Introduction of a new definition for effectiveness of desiccant wheels

In common air conditioning methods, both sensible and latent loads are handled by cooling systems. Utilizing desiccant, individual cooling and dehumidification processes are possible. In this paper, desiccant wheel operation has been investigated by experimental study. Experimental conditions include different climates (hot dry and hot humid) at different operating parameters (regeneration temperature and wheel speed). Due to the temperature and humidity measurements of inlet and outlet streams, the desiccant wheel efficiency in each definition was calculated. All experiments show that enthalpy of the outlet process air is notably higher than that of the inlet air. This event leads to a novel efficiency definition which presents the deviation of the outlet process air enthalpy from the inlet air enthalpy. By increase in the dehumidification efficiency, the adiabatic efficiency decreases, whereas it increases by the regeneration efficiency. Hence in some situations the adiabatic efficiency will have an optimum value. According to the adiabatic efficiency concept, it seems to be related to the coefficient of performance of the desiccant cooling systems.     

Effect of Desiccant Isotherm on the Performance of a Desiccant Wheel at Different Operating Conditions

A one‐dimensional mathematical model has been developed to investigate the effect of desiccant isotherm (adsorptive material) on the performance of the desiccant wheel. The model consists of four governing heat and mass transfer equations with some auxiliary conditions which are then solved using FlexPDE Software. The model shows good agreement with the experimental results obtained from the literature as well as with the experiment conducted by the author. In this paper the different adsorption isotherms have been characterized by a single parameter called the separation factor. The separation factors, R = 0.05, 0.1, 0.5, and 1 are analyzed under a different range of operating conditions. Investigation of these parameters is based on the wheel performance, index, that is moisture removal (MR) and dehumidification coefficient of performance (DCOP), as DCOP is more appropriate to reflect the utilization of heat energy in the regeneration process. The result of this study shows that there is a significant effect on the operating parameter on the isotherm shape. Across the entire range of operating condition single isotherm (R = 0.5 or R = 0.1) there is not a better adsorptive material choice but it varies with change in the operating conditions. The results of this study shows that the isotherm shape of R = 0.1 or 0.5 yields better results depending upon whether the given range of operating conditions are higher or lower.     

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.     

Evaluation of rotary dehumidifier performance with and without heated purge

In this paper we evaluate the potential benefits from separating process air stream at the exit of rotary dehumidifier into two streams. One air stream, hot and humid, is called purge air stream and other is remaining process air stream. The remaining process air stream has a lower temperature and humidity ratio as result of separation of initial hot process air stream. It is found that as the purge angle increases the exit humidity ratio of remaining process air stream decreases up to a point where it reaches a minimum. The purge angle for which this occurs is named “effective purge angle”. The effective purge angles for different splits between adsorption and desorption side of the rotary dehumidifier, various regeneration temperatures, non-dimensional lengths and their corresponding optimum non-dimensional times are determined. An existing finite-difference model, developed by the authors of this paper, for simulation of desiccant wheel performance is extended to account for the separation of the process air stream at the exit of rotary dehumidifier and later mixing of purge air stream and outside air to form the regeneration air stream. The performance of desiccant wheel with heated “effective purge angle” is evaluated and compared with performance of the same wheel without purge angle at all. It is found, for all cases considered in this study, that having heated “effective purge angle” has overall positive effect on the performance of the rotary dehumidifier.     

A simulation study of heat and mass transfer in a honeycombed rotary desiccant dehumidifier

A one-dimensional coupled heat and mass transfer model, which is expected for use in designing and manufacturing of a honeycombed rotary desiccant wheel, is presented in this paper. The mathematical model has been validated using a real desiccant wheel, and the calculation results are in reasonable agreement with the experimental data. Based on this model, the temperature and humidity profiles in the wheel during both the dehumidification and the regeneration processes are analyzed and verified by experimental data. The numerical results indicate that in the regeneration process a hump curve of air humidity ratio along the channel exists all the time. In the regeneration process the hump of air humidity ratio moves from the duct entrance to the duct exit and increases gradually until the hump reaches the duct exit, where the hump will drop subsequently. The effects of velocity of regeneration air V_reg inlet temperature of regeneration air T_reg and velocity of process air V_ad on the hump moving speed are investigated. To improve the performance of desiccant wheel, it is essential to accelerate the hump moving from the duct entrance to the duct exit as soon as possible.     

Numerical and experimental investigation of operating parameters of solar–powered desiccant wheel in India

A mathematical model has been used to predict the operating parameters of a desiccant wheel for performance analysis of the desiccant wheel. The model considered both gas and solid side resistance. The model shows a good agreement with experimental data. An experimental setup was fabricated using an evacuated tube solar air collector with a desiccant wheel. The hot air needed for regeneration is produced by the evacuated tube solar air collector, which has a collector surface area of 4.44 m² . The regeneration can be started from 40 °C. The temperature of outlet air obtained is in the range of 40–65 °C in this evacuated tube solar air collector. The experimental setup was installed at NIT Kurukshetra, India, 29° 58′ (latitude) North and 76° 53′ (longitude) East. Numerical results showed that both the moisture removal and the temperature increment of the process air increases with an increasing regeneration air temperature, regeneration air inlet velocity, and process inlet moisture. But both the moisture removal and the temperature increment of the processed air decreases by increasing the process air inlet velocity and regeneration air moisture at the inlet. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21031     

Parametric analysis of desiccant wheel for air conditioning application

A one‐dimensional mathematical model is developed to evaluate the operating and design parameters of the desiccant wheel for air conditioning application. In this paper, dehumidification coefficient of performance (DCOP) and sensible energy ratio (SER) are adopted as a combined performance index to reflect the dehumidification and thermal performance of the desiccant wheel. The analysis of the results reveals that for lower SER, suitable wheel length, wall thickness, channel pitch, and channel height should be 100 mm, 0.2 mm, 3 mm, and 5 mm, respectively. These design parameters have been analyzed under different operating conditions and it was found that for higher DCOP, rotational speed, regeneration temperature, process and regeneration velocity should be 20 rph, 60°C, and 2 m/s.     

Optimization of desiccant wheel speed and area ratio of regeneration to dehumidification as a function of regeneration temperature

Numerical simulation has been conducted for the desiccant wheel, which is the crucial component of a desiccant cooling system. The mathematical model has been validated by comparing with previous experimental data and numerical results. The calculation results are in reasonable agreement with both, experimental and numerical results. As the key operating/design parameters, the wheel speed and the area ratio of regeneration to dehumidification have been examined for a range of regeneration temperatures from 60 °C to 150 °C. Optimization of these parameters is conducted based on the wheel performance evaluated by means of its moisture removal capacity (MRC) which is more appropriate than effectiveness as a performance index of unbalanced flows. Also the effects of the outdoor air temperature and humidity on the optimum design parameters are examined.     

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.     

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.     

Update on desiccant wheel model

This short communication presents the update on the desiccant wheel models, developed earlier by the present authors. It is to be mentioned here that the psycrometric model presented earlier (Int. J. Energy Res., 2003; 27 :17) is valid only for the desiccant wheel running with identical volume air‐flows in supply and regeneration sides. However, when system runs with volume air flow ratio between supply and regeneration side (α), different from 1 (one), the model need further modifications. In the present work, correction factors were developed and incorporated in order to update the earlier model for predicting correctly the temperature and humidity of processed air at the outlet of desiccant wheel. The modified model were verified against the experimental results corresponding to different values of α, obtained from the industry for three different kinds of desiccant wheels. Copyright © 2004 John Wiley & Sons, Ltd.     

Simulation–optimization of solar-assisted desiccant cooling system for subtropical Hong Kong

Solar cooling is a novel approach, which primarily makes use of solar energy, instead of electricity, to drive the air-conditioning systems. In this study, solar-assisted desiccant cooling system (SADCS) was designed to handle the cooling load of typical office in the subtropical Hong Kong, in which half of the building energy is consumed by the air-conditioning systems. The SADCS mainly consisted of desiccant wheel, thermal wheel, evaporative coolers, solar air collectors and gas-fired auxiliary heater, it could directly tackle both the space load and ventilation load. Since the supply air flow is same as the outdoor air flow, the SADCS has a feature of sufficient ventilation that enhances the indoor air quality. Although it is inevitable to involve the auxiliary heater for regeneration of desiccant wheel, it is possible to minimize its usage by the optimal design and control scheme of the SADCS. Through simulation–optimization approach, the SADCS can provide a satisfactory performance in the subtropical Hong Kong.     

转轮复合式空调系统的数值计算及能耗分析

主要研究了以太阳能作为再生热源的转轮除湿和蒸气压缩制冷相结合及转轮除湿、蒸气压缩和蒸发冷却相结合的2种复合式空调系统,同时对电能作为再生热源的上述空调系统进行研究,建立了系统的物理模型,并对系统性能参数进行数学描述。通过与相同条件下常规蒸气压缩空调系统的比较分析,得出复合式空调系统制冷剂质量流量分别减少50.20％和66.67％；压缩系统性能系数COP分别提高了26.49％和32.16％；压缩机能耗分别节省了62.64％和76.92％。电能作为再生热源时,总负荷能耗分别节省了32.68％和42.00％；当采用太阳能作为再生热源时,总负荷能耗节省更多的能量,分别为61.86％和71.16％(认为1kW电能等价于3kW热能)。研究还发现,室内相对湿度相同,随室内设计温度的提高,复合式系统压缩机能耗明显减少,节能率呈上升趋势；相反总负荷能耗的节能率呈下降趋势。干热气候条件下,系统节能较为明显：71.75％和85.96％(电能再生)。热湿气候条件下,系统节能不明显,甚至消耗更多能量,而采用太阳能时,复合式系统均具有明显节能效果。     

An analytical solution for air dehumidification by liquid desiccant in a packed column

This paper presents a new analytical solution of heat and mass transfer processes in a packed bed liquid desiccant dehumidifier based on the equilibrium humidity on the interface is assumed to be constant. In order to maintain the partial pressure difference on the interface, a high liquid desiccant flow rate is often applied in the practical absorber. Therefore, for a narrow range of operating conditions for practical dehumidification process, we can assume that the equilibrium humidity ratio on the interface is constant. The assumption of constant humidity ratio is applied in this paper for derivation of the analytical solution. The model and the analytical solution predictions were compared against a reliable set of experimental data available in the literature, with very good agreement. According to the Lewis definition in this present study, the Lewis number obtains 0.9. The effects of variables such as air and liquid desiccant flow rate, air temperature and humidity, desiccant temperature and concentration have been investigated on the condensation rate. The results present that design variables such as desiccant concentration, desiccant temperature, air flow rate, and air humidity ratio have the greatest impact on the performance of the dehumidifier. The liquid flow rate and the air temperature have not a significant effect. Furthermore, the effects of air and liquid desiccant flow rate have been reported on the humidity effectiveness of the column.     

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.     

Comparative analysis of different design of rotary dehumidifier

A one‐dimensional mathematical model has been developed to do the comparative analysis of four design of the desiccant wheel. The mathematical model has been validated with experimental results and shows good agreements with experimental results. In this study, a two‐sector desiccant wheel is modified into a three‐sector wheel by introducing a purge section in the desiccant wheel. Similarly, a four‐sector desiccant wheel is reformed into a six‐sector wheel by installing two purge section in the desiccant wheel and comparative study of these designs have been carried for given ranges of operating conditions. The effect of purge conditions has been also investigated and found that in all operating conditions six‐sector design is a better choice for optimum MRC and ?T.     

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.     

Experimental investigation of a solar desiccant cooling installation

Desiccant cooling is a technique based on evaporative cooling and air dehumidification using desiccant regenerated by thermal energy. It is particularly interesting when it is driven by waste or solar heat making this technique environmentally friendly.In this paper, an experimental investigation is carried on a desiccant air handling unit powered by vacuum-tube solar collectors. First, the components are studied under various operating conditions. Then overall performance of the installation is evaluated over a day for a moderately humid climate with regeneration solely by solar energy. In these conditions the overall efficiency of the solar installation is 0.55 while the thermodynamic coefficient of performance is 0.45 and the performance indicator based on the electrical consumption is 4.5. Finally, the impact of outside and regeneration conditions on the performance indicators is studied.     

Performance comparison between a solar driven rotary desiccant cooling system and conventional vapor compression system (performance study of desiccant cooling)

Solar driven rotary desiccant cooling systems have been widely recognized as alternatives to conventional vapor compression systems for their merits of energy-saving and being eco-friendly. In the previous paper, the basic performance features of desiccant wheel have been discussed. In this paper, a solar driven two-stage rotary desiccant cooling system and a vapor compression system are simulated to provide cooling for one floor in a commercial office building in two cities with different climates: Berlin and Shanghai. The model developed in the previous paper is adopted to predict the performance of the desiccant wheel. The objectives of this paper are to evaluate and compare the thermodynamic and economic performance of the two systems and to obtain useful data for practical application. Results show that the desiccant cooling system is able to meet the cooling demand and provide comfortable supply air in both of the two regions. The required regeneration temperatures are 55 °C in Berlin and 85 °C in Shanghai. As compared to the vapor compression system, the desiccant cooling system has better supply air quality and consumes less electricity. The results of the economic analysis demonstrate that the dynamic investment payback periods are 4.7 years in Berlin and 7.2 years in Shanghai.     

Performance comparisons of desiccant wheels for air dehumidification and enthalpy recovery

Desiccant wheels have two major applications: air dehumidification and enthalpy recovery. Since the operating conditions are different, heat and mass transfer behaviors in the wheels are quite different. In this paper, the performances of desiccant wheels used in air dehumidification and enthalpy recovery are compared with each other. To accomplish this task, a two-dimensional, dual-diffusion transient heat and mass transfer model which takes into account the heat conduction, the surface and gaseous diffusion in both the axial and the thickness directions is presented. Effects of the rotary speed, the number of transfer units, and the specific area on the performance of the wheel are investigated and compared in the two situations. The cycles that the desiccant and air undergo in the wheel are plotted in psychrometric charts to demonstrate the different heat and moisture transfer mechanisms during the dehumidification and enthalpy recovery processes.