Determination of the optimum tilt angle of solar collectors for building applications

Solar energy technologies offer a clean, renewable and domestic energy source, and are essential components of a sustainable energy future. This paper deals with the determination of the optimum tilt angle of solar collectors for building applications. The optimum angle is calculated by searching for the values for which the total radiation on the collector surface is a maximum for a particular day or a specific period. An application of the model is done using the experimental data measured for Izmir in Turkey. The best orientation for solar collectors in Izmir is due south. For increasing the utilization efficiency of solar collectors, it is recommended that, if it is possible, the solar collector should be mounted at the monthly average tilt angle and the slope adjusted once a month.     

Optimum slope angles and the corresponding uncertainties for a solar collector

Determination of the optimum slope angle of a solar collector is highly dependent on the incident solar radiations on the collector surface. As collected instantaneous data of incident solar radiation values are averaged, more attention must be directed towards these figures by determining the uncertainties in these measurements as this allows the calculation of the optimum slope angle. These average solar radiations give a definite optimum slope angle if they come along with the lowest uncertainties. Hence, this study aimed to find the optimum slope angles of solar collectors with corresponding uncertainties. For doing this, the solar radiation data borrowed from the Iranian Meteorological Organization which were measured on a horizontal surface in a period of 20 years (1986–2005), were employed. The results showed that the uncertainties of the optimum slope angles in some cities were quite high and it indicated that in this case more attention should be paid to select the appropriate optimum slope angle. These changes were more in cold regions compared to hot and dry regions because the weather in the colder climates is typically more transient than the weather in hot and dry climates.     

Optimising the tilt angle for solar surfaces using different solar irradiation models in Yazd,Iran

This paper deals with the determination of the optimum tilt angle for south-facing solar surfaces to collect the maximum solar irradiation in Yazd, Iran. In order to increase the accuracy of results, different models are used in this study. The accuracy of the models is compared in terms of the statistical indicators, RMSE and MBE. The results indicate that the Klucher model gives the best results for estimating the solar radiation reaching the solar surfaces. In comparison to the horizontal surface, on average the gain of energy during the year will be 22.82% if a solar surface is installed at yearly optimum tilt angle. For higher efficiency, it is suggested that the solar surface mounts at the seasonal tilt angle. This leads to an increase in the efficiency of the solar surface of more than 8.86% over that of a similar fixed solar surface at the annual tilt angle.     

建筑日照分析中太阳位置计算公式的改进研究

指出现有太阳高度角和方位角计算公式的使用在建筑物日照分析中存在的不便与不足。构建辐射历计时系统,并重新定义太阳高度角和方位角的取值范围,使其随时角的日变化和年变化具备单值性从而改进了现有计算公式。根据改进后的计算公式绘制不同纬度处太阳高度角和方位角随时角的变化曲线,讨论了这些曲线的数学特征所描述的天文学和地理学现象,验证了改进后公式的正确性和通用性,对建筑、空调、结构设计人员进行建筑日照分析都具有一定的参考价值。     

北京地区高层住宅太阳能立面集热一体化设计

在高层住宅太阳能立面集热一体化设计中,集热器立面最佳倾角与布置高度是建筑师在方案初始阶段就要考虑的因素。该文在对高层住宅立面太阳辐射量与有效日照时间段进行计算的基础上,提出太阳能集热与窗口遮阳功能的一体化设计方案,定量研究了集热器倾角与窗口遮阳的平衡关系;针对立面结合典型高层住宅布局模式,研究了不同类型高层住宅对北向住宅的日照影响,得出集热器最低悬挂高度的影响规律。研究成果将为建筑师在高层住区规划与集热器在立面的布置方案中提供依据。     

Power output analysis of transparent thin-film module in building integrated photovoltaic system (BIPV)

The power output of PV module was characterized depending on incidence angle and the azimuth using a transparent thin-film solar cell in a mock-up model at various slopes to the south, as a building integrated photovoltaic system. Simulated data was also evaluated to determine the influence of the inclined angles and the azimuth on the power performance of the PV module. The experimental and computed data fitted comparatively well through the relative error estimation after calibration. It was found that the PV module with a slope of 30°, facing south, provided the best power performance according to an annual power output, producing about 2.5 times higher power output than that with the vertical module. Furthermore, the PV module facing south showed higher power output than that to the east. The varying power output of the PV module with inclined angles can be explained by the impact of the incidence angle modifier of the glass on the PV module. Specifically, the increased inclined slope of the PV module resulted in the reduced solar energy transmission, which producing a significant reduction of power output for the PV module with a slope over 70°.     

Glass–azimuth modification to reform direct solar heat gain

A means of reforming the solar heat gain, and hence, energy conservation in air-conditioning systems is suggested and studied. The main idea is to control the solar gain by horizontal rotation of glass windowpanes, i.e. modifying the glass–azimuth irrespective of the original orientation of the building wall. The influence of the rotation angle, magnitude and direction, on the instantaneous, daily and seasonal solar heat gain is investigated in detail. A computer program is coded to carry out the extensive computations. This program is prepared in a general and versatile form, in the sense that it can be used at any location and time, for any wall orientation and any type of glazing characterized by the extinction coefficient, index of refraction, and thickness. The study helps the designer to select an optimum glass rotation angle which increases or decreases the heat gain depending on the objective. There may be two optimum rotation angles, the first results in reducing the air-conditioning system size, while the second angle decreases the energy consumption.     

Design of a solar absorption cooling system in a Greek hospital

Air conditioning of buildings is responsible for a large percentage of the greenhouse and ozone depletion effect, as refrigerant harmful gases are released into the atmosphere from conventional cooling systems. The need to implement advanced new concepts in building air conditioning systems is more crucial than ever today.Solar cooling systems (SCS) have the advantage of using absolutely harmless working fluids such as water, or solutions of certain salts. They are energy efficient and environmentally safe. They can be used, either as stand-alone systems or with conventional AC, to improve the indoor air quality of all types of buildings. The main goal is to utilize “zero emissions” technologies to reduce energy consumption and reduce CO₂ emissions.Amongst cooling technologies, absorption cooling seems to have a promising market potential.In this paper, the performance and economic evaluation of a solar heating and cooling system of a hospital in Crete, is studied using the transient simulation program (TRNSYS). The meteorological year file exploited the hourly weather data where produced by 30-year statistical process. The required data were obtained by Hellenic National Meteorological Service.The objective of this study is to simulate a complete system comprised of a solar collector, a storage tank, a backup heat source, a water cooling tower and a LiBr-H₂O absorption chiller. The exploitation of the results of the simulation provided the optimum sizing of the system.     

Mathematical modelling of solar radiation incident on tilted surface for photovoltaic application at Bhopal,M.P., India

In general, solar radiations are the combination of beam plus diffuse and ground-reflected radiation. The availability of recorded data on a tilted surface is very rare due to lack of measuring equipment and techniques involved. In this study, a standard procedure is adopted for estimation of solar radiation on a tilted surface for a location in Central region of India. Solar radiation is estimated for three tilted positions: First, solar collector tilt equal to latitude angle, second, solar collector tilt equal to latitude angle +15° and third, solar collector tilt at latitude ?15°. Total global solar radiation estimated on the inclined surface for various photovoltaic (PV) modules was used to obtain the annual energy yield based on the estimated value. It was found that on an average, 14?kWh/m² of annual energy output can be obtained for monocrystalline solar PV module corresponding to the inclination of 23.26° latitude at Bhopal.     

A novel experimental method to find dust deposition effect on the performance of parabolic trough solar collectors

One main reason why the performance of a parabolic trough collector measured experimentally differs from the simulation results is inaccurate prediction of absorbed solar energy. The amount of absorbed energy of such systems mainly depends on optical properties of mirrors, absorber tube and transmissivity of glass cover of absorber tube. This study examines the thermal performances of three collectors during different periods regarding the weight and thickness of deposited dust. A transient computer code is prepared to estimate thermal performances of clean collectors and compare the results with dusty collectors. A special optical instrument is designed and constructed to measure the reflectivity of mirrors with respect to the thickness of dust deposited on the surface of the mirrors. Based on the measurements a new correlation is developed to find thermal performance of a parabolic collector for various dust thicknesses in comparison to a clean collector. Different cleaning schedules are proposed for the collectors’ field; and thermal performance and yearly solar energy absorption of collectors according to each cleaning schedule are also determined. The performance of a solar-fossil hybrid power plant is studied and the impact of different cleaning schedules on reducing fossil fuel consumption and CO₂ emission is investigated.     

太阳能工程中几种相关角度的计算及应用

如今太阳能光伏技术在全球得到广泛应用和发展,而在技术研究和产品开发过程中,确定不同时刻太阳的位置十分重要,这就要求我们要熟知各种参数的计算方法。本文结合当前最新研究成果,对常用的几种相关太阳能角度进行梳理,介绍一系列确定太阳方位的计算程序,及其在太阳能集热器检测中的应用,以供参考。     

基于遮阳理论冻土区路基及周边冻土表面辐射分析

 冻土区路基各表面间太阳辐射的差异引起路基发生横向非均匀变形。目前所采用的基于太阳入射角的分析方法,未能充分考虑到冻土区高路基遮阳效应对周边冻土的影响,特别是在对路基坡脚附近冻土分析时与实际工况存在很大的偏差。基于太阳辐射强度和路基影子轨迹随时间的变化规律,提出利用遮阳理论分析路基表面太阳辐射的分析方法。通过对比K. Y. Kondratyev关于倾斜面接收太阳辐射的描述和工程实测数据,验证遮阳理论分析方法的正确性。基于遮阳理论提出直射率概念,并获得路基表面温度计算的经验方程。分析表明,冻土区路基各表面的太阳辐射之间存在明显的差异,其差异性与路基走向、坡度等影响因素密切相关。对于坡度较大的高路基,路基的遮阳效应也会引起路基周边冻土表面出现明显的非均匀太阳辐射,表现为越靠近路基坡脚辐射量越小,阴坡坡脚处辐射量小于阳坡坡脚处辐射量,这种太阳辐射的非均匀分布在路基的稳定性分析中应予以考虑。     

Effect of solar chimney inclination angle on space flow pattern and ventilation rate

The solar chimney is a simple and practical idea that is applied to enhance space natural ventilation. The chimney could be vertical or inclined. The chimney inclination angle is an important parameter that greatly affects space flow pattern and ventilation rate.In the present study, the effect of chimney inclination angle on air change per hour and indoor flow pattern was numerically and analytically investigated. A numerical simulation using Ansys, a FEM-based code, was used to predict flow pattern. Then the results were compared with published experimental measurements. A FORTRAN program was developed to iteratively solve the mathematical model that was obtained through an overall energy balance on the solar chimney.The analytical results showed that an optimum air flow rate value was achieved when the chimney inclination is between 45° and 70° for latitude of 28.4°. The numerically predicted flow pattern inside the space supports this finding. Moreover, in the present study a correlation to predict the air change per hour was developed. The correlation was tested within a solar intensity greater than or equal to 500 W/m², and chimney width from 0.1 m to 0.35 m for different inclination angles with acceptable values.     

Analysis on the optimum matching of collector and storage size of solar water heating systems in building space heating applications

The overall thermal performance of a solar water heating (SWH) system is significantly affected by the mismatch between the temporal distribution of solar radiation and the heating load. Therefore, a favorable correlation between the collector and storage size should be generated based on the dynamic characteristics of the system. This study focuses on the optimal matching of solar collector area with storage volume for an SWH system (with short-term heat storage capability) for a space heating application. A simplified model of an SWH system based on hourly energy flow is established. System control strategy is integrated into the model in a simple manner without sacrificing computing speed. Based on this model, the combined effect of collector area and storage volume on system thermal performance and economy is analyzed, and a simple procedure for determining the optimal system size is illustrated. A case study showed that for an SWH system utilized for space heating application, the optimized ratio between storage volume and collector area is dependent on the total collector area of the system, and is dominated by the requirement of overheating prevention. The minimum storage volume for a specific collector area that can prevent the storage tank from being overheated can be adopted as the optimum storage volume for that collector area. The optimum ratio between storage volume and collector area increases as the collector area increases. Therefore, a trade-off between heat collection and heat loss has to be made while attempting to increase solar fraction by improving collector area.     

Estimation of direct solar irradiance intercepted by a solar concentrator in different modes of tracking (case study: Algeria)

While reasonably accurate knowledge of solar radiation data at the location of interest is necessary for the design of any solar energy conversion system, the use of concentrating solar collectors implies that these systems only work with the direct solar irradiance. In the present study, by using a methodology based on simple equations, the total hourly, daily, monthly and annual direct radiations incident on four different oriented solar concentrators were calculated only from the recorded data of monthly mean daily global and diffuse horizontal solar irradiance for three different Algerian sites, in order to choose the best mode of tracking for concentrating solar thermal power systems and concentrating solar photovoltaic power systems applications. The model predictions were found to be consistent with ground measurements and prior studies of Kalogirou. It was found that the greatest energy gains were achieved by using full tracking. Furthermore, the model can be included for application to other Algerian and worldwide locations for estimating direct solar irradiance intercepted by a solar concentrator in different modes of tracking.     

Thermal performance analysis of the glass evacuated tube solar collector with U-tube

In this paper, based on the energy balance for the glass evacuated tube solar collector with U-tube, the thermal performance of the individual glass evacuated tube solar collector is investigated by analytical method. The solar collector considered in this study is a two-layered glass evacuated tube, and the absorber film is deposited in the outer surface of the absorber tube. The heat loss coefficient and heat efficiency factor are analyzed using one-dimensional analytical solution. And the influence of air layer between the absorber tube and the copper fin on the heat efficiency is also studied. The results show that the function relation of the heat loss coefficient of the glass evacuated tube solar collector with temperature difference between the absorbing coating surface and the ambient air is nonlinear. In the different ambient temperatures, the heat loss coefficient of the solar collector should be calculated by different expressions. The heat efficiency factor will be subject to influence of air layer between absorber tube and the copper fin. Specially, the influence is remarkable when the heat loss coefficient of the collector is large. When the synthetical conductance amounts to 5 W/m K, the solar collector efficiency decreases 10%, and the outlet fluid temperature decreases 16% compared with the case which the air thermal resistance is neglected. And the surface temperature of the absorbing coating increases 30 °C due to the effect of air thermal resistance. So the surface temperature of the absorbing coating is an important parameter to evaluate the thermal performance of the glass evacuated tube solar collector.     

太阳能跟踪器光伏面板风荷载体型系数的数值模拟研究

风荷载对太阳能跟踪器结构的稳定性以及跟踪的精确性有很大的影响。首先对各国可用于光伏支架风荷载计算的规范及经验公式进行介绍,然后通过CFD数值风洞对太阳能跟踪器光伏面板简化模型的风荷载体型系数进行了模拟,并把所得到的结果与相应的各国规范及经验公式进行了对比。分析结果表明,在各风向角下,随着光伏面板仰角的增加,其体型系数的均值会不断增大;仰角同为30°的平板与抛物面板的体型系数有较大的差异;ASCE7-10规范不能很好地适用于光伏面板风荷载计算;使用我国规范进行光伏支架设计偏保守,用钢量偏大。根据数值模拟分析以及与不同规范对比分析,提出了适用于现阶段我国太阳能跟踪器光伏面板风荷载体型系数取值的方法。     

铅垂面上太阳辐射计算方法探讨

在建筑物的太阳能利用中常涉及铅垂面上太阳辐射量的计算问题，其理论计算方法中，Perez模型计算较准确但是较繁杂，Hay模型较为简明实用。在工程应用中，通过计算铅垂面上日平均辐射量而计算月总辐射量，其中Hay模型较准确。     

槽式太阳能集热器传热模型及性能分析

槽式太阳能集热器一维和二维传热数学模型是一组非线性代数方程,为改进求解的稳定性和计算精度,将槽式太阳能集热器一维和二维传热模型的求解看作有约束优化问题,建立了集热器传热过程求解的有约束优化数学模型,应用MATLAB软件优化函数fmincon进行求解。分析了传热流体入口温度及太阳能辐射热流密度变化对集热器性能的影响。采用fmincon函数求解集热器传热过程,计算速度快,计算过程稳定。分析表明,传热流体温度变化对集热器效率的影响大于太阳能辐射热流密度对集热器效率的影响。     

福建省太阳辐射接受面最佳方位角及倾角的计算

本文介绍了一种计算太阳辐射接受面最佳方位角及倾角的方法和工具,并以此计算得到福建省7个县市的最佳安装角度。本研究选择天空各向异性的Reindl模型作为太阳辐射计算模型;在气象数据上,采用SWERA和CSWD两种数据源,并优先采用准确度较高的SWERA数据源。在这基础上,本研究研制出“太阳能辐射接受面角度计算器”,该软件可可以为太阳能利用的研究、标准编制,以及设计提供数据支持。最后,福建省7个县市太阳辐射接受面的最佳安装角度被计算得到,以便于实际工程应用。