A photographic method to estimate the shading effect of obstructions

A new, simplified photographic method is proposed to estimate the shading effects of obstructions. In spite of the complex mathematical background of the new tool, its practical application is very simple, and only requires the measurements of three angles for each photograph. The proposed method is particularly useful for overcoming the difficulties caused by the need to accurately describe surrounding objects to estimate shading effects. The new method uses the information contained in a picture and allows one to draw daily solar paths directly on the photograph. The procedure is useful to verify the suitability of a generic site for solar exploitation; its main benefit is the simplicity of use and the transparency of the obtained results. This simple method is also particularly suitable to evaluate the technical feasibility of small solar equipment, which would not require complex software to define the daily solar path.     

An improved photographic method to estimate the shading effect of obstructions

A new photographic method is presented to evaluate the shading effects of obstructions on surfaces exposed to the sun. The method overcomes the difficulties caused by the need to accurately describe the surrounding objects to estimate the shading effects by means of the usual tools that use the spatial reconstruction of obstructions or cylindrical or polar suncharts. The photographs of the surrounding objects are used as the background on which the solar disc is depicted at the various hours of the day. In this way it is easily detectable if the sun is visible from the place where the photographs were taken or if the surrounding obstructions obscure the sun.In spite of the complex mathematical background of the new method, the practical application of the procedure is very simple, and only requires the measurements of three angles for each photograph. The procedure permits to verify the suitability of a generic site for solar exploitation; its main benefit is the simplicity of use and the transparency of the obtained results. This method is particularly useful to evaluate the technical feasibility of small solar systems installed on the buildings of densely urbanised cities.The accuracy of the method was tested by performing an experimental verification in the field. For this purpose, the sun was photographed at different hours of the day. The photographed solar discs and the calculated sun’s positions were compared. The differences between the photographed and calculated sun’s positions corresponded to small time lags that do not exceed few minutes in the worst case. To further investigate the reliability of the proposed method, the impact of image distortion, which always affects all methods that use cameras to get information about the photographed reality, was also examined.     

Coupling satellite images with surface measurements of bright sunshine hours to estimate daily solar irradiation on horizontal surface

Satellite images are heavily used for the estimation of solar irradiation at the Earth's surface. The accuracy yet should be improved to attain more reliable input values for the use of all types of solar energy systems. This paper presents two new alternative approaches to increase the estimation accuracy of daily solar irradiation by coupling the satellite images with surface bright sunshine hour measurements. Two different approaches are described for the estimation of global solar irradiation on daily base, by using the data for some locations in Turkey and Germany. These approaches are compared with the estimation of a satellite model (HELIOSAT), Angstrom models and ground measured daily global solar irradiation by using regressions and error analyses. For nine out of ten stations the relative RMSE values of the proposed models slightly decrease in the range of only 2% in comparison with the direct satellite model for the daily global solar irradiation. The results obtained for the new approaches did not considerably improve the performance of the satellite model. However, it is possible to recommend new coupled approaches to estimate daily global solar irradiation because of their simpler calculation procedure. The results are encouraging for the future works to use long and short-term satellite image data together with the surface measured data to estimate the solar irradiation values.     

Using the competitive storage model to estimate the impact of ethanol and fueling investment on corn prices

The impact of biofuel mandates on the prices of commodities used to produce food continues to be a major consideration by policy makers. More recently concern about high compliance costs of oil companies due to mandates led to reductions in US biofuel mandates. To gain insights into the policy impacts of mandates on agricultural commodity prices and compliance costs requires development of dynamic models that can capture the fact that many agricultural commodities used to produce biofuels can be stored and that flexible compliance mechanisms allow for banking and borrowing of tradable permits. US biofuel mandates are enforced using a system of tradable permits, called RINs. RIN market dynamics are important because RINs can be banked for the future or borrowed from the future. Corn is the primary U.S. biofuel feedstock and is storable. Rational expectations competitive storage models are well suited to capture the dynamic behavior of commodity markets. Such a model is developed here for corn and RIN markets to estimate the impacts of alternative future ethanol mandate levels. The model considers corn use for ethanol, storage and all other uses in each period, accounting for two random variables: oil prices and corn yields. Borrowing and banking provisions of the Renewable Fuels Standard mandate are explicitly integrated into the model. We use the model to provide estimates of the impact on corn prices, corn plantings and ethanol production under two ethanol mandate scenarios for six marketing years from 2016/17. Our scenarios are a combination of volume requirements and infrastructure investment. The first scenario is one in which corn ethanol mandates stay the same as required in the current and proposed RFS and additional E85 stations are introduced that allow for compliance with higher mandates. The second scenario is one in which no investment occurs and the Environmental Protection Agency reduces the mandate to 13.87 billion gal, a level that can just be met with 10 percent ethanol blends. Comparing the results for two scenarios, we find that average corn prices are about 5 to 6% lower with the reduced mandates or about 22 to 30 cents per bushel, while average RIN prices drop from around 60 to 40 cents per RIN.     

Convective heat transfer on leeward building walls in an urban environment: Measurements in an outdoor scale model

Convection over the building envelope is a critical determinant of building cooling load, but parameterization of convection in building energy models and urban computational fluid dynamics models is challenging. An experimental investigation intended to clarify the heat transfer mechanism of a convective wall boundary layer (WBL) on a leeward, vertical building wall was conducted at the Comprehensive Outdoor Scale Model (COSMO) facility for urban atmospheric research. Comparison of mean and turbulent temperature fluctuation intensity profiles showed that the dominant regime of the WBL flow was turbulent natural convection. Implications for parameterization of convective heat fluxes in urban areas are discussed.     

Theoretical model to estimate the thermal performance of an evaporative wind tower placed in an open space

A theoretical model to evaluate the thermal performance of an evaporative wind tower installed in open spaces with hot and dry climates has been developed. It was based on the laws of conservation of mass and energy and used TRNSYS as a simulation tool. Evaporative wind towers produce an adiabatic cooling which has been modelled taking into account all the heat and mass exchanges between the airflow and the injected water, and also considering the processes of radiation, convection and conduction. The system analyzed has a special design based on an existing installation placed in Madrid, which is composed of sixteen evaporative wind towers with one fan and six nozzles on the top of each one. A first validation of this theoretical model was done by comparing calculated results obtained through numerical simulation with experimental data. These last data were previously registered in a campaign carried out during the summer 2008 to evaluate the thermal behaviour of the system. To contrast both results, the same initial assumptions in fan and water operation as well as environmental conditions were considered. The comparison between them during the period of 18th to 20th July 2008, show an average temperature drop of 6.5 °C and an average increase of relative humidity of 27%. These values present a high correlation, up to 0.79, between experimental and calculated wet bulb depression. The average cooling power achieved by this system varies from 13 to 16 kW, with maximum peaks around 20 kW. So this theoretical model could be used for future energy estimations of wind towers design with similar constructive characteristics.     

Using advanced cool materials in the urban built environment to mitigate heat islands and improve thermal comfort conditions

The present paper is a review article aiming to present the actual state of the art on the development and the assessment of cool materials (i.e. materials with high solar reflectance and infrared emittance) for buildings and urban structures. The research in this field is roughly divided in four phases and includes the development and assessment of: (a) highly reflective and emissive light colored materials, (b) cool colored materials, i.e. colored materials with increased near infrared and thus overall solar reflectance compared to similarly colored conventional ones, (c) phase change materials and (d) dynamic cool materials. All major aspects related to each technology are described and the benefits and impacts of these materials at building, city and global scale are reported. The wide use of such materials can significantly contribute to the mitigation of the heat island effect and the improvement of urban environmental quality.     

Building cluster and shading in urban canyon for hot dry climate: Part 1: Air and surface temperature measurements

Under low latitude conditions, minimization of solar radiation within the urban environment may often be a desirable criterion in urban design. The dominance of the direct component of the global solar irradiance under clear high sun conditions requires that the street solar access must be small. It is well known that the size and proportion of open spaces has a great influence on the urban microclimateThis paper is directed towards finding the interaction between urban canyon geometry and incident solar radiation. The effect of building height and street width on the shading of the street surfaces and ground for different orientations have been examined and evaluated. It is aimed to explore the extent to which these parameters affect the temperature in the street. This work is based on air and surface temperature measurements taken in different urban street canyons in EL-Oued City (hot and arid climate), Algeria.In general, the results show that there are less air temperature variations compared to the surface temperature which really depends on the street geometry and sky view factor. In other words, there is a big correlation between the street geometry, sky view factor and surface temperatures.     

The effect of urban layout,street geometry and orientation on shading conditions in urban canyons in the Mediterranean

The paper presents the results of shading analysis which was carried out as part of a wider comparative analysis of two sites with different characteristics in terms of street geometry and urban density. The first experiment site was a traditional settlement in the island of Tinos, Greece, and the second was a relatively newly built part of the capital city of the island. Also a parametric shading analysis was carried out in order to examine a number of parameters that influence shading conditions in urban canyons.The paper aims in analyzing the effect of parameters such as urban layout, street geometry and orientation on solar access and shading conditions, which strongly affect urban canyon microclimate. The results of shading simulations are compared to the results of experimental measurements of air and surface temperatures and to parametric thermal analysis results. The conclusions can contribute in the formulation of urban design guidelines.     

Using an equation based on flow stress to estimate structural integrity of annealed Type 304 stainless steel plate and pipes containing surface defects

An accurate assessment of the influence of defects on structural component integrity is needed. Generally accepted analytical techniques are not available for the very ductile materials used in many nuclear reactor components. This paper presents some results from a test programme to obtain data by which to evaluate proposed models. Plate and pipe specimens containing surface flaws were fabricated from annealed Type 304 stainless steel and tested at room temperature. An evaluation of an empirical equation based on flow stress is presented. In essentially all instances the flow stress is not a constant but varies as a function of the size of the surface flaw.     

Microclimatic modeling of the urban thermal environment of Singapore to mitigate urban heat island

This study investigates the urban heat island effect in Singapore and examines the key factors causing this effect. The possibilities of improving heat extraction rate by optimizing air flow in selected hot spots were explored. The effect of building geometry, façade materials and the location of air-conditioning condensers on the outdoor air temperature was explored using computational fluid dynamics (CFD) simulations. It was found that at very low wind speeds, the effect of façade materials and their colours was very significant and the temperature at the middle of a narrow canyon increased up to 2.5 °C with the façade material having lower albedo. It was also found that strategically placing a few high-rise towers will enhance the air flow inside the canyon thereby reducing the air temperature. Adopting an optimum H/W ratio for the canyons increased the velocity by up to 35% and reduced the corresponding temperature by up to 0.7 °C.     

Simulating the thermal behaviour of a building during summer period in the urban environment

In this study an attempt is made to investigate the impact of the urban heat island (UHI) effect on the energy consumption of a building using both, an accurate, extensively validated, transient simulation model and a neural network one. The energy consumption is calculated for a typical building during the summer period at 20 different sites of the Greater Athens area with the aid of the TRNSYS model. It was found that the UHI affects significantly the thermal behaviour of the building, implying much lower cooling load cost and energy cost in the suburban areas. A neural network model was then appropriately designed and tested for the estimation of the energy consumption, employing as an input, the UHI intensity. The results of both methods were tested and compared and it was found that there is a good agreement between the cooling rate values calculated by the TRNSYS programme and those derived by the neural model. Moreover, the employment of the neural model helped in quantifying the contribution of each input parameter in the calculation of the cooling rate, and it was demonstrated that the UHI effect is a predominant parameter, affecting considerably the energy consumption of a building in the Mediterranean region.     

Airflow in courtyard and atrium buildings in the urban environment: a wind tunnel study

A wind tunnel study was carried out to investigate the airflow through courtyard and atrium building models located within an urban setting and exposed to an urban atmospheric boundary layer. Ventilation strategies resulting from the use of different courtyard and atrium pressure regimes (positive pressure and suction) were examined. The model buildings were monitored both in isolation and in idealised urban environments of varying group layout densities. The effect of wind direction was also observed. The results from the study suggest that the open courtyard in an urban environment had a poor ventilation performance whilst an atrium roof with many openings operating under a negative (suction) pressure regime was the most effective. Changing the wind direction from perpendicular to the building façades to a 45° incidence angle had the effect of making the differences in the observed flows between all the models much smaller.     

Daylight, sunlight and solar gain in the urban environment

In cities, site layout has a substantial impact on the daylight, sunlight and solar radiation received by windows. This paper discusses ways to ensure solar access in obstructed situations, both within new developments and in existing buildings nearby.     

Relationships between the properties of an urban street canyon and its radiant environment: Introduction of appropriate urban heat island mitigation technologies

The relationships between the properties of urban canopy components and the radiant environment in an urban street canyon are examined considering the introduction of appropriate urban heat island mitigation technologies. Radiant heat transfers between walls and roads are calculated according to Gebhart’s radiant absorption coefficients and using the Monte Carlo method. Roads are classified as either north–south or east–west; intersections are also considered. The key property of an urban street canyon is expressed by its aspect ratio W/H. A simple street canyon model and two actual urban street canyon areas are used as the objects of examination. Distributions of surface temperatures and solar radiation gains on street canyon roofs, roads, and walls are analyzed. The top priority for the implementation of urban heat island mitigation measures concerns the buildings with large roof areas. The other high-priority areas for implementing mitigation measures focus on smaller roofs and roads for which the street canyon aspect ratio W/H is greater than 1.5; the lowest-priority area is the walls.     

某城市商业综合体公共空间热环境数值模拟

用数值模拟方法,对北京某城市商业综合体内部公共空间空调工况热环境进行了研究,分析了各层不同送风比例下,综合体内部公共空间纵截面温度分布情况和人员活动区域内的温度分布情况。通过比较,确定了满足综合体内部公共空间各层热舒适要求的空调送风方案。数值模拟结果表明,对于城市商业综合体内部公共空间采用各层均匀送风,顶层与公共空间相连的走廊区域温度在夏季不能满足热舒适要求,适当减少内部公共空间下层的送回风量,增加至顶层,在冬夏工况下均可有效改善内部公共空间热环境,增加舒适度,有效解决顶层走廊温度偏高及温度不均等问题。     

Soret and Dufour effects for three-dimensional flow in a viscoelastic fluid over a stretching surface

This article deals with the Soret and Dufour effects on three-dimensional boundary layer flow of viscoelastic fluid over a stretching surface. The governing partial differential equations are transformed into a dimensionless coupled system of non-linear ordinary differential equations and then solved analytically by the homotopy analysis method (HAM). Graphs are plotted to analyze the variation of different parameters of interest on the velocity, concentration and temperature fields.     

A comparison of methods to estimate daily global solar irradiation from other climatic variables on the Canadian prairies

Historic estimates of daily global solar irradiation are often required for climatic impact studies. Regression equations with daily global solar irradiation, H, as the dependent variable and other climatic variables as the independent variables provide a practical way to estimate H at locations where it is not measured. They may also have potential to estimate H before 1953, the year of the first routine H measurements in Canada. This study compares several regression equations for calculating H on the Canadian prairies.Simple linear regression with daily bright sunshine duration as the dependent variable accounted for 90% of the variation of H in summer and 75% of the variation of H in winter. Linear regression with the daily air temperature range as the dependent variable accounted for 45% of the variation of H in summer and only 6% of the variation of H in winter. Linear regression with precipitation status (wet or dry) as the dependent variable accounted for only 35% of the summer-time variation in H, but stratifying other regression analyses into wet and dry days reduced their root-mean-squared errors. For periods with sufficiently dense bright sunshine observations (i.e. after 1960), however, H was more accurately estimated from spatially interpolated bright sunshine duration than from locally observed air temperature range or precipitation status.The daily air temperature range and precipitation status may have utility for estimating H for periods before 1953, when they are the only widely available climatic data on the Canadian prairies. Between 1953 and 1989, a period of large climatic variation, the regression coefficients did not vary significantly between contrasting years with cool-wet, intermediate and warm-dry summers. They should apply equally well earlier in the century.