Measuring and evaluating solar radiative properties of plastic shading nets

Plastic shading nets are being used extensively in hot and sunny regions to protect plants from intense solar radiation. Different varieties of these nets are commercially available. However, the choice of net to fulfill specific shading requirements often depends on empirical or economic criteria and not on scientific considerations because essential information used to characterize the different types of nets is not available.In order to replace expensive measuring facilities that use artificial lighting to investigate the radiative properties of plastic shading nets, a simple method was presented to investigate these properties under global, diffuse and direct beam solar radiation conditions. Measurements were performed on clear sunny days, (December 28, 2008-February 10, 2009). Nets with colors and shading factors that are most commonly used in hot regions were selected for the study. The results showed that the behaviors of the plastic nets under solar radiation conditions were similar to those of translucent materials. Accordingly, it is possible to (i) treat plastic nets as translucent materials and (ii) investigate the equivalent optical constants (i.e., refractive indexes and extinction coefficients) for plastic nets as functions of the net solidity, texture type and color. Solidity and color of the net had significant effects on the radiative properties and the effect of color was much more than the effect of solidity. The shading factor of a plastic net, is a function of the daytime, depends on several design and meteorological parameters, so it cannot be used to describe a net. However, the net solidity together with color, and the daily integrals of the shading factor and of the radiative properties are appropriate parameters for describing a net.     

Solar thermal modelling of a non-airconditioned building: Evaluation of overall heat flux

This paper presents an investigation of the thermal behaviour of a non-airconditioned building with walls/roof being exposed to periodic solar radiation and atmospheric air while the inside air temperature is controlled by an isothermal mass, window and door in the walls of the room. The effects of air ventilation and infiltration, the heat capacities of the isothermal storage mass inside air and walls/roof, heat loss into the ground, and the presence/absence of the window/door have been incorporated in the realistic time dependent periodic heat transfer analysis to evaluate the overall heat flux coming into the room and the inside air temperature. A numerical computer model using typical weather data for Delhi has been made to appreciate the analytical results quantitatively. It is found that the heat fluxes through different walls have different magnitudes and phase lags w.r.t. the corresponding solair temperatures. The overall heat flux coming into the room as well as the room air temperature are sensitive functions of the number of air changes per hour, closing/opening of the window and the door ventilation. The effects of the heat capacity of the isothermal mass and the basement ground are found to reduce the inside air temperature swing and the presence of a window is found to increase the inside air temperature even when the window area is much smaller than the wall/roof area. The model presented would be an aid to a building architect for good thermal design of non-airconditioned buildings.     

Analytically calculating shading in regular arrays of sun-pointing collectors

A method is presented for deriving an algorithm for analytically calculating shading of sun-pointing solar collectors by other identical collectors in the field. The method is particularly suited to regularly-spaced collectors, with convex aperture shapes. Using this method, an algorithm suitable for circular-aperture collectors is derived. The algorithm is validated against results obtained using an existing algorithm, and an example for usage of the algorithm as a tool for validating assumptions of an existing algorithm is presented.     

Evaluation of effective shading factor by fitting a clear-day pattern obtained from hourly maximum irradiance data

The authors have developed the sophisticated verification (SV) method, which can evaluate involving performance ratio, power conditioner efficiency, temperature factor, shading factor, load matching factor and other array parameter. So, we also have ensured adequacies of the evaluation of PV systems. However, the originally proposed method accepts PV installations facing the south only. In this paper, we have modified the SV method by taking into account the arbitrary orientation and inclination, because of improved the accuracy of evaluation. Therefore, the shading effect can be intelligible for each hour. The maximum value of shading losses reached 13.1%.     

On the boundary conditions applicable in the analysis of a matrix air heater

A dimensionless analysis of matrix air heaters has been developed to study the effect of different boundary conditions on its performance. The theoretical results have been compared with measurements of an experiment performed with one type of matrix solar air heater. For the fourth set of boundary conditions, the theoretical results agree fairly well with experimental data, thus providing confidence in particular boundary conditions. For this particular set of boundary conditions, the thermal performance of the system has been studied for different mass flow rates of air and as a function of other physical parameters.     

Molten-salt thermal energy storage in thermoclines under different environmental boundary conditions

Operation during the charge and discharge cycles of molten-salt thermoclines used for solar thermal energy storage depends strongly on the environmental boundary conditions to which the tanks are exposed. A comprehensive model which accounts for thermal transport in the molten-salt heat transfer fluid and the filler material in the tank is developed for exploring the effects of boundary conditions on thermocline performance. Heat loss from the tank under non-adiabatic boundary conditions is found to distort the temperature and salt flow distributions relative to the uniform conditions found in adiabatic thermoclines; as a result, the outflow temperature drops more rapidly in the former case. Such effects of non-adiabatic boundaries become insignificant at large salt-flow Reynolds numbers. As the Reynolds number increases beyond 250, the discharge efficiency of non-adiabatic thermoclines approaches that of the adiabatic counterparts. In the case of significant heat loss at the walls, the discharge efficiency of thermoclines increases with increasing Reynolds number, a trend that is opposite to that in adiabatic thermoclines.     

Performance of a solar crop dryer under PNG climatic conditions

A small solar crop dryer, consisting of a drying unit, thermal storage and solar collector, was designed for the climatic conditions of Papua New Guinea, and was constructed and tested at the Energy Research site of the University of Papua New Guinea. Detailed experimental studies were carried out for drying of tapioca, as well as the testing of the drying unit with and without thermal storage. Based on economic analysis, it is observed that the annual cost per kilogram of dried tapioca is about K0.27 ≠.     

Effect of louver shading devices on building energy requirements

External louvers are increasingly used to provide solar protection for building glazed surfaces. In this work, a general study of the effect of louver shading devices applied to different façades of a building is carried out, for different locations (latitudes). Building energy requirements for a building in the cooling and heating seasons is quantified for different window and louver areas, under climatic conditions of Mexico (Mexico), Cairo (Egypt), Lisbon (Portugal), Madrid (Spain) and London (UK). Also, operative and indoor temperatures were calculated through simulations using TRNSYS software, whereas the model for the shading geometry study was solved with EES software. Both horizontal and vertical louver layouts were considered. The results show that the integration of louver shading devices in the building leads to indoor comfortable thermal conditions and may lead to significant energy savings, by comparison to a building without shading devices.     

Experimental investigation of a solar energy heating system under the climatic conditions of Edirne

Seasonal storage of solar energy to supply the heat requirement of buildings in Edirne (41°39′54″N) has been examined experimentally. Solar energy has been stored in a cylindrical underground storage unit. Measurement values have been recorded per hour by means of a computerized recorder between July 2005 and May 2006. Monthly average temperature values of the heat storage unit and the surrounding ground have been calculated through the measurement results. The transient heat transfer which takes place between the heat storage unit and the surrounding ground has been calculated by means of the QuickField finite-element analysis program. It has been determined that the most significant deviations between the theoretical and the experimental temperature values turn out to be in question during the heating period. The annual solar fraction of the solar energy heating system has been determined as 53% for space heating and 85% for domestic water heating.     

STUDIES OF THE THERMALPERFORMANCE OF BUILDINGSIN SUMMER IN SOUTHERN EUROPE

This paper describes the studies carried out in project PASCOOL to document and assess the thermal performance of selected buildings in Southern Europe and to identify possible improvements. Twenty-three buildings from nine European countries were surveyed and monitored for a short period during summertime. The measurements and survey data obtained formed the basis for sensitivity studies with thermal simulation. The selection encompassed buildings of typical construction, including both recent and traditional, residential and non-residential. The sensitivity studies addressed the effects of shading, thermal inertia, ventilation, window orientation and colour of external surfaces, envelope insulation, and occupancy patterns. The paper presents the methodology adopted for the measurements and sensitivity studies and discusses the main conclusions drawn from the studies- It identifies options for improving occupant thermal comfort with little or no mechanical cooling.     

Optimizing the energy and exergy of building integrated photovoltaic thermal (BIPVT) systems under cold climatic conditions

Building integrated photovoltaic thermal (BIPVT) system has the potential to become a major source of renewable energy in the urban environment. In this paper, the system has been used as the roof top of a building to generate higher electrical energy per unit area and to produce necessary thermal energy required for space heating. One-dimensional transient model has been developed using basic heat transfer equations. On the basis of this model, an analysis has been carried in order to select an appropriate BIPVT system suitable for the cold climatic conditions of India. The PV performances, net energy gain and exergy of the building are determined. The results show that for a constant mass flow rate of air the system connected in series gives a better performance whereas for a constant velocity of air flow the system connected in parallel gives a better performance. The BIPVT system, fitted on the rooftop in an effective area of 65 m², is capable of annually producing the net electrical and thermal exergies of 16,209 kW h and 1531 kW h, respectively, at an overall thermal efficiency of 53.7%.     

一种阴影情况下光伏组件输出特性的计算方法

提出了一种计算阴影遮挡情况下组件输出特性的方法。该方法首先根据流经组件的电流对被遮挡电池及其所在电池串的输出特性进行分析,在此基础上对旁路二极管的伏安特性进行理论分析,进而判定旁路二极管导通状态,从而计算出光伏组件在阴影遮挡情况下的多峰特性。经试验验证,此种方法可精确地模拟复杂遮挡情况下光伏组件的输出特性,对于各种阴影遮挡情况下的峰值点的最大误差在3%以内。该方法较传统的失配情况下基于一个电池单元并联一个保护旁路二极管的计算方法更符合实际,具有较强的实用价值。     

Thermal behavior of a small salinity-gradient solar pond with wall shading effect

The thermal behavior of a small-scale salinity-gradient solar pond has been studied in this paper. The model of heat conduction equation for the non-convective zone has been solved numerically with the boundary conditions of the upper and lower convective zones. The variation of the solar radiation, during a year, and its attenuation in the depth of the pond has been discussed. The wall shading area for a vertical wall square pond has been elaborated and its effect on the reduction of the sunny area has been included in the model. The temperature variation of the storage zone has been calculated theoretically and compared with the experimental results. The sensitivity analysis demonstrates the importance of the side and bottom insulation and the thickness of the non-convective zone, as well as the wall shading effect on the performance of the pond. The application of several loading patterns gives an overall efficiency of 10% for the small pond.     

A general procedure for the small-scale modelling of buildings

A procedure for the development of physically similar small-scale model buildings, based on dimensional analysis, is outlined. Theoretical considerations are discussed, and the example of a scale of model, based on an existing prototype test building in Nigeria, is presented. The model will be built and tested at the University of Trieste in Italy. Test results should assist in verifying and improving the modelling technique.     

A numerical method to estimate time-varying values of diffuse irradiance on surfaces in complex geometrical environments

The estimation of diffuse irradiance that impinges on a window or photovoltaic panel is of major importance for the determination of their performance. The present paper introduces the concept of angular shading factors (ASF) for the calculation of time-varying diffuse irradiances to be performed without the need to repeat time consuming intereflection calculations at every time step. For the calculation of ASF, a Monte Carlo backward ray tracing technique was used. As a result, geometrically complex scenes consisting of surfaces with diffuse and specular reflectances, while transparent and translucent surfaces can be modeled as well. After that, the calculation of diffuse irradiance can be performed in a dynamic way, by taking into consideration the sky radiance distribution variability.     

聚焦太阳能建筑外观的能源论坛

第七届太阳能建筑外观能源论坛于2012年12月6～7日在意大利布列瑟农举行。本届论坛旨在为建筑师、工程师、科学家、能源管理者和光伏设备制造商创造对话机会,提供一个国际交流的平台,研究讨论出一种多学科的综合规划办法,在改善居住者舒适性和身心健康的同时减少能源消耗。能源论坛总经理Mr.Andreas Karweger在开幕式上致辞。     

Application and development of solar energy in building industry and its prospects in China

China is the second largest country in energy consumption. More and more energy demand pressures cause the Chinese government to review its economy and energy policies in order to support the sustainable development. In China, the building sector amounts to 27.8% total energy consumption, which is only behind the industry sector. China has abundant solar energy resource, which is extensively applied to buildings. Therefore, solar energy utilization in buildings has become one of the most important issues to help China optimize the energy proportion, increasing energy efficiency and protecting the environment. Solar energy resource and its district distribution in China are introduced in detail in this paper, and the representative solar energy application to the building sector is highlighted as well. The solar energy utilization obstacles, especially policy disadvantages in building sector in China, are reviewed. Moreover, the application prospects of solar energy in building sector are presented in combination with the China economic and household industry growth.     

Evaluation of a solar thermal system using building louvre shading devices

External louvres are increasingly used to provide solar protection for building glazed surfaces. The integration of collectors into the external louvres of buildings offers a means of reducing system cost as well as providing architects with more freedom to integrate the technology into their designs. This work concerns the modification of existing louvre designs to integrate a solar collector in the shading device. The evaluation of a thermal solar system for water heating is assessed in this paper. A numerical model for the integrated solar collector was developed for different configurations and the collector efficiency is quantified for each configuration. System thermal performance was obtained for the climatic conditions of Lisbon (Portugal) and Tenerife (Spain). Economic and environmental viability of the system is assessed.