon to expanded natural graphite. Thermal conductivity increased as the ratio of expanded graphite increased. Considering that the density of activated carbon for the composite adsorbent should not be lower than 200 kg/m³, otherwise the volumetric cooling capacity would be unacceptably low, the highest thermal conductivity obtained from experiments was 2.47 W m^?1 K^?1. The permeability was also measured, and the best result obtained was 4.378 × 10^?12 m². In order to evaluate the influence of heat and mass transfer on adsorption performance, the adsorption rate was tested using a Rubotherm magnetic suspension balance, and results showed that for the freezing conditions lower than ?10 °C the performance of granular activated carbon was better than that of solidified adsorbent because of the reduced mass transfer of ammonia at low saturated pressure. The adsorption performance of consolidated adsorbents increased rapidly when the evaporating temperature was higher than ?10 °C. When the evaporating temperature was 8 °C, the adsorption rate of consolidated adsorbent was improved by 29% if compared with that of granular adsorbent.     

Heat and mass transfer in adsorbent bed with consideration of non-equilibrium adsorption

In the solid adsorption refrigeration cycles, the actual adsorption processes are all non-equilibrium. To investigate the heat and mass transfer in adsorbent bed, mathematical model is established and solved by a numerical method. The relations between adsorption temperature, adsorption velocity, adsorption quantity, coefficient of performance (COP), specific cooling power (SCP) and time are discussed during the process of cooling the adsorbent bed. The relations between desorption temperature, desorption velocity, desorption quantity and time are discussed during the process of heating the adsorbent bed. It indicates that there is a peak value for adsorption velocity in the adsorption process and there is also a peak value for desorption velocity in the desorption process. It also shows that the changing rate of the adsorbent temperature tends to let up, and the coefficient of performance value grows nearly linearly in the adsorption process and there is a peak value of SCP in the adsorption process.     

Experimental studies of the performance of adsorbed natural gas storage system during discharge

In this work, comparative experiments were carried out to study the thermal effect of the adsorption heat on the discharge performance of an adsorbed natural gas (ANG) storage system. A storage vessel with u-shaped heat exchanging pipe round the central region was designed according to the temperature field determined by the finite element analysis of the adsorbent bed within 1 L cylinder. Discharge performances of the ANG on the activated carbon within the storage vessel were tested on a volumetrically built experimental unit. Results show that the central region of the adsorbent bed suffers from the severest temperature fluctuation in a short period of the initial discharge state; introduction of the hot water whose temperature is similar to that of the cooling water of a vehicular engine can significantly moderate the temperature fluctuation of the adsorbent bed, shorten the discharge process for about 60 % in comparing with that without the application of the supplemental heat. Conclusions are drawn that the heat from the cooling water of a vehicular engine should be a consideration to improve the discharge performance of the ANG storage system.     

Experimental study of a solidified activated carbon-methanol adsorption ice maker

In this paper, the experiments are performed on an adsorption ice maker driven by waste heat, which uses up to two beds. Each bed uses methanol as refrigerant and solidified activated carbon (120 kg adsorbent totally, 60 kg adsorbent per bed) as adsorbent. This system is designed to be driven by the waste heat of a 100 kW diesel engine. The experiments show that the cooling power could be enhanced by the mass recovery process up to 11%, and the heating power could be lowered by the heat recovery process up to 30%. The optimal cooling power of this prototype is about 2.0 kW and corresponds to a specific cooling power (SCP) is about 17 W/kg with both heat and mass recoveries between two beds. Considering the optimal adsorption time is much longer than optimal desorption time at the condition of ice making, the experiments are operated on a single bed (60 kg adsorbent per bed) and the adsorption time used in experiments is two times of desorption time, then the performance of a three-bed adsorption ice maker (120 kg adsorbent totally, 40 kg adsorbent per bed) is predicted by the results of experiments on this single bed. The results of prediction show that both COP and cooling power of three-bed operation could be enhanced greatly compared to the two-bed operation; optimal SCP and COP are respectively 22 W/kg and 0.239 when mass and heat recoveries proceed between three beds. Optimal ice productivity of this three-bed system is 21 kg/h when the water temperature is 25 °C and ice temperature is −7 °C.     

Numerical study of a combined heat and mass recovery adsorption cooling cycle

A new transient two-dimensional model for the simulation of a combined heat and mass recovery adsorption cooling cycle based on the zeolite NaX/water working pair is proposed in this paper. The model describes the transfer phenomena in the adsorber in detail and is solved by control volume method. Internal and external mass transfer limitations which are neglected by many researchers are considered in the model since they have significant effects on the performance of the adsorption cooling cycle. The numerical results show that the combined heat and mass recovery cycle between two adsorbent beds can increase the coefficient of performance (COP) of an adsorption cooling system by more than 47% compared to the single bed cycle. This numerical model can be used in system optimization and design of adsorption cycles.     

A new type adsorber for adsorption ice maker on fishing boats

A new type of adsorber for an adsorption ice maker on fishing boats, which uses a compound adsorbent (activated carbon and CaCl₂) and ammonia working pair, is designed. This type of heat pipe adsorber solves the problem of incompatibility between ammonia, copper, seawater and steel. The heating/cooling power for the adsorption/desorption process of the adsorbent, which is required to be transferred by one heat pipe in the adsorber, is computed by the test results of the adsorbent, and the heat transfer performance of one heat pipe in the adsorber is simulated according to the theory of the two phase closed thermosyphon. The heat transfer performance of the heat pipe can meet the heat demands for adsorption/desorption of the adsorbent when the evaporating temperature is −15 °C and the cycle time is 10 min. A test unit is set up to test the heating/cooling performance of the heat pipe type adsorber, and the experimental results are coincident with the simulation. The performance of a two bed adsorption ice maker with heat pipe adsorbers is predicted, and the cooling power is about 17.1–17.8 kW at the evaporating temperature of −15 °C and cycle time of 10 min with mass recovery between two 29 kg compound adsorbent beds.     

Theoretical and experimental investigation of a constant-pressure adsorption process

This paper presents a theoretical and experimental analysis of a constant pressure adsorption process. The governing heat and mass transfer equations derived from local thermodynamic equilibrium and energy balance are solved numerically. The model is validated by comparison with experimental results. It is then used to analyze the effect of some operating and design parameters on a constant-pressure sorption process. The adsorbent thickness and heat transfer coefficient between the adsorbent and the heating/cooling fluid have the strongest influence on sorption kinetics and on the cooling capacity of adsorption systems.     

Isothermal composite adsorbent. Part I: Thermal characterisation

Adsorption and desorption are respectively exo and endothermic phenomena leading to significant temperature changes in adsorption columns. Enhanced efficiency of a sorption process could be obtained under isothermal conditions, either for gas storage, purification or separation applications. The heat transfer within the adsorbent beds can be managed in situ, using thermal energy storage material: a phase change materials (PCM) for example. The thermal behaviour of a mixture of activated carbon and PCM during CO₂ adsorption has been studied. The thermal characteristics of the involved materials have been determined and experiments carried out to highlight the positive effect of the PCM to reduce the CO₂ adsorption heat effects on an activated carbon bed. Calorimetry was the technique used for all the thermal characterisations. It appears that the heat effects induced by CO₂ adsorption are reduced by the presence of the PCM together with the adsorbent. The endothermic effect of fusion balances the heat effect of adsorption and significantly reduces the temperature changes.     

Analysis of the performance of a multi-bed adsorption heat pump using a solid-side resistance model

An analysis of the coefficient of performance and specific cooling power for a four-bed adsorption heat pump, using a solid-side resistance model, is presented. Methanol and an activated carbon are the adsorption pair. An Arrhenius form of solid-side mass diffusivity was adopted. A plate-fin type insert was considered as the heating/cooling element in adsorbers. The result shows that, for large grain-size activated carbon, the intra-particle mass diffusion resistance significantly affects the adsorption and regeneration rates. Both the coefficient of performance and the specific cooling power increase with the overall heat transfer coefficient, regeneration and evaporation temperatures, but decrease with an increase of the condensing temperature and time constant of the insert. The coefficient of performance considerably increases with a decrease of the insert heat capacitance. An optimum cycle time, corresponding to a maximum specific cooling power, was found. To achieve a high specific cooling power for short cycle time operations, small grain-size activated carbon should be selected as the adsorbent. In addition, a small time constant of the insert and a large overall heat transfer coefficient are also highly recommended.     

Modelling and performance study of a continuous adsorption refrigeration system driven by parabolic trough solar collector

This article suggests a numerical study of a continuous adsorption refrigeration system consisting of two adsorbent beds and powered by parabolic trough solar collector (PTC). Activated carbon as adsorbent and ammonia as refrigerant are selected. A predictive model accounting for heat balance in the solar collector components and instantaneous heat and mass transfer in adsorbent bed is presented. The validity of the theoretical model has been tested by comparison with experimental data of the temperature evolution within the adsorber during isosteric heating phase. A good agreement is obtained. The system performance is assessed in terms of specific cooling power (SCP), refrigeration cycle COP (COP_cycle) and solar coefficient of performance (COP_s), which were evaluated by a cycle simulation computer program. The temperature, pressure and adsorbed mass profiles in the two adsorbers have been shown. The influences of some important operating and design parameters on the system performance have been analyzed.The study has put in evidence the ability of such a system to achieve a promising performance and to overcome the intermittence of the adsorption refrigeration systems driven by solar energy. Under the climatic conditions of daily solar radiation being about 14 MJ per 0.8 m² (17.5 MJ/m²) and operating conditions of evaporating temperature, T_ev = 0 °C, condensing temperature, T_con = 30 °C and heat source temperature of 100 °C, the results indicate that the system could achieve a SCP of the order of 104 W/kg, a refrigeration cycle COP of 0.43, and it could produce a daily useful cooling of 2515 kJ per 0.8 m² of collector area, while its gross solar COP could reach 0.18.