Feasibility study on power ultrasound for regeneration of silica gel—A potential desiccant used in air-conditioning system

A new regeneration method using power ultrasound was put forward to overcome the limitations of silica gel in air-conditioning applications, such as high regeneration temperature and low regeneration efficiency. The technical feasibility of the new method was validated experimentally and demonstrated in detail from different sides. The experiments were performed under different regeneration temperatures, i.e. 45 °C, 55 °C, 65 °C and 75 °C. The power and frequency of ultrasound applied in this experimental study was set as 40 W and 26 kHz, respectively. The three indicators, including the regeneration degree (RD), enhanced rate of regeneration (ER) and energy-saving rate (ESR), were suggested to evaluate the effect of power ultrasound in the regeneration. The Crank’s diffusion model was used for the calculation of the moisture diffusivity in silica gel, and the Arrhenius equation for the determination of energy activation of moisture desorption on silica gel. The analysis results prove that the introduction of high-intensity ultrasound to the regeneration of silica gel can help to improve the regeneration efficiency and reduce regeneration energy. The benefits should owe to the special ‘heating effect’ and ‘micro-vibration effect’ caused by power ultrasound that can enhance the moisture diffusivity in silica gel and lower the activation energy of moisture desorption on silica gel.     

Experimental study of the transient adsorption/desorption characteristics of silica gel particles in fluidized bed

Transient adsorption/desorption characteristics of spherical particles of silica gel (about 3 mm in diameter) in a fluidized bed have been studied experimentally. To control the humidity of inlet air, a humidifier is designed and fitted in a proper location in the system. The system is well instrumented to measure the inlet and outlet air parameters as well as bed temperatures during the experiments.     

Solar/waste heat driven two-stage adsorption chiller: the prototype

Nowadays, adsorption heat pumps receive considerable attention as they are energy savers and environmentally benign. In this study silica gel–water is taken as the adsorbent refrigerant pair. To exploit solar/waste heat of temperatures below 70°C, staged regeneration is necessary. A new two-stage non-regenerative adsorption chiller design and experimental prototype is proposed. Experimental temperature profiles of heat transfer fluid inlets and outlets are presented. The two-stage cycle can be operated effectively with 55°C solar/waste heat in combination with a 30°C coolant temperature. In this paper the physical adsorption of silica gel, working principle and features of a two-stage chiller are described.     

Effect of internal cooling/heating coil on adsorption/regeneration of solid desiccant tray for controlling air humidity

Thermal performances of solid desiccant tray having internal cooling/heating coil for air humidity adsorption and desiccant regeneration are investigated. Three units of desiccant tray each of 48 cm × 48 cm cross‐sectional area and 2.5 cm thickness filled with silica gel are tested in a wind tunnel. For adsorption process, an air stream is flowing through the desiccant trays and the air humidity is captured by the silica gel. Approximately 10–40% of air humidity could be adsorbed more in case of the internal cooling. Besides, the outlet air temperature increases only slightly. In regeneration process, a hot air stream is used to repel the moisture in the silica gel. With the internal heating, the regeneration time is shorter compared with that without internal water heating. In addition, a correlation for calculating the adsorption/regeneration performance of the silica gel trays is developed and the results from the model agree well with the experimental data. Copyright © 2008 John Wiley & Sons, Ltd.     

Study on a dual-mode, multi-stage, multi-bed regenerative adsorption chiller

In this paper, a detailed parametric study on a dual-mode silica gel–water adsorption chiller is performed. This advanced adsorption chiller utilizes effectively low-temperature solar or waste heat sources of temperature between 40 and 95 °C. Two operation modes are possible for the advanced chiller. The first operation mode will be to work as a highly efficient conventional chiller where the driving source temperature is between 60 and 95 °C. The second operation mode will be to work as an advanced three-stage adsorption chiller where the available driving source temperature is very low (between 40 and 60 °C). With this very low driving source temperature in combination with a coolant at 30 °C, no other cycle except an advanced adsorption cycle with staged regeneration will be operational. In this paper, the effect of chilled-water inlet temperature, heat transfer fluid flow rates and adsorption–desorption cycle time effect on cooling capacity and COP of the dual-mode chiller is performed. Simulation results show that both cooling capacity and COP values increase with the increase of chilled water inlet temperature with driving source temperature at 50 and 80 °C in three-stage mode, and single-stage multi-bed mode, respectively. However, the delivered chilled-water temperature increases with chilled-water inlet temperature in both modes.     

Performance evaluation of multi-stage, multi-bed adsorption chiller employing re-heat scheme

This paper deals with the performance investigation of a silica gel/water-based multi-stage, multi-bed, six-bed adsorption chiller employing re-heat scheme. The innovative chiller is powered by waste heat or renewable energy sources of temperature between 50 and 70 °C along with a coolant of inlet temperature at 30 °C for air-conditioning purpose. The performance of the six-bed adsorption chiller using re-heat scheme is compared with that of the six-bed chiller without re-heat. With the same operating conditions, such as the heat transfer fluid inlet (HTF) temperatures, HTF flow rates, adsorption/desorption cycle time and same chiller physical dimension, it is found that both the cooling capacity (CC) and the coefficient of performance (COP) of the three-stage chiller with re-heat scheme are superior than those of the three-stage chiller without re-heat scheme.     

An experimental investigation into a solar assisted desiccant-evaporative air-conditioning system

In this experimental investigation a solar assisted open adsorption cooling system has been designed and tested under the local weather conditions of Basrah, Iraq. Data were obtained from June to September, inclusive, 1984. Tests were carried out hourly with a directly supply of hot air from a corrugated absorber solar air heater for regeneration. Also, tests were conducted at a constant regeneration temperature of 70°C using auxiliary heat. Adsorption was carried out by a rotary disk of silica gel. Three mass flow rates of process air were employed without recirculation. The performance of the solar air heater was obtained for both seasons, and the instantaneous efficiency was evaluated experimentally and analytically with results compared. Daily and seasonal coefficients of performance were obtained for the cooling system for the mass flow rates employed. A maximum seasonal average value of 2.8 was obtained for a mass flow rate of 0.075 kg/s. The system performance improved with higher regeneration temperature, higher process air mass flow rate and dry weather. It was possible to generate a cool supply of air at satisfactory conditions using solar energy only for all clear days under the local weather conditions.     

Feasibility study of using agriculture waste as desiccant for air conditioning system

This research was aimed at investigating the feasibility of using dried agricultural waste as desiccant for an open cycle air conditioning system. The natural fibers are, therefore, intended to replace chemical desiccant such as silica gel, molecular sieves etc. The investigation was limited to Coconut coir (Cocos nucifera) and Durian peels (Durio zibethinus).Experimental results confirmed that dry coconut coir and durian peel can absorb 30 g and 17 g H₂O per 100 g dry product, respectively, from air at the average condition of 32°C and 75% relative humidity. The optimum airflow rate is about 84 and 98 m³/hr-100 g dry product, respectively. Therefore, the dry coconut coir is more suitable than the dry durian peel.Comparison between the dry coconut coir and silica gel showed that the average adsorption rate of coconut coir is less than that of silica gel by about 5 g/h-100 g dry product at an airflow rate of 84 m³/h and 60 min operating time. However, it is still an interesting option to replace silica gel in open cycle air conditioning system, as the decrease of average adsorption rate is rather small.The other extremely interesting advantage of coconut coir is that during moisture absorption the heat generated during the process is less important. That means the air leaves the coconut coir bed at a lower temperature compared to that with a silica gel. Therefore, the saving of cooling energy is much more important.     

Regeneration of silica gel in multi-shelf regenerator

In the present paper an investigation on the regeneration of solid desiccant (silica gel) in a modified design of dehumidifier called “Multi-shelf Dehumidifier”, has been reported. A “Packed Bed Dehumidifier” was used as control. Both the dehumidifiers were fabricated and tested simultaneously. The effect of regeneration air temperature, bed-air velocity and number of shelves on regeneration of silica gel was investigated. The range of regeneration air temperature was 42–72°C, while bed-air velocity varied from 0.175-0.55 m/s and number of shelves from 2 to 4. The regeneration time decreased with the increase in regeneration air temperature, bed-air velocity as well as number of shelves. The values of regeneration air temperature and bed-air velocity for minimum energy input were found to be 52°C and 0.175 m/s irrespective of the number of shelves. The effect of the rest period was investigated for these optimum values of regeneration air temperature and air velocity for a number of shelves varying from 2 to 4. The rest periods used were 30, 60 and 180 mins. The regeneration time reduced with increase in rest period for all the values of number of shelves, but the reduction in regeneration time was not appreciable.     

Improved utilization of desiccant material in packed bed dehumidifier using composite particles

Solid desiccant dehumidifiers are widely used in drying processes. In most of these dehumidifiers, the desiccant material is used as packed bed of granule or spherical particles. Investigations of intra-particle heat and mass transfer processes has shown that the entire portion of the particle is not participating effectively during adsorption as well as desorption processes [Pesaran AA, Mills F. Moisture transport in silica gel packed beds-I. Theoretical study. International Journal of Heat and Mass Transfer 1987; 30: 1037–49]. This is because the diffusion rate is very small compared to that of convection. In the present work, a new desiccant composite particle, in which the unutilized portion of the spherical desiccant particle is replaced with an inert particle, is proposed. By replacing the conventional particles with composite particles for the same mass of desiccant material, the available area for heat and mass transfer increases and more amount of desiccant material is effectively utilized. Further, in order to ascertain the improvement in the performance of the desiccant bed using the composite particles, various factors like thermo-physical properties of the inert material, composite particle thickness ratio, bed configuration, bed volume, the pressure drop and the increase in total adsorbed or desorbed mass have to be considered. In view of this, a theoretical investigation of the operation of vertical solid desiccant packed bed dehumidifier, using both conventional silica gel particles as well as the new proposed composite silica gel particles has been reported. A modified solid side resistance (MSSR) model is developed for the prediction of intra-particle temperature and water content profiles. Results of the present theoretical models, when applied to packed bed of conventional silica gel particles, agree well with the experimental results from the literature for both desorption and adsorption processes. From the theoretical results, more utilization for the desiccant material is obtained when ordinary silica gel particles are replaced by composite silica gel particles. For the same amount of desiccant material and same mass flow rate of air, using particles of 0.2 thickness ratio the pressure drop decreases by about 60% for the case investigated. In addition, an increase of about 11.07% and 20.46% in total mass adsorbed and desorbed respectively are obtained. At the time when adsorption process ends, an increase of 15.5% in the bed effectiveness has been obtained. In addition, the expected improvement in total mass adsorbed and desorbed is observed to be dependent on the inert material thermo-physical properties for thickness ratio less than 0.5. An optimization technique relating the composite particle design, resulting savings in pressure drop and bed volume increase is proposed.     

Influence of ultrasonic frequency on the regeneration of silica gel by applying high-intensity ultrasound

Ultrasonic frequency is the key parameter considered in ultrasonic applications. In order to provide a basic knowledge about the influence of ultrasonic frequency on the regeneration of silica gel assisted by power ultrasound, the experiments about silica gel regeneration under the radiation of constant-power (60 W) ultrasound with different frequencies (i.e., 23, 27, and 38 kHz) and that without ultrasound were carried out at different regeneration temperatures (i.e., 35, 45, 55, and 65 °C). The experimental results showed that the lower frequency was beneficial for the application of power ultrasound in the regeneration of silica gel. The fact was theoretically explained by the ultrasonic power attenuation model which indicates that the ultrasound of lower frequency will lead to more uniform energy distribution and hence achieve higher efficiency of utilization. Meanwhile, the effect of ultrasonic frequency on silica gel regeneration would be influenced by the regeneration temperature and the moisture ratio in silica gel. As investigated in this study, the effect of ultrasonic frequency on the regeneration would be more significant at the lower regeneration temperature or at the higher moisture ratio in silica gel. In addition, the mean regeneration speed model of silica gel dependent of the regeneration temperature and the ultrasonic frequency was established according to the experimental data.     

Moisture transport in silica gel packed beds—II. Experimental study

Experiments have been performed to obtain the transient response of a thin adiabatic packed bed of silica gel after a step change in inlet air conditions. Comparisons are made with predictions using a solid-side resistance model and a pseudo-gas-side controlled model and better agreement obtained with the former model. An apparent dynamic hysteresis for adsorption/desorption with microporous silica gel is clearly in evidence, which could be due to a solid-side effective diffusion coefficient which decreases with increasing moisture content, or to a lesser extent to a hysteresis in the adsorption isotherm itself.     

The regeneration of silica gel desiccant by air from a solar heater with a compound parabolic concentrator

The regeneration of silica gel desiccant by a solar air heater for use in an air-conditioning system has been investigated. The hot air is produced by a compound parabolic concentrator collector (CPC), which has aperture and receiver areas 1.44 and 0.48 m², respectively. The regeneration temperature can be started at 40 ^oC. The regeneration rate and the regeneration efficiency were greatly affected by the solar radiation, but depended only slightly on the different initial moisture contents of silica gel and the number of silica gel beds. The regeneration of silica gel provided by the CPC collector is suitable for a tropical climate where the diffuse solar radiation is high all the year round.     

Adsorption cooling cycles for alternative adsorbent/adsorbate pairs working at partial vacuum and pressurized conditions

This article presents the performance analysis of both ideal single-stage and single-effect double-lift adsorption cooling cycles working at partially evacuated and pressurized conditions. Six specimens of adsorbents and refrigerant pairs, i.e., ACF (A-15)/ethanol, ACF (A-20)/ethanol, silica gel/water, Chemviron/R134a, Fluka/R134a and MaxsorbII/R134a have been investigated. The relationships between equilibrium pressures, adsorbent temperatures and equilibrium adsorption concentrations (Dühring diagram) are presented. Parametric analyses have been carried out with various regeneration (desorption) and evaporation temperatures. Theoretical analysis for adsorption cycles working in single-stage mode shows that ACF (A-20)/ethanol can achieve a specific cooling effect (SCE) of 344 kJ/kg_ads, which is followed by the silica gel/water pair with 217 kJ/kg_ads at a regeneration temperature of 85 °C. On the other hand, when the regeneration temperature is below 70 °C, single-effect double-lift cycle has a significant advantage over single-stage cycle, at which the SCE is higher due to the reduction in adsorption bed pressure in single-effect double-lift cycle.     

Theoretical model on the heat and mass transfer in silica gel packed beds during the regeneration assisted by high-intensity ultrasound

In the present work, a theoretical model is reported on the heat and mass transfer in silica gel packed bed during the regeneration process by using hot air combined with high-intensity ultrasound. The model consists of two parts: one is about the sound propagation in porous media; the other is about the fundamental heat and mass transfer process in the silica gel (particle) packed bed. The theoretical model is then validated by experiments in terms of the exit air temperature and humidity (kg/(kg dryair)) under different conditions of regeneration. The experimental error due to the measuring instruments is estimated to be within 0.4% and 3.5%, respectively, for the exit air temperature and humidity. The comparison between theoretical and experimental data shows that the mean relative errors (MREs) of the calculated exit air temperature and humidity compared with the experimental ones are mostly within 2.0%, which manifests the model developed in this study has a favorite agreement with the experiments. The theoretical model will help us conduct a further parametric analysis on the regeneration process in the presence of an ultrasonic field, and have a better understanding of the mechanism of enhancement of silica gel regeneration brought by the high-intensity ultrasound.     

Evaluation and optimization of solar desiccant wheel performance

This paper presents the evaluation and optimization of a solar desiccant wheel performance. A numerical model is developed to study and discuss the effect of the design parameters such as wheel thickness, wheel speed, regeneration to adsorption area ratio, wheel porosity, and the operating parameters such as air flow rate, inlet humidity ratio of the air and regeneration air temperature on the wheel performance. It is also used to draw the performance curves of the desiccant wheel to quantify the optimum design parameters for certain operating conditions.Also, an open test loop for the desiccant wheel is constructed with appropriate control devices and measuring instruments. A perforated plate solar air heater of 2 m² area, together with an electric heater, is used as a source of energy to regenerate the desiccant material. The experimental tests are used to validate the numerical model and to evaluate the performance of the solar system and the desiccant wheel under actual conditions of Cairo climate (30° latitude).Comparison between numerical and experimental results shows good agreement between them, especially at low flow rates of air. Numerical results show that there is a maximum value of each design parameter at each operating condition, and above that no remarkable changes in the wheel performance are noticed. The results also show that there is an effective range of the air flow rate, due to which wheel performance becomes inefficient. This range is found to be between 1 and 5 kg/min. The performance curves of the wheel, which help to determine the humidity reduction ratio, are drawn for wheel speeds between 15 and 120 rev/h, dimensionless wheel thickness between 0.15 and 0.5, air flow rate equal to 1.9 and 4.9 kg/min, and regeneration temperature equal to 60 and 90 °C. These curves show that there is an optimum value of the wheel speed for each wheel thickness to obtain the best wheel performance for certain operating conditions.Experimental results show that the perforated plate solar air heater of 2 m² area can share about 72.8% of the total regeneration energy required at 1.9 kg/min air flow rate and 60 °C the regeneration air temperature. This value decreases to about 13.7% at a flow rate equal to 9.4 kg/min and regeneration temperature equal to 90 °C. The perforated plate solar air heater area required to completely fulfill the regeneration energy during the daytime is also calculated.     

Experimental investigation of the silica gel–water adsorption isotherm characteristics

In designing adsorption chillers that employs silica gel–water as adsorbent-adsorbate pair, the overriding objective is to exploit low temperature waste-heat sources from industry. This paper describes an experimental approach for the determination of thermodynamic characteristics of silica gel–water working pair that is essential for the sizing of adsorption chillers. The experiments incorporated the moisture balance technique, a control-volume-variable-pressure (CVVP) apparatus and three types of silica gel have been investigated, namely the Fuji Davison Type A, Type 3A and Type RD. As evidenced by the experimental results, the Henry-type equation is found to be suitable for describing the isotherm characteristics of silica gel–water working pair at the conditions of adsorption chiller. The regeneration of adsorbent depends on the correct allocation of temperature as well as the amount of regeneration time. From the experiments, the isotherm characteristics of silica gel–water in the low- to high-pressure regimes and hence, its isosteric heat of adsorption will be determined. Key parameters for optimizing the amount of heat recovery such as the cycle and switching time of chiller can also be implied from the measured results.     

Novel, low cost CaCl2 based desiccants for solar crop drying applications

Drying with solar-heated air is satisfactory so long as the sun is shining. To continue this process through the night-time and periods of cloud cover, it is necessary to either store some of this energy in a thermal mass or incorporate desiccants within the drying system. This paper reports the results from studies undertaken to develop three low cost, solar regenerative clay–CaCl₂ based solid desiccant materials; establish their moisture sorption and regeneration characteristics; assess their performance when compared with commercial desiccants; and integrate these within a low cost solar drying system for small-scale village-based crop drying. The moisture sorption and desorption performance of the desiccants were characterised in a Fison Environmental Cabinet at conditions of 85% (RH) and 25°C for 120 h for moisture sorption and 50°C and 20% (RH) for 8 h for regeneration. These conditions were representative of the environmental conditions monitored in the solar drying system. The bentonite–CaCl₂ (type 1) desiccant gave a maximum moisture sorption of 45% dry weight basis (dwb) while bentonite–CaCl₂ (type 2) and kaolinite–CaCl₂ (type 3) solid desiccants each gave moisture sorption values of 30% (dwb). It was concluded from the moisture sorption and regeneration characteristics that their application in solar crop drying and air dehumidification is highly useful due to their low regeneration temperatures, sub 100°C.     

Performance analysis of dehumidification rotating wheel using liquid desiccant

In the present work, theoretical and experimental evaluation of the effect of bed configuration and operating conditions on the performance of desiccant dehumidification system has been carried out. A new rotating absorption disk has been designed and constructed to be tested in the experimental work. The desiccant wheel has a cylindrical shape of 50-cm diameter and 10 cm thickness. The flow area of this bed is consisted of 350 narrow slots, which are uniformly distributed over the cross section of the cylindrical bed. Each slot has a cylindrical shape and constructed from a steel spring of 100 mm length and 20 mm inside diameter. To form the absorbing surface in the bed, each spring is coated with a thick cloth layer impregnated with lithium chloride solution, which is used as the working desiccant in these experiments.In the theoretical part of this study, a mathematical model has been developed where its output results are compared with the experimental data. The effect of different design parameters and operating conditions on the absorption and regeneration processes is discussed. The effect of regeneration air temperature, the process air and regeneration air inlet humidity, the rotational speed, the process and regeneration air velocity (or flow rates), the bed length, etc. on the amount of water absorbed/desorbed in a cycle is investigated.For the specific bed design parameters, actual recorded data show that an amount of 95 g of water can be absorbed in the absorption cycle per hour. This value changes with varying the operating conditions. From the theoretical investigation, it is found that at regeneration temperature of 85 °C, the amount of water absorbed is nearly equal to the amount of water desorbed (i.e. equilibrium condition) for a complete cycle. It is seen also that for moderate operating conditions (50% RH, 30 °C) and lower regeneration temperature which is suitable for solar energy application, the reduction in the humidity ratio of the process air reaches about 13% of its initial value. Finally, comparisons between theoretical and experimental results show good agreement.     

The energy-saving characteristic of silica gel regeneration with high-intensity ultrasound

The energy-saving characteristic of silica gel regeneration with power ultrasonic was analyzed by introducing the conception of specific energy consumption. For the purpose, the experiments of silica gel regeneration with 21-kHz power ultrasound were performed under different drying air temperatures (i.e., 35, 45, 55 and 65 °C) combined with different acoustic power levels (i.e., 0, 20, 40 and 60 W). And the energy saving ratios of the ultrasonic-assisted regeneration were studied by the method of ANOVA (Analysis of Variance) and compared among different conditions. The influences of acoustic power and drying air temperature as well as the target moisture ratio (at which the regeneration process ended) on the total specific energy consumption (TSEC) and the excess specific energy consumption (ESEC) were also discussed. The results indicate that all the factors (drying temperature, ultrasonic power level and the interaction between the drying temperature and the power level) have a significant (P < 0.05) influence on the energy saving ratio, among which the influence of drying temperature is the most significant (P < 0.05). According to the analysis of specific energy consumption, the optimal drying conditions aiming at the minimum energy use can be obtained. For the present experimental conditions, the condition of 55 °C (drying temperature) and 60 W (acoustic power level) can achieve the lowest TSEC and ESEC. In addition, different thresholds of power level are required to achieve the energy-saving effect due to the application of ultrasonic in the regeneration. The method of specific energy consumption can be also used for the energy analysis of the new regeneration technology in the scale-up study.