Experimental and theoretical performance of a demonstration solar chimney model—Part I: mathematical model development

The solar chimney is a natural draft device which uses solar radiation to provide upward momentum to the in-flowing air, thereby converting the thermal energy into kinetic energy. A study was undertaken to evaluate the performance characteristics of solar chimneys both theoretically and experimentally. In this paper, a mathematical model which was developed to study the effect of various parameters on the air temperature, air velocity, and power output of the solar chimney, is presented. Tests were conducted on a demonstration model which was designed and built for that purpose. The mathematical model presented here, was verified against experimental test results and the overall results were encouraging. © 1998 John Wiley & Sons, Ltd.     

Performance evaluation of solar chimney power plants in Iran

The solar chimney power plant is a simple solar thermal power plant that is capable of converting solar energy into thermal energy in the solar collector. In the second stage, the generated thermal energy is converted into kinetic energy in the chimney and ultimately into electric energy using a combination of a wind turbine and a generator. The purpose of this study is to evaluate the performance of solar chimney power plants in some parts of Iran theoretically and to estimate the quantity of the produced electric energy. A mathematical model based on the energy balance was developed to estimate the power output of solar chimneys as well as to examine the effect of various ambient conditions and structural dimensions on the power generation. The solar chimney power plant with 350 m chimney height and 1000 m collector diameter is capable of producing monthly average 1-2 MW electric power over a year.     

Simulation of a pilot solar chimney thermal power generating equipment

A pilot experimental solar chimney thermal power generating equipment was set up in China. A simulation study was carried out to investigate the performance of the power generating system based on a developed mathematical model. The simulated power outputs in steady state were obtained for different global solar radiation intensity, collector area and chimney height. By intercomparison, it is found that the simulated power outputs are basically in agreement with the results calculated with the measurements, which validates the mathematical model of the solar chimney thermal power generating system. Furthermore, based on the simulation and the specific construction costs at a specific site, the optimum combination of chimney and collector dimensions can be selected for a required electric power output.     

Counter-rotating turbines for solar chimney power plants

An efficiency model at design performance for counter-rotating turbines is developed and validated. Based on the efficiency equations, an off-design performance model for counter-rotating turbines is developed. Combined with a thermodynamic model for a solar chimney system and a solar radiation model, annual energy output of solar chimney systems is determined. Two counter-rotating turbines, one with inlet guide vanes, the other without, are compared to a single-runner system. The design and off-design performances are weighed against in three different solar chimney plant sizes. It is shown that the counter-rotating turbines without guide vanes have lower design efficiency and a higher off-design performance than a single-runner turbine. Based on the output torque versus power for various turbine layouts, advantageous operational conditions of counter-rotating turbines are demonstrated.     

A review for the applications of solar chimneys in buildings

As a simple and practical bioclimatic design methodology, solar chimneys are receiving considerable attention for reducing heat gain and inducing natural cooling or heating in both commercial and residential buildings because of their potential benefits in terms of operational cost, energy requirement and carbon dioxide emission. In practical civil buildings, solar chimneys can be installed on the walls and roofs. For the purpose of improving natural ventilation performance and achieving better indoor thermal comfort, solar chimneys are always applied in the form of integrated configurations. Solar chimneys can also be used to combine with natural cooling systems so as to enhance the cooling effect inside buildings. Besides, active solar systems may be utilized to enhance the ventilation performance of solar chimneys. In this paper, the main configurations and the integrated renewable energy systems based on solar chimneys were summarized. Then the suggestions were given. Generally, solar chimney technology has been regarded as an effective and economical design method in low carbon buildings. As for the integrated energy systems based upon solar chimneys, it is still necessary to carry out more experimental investigations to acquire objective data for the system design. Besides, it is suggested to further study the optimization and control strategy of such integrated systems in different climates.     

Study of the natural convection phenomena inside a wall solar chimney with one wall adiabatic and one wall under a heat flux

Four wall solar chimneys have been constructed and put at each wall and orientation of a small-scale test room so as to be used for the evaluation and measurement of their thermal behavior and the certification of their efficiency. At this stage, research focuses on the study of the buoyancy-driven flow field and heat transfer inside them. A numerical investigation of the thermo-fluid phenomena that take place inside the wall solar chimneys is performed and the governing elliptic equations are solved in a two-dimensional domain using a control volume method. The flow is turbulent and six different turbulence models have been tested to this study. As the realizable k–ε model is likely to provide superior performance for flows boundary layers under strong adverse pressure gradients, it has been selected to be used in the simulations. This is also confirmed by comparing with the experimental results. Predicted velocity and temperature profiles are presented for different locations, near the inlet, at different heights and near the outlet of the channel and they are as expected by theory. Important parameters such as average Nusselt number are also compared and calculated at several grid resolutions. The developed model is general and it can be easily customised to describe various solar chimney’s conditions, aspect ratios, etc. The results from the application of the model will support the effective set-up of the next configurations of the system.     

Performance analysis of water cooled concentrated photovoltaic (CPV) system

The concentrated photovoltaic (CPV) system focuses solar radiation on the solar cells. CPV systems need to track the sun for keeping the reflected radiation focussed on the solar cell. A CPV module and its active water-cooling system are developed at the School of Energy and Environment, Southeast University, China and its performance has been reported here. This developed system has been used for testing the PV module's performance for different parameters such as operating temperature, power output, and efficiency. The experimental results show that the operating temperature of the CPV module under water cooling is reduced under 60 °C and therefore the efficiency of the CPV has increased and produced the more electric power output. The effect of water flow rate has been analyzed for the CPV efficiency and output.     

Analysis of solar chimney power plant utilizing chimney discrete model

The paper presents a mathematical thermal model for steady state airflow inside a solar chimney power plant using modified Bernoulli equation with buoyancy effect and ideal gas equation. The study evaluates the use of constant density assumption across the solar chimney and compares it with more realistic chimney mathematical discrete model that allows density variation across the chimney. The result shows that using a constant density assumption through the solar chimney can simplify the analytical model however it over predicts the power generation. The results show that the chimney height, the collector radius, the solar irradiance, and the turbine head are essential parameters for the design of solar chimneys. The maximum power generation depends on the turbine head and the relation is not monotonic.     

基于多传感器信息融合的太阳电池动态建模

为获取运动状态下太阳电池输出特性,提出一种基于多传感器信息融合的太阳电池动态建模方法。基于多传感器信息融合测量模型、欧式空间旋转理论、光照强度与太阳电池空间位置关系和太阳电池数学模型及其参数与光照强度之间耦合关系,构建太阳电池动态模型得到其动态输出特性。通过SIMPACK及Matlab/Simulink仿真软件建立太阳电池动态发电仿真平台验证了方法的有效性。结果表明,载体的运动明显影响了太阳电池输出特性,且在太阳电池输出最大功率点处的功率最大减少了18.038%。     

Analysis of some available heat transfer coefficients applicable to solar chimney power plant collectors

A solar chimney power plant consists of a translucent collector which heats the air near the ground and guides it into the base of a chimney at its centre. The buoyant air rises in the chimney and electricity is generated through one or more turbines in or near the base of the chimney. Various studies about solar chimney power plant performance have been published. Different calculation approaches with a variety of considerations have been applied to calculate chimney power plant performance. In particular, two comprehensive studies are relevant, namely those of (Bernardes, M.A.d. S., Voß, A., Weinrebe, G., 2003. Thermal and technical analyses of solar chimneys. Solar Energy 75, 511-524; Pretorius, J.P., Kröger, D.G., 2006b. Solar chimney power plant performance. Transactions of the ASME 128, 302-311). The paper compares the methods used to calculate the heat fluxes in the collector, and their effects on solar chimney performance. Reasons for the discrepancies between the predictions of the two models are given. In general the Pretorius model produces higher heat transfer coefficients and higher heat rate fluxes for both the roof and for the ground surfaces. The two approaches lead to very similar air temperature rises in the collector and thus, similar produced power.     

Experimental and theoretical performance of a demonstration solar chimney model—Part II: experimental and theoretical results and economic analysis

This paper describes details of the experimental program conducted to assess the viability of the solar chimney concept. A demonstration model was designed and built and its theoretical and experimental performance was examined. Two experimental modifications were tried on the collector: (1) extending the collector base and (2) introducing an intermediate absorber. The former modification helped in enhancing the air temperature, while the latter contributed to increasing the air temperature as well as the mass flow rate inside the chimney. Both enhancements helped to increase the overall chimney power output. Theoretical and experimental performance results of this demonstration model are presented in this paper, while the mathematical model developed in Part I was used to predict the performance of much larger systems. Mathematical model results were validated by comparing them to published data on the solar chimney system built in Manzanares, Spain. Also, an economic assessment of the system costs are presented. © 1998 John Wiley & Sons, Ltd.     

Evaporation Cooling Using a Bionic Micro Porous Evaporation System

The temperature increase due to incident solar radiation has an adverse impact on the electrical output of photovoltaic (PV) modules. A theoretical model of the fabricated and tested bionic evaporation backside cooling was established and verified by experimental investigation. A microfluidic structure featuring micropores consists of two polymer layers attached on the backside of a PV cell model. The thermal performance of roof-mounted PV modules with rear panel air ventilation was mathematically described and extended by the cooling capabilities of the developed bionic evaporation foil. The results of experimental investigations performed in a roof equivalent test environment consisting of a wind tunnel within a climate chamber are in good accordance to the established model. Experimentally, temperature reductions at low incident solar power of less than 575 W causing an efficiency gain for up to 4.8% have been demonstrated while the model implicates an efficiency increase of 10% for real roof systems at an incident solar radiation of 1000 W.     

Questionable effects of antireflective coatings on inefficiently cooled solar cells

A model for temperature effects in p–n junction solar cells is introduced. The temperature of solar cells and the losses in the solar cell junction region caused by elevated temperatures are discussed. The model developed is examined for low-cost silicon solar cells. Plots of output power and efficiency as functions of the time throughout the day are presented. In order to improve the shape of the output power and efficiency curves throughout the day the coherence between technical parameters of the solar cells and the climate in the operation region is observed and examined. It is shown how the drop in output power around noon can be avoided by fitting technical parameters of the solar cells to the climate.     

Highly efficient maximum power point tracking using DC–DC coupled inductor single-ended primary inductance converter for photovoltaic power systems

Solar photovoltaics (PVs) have nonlinear voltage–current characteristics, with a distinct maximum power point (MPP) depending on factors such as solar irradiance and operating temperature. To extract maximum power from the PV array at any environmental condition, DC–DC converters are usually used as MPP trackers. This paper presents the performance analysis of a coupled inductor single-ended primary inductance converter for maximum power point tracking (MPPT) in a PV system. A detailed model of the system has been designed and developed in MATLAB/Simulink. The performance evaluation has been conducted on the basis of stability, current ripple reduction and efficiency at different operating conditions. Simulation results show considerable ripple reduction in the input and output currents of the converter. Both the MPPT and converter efficiencies are significantly improved. The obtained simulation results validate the effectiveness and suitability of the converter model in MPPT and show reasonable agreement with the theoretical analysis.     

A comprehensive study of dense-array concentrator photovoltaic system using non-imaging planar concentrator

A special modeling method using Simulink has been developed to analyze the electrical performance of dense-array concentrator photovoltaic (CPV) system. To optimize the performance of CPV system, we have adopted computational modeling method to design the best configuration of dense-array layout specially tailored for flux distribution profile of solar concentrator. It is an expeditious, efficient and cost effective approach to optimize any dense-array configuration for any solar concentrator. A prototype of non-imaging planar concentrator (NIPC) was chosen in this study for verifying the effectiveness of this method. Mismatch effects in dense array solar cells caused by non-uniform irradiance as well as sun-tracking error normally happens at the peripheral of the array. It is a crucial drawback that affects the electrical performance of CPV systems because maximum output power of the array is considerably reduced when a current–voltage (I–V) curve has many mismatch steps and thus leads to lower fill factor (FF) and conversion efficiency. The modeling method is validated by assembling, installing and field testing on an optimized configuration of solar cells with the NIPC prototype to achieve a conversion efficiency of 34.18%. The measured results are in close agreement with simulated results with a less than 3% deviation in maximum output power.     

Thermodynamic multi-objective optimization of a solar-dish Brayton system based on maximum power output,thermal efficiency and ecological performance

Solar-dish Brayton system driven by the hybrid of fossil fuel and solar energy is characterized by continuously stable operation, simplified hybridization, low system costs and high thermal efficiency. In order to enable the system to operate with its highest capabilities, a thermodynamic multi-objective optimization was performed in this study based on maximum power output, thermal efficiency and ecological performance. A thermodynamic model was developed to obtain the dimensionless power output, thermal efficiency and ecological performance, in which the imperfect performance of parabolic dish solar collector, the external irreversibility of Brayton heat engine and the conductive thermal bridging loss were considered. The combination of NSGA-II algorithm and decision makings was used to realize multi-objective optimization, where the temperatures of absorber, cooling water and working fluid, the effectiveness of hot-side heat exchanger, cold-side heat exchanger and regenerator were considered as optimization variables. Using the decision makings of Shannon Entropy, LINMAP and TOPSIS, the final optimal solutions were chosen from the Pareto frontier obtained by NSGA-II. By comparing the deviation index of each final optimal solution from the ideal solution, it is shown that the multi-objective optimization can lead to a more desirable design compared to the single-objective optimizations, and the final optimal solution selected by TOPSIS decision making presents superior performance. Moreover, the fitted curve between the optimal power output, thermal efficiency and ecological performance derived from Pareto frontier is obtained for better insight into the optimal design of the system. The sensitivity analysis shows that the optimal system performance is strongly dependent on the temperatures of absorber, cooling water and working fluid, and the effectiveness of regenerator. The results of this work offer benefits for related theoretic research and basis for solar energy industry.     

基于太阳能热驱动甲烷重整制氢的燃料电池发电系统性能研究

该文采用Aspen Plus软件建立膜反应器重整制氢及燃料电池模型,根据拉萨某日太阳能直接辐射强度（DNI）变化计算太阳能可供使用的能量,作为外热源输入重整系统,并分析反应温度、水碳比（S/C）及DNI对该系统各性能指标的影响,性能指标包括甲烷转化率、H₂收率、电池功率及电压、太阳能转换为氢能的效率。结果表明：反应温度为500 ℃,S/C为2.5时有利于太阳能甲烷湿重整反应;系统日性能结果显示在某日10:00—20:00时,电池输出功率120 kW,太阳能-化学能转化效率0.368,系统发电效率0.225。     

ANN and ANFIS models to predict the performance of solar chimney power plants

A precise model of the behavior of complex systems such as solar chimney power plants (SCPP) would be much beneficial. Also, such a model would be quite contributing to the control of solar chimney operation. In this paper, the identification and modeling of SCPP utilizing ANN and Adaptive Neuro Fuzzy Inference System (ANFIS) are discussed. The modeling is based on the data of three working days which were taken of a built pilot in University of Zanjan, Iran. The input parameters are time, radiation and ambient temperature, while the output is the air velocity at the inlet of the chimney. The results of ANN model and ANFIS model were compared; it was found that ANFIS model exhibited better performance than ANN. The R-Square error of testing in ANFIS is about 0.91, therefore there is good agreement between the ANFIS model and experimental data. Therefore the ANFIS model used to predict the SCPP performance for coming days. A numerical simulation of the problem is conducted to provide a comparison between the conventional method and the presented approach. The results indicated that the performance of solar chimney power plants will be accurately predictable via such a method providing less computational cost.     

On the investigation of photovoltaic output power reduction due to dust accumulation and weather conditions

Certain environmental conditions such as accumulation of dust and change in weather conditions affect the amount of solar radiation received by photovoltaic (PV) panel surfaces and thus have a significant effect on panel efficiency. This study conducted an experimental investigation in Surabaya, Indonesia, on the effect of these factors on output PV power reduction from the surface of a PV module. The module was exposed to outside weather conditions and connected to a measurement system developed using a rule-based model to identify different environmental conditions. The rule-based model, a clear sky solar irradiance model that included solar position, and a PV temperature model were then used to estimate the PV output power, and tests were also conducted using an ARM Cortex-M4 microcontroller STM32F407 as a standalone digital controller equipped with voltage, current, temperature, and humidity sensors to measure real time PV output power. In this system, humidity was monitored to identify dusty, cloudy, and rainy conditions. Validated test results demonstrate that the prediction error of PV power output based on the model is 3.6% compared to field measurements under clean surface conditions. The effects of dust accumulation and weather conditions on PV panel power output were then analyzed after one to four weeks of exposure. Results revealed that two weeks of dust accumulation caused a PV power output reduction of 10.8% in an average relative humidity of 52.24%. Results of the experiment under rainy conditions revealed a decrease in PV output power of more than 40% in average relative humidity of 76.32%, and a decrease in output power during cloudy conditions of more than 45% in an average relative humidity of 60.45% was observed. This study reveals that local environmental conditions, i.e., dust, rain, and partial cloud, significantly reduce PV power output.     

基于系统辨识的地表太阳辐射Box-Jenkins模型的建立

利用天文辐射作为输入数据,采用系统辨识的方法得到地表太阳辐射的BJ(Box-Jenkins)模型,并通过残差分析和零极点检验。该方法可用于预测5～15min时间间隔的地表太阳辐射,为太阳能电站的功率输出预测提供太阳能辐射数据。