Simplified models for the performance evaluation of desiccant wheel dehumidification

In the present communication, simple models have been presented to evaluate the performance of rotary desiccant wheels based on different kind of solid desiccants e.g. silica gel and LiCl. The first part of the paper presents ‘Model 54’ which is developed for silica gel desiccant rotor. The model has been derived from the interpolation of experimental data obtained from the industry and the correlations have been developed for predicting outlet temperature and absolute humidity. The ‘Model 54’ consists of 54 coefficients corresponding to each correlation for outlet absolute humidity and temperature and it is found that the model predicts very well the performance of silica gel desiccant rotor (Type-I). In the second part of the paper, a psychrometric model has been presented to obtain relatively simple correlations for outlet temperature and absolute humidity. The developed psychometric model is based on the correlations between the relative humidity and enthalpy of supply and regeneration air streams. The model is used to predict the performance of three type of desiccant rotors manufactured by using different kind of solid desiccants (Type I, II and III). The model is tested corresponding to a wide range of measurement data. The developed psychometric model is simple in nature and able to predict very well the performance of different kind of desiccant rotors. Copyright © 2002 John Wiley & Sons, Ltd.     

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.     

A simplified mathematical model for predicting the nocturnal output of a solar still

The production capacity of a solar still which converts saline water to fresh water can be increased by introducing hot feed water into the unit at night. A waste heat source, such as cooling water from a power plant, can be used to preheat the feed. The nocturnal production, i.e. the distilled water produced at night, seems to be influenced by several parameters. However, a simplified mathematical model suggests that the distillate depends only on the initial brine temperature, the drop in brine temperature and the brine depth. This was experimentally verified for different brine depths and for initial brine temperatures up to 150°F.     

A simulation model for the performance of a hybrid liquid desiccant system during cooling and dehumidification

A finite difference model describing simultaneous heat and mass transfer in a hybrid liquid desiccant cooling system is presented in this paper. This type of system provides cooled and dehumidified air by a combination of a packed absorber tower and a conventional vapour compression system. The model will allow the prediction of the rate of condensation in the dehumidification tower and the evaporator of the vapour compression system, the conditions of the air and liquid desiccant leaving the tower, the conditions of the air leaving the evaporator, and other important parameters. The mathematical model results were validated with existing experimental data for a similar system with an uncertainty of 10.5% for the total rate of condensation and 0.8°C for the evaporator air exit temperature. Copyright © 2005 John Wiley & Sons, Ltd.     

A stochastic model for predicting solar system performance

A method for predicting long term performance of solar systems is presented. The method uses a stochastic approach and is based on some statistical properties of monthly averages of daily insolation and dry bulb temperatures. Application of this method to solar heating and hot water systems yields results that agree with those obtained using the f-chart and with experimental observations.     

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.     

The use of psychrometric charts for the optimisation of a thermal swing desiccant wheel

An effective prediction is proposed to estimate the optimal rotation speed and performance of a rotary adsorber, in which simultaneous enthalpy and humidity changes are dealt with separately by visualising changes of state of product or exhaust air on a psychrometric chart. Assuming that the adsorbent rotor is completely regenerated to equilibrium with the regeneration air during the corresponding period, the optimal rotation speed corresponds to the region of the short time adsorption in which penetration theory holds and enthalpy exchange between both streams through the adsorbent rotor follows the behaviour of a rotary sensible heat exchanger at lower revolution rates. The change of the product/exhaust air condition with increasing rotational speed is presented as a set of simple equations. Also, by considering the relative humidity of product air and that of regeneration air to be almost the same at a sufficiently high flow rates of regeneration air, an optimal rotation speed and the product air condition are easily found by simple calculation. In comparison with experiments, the proposed method gives a rotational speed near the “optimum” and the humidity and temperature of the product air are predicted almost exactly.     

A mathematical analysis of a model of structured population (II)

In this work, we complete [4] in which we have mathematically analyzed an age-cycle structured population endowed with a general biological rule. We describe the asymptotic behavior of the generated semigroup in the uniform topology.     

Development of a mathematical model for predicting water vapor mass generated in micro-explosion

This paper describes a mathematical model for predicting the mass of water vapor generated in micro-explosion. First, a single droplet experiment was carried out. A W/O (water/oil) emulsified fuel droplet suspended by a thermocouple was heated by a halogen spot heater, and micro-explosion was observed using a high-speed video camera. The progress of the coalescence of the dispersed water droplet was observed while droplet was heated, and an aggregated water droplet was formed in the oil layer. Based on the measured micro-explosion characteristics, a mathematical model for predicting water vapor mass generated in micro-explosion was proposed. The size of the aggregated water droplet just before micro-explosion was measured to verify the proposed mathematical model. Under certain assumptions, mass and energy conservation equations were applied to micro-explosion process, and an equation to calculate water vapor mass generated in micro-explosion was derived. The derived equation and some measurement results provide enough information to calculate water vapor mass generated in micro-explosion. The calculated diameter of the water droplet, which changed to vapor in micro-explosion, was compared to that of the aggregated water droplet just before micro-explosion. The calculated results roughly agreed with experimental ones, and the validity of the proposed model was verified.     

A simulation model for predicting the performance of a solar photovoltaic system with alternating current loads

This paper describes the development of a simulation model for predicting the performance of a solar photovoltaic (PV) system under specified load requirements and prevailing meteorological conditions at the site location. This study is aimed at situations where the loads are provided by alternating current (AC) electrical devices. The model consists of several submodels for each of the main components of the PV system; namely, PV array, battery, controller, inverter and various loads. Mathematical equations developed for modeling the performance of each component are explained along with the methodology to determine the performance coefficients. In order to validate the developed simulation model, an experimental system has been set up and tested under a variety of climatic conditions. Simulated results from the model under the same operating and environmental conditions are compared with those observed from the experimental tests. Good agreement is found in the comparison. Slight discrepancies appearing in the results are described and recommendations are given for further improvement. The simulation model developed can be used not only for analyzing the PV system performance, but also for sizing the PV system which is most suitable to the load requirements at any specified location provided that the local meteorological data is available.     

A mathematical model for solar radiation in South-East Asia (Thailand)

A compact first order random model for simulating daily totals of solar radiation and hourly solar radiation fluxes by computer is described. It is based on detailed studies of solar radiation data for Thailand and is expected to be applicable over the entire South-East Asian peninsula. Empirical formulae (binomial distributions) have been found to give satisfactory approximations to the probability distributions needed. Consequently the model can be operated with only a modest amount of input information related to the statistics of the daily totals of solar radiation throughout the year. Provision is made for estimating the separation of the solar radiation fluxes into their direct and diffuse components.     

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.     

A methodology for the performance evaluation of an experimental desiccant cooling system

An experimental investigation was conducted in an open cycle desiccant cooling system (DCS) operating on the ventilation mode in the laboratory site [M. Yıldırım, An experimental investigation on heat and mass transfer in a desiccant cooling system, PhD thesis, Gaziantep University, Turkey (2002). [1]]. Although the operation of DCS is presumably affected by the design of primary components of rotary regenerator (RR) and desiccant wheel (DW) the methodology used in the analysis of experimental data is presented in this paper to set a different approach for the performance evaluation of similar systems.

The rotational speeds of RR and DW (N_RR and N_DW), air mass flow rate (m_a) in process and regeneration lines, and the regeneration temperature (T_R) were defined as operation parameters. Meanwhile coefficient of performance (COP) and cooling capacity (CC) of the system were called as the performance parameters. The system operation with a variety of experimental conditions resulted in an extensive data set covering the ranges of N_RR, N_DW, m_a and T_R as 5 rpm ≤ N_RR ≤ 20 rpm, 0.1 rpm ≤ N_DW ≤ 0.4 rpm, 0.05 kg/s ≤ m_a ≤ 0.139 kg/s and 60 °C ≤ T_R ≤ 90 °C, respectively. The interactive influence of the operation parameters was determined through the realization of the psychrometric cycle in deviation from an ideal cycle. A dimensional analysis based on a trial and error procedure was followed to determine the functional relationship of COP and CC.

The proposed correlations between COP and CC and the introduced system performance parameter (PP) were determined to be a sole function of m_a independent of N_RR, N_DW and T_R in their covered ranges.     

A dynamic model for predicting solar plant performance and optimum control

Planning over the intermediate and long terms for the operation of solar energy systems must be based on the use of measured data and performance models of system components. We present a dynamic model for predicting the thermal performance of a low-temperature solar power plant. We analyse the optimal control regime for energy management. Validation of the model was performed on a large solar plant located in the north of Tunisia. The results showed good agreement between measured and model values. During the cold season, the plant efficiency increases considerably when a greenhouse is connected to the power plant.     

Simulation and energy efficiency analysis of desiccant wheel systems for drying processes

In drying processes it is necessary to appropriately control air humidity and temperature in order to enhance water evaporation from product surface. The aim of this work is to investigate several HVAC configurations for product drying based on desiccant wheels, in order to find systems which reach high primary energy savings through the appropriate integration of refrigerating machines, adsorption wheels and cogenerative engines. Simulations are carried out for different values of sensible to latent ambient load ratio and the effect of ambient and outside air conditions is evaluated for each configuration. It is shown that primary energy savings can reach 70–80% compared to the reference technology based on a cooling coil. With respect to works available in literature, the results of this study keep a general approach and they can be used as a simple tool for preliminary assessment in a wide range of applications.     

A mathematical model of thermal performance of a solar air heater with slats

A mathematical model for computing the thermal performance of a single pass flat-plate solar air collector is presented. Air channels were formed by providing metal slats running along the circulated air passage linking the absorber plate by the bottom one in an endeavor to enhance the thermal efficiency of the solar air collector. A mathematical model, therefore, is developed by which the influence of the addition of the metal slats on the efficiency of the solar collector is studied. A computer code that employs an iterative solution procedure is constructed to solve for the governing energy equations to estimate the mean temperatures of the collector. The effect of volume airflow rate, collector length, and spacing between the absorber and bottom plates on the thermal performance of the present solar air heater was investigated. Furthermore, a numerical comparison of the present design with the most common type of solar air heaters is conducted. The results of the comparison have indicated that better thermal performance was obtained by the modified system.     

A simplified model for air dehumidification with liquid desiccant

This paper describes a relatively simple model for the preliminary design of an air dehumidification process occurring in a packed bed using liquid desiccant through dimensionless vapor pressure and temperature difference ratios. An expression is derived using the aforementioned ratios to predict the water condensation rate from the air to the desiccant solution in terms of known operating parameters. The model predictions were compared against a reliable set of experimental data available in the literature, with very good agreement. The effects of the cooling water inlet temperature and the desiccant-to-water heat exchanger effectiveness on the performance of the dehumidifier are also studied and the results are presented in this paper.     

基于人工神经网络的转轮除湿系统建模方法研究

转轮除湿系统具有除湿精确度高且能耗低的优点,但其除湿机理相对复杂,除湿过程具有很强的非线性和时变性,对其建模具有一定难度。现有的建模手段以物理方法为主,建模过程复杂且模型误差大。本文引入了神经网络技术,建立了转轮除湿系统的两种神经网络模型——反向传播(BP)网络模型和径向基函数(RBF)网络模型,通过对两种模型进行运算分析,可知两种神经网络对于转轮除湿系统具有良好的适用性,同时RBF网络模型具有较高的精度,应用前景较好。     

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.     

A cycle simulation model for predicting the performance of a diesel engine fuelled by diesel and biodiesel blends

Among the alternative fuels, biodiesel and its blends are considered suitable and the most promising fuel for diesel engine. The properties of biodiesel are found similar to that of diesel. Many researchers have experimentally evaluated the performance characteristics of conventional diesel engines fuelled by biodiesel and its blends. However, experiments require enormous effort, money and time. Hence, a cycle simulation model incorporating a thermodynamic based single zone combustion model is developed to predict the performance of diesel engine. The effect of engine speed and compression ratio on brake power and brake thermal efficiency is analysed through the model. The fuel considered for the analysis are diesel, 20%, 40%, 60% blending of diesel and biodiesel derived from Karanja oil (Pongamia Glabra). The model predicts similar performance with diesel, 20% and 40% blending. However, with 60% blending, it reveals better performance in terms of brake power and brake thermal efficiency.