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.     

Experimental Studies for Convective Drying of Potato

An experimental facility was built at the Indian Institute of Technology Delhi in order to examine the characteristics of convective drying of a moist object. The test facility consists of an inlet section, a divergent and convergent section, a settling chamber, a test section, and an outlet section. Initial moisture content and time-dependent moisture content of a rectangular shaped moist object (4 cm × 2 cm × 2 cm) are measured by this test facility. The potato slice was selected as a sample moist object. Moisture content was measured at different air temperatures of 40, 50, 60, and 70°C with an air velocity of 2 m/sec. The density of potato slice was determined for various drying temperatures. The volume shrinkage during drying decreased almost linearly with moisture content. The percentage air pores and porosity increased gradually with decreasing moisture content and increasing drying air temperature. Volumes of water, air, and solid content of potato were determined at different drying air temperatures. The results are validated with theoretical data.     

Experiments in a solar simulator on solid desiccant regeneration and air dehumidification for air conditioning in a tropical humid climate

This paper presents an indoor and analytical study to evaluate the performance of a desiccant cooling system that uses silica gel as desiccant, electric light bulbs to simulate solar radiation, and forced flow of air through an IDC (integrated Desiccant/Collector). In the regeneration process, the rate at which water is removed from the desiccant increases with irradiation and decreases with air flowrate. In the air dehumidification process, the adsorption rate decreases with irradiation and increases slightly with flowrate. Comparisons between analytical calculations and experimental data show good agreement, and the calculations show that it should be possible to operate this system in tropical humid climates using the regeneration process in the day and the air dehumidification in the night time.     

Experimental investigation on desiccant air-conditioning system in India

An experimental investigation in India was presented to evaluate the performance and energy saving capacity of a desiccant air-conditioning system composed of a silica gel bed, a split type air-conditioner (1.0 ton refrigeration) installed in a room with a volume of 86.4 m³, air ducts and a blower. The experiment was made in such a way that the percentages of return air, outdoor air and indoor air mixed with the air leaving the desiccant and desiccant bed thickness could be adjusted. Tests were conducted on several days with relatively similar ambient conditions. Under the test conditions in this experiment, a 7cm bed thickness is recommended with a maximum adsorption rate of 403g/h. The optimum percentages of air ratios were as follows: 10% of outdoor air, 10% of return air (mixed together at the desiccant bed inlet) and 80% of indoor air mixed with the dry air leaving the desiccant. The corresponding electricity saving was about 19%. As expected, simple economic analysis indicates that the desiccant air-conditioning is not viable for smaller cooling capacities.     

Experimental investigation on desiccant air-conditioning system in India

An experimental investigation in India was presented to evaluate the performance and energy saving capacity of a desiccant air-conditioning system composed of a silica gel bed, a split type air-conditioner (1.0 ton refrigeration) installed in a room with a volume of 86.4 m³, air ducts and a blower. The experiment was made in such a way that the percentages of return air, outdoor air and indoor air mixed with the air leaving the desiccant and desiccant bed thickness could be adjusted. Tests were conducted on several days with relatively similar ambient conditions. Under the test conditions in this experiment, a 7 cm bed thickness is recommended with a maximum adsorption rate of 403 g/h. The optimum percentages of air ratios were as follows: 10% of outdoor air, 10% of return air (mixed together at the desiccant bed inlet) and 80% of indoor air mixed with the dry air leaving the desiccant. The corresponding electricity saving was about 19%. As expected, simple economic analysis indicates that the desiccant air-conditioning is not viable for smaller cooling capacities.     

Enhancement of hydrogen storage capacity of multi-walled carbon nanotubes with palladium doping prepared through supercritical CO2 deposition method

Pd doped Multi-Walled Carbon Nanotubes were prepared via supercritical carbon dioxide deposition method in order to enhance the hydrogen uptake capacity of carbon nanotubes at ambient conditions. A new bipyridyl precursor that enables reduction at moderate conditions was used during preparation of the sample. Both XRD analyses and TEM images confirmed that average Pd nanoparticle size distribution was around 10 nm. Hydrogen adsorption and desorption experiments at room temperature with very low pressures (0–0.133 bar) were conducted together with temperature programmed desorption (TPD) and reduction (TPR) experiments on undoped and doped materials to understand the complete hydrogen uptake profile of the materials. TPD experiments showed that Pd nanoparticles increased the hydrogen desorption activity at moderate temperatures around at 38 °C while for undoped materials it was determined around at 600 °C. Moreover, a drastic enhancement of hydrogen storage was recorded from 44 μmol/g sample for undoped material to 737 μmol/g sample for doped material through adsorption/desorption isotherms at room temperature. This enhancement, also verified by TPR, was attributed to spillover effect.     

INFLUENCE OF ELEVATED PRESSURE ON SORPTION IN DESICCANT WHEELS

Rotary desiccant wheels have been employed to dry pressurized air streams. In these systems, depending on the moisture content in the air stream and the operating pressure, condensation can occur in the regeneration portion of the wheel. In this article, a numerical method using an implicit finite-difference scheme is developed and applied that enables condensation to be detected and simulated in the regeneration portion of a desiccant wheel operating at high pressures. Using this model, performance analysis of desiccant wheel under these conditions is investigated. It is found that, depending on the value of the separation factor and regeneration temperature, condensation could occupy as much as 40% of regeneration section of the wheel. In this region, regeneration of the desiccant is not possible and usually dehumidification of regeneration air occurs. Also, as the operating pressure increases, the adsorption and desorption characteristics are dramatically affected and the optimum separation factor of desiccant material increases with operating pressure.     

On the behaviour of hygroscopic wheels: Part I – channel modelling

In this paper a detailed mathematical formulation is developed for the numerical modelling of the behaviour of a channel of a hygroscopic compact matrix. A comparison between the detailed version and a simplified one is performed considering a two-dimensional airflow between desiccant parallel plates. The distinct heat and mass transfer phenomena are strongly coupled, and some properties of the airflow and of the desiccant medium exhibit important changes during the sorption processes. Both physical models take into account the gas side and solid side resistances to heat and mass transfer. The wall domain is treated similarly in both models, by taking into account the simultaneous heat and mass transfer together with the water adsorption/desorption process. Two phases co-exist in equilibrium inside the desiccant porous medium, the equilibrium being characterized by sorption isotherms without hysteresis. The detailed model is based on the solution of the differential equations for the conservation of mass, energy and momentum, assuming that no momentum transport exists in the porous wall domain. In the simplified model, the airflow is treated as a bulk flow, the interaction with the wall being evaluated by using appropriated convective coefficients.Both models are compared in the simulation of a parallel plate channel during an adsorption process. The results show a good agreement for channel lengths greater than 0.1 m. In part II of the paper, the simplified model is adapted to the simulation of the three-dimensional problem in the channel of a hygroscopic rotor, and it is used to perform parametric studies.     

Sorption and desorption characteristics of a packed bed of clay-CaCl2 desiccant particles

Desiccants can be used in conjunction with solar energy to provide a viable alternative to traditional air conditioning techniques. A desiccant consisting of clay and calcium chloride was developed and tested using multiple sorption and desorption cycles. During sorption, inlet air temperatures from 23 to 36 °C with corresponding relative humidities of 42-66% were tested. Additionally, superficial air velocities from 0.17 to 0.85 m/s were tested. During desorption, inlet air temperatures from 50 to 57 °C and superficial air velocities of approximately 0.30 and 0.60 m/s were tested. A regression equation was determined for the mass of water sorbed by the clay-CaCl2 desiccant with a R² value of 0.917. The desorption data was regressed to an exponential function and significant k-values were determined. An equation for pressure drop through the desiccant was determined and compared to existing models. The desiccant was found to perform well during the repeated test cycles though small masses of desiccant were lost due to surface disintegration of the desiccant spheres.     

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.     

Experimental investigation on the adsorption/desorption processes using solid desiccant in an inclined-fluidized bed

This paper presents an experimental investigation on the adsorption and desorption operations in an inclined-fluidized bed using silica gel as the working desiccant. The experimental system involves a circular glass tube containing the particles of silica gel, which is tested at an inclination angle of 45°. The moisture capacity of the bed is measured using a gravimetric technique. Process air at nearly constant ambient parameters (humidity and temperature) and different values of flow rate are used during adsorption. Moisture concentration in the bed is analyzed through visual observation of the color of silica gel particles. Experimental measurements indicate that the regeneration and adsorption rates are highly dependent on the air stream velocity. A satisfactory regeneration rate is confirmed at regeneration temperature as low as 90 °C when inclined-fluidized bed is applied. The transient-state moisture transfer rates during adsorption and desorption are presented. Finally, observation of the movement and color of the particles in the bed show regular circulation and homogenous distribution of moisture concentration.     

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.     

Review of solid/liquid desiccant in the drying applications and its regeneration methods

Desiccant material has been used in drying applications because of its low energy consumption, among other advantages. Desiccant material can produce hot and dry air that is beneficial for the drying process. The advantages of using desiccant material in a drying system include continuous drying even during off-sunshine hours, increased drying rate due to hot and dry air, more uniform drying, and increased product quality especially for heat-sensitive products. Some problems in desiccant system such as pressure drop in solid desiccant, carry over of liquid desiccant by air stream and low moisture adsorption capacity may be improved by optimization of the design of desiccant system. Numerous researchers have studied the low cost and low regeneration temperature of desiccant material, and the optimization of desiccant application to produce more competitive energy. The use of heat to regenerate desiccant material in a drying system has limitations in energy saving. However the use of low energy or free available energy such as solar energy and waste heat from industrial processes for regeneration of desiccant material will make the system more cost-effective. This paper presents several works on the regenerative method of the desiccant system and its application in the drying system for both solid and liquid desiccant materials.     

Performance prediction of PEM fuel cell with wavy serpentine flow channel by using artificial neural network

Effects of serpentine flow channel having sinusoidal wave at the rib surface on performance of PEMFC having 25 cm² active area are investigated at different flow rates, three different amplitudes changing from 0.25 mm to 0.75 mm and three different cell operation temperatures. A proton exchange membrane fuel cell (PEMFC) is modeled for the prediction of the output current by using artificial neural network (ANN) that is utilized the aforementioned experimental parameters. Effect of hydrogen and air flow rate, the fuel cell temperature, amplitude of channel is tested. The results indicated that model C1 having lowest amplitude is enhanced maximum power output up to 20.15% as compared to indicated conventional serpentine channel (model C4) for 0.7 SLPM H₂ and 1.5 SLPM air and also model C1 has better performance than C2, C3 and C4 models. The maximum power output is augmented with increasing the cell temperature due to raising the fuel and oxidant diffusion ratio. Cell temperature, amplitude, H₂ and air flow rate and input voltage is used as input variables in train and test of the developing ANN model. MAPE of training and testing is determined as 2.89 and 2.059, respectively. Prediction results of developed ANN model including two hidden layer shows similar trend with experimental results. Developed ANN model can be used to both decrease the number of required experiments and find the optimum operation condition within the range of input parameters.     

Cooling potential of ventilated PV façade and solar air heaters combined with a desiccant cooling machine

Thermal energy collected from a PV-solar air heating system is being used to provide cooling for the Mataro Library, near Barcelona. The system is designed to utilise surplus heat available from the ventilated PV facade and PV shed elements during the summer season to provide building cooling. A desiccant cooling machine was installed on the library roof with an additional solar air collector and connected to the existing ventilated PV façade and PV sheds. The desiccant cooling cycle is a novel open heat driven system that can be used to condition the air supplied to the building interior. Cooling power is supplied to the room space within the building by evaporative cooling of the fresh air supply, and the solar heat from the PV-solar air heating system provides the necessary regeneration air temperature for the desiccant machine. This paper describes the system and gives the main technical details. The cooling performance of the solar powered desiccant cooling system is evaluated by the detailed modelling of the complete cooling process. It is shown that air temperature level of the PV-solar air heating system of 70 °C or more can be efficiently used to regenerate the sorption wheel in the desiccant cooling machine. A solar fraction of 75% can be achieved by such an innovative system and the average COP of the cooling machine over the summer season is approximate 0.518.     

Theoretical and experimental investigation on the application of solar water heater coupled with air humidifier for regeneration of liquid desiccant

Theoretical and experimental investigation on the application of flat plate solar water heater coupled with air humidifier for regeneration of liquid desiccant has been presented in this work. The heated water from the storage tank of the solar heating system is circulated in a finned tube air heater. Hot air from the air heater is blown through a packing of a honeycomb type for the purpose of regeneration of calcium chloride (CaCl₂) solution. An experimental system has been designed and installed for this purpose. The system comprises a solar water heater with a storage tank connected to an air/water heat exchanger. Hot air from the heat exchanger is blown to the air humidifier, which functions in this study as a regenerator. Calcium chloride solution is applied as the working desiccant in this study. Solution concentration is determined at the end of regeneration process and the mass of evaporated water is evaluated. It is observed that the heating temperature varies, at day time, in a range of about 5 °C. This limited variation in hot water temperature demonstrates the importance of the storage tank to attain a nearly steady state operation of the system. Experimental results show that solution with 30% concentration can be regenerated up to 50% using solar energy. In the theoretical part of this study, a multiple-layer artificial neural network (ANN) model has been applied to study the performance of a solar liquid-desiccant dehumidification/regeneration system when calcium chloride solution is applied as the working desiccant. The experimental results of the present study are used to construct and test the ANN model. Then the model has been utilized to describe and analyze the effect of the inlet conditions of air on the regeneration process. Good agreement between the outputs from the ANN model and the corresponding results from the experimental data has been found. The proposed model can work well as a predictive tool to complement the experiments.     

Study of methane and carbon dioxide adsorption capacity by synthetic nanoporous carbon based on pyrogallol-formaldehyde

Nanoporous carbons were synthesized at certain conditions by sol–gel method combined with furnace firing in inert atmosphere from pyrogallol-formaldehyde (PF) mixtures in water using perchloric acid as catalyst. Their morphology was studied experimentally to examine their adsorption capacity for greenhouse gases. The preparation conditions of the nanoporous carbons were explored by changing the pyrolysis temperature. The effect of this factor on determining the pore structures and the adsorption capacities were evaluated. The synthesized xerogels were characterized by X-ray diffraction, nitrogen adsorption–desorption isotherms, thermogravimetric analysis (TGA), differential thermal analysis (DTA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that surface areas and nitrogen adsorption capacity are dependent completely on the pyrolysis temperature. Equilibrium and enthalpies studies for the CO₂ and CH₄ adsorption on PF were measured at room temperature and up to 25 bar. The adsorption capacity on PF was highest for CO₂ and then CH₄. The best sample shows maximal adsorption capacities as follows 5.50 mmol g^?1 of CH₄ and 7.62 mmol g^?1 of CO₂ at 25 bar and 30 °C.     

An experimental study on multi‐purpose desiccant integrated vapor‐compression air‐conditioning system

In this paper, a multi‐purpose hybrid desiccant integrated vapor‐compression air‐conditioning system of a small capacity is experimentally investigated. The system, referred as hybrid desiccant‐assisted air conditioner (HDAC), is designed to meet the cooling load of spaces having large latent heat portions and at the same time to extract water from atmospheric air. The system is mainly consisted of a liquid‐desiccant dehumidification unit integrated with a vapor‐compression system (VCS). The dehumidification unit uses lithium chloride (LiCl) solution as the working material. The effect of different parameters, such as desiccant solution flow rate, process airflow rate, evaporator and condenser temperatures, strong solution concentration and regeneration temperature on the performance of the system, is studied. This system has a water recovery rate of 6.7 l/h TR (1.91 l/h kW) of pure water at typical north Egyptian climate (20–30°C dry bulb and 35–45% relative humidity). The HDAC system has a COP as high as 3.8 (an improvement of about 68% over the conventional VCS). The system offers a total cooling capacity of about 1.75 TR (6.15 kW) using a 0.75 TR (2.6 kW) VCS unit. Finally, the proposed system is found to have a payback time of about 10 months without any considerable extra capital cost compared with the known split air‐conditioning system. The results emphasize the potential benefits of the HDAC system. Copyright © 2010 John Wiley & Sons, Ltd.     

CONJUGATE COMPUTATION OF TRANSIENT FLOW AND HEAT AND MASS TRANSFER BETWEEN HUMID AIR AND DESICCANT PLATES AND CHANNELS

This is a numerical study of dehumidification of humid air in laminar and turbulent flows (333 ≤ Re ≤ 6,000) over desiccant (silica gel)-lined finite flat plates and in channels. The problem is treated as conjugate flow, heat, and mass transfer, and solved by using a finite control-volume method. The effects of the plate thickness (3 ≤ b ≤ 7 mm), the Reynolds number (333 ≤ Re ≤ 3,333), and the turbulence intensity (1 ≤ TI ≤ 10%) on the dehumidification process are investigated. The results show that increasing the desiccant plate thickness decreases the heat and mass transfer coefficients by 25% and 22% at t = 10 s and x = 0.11 m, respectively, in comparison to a thin plate. Mass transport rates increase with Re, e.g., at t = 20 s, W_ave increases by 50% as Re is increased 10-fold from 333 to 3,333. Turbulent flow in channel desiccants increases the rate of dehumidification, e.g., an increase in Re from 600 (laminar) to 6,000 (turbulent) results in an increase in W_ave by 22% at t = 20 s. Also, increasing the turbulence intensity from 1% to 10% increase W_ave by 7%.     

Application of a solar desiccant/collector system for water recovery from atmospheric air

An integrated desiccant/solar collector system for production of fresh water from atmospheric air is described. The solar driven system provided about 1.5 l of fresh water per square meter per day. The system involves the absorption of water vapor from ambient air during the night and simultaneous desiccant regeneration and water vapor condensation during the day. To enhance the mass transfer surface, a thick corrugated layer of cloth was used as a bed to carry the liquid absorbent. In the nocturnal phase of operation, air is allowed to penetrate the desiccant bed. The airflow is driven by fans supported on one side of the desiccant/solar collector unit. In this study, the effects of different parameters on the absorption and regeneration processes are discussed, and operational conditions for the proposed equipment evaluated. Radiation intensity, ambient temperature, bed temperature and temperature of the glass surface were recorded. Also, the productivity of the system during the day and under the given operation conditions was plotted. A mathematical model was prepared and its output compared with the analyzed experimental data.