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 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.     

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.     

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.     

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 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.     

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 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.     

A review of the mathematical models for predicting solar air heaters systems

A mathematical model of the closed solar air heaters is used in particular, to assist in interpreting the observed phenomena in the solar air heaters, to design the system, to predict the trends, and to assist in optimization. In this paper, various mathematical models, mainly analyzing the heat transfer process of solar air heaters, are reviewed and classified based on the model, the number of the cover, the shape of the absorber and the presence or not of the packing bed. Although the models have evolved to a point where several features of the process can be predicted, more effort is required before the models can be applied to define actual operating conditions as well as to further investigate new closed solar air heaters. It is shown that the major governing equations in the models are based on the first law of thermodynamics.     

A typical meteorological day (TMD) approach for predicting the long-term performance of solar energy systems

A method for predicting the long-term performance of solar energy systems, based on the analysis of system performance for one particular day—the typical meteorological day (TMD)—is presented. The TMD is constructed from the cumulative time distribution of insolation values on the collector aperture. The TMD method requires little calculational effort and a small data base relative to standard yearly computer simulations. Good agreement is found between the predictions of the new method and the corresponding results of the -f-chart method. The TMD method is of particular value for cases that may often be treated inaccurately by simple calculational methods: (1) high threshold problems; (2) systems with short response times (e.g., due to small storage); and (3) systems in which collectors other than flat plates are used (the method is applicable to all solar collector types).     

Application of a lumped model for predicting energy performance of a variable-speed vapour compression system

This work presents a model for a variable-speed vapour compression system that is able to predict accurately the system performance using data easily obtained from an industrial facility. The model uses information on the secondary fluids input conditions and the compressor speed to predict the secondary fluids output temperatures, the operating pressures, the compressor power consumption and the system overall energy performance. This model has been validated experimentally with steady state tests, presenting a prediction error lower than 10%. Finally, an application of the model to evaluate the influence of the operating variables on the energy performance of a chiller is presented.     

A mathematical model for lead-acid batteries

A mathematical model of a lead-acid battery is presented. This model takes into account self-discharge, battery storage capacity, internal resistance, overvoltage, and environmental temperature. Nonlinear components are used to represent the behavior of the different battery parameters thereby simplifying the model design. The model components are found by using manufacturers specifications and experimental tests. A comparison between the model and experimental results obtained from a battery evaluation test system was used for verification. This model can be used to accurately evaluate battery performance in electrical systems     

A mathematical model for shading calculations

A mathematical model for shading calculations is developed. Closed form expressions for shadow position of an isolated point on a plane surface with arbitrary orientation are derived. By parallel projection methods these expressions can be applied for shadow calculations of relatively complex objects. Furthermore, for some particular plane surface orientations the shadow cast is presented by the shading diagrams. The use of such diagrams for analysis is also discussed on few illustrative examples.     

Theoretical and experimental investigation of the performance of a desiccant air-conditioning system

Solid desiccant air-conditioning systems present an interesting alternative solution with regard to the conventional vapour compression systems, in the sense that they do not use any refrigerants and they present the opportunity to exploit thermal energy, and more specifically solar thermal energy, instead of electrical energy. In the present work a theoretical model is presented for the operation of a desiccant air-conditioning system, developed on the basis of existing approaches for the modelling of the main subsystems of such a device. The model is experimentally validated on a real scale system, through the exploitation of a significant number of measurements, which correspond to a typical range of operation conditions for these systems. The proposed model is used for the investigation of the performance of a system with a typical set-up, examining the influence of parameters such as the weather conditions, the level of the imposed cooling load, the efficiency level of the main subsystems of the set-up, the air flow rate and the regeneration temperature. The results confirm the potential of the examined technology to satisfy actual cooling loads, and at the same time they lead to specific conclusions for the operation of these systems.