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.     

Performance evaluation of a solar-powered flat-plate multichannel liquid desiccant air conditioning system

There has been a limited application of liquid desiccant (LD) dehumidification systems in space air conditioning until now. The key elements responsible for this restricted implementation are leakage of desiccant solution, corrosion of components, and solution carryover along with the processed air to the space to be conditioned. To remove these problems, an evacuated tube solar heat collector-driven multichannel liquid desiccant air conditioning system has been proposed and experimentally investigated. In this study, dehumidification and regeneration rate, their effectiveness, cooling effect of the dehumidifier, and indirect evaporative cooling unit have been analyzed. The results obtained indicate that the process air has been dehumidified and cooled by 6.32 g kg⁻¹ and 5.26°C, respectively. The regeneration rate and effectiveness have been obtained to be 0.26 g s⁻¹ and 0.31, respectively. In terms of the cooling effect, the system output of 0.703 and 0.130 kW has been obtained from the dehumidifier and indirect evaporative cooling unit of the system, respectively. The proposed system validates the possibility of the novel solar-powered liquid desiccant air conditioning system concept and provides growth and development of the LD air conditioning technology for space air conditioning.     

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.     

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 comparison and analysis on silica gel and polymer coated fin-tube heat exchangers

In this paper, two desiccant-coated heat exchangers, which are actually fin-tube heat exchanging devices coated with silica gel and polymer materials respectively, are investigated experimentally. Due to the hygroscopic properties of the desiccant materials, both the sensible heat and the latent heat of the process air can be handled by using this kind of heat exchanger. An experimental setup was designed and built to test the performance of this unit. It is found that this desiccant-coated fin-tube heat exchanger well overcomes the side effect of adsorption heat which occurs in desiccant dehumidification process, and achieves good dehumidification performance under given conditions. The silica gel coated heat exchanger behaves better than the polymer one. The influences of regeneration temperature, inlet air temperature and humidity on the system performance in terms of average moisture removal rate D_avg and thermal coefficient of performance COP_th were also analyzed.     

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.     

Predictions of moisture removal rate and dehumidification effectiveness for structured liquid desiccant air dehumidifier

In hot and humid climates such as in the Sultanate of Oman, the humidity puts extra load on the electric vapor-compression air conditioning (VAC) systems. Liquid and solid desiccants can reduce the moisture content of humid air and thus reduce the latent load imposed on the VAC systems. In the present work, the performance of air dehumidifiers using triethylene glygol (TEG) as desiccant was investigated. Three differently structured packing densities were used (77, 100 and 200 m²/m³). The performance of the dehumidifier was evaluated and expressed in terms of the moisture removal rate (m_cond) and the dehumidifier effectiveness (ε_y). The experimental work was undertaken to study the effects of several influencing design factors on this performance. The design factors covered included the air and TEG flow rates, air and TEG inlet temperatures, inlet air humidity and the inlet TEG concentration. The desiccant flow rate investigated was much less than that covered in previous studies and the range of the inlet temperatures of air and desiccant was significantly wider. The objective this study was to use the multiple regression method and the principal component analysis to obtain statistical prediction models for the water condensation rate and the dehumidification effectiveness in terms of these design factors. The results of both techniques agree with each other affirmed that the desiccant flow rate, desiccant inlet concentration and air inlet temperature are the most significant variables in predicting m_cond, whereas desiccant flow rate, air inlet temperature and packing density are the most significant variables in predicting ε_y.     

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.     

An experimental study of structured packing dehumidifier/regenerator operating with liquid desiccant

The present work presents an experimental investigation on the performance of the structured packing cross flow desiccant dehumidification system (DDS). This system is referred as DDS; its heart is the dehumidifier/regenerator. It is used to meet a latent heat load by air dehumidification. Calcium chloride (CaCl₂) solution is used as the working desiccant material in this system. The structured packing has a density (specific surface area) of 390 m²/m³, corrugation angle of 60° and void fraction of 0.88. The effect of relevant parameters such as air flow rate, desiccant solution flow rate, desiccant solution temperature and concentration and packing thickness on the performance of the system is studied. The performance of the system is evaluated using the mass transfer coefficient, moisture removal rate (MMR), effectiveness and the coefficient of performance (COP). The remarkable increase of mass transfer coefficient and MRR for both deh/reg is observed by increasing both air and solution flow rates. Eventually, the payback period (PP) of the DDS is 11 months with annual running cost savings (ΔC_RC) of about 31.24% compared with vapor compression system (VCS) dehumidification. The overall environmental impacts of DDS are nearly 0.63 of VCS. This may emphasize the need of incorporating a desiccant system along with air conditioning applications.     

Dehydration of natural gas using solid desiccants

Natural gas is an important source of primary energy that, under normal production conditions, is saturated with water vapor. Water vapor increases natural gases' corrosivity, especially when acid gases are present. Several methods can be used to dry natural gas and, in this paper, a solid desiccant dehydrator using silica gel is considered due to its ability to provide extremely low dew points. The design analysis of a two-tower, silica gel dehydration unit to dry one million standard m³ of natural gas per day is presented in this paper and the effects of various operating parameters on the design of the unit are discussed. The study also covers the analysis of energy requirements for the regeneration of the weak desiccant bed based on some simplified assumptions and it is found that the higher the regeneration temperature, the smaller are the required quantities of regeneration gas.     

Experimental investigation of forced convection and desiccant integrated solar dryer

An indirect forced convection and desiccant integrated solar dryer is designed and fabricated to investigate its performance under the hot and humid climatic conditions of Chennai, India. The system consists of a flat plate solar air collector, drying chamber and a desiccant unit. The desiccant unit is designed to hold 75 kg of CaCl₂-based solid desiccant consisting of 60% bentonite, 10% calcium chloride, 20% vermiculite and 10% cement. Drying experiments have been performed for green peas at different air flow rate. The equilibrium moisture content M_e is reached in 14 h at an air flow rate of 0.03 kg/m² s. The system pickup efficiency, specific moisture extraction rate, dimensionless mass loss, mass shrinkage ratio and drying rate are discussed in this paper.     

Dynamic hygroscopic effect of the composite material used in desiccant rotary wheel

In this study, silica gel (SG), calcium chloride (CaCl₂) and composite desiccant (SG–CaCl₂) applied to a corrugated paper (CP) based desiccant rotary wheel are compared for their abilities to remove moisture from wet air. The experimental data shows that the CP–SG–CaCl₂ material could attain equilibrium within a very short period, and its hygroscopic capacity is much higher than that of CP–SG. Also, it exhibits a remarkable increase in moisture removal compared with the silica gel wheel.     

Performance characterisation of liquid desiccant columns for a hybrid air-conditioner

Use of liquid desiccant-vapor compression hybrid system is encouraged for low humidity applications. The liquid desiccant is primarily used to further dehumidify the supply air. In the present study, by using psychrometric equations and liquid desiccant property data, heat and mass transfer analysis for the dehumidifier and regenerator columns in counter flow configuration has been carried out. The simulation of the columns corresponds to low solution to air (S/A) flow ratio where precooled air gets dehumidified in the absorber while preheated air is used for regeneration of the solution. A detailed study of the performance characteristics for the absorber and regenerator columns confirms the requirement of the desiccant loop for additional dehumidification of the conditioned air. This need develops the main motive towards the concept of hybrid air conditioning.     

Measurements of effectiveness in a silica gel rotary exchanger

Desiccant air conditioning systems that incorporate rotary heat and mass exchangers are getting a closer look in view of the pressure on the industry to decrease the use of chlorofluorocarbons and for fuel economies. Heat and mass exchangers that utilize LiCl, silica gel and other desiccants have been considered as possibilities. The performance features of a silica gel rotary mass exchanger wheel, which is the primary component in a desiccant system, has been measured. The process air flow velocities and temperatures ranged from 0.5 to 2.5 m/s, and 20 to 30°C with 30 to 100% relative humidities. Similar velocity and relative humidity conditions were used for regeneration air. The desiccant data are compared to two different theories from the literature; the method of characteristics and a numerical approach. The method of characteristics appears to be good for only a small range of conditions, generally for low values of specific capacities. The numerical approach appears to predict the trends well, though sometimes with large errors.     

Solar powered air conditioning system using rotary honeycomb desiccant wheel

A solar powered air conditioning system using liquid desiccant is proposed. A solar air heater containing a porous material is used for regeneration purpose in the proposed system. The honeycomb desiccant rotary wheel is constructed from iron wire and clothes layer impregnated with calcium chloride solution, in honeycomb form, is utilized for the regeneration and absorption processes. The effect of airflow rate and solar radiation intensity on the system regeneration and absorption processes are studied. The obtained results show that the system is highly effective in the regeneration process. An empirical equation to calculate the removed moisture as a function of air flow rate at solar noon is obtained. Also empirical equation for wheel effectiveness as a function of air flow rate for regeneration and absorption process was obtained.     

Water generation through desiccant using novel-designed solar still coupled with heat pipe vacuum tube collector: An experimental observation

In this article, an experiment has been carried out with heat pipe vacuum or evacuated tube collector to produce water from atmospheric air. In this experiment, the regeneration and adsorption method has been adopted, that is, water has been produced through the adsorption and regeneration of desiccants. The desiccant is heated through a hot surface to facilitate its regeneration. Limited experiments have been conducted to obtain water through the regeneration of desiccant using a hot surface. For the condensation of water vapor, a novel box has been designed, named the “novel-designed acrylic box.” The water is collected in a measuring flask or beaker to determine its quantity. Silica gel desiccant has been used for the adsorption and regeneration of water vapors. In this experiment, the adsorption process for silica gel was carried out in two different ways. In the first method, 1 kg of silica gel was scattered on the copper tray, that is, inside the system, while in the second method, 1 kg of silica gel was scattered on the paper, that is, outside of the system. In the first case silica gel adsorbed 137 g water vapor, and in the second case, it adsorbed 232 g water vapor. In the first case of adsorption, 70 mL water was produced while in the second case of adsorption, 175 mL water was produced from ambient air. The system's maximum efficiency was found to be 4.9%. Effects of various parameters, such as solar intensity, ambient temperature, wind speed, and so forth, have been studied.     

Numerical study of the influence of the atmospheric pressure on the heat and mass transfer rates of desiccant wheels

A study was conducted to investigate the influence of the atmospheric pressure, from 101,325 to 60,000 Pa (corresponding from 0 to 4217 m of altitude), on the heat and mass transfer rates of desiccant wheels (hygroscopic matrix of silica gel RD), assuming fixed-mass airflow rates and fixed inlet states for the process and regeneration airflows, defined by the respective inlet temperatures and water vapour contents. The results show a generic reduction of the heat and mass transfer rates as the atmospheric pressure decreases, an effect that is more pronounced for short channel wheels and high airflow rates. Correlations based on the numerical results are presented for the correction of the heat and mass transfer rates when the desiccant wheels are operating at non-standard atmospheric pressure.     

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.     

Moisture transport in silica gel packed beds—I.Theoretical study

Diffusion mechanisms of moisture within silica gel particles are investigated. It is found that for microporous silica gel surface diffusion is the dominant mechanism of moisture transport, while for macroporous silica gel both Knudsen and surface diffusions are important. A model is proposed for simultaneous heat and mass transfer in a thin packed bed of desiccant particles, which accounts for diffusion of moisture into the particles by both Knudsen and surface diffusions. Using finite difference methods to solve the resulting partial differential equations, predictions are made for the response of thin beds of silica gel particles to a step change in air inlet conditions, and compared to a pseudo-gas-side controlled model commonly used for the design of desiccant dehumidifiers for solar desiccant cooling applications.     

Optimized solar-powered liquid desiccant system to supply building fresh water and cooling needs

This paper studies the feasibility of using a solar-powered liquid desiccant system to meet both building cooling and fresh water needs in Beirut humid climate using parabolic solar concentrators as a heat source for regenerating the liquid desiccant. The water condensate is captured from the air leaving the regenerator. An integrated model of solar-powered calcium chloride liquid desiccant system for air dehumidification/humidification is developed. The LDS model predicted the amount of condensate obtained from the humid air leaving the regenerator bed when directed through a coil submerged in cold sea water. An optimization problem is formulated for selection and operation of a LDS to meet fresh water requirement and air conditioning load at minimal energy cost for a typical residential space in the Lebanon coastal climate with conditioned area of 80 m² with the objective of producing 15 l of fresh drinking water a day and meet air conditioning need of residence at minimum energy cost. The optimal regeneration temperature increases with decreased heat sink temperature with values of 50.5 °C and 52 °C corresponding to sink temperatures of 19 °C and 16 °C.