Solar irradiance on flat-plate collectors in urban environments

A numerical climatic model has been developed in order to simulate the solar irradiance that would be available to a flat-plate collector sited at street level within a typical urban setting. Results confirm that the available irradiance can be substantially reduced by the presence of horizonal obstructions. It is additionally evident that the optimal collector azimuth may vary by as much as 60° from due south—in the mid-latitude northern hemisphere. Losses of available irradiance, associated with the popular practice of facing collectors due south and tilting them as a simple function of latitude, appear to be as high as 10–15% when compared to the total solar irradiance available at an optimal collector orientation as determined by simulation.     

基于Airpak的锻铸车间环境综合治理研究

锻铸车间气温高,热辐射强度大,易形成干热环境,对周围工作区作业的工人产生了危害。以西安某锻铸车间为例,利用CFD软件Airpak对隔绝热源、局部通风这2种改善车间环境方式进行模拟研究。将得出的辐射温度分布图与风速分布图,进行对比分析,总结出2种方案的优缺点,为今后的锻铸车间的优化设计以及气流组织设计等方面提供借鉴与指导。     

Intercomparison of irradiance measurements based on WRR and ETL irradiance scales

We report the corrected intercomparison of the World Radiometer Reference (WRR) irradiance scale and the Electrotechnical Laboratory (ETL) spectral irradiance scale. In addition, we confirm the intercomparison precision using the test facility where the irradiance of ETL 500 W standard lamp can be measured directly with the cavity radiometer. The results showed that the irradiance based on the WRR scale was 0.5–0.7% lower than the one based on the ETL scale.     

Solar radiation modelling in the urban context

This paper describes alternative methods for predicting surface irradiance in the urban context. In this the focus is on means of accounting for the effects of nearby obstructions on reducing direct sky radiation and on contributing reflected radiation. The first two methods involve abstracting the urban skyline into an effective canyon using isotropic and anisotropic tilted surface irradiance models. The third predicts the irradiance contribution from two hemispheres which are discretised into patches––given the radiance of the sky and dominant obstructions (if these exist) and associated view factors––so that we have a new simplified radiosity algorithm (SRA). Results from the three methods (isotropic canyon (IC), anisotropic canyon (AC) and simplified radiosity algorithm (SRA)) are compared with a ‘truth model' under the following circumstances: (i) unobstructed sky, (ii) sky obstructed by black surfaces, (iii) sky obstructed by grey diffusely reflecting surfaces. Results show conclusively that the SRA offers superior accuracy at comparable speed to the canyon models. The SRA also compares well with a ray tracing program, it can handle urban scenes of arbitrary geometric complexity and is readily amenable for inclusion into standard computer programs that require surface irradiance as an input.     

K-factor data of interacting duct fittings based on measurement and CFD modelling

Energy use and the environment in mechanically-ventilated buildings are strongly influenced by the performance of heating, ventilation and air-conditioning (HVAC) systems, which is in turn governed by the accurate prediction of pressure loss. This paper presents the results of an investigation of pressure loss and associated loss coefficient (k-factor) for a number of interacting duct fittings in close proximity as commonly found in HVAC systems. The constant-injection tracer-gas technique and pitot-tube were used to measure the mean air velocity in ducts and the pressure distribution along the ducts was measured using static pressure tappings. k-Factors were calculated from the measured pressure loss and air velocity for each interacting duct fitting. Computational fluid dynamics (CFD) was used for prediction of air flow and pressure distribution in ducts. Predictions were compared with results obtained experimentally and CFD was found to be a useful method for the prediction of the k-factors of duct fittings. It will also be shown that duct fittings in close proximity interact in such a way that it is difficult to predict their behaviour based on evidence of the fittings' individual characteristics. The behaviour of interacting duct fittings as described in this investigation appears to be isolated to fittings of similar individual loss coefficients placed in close proximity without the effect of spacers. To establish this characteristic and fully to understand the complexity of interacting duct fittings there is an obvious need for further work in this area.     

Predicting solar irradiance on inclined surfaces using sky radiance data

The availability of more comprehensive solar irradiance data is invaluable for the reduction of cooling load in buildings and for the evaluation of the performance of photovoltaic plants. In many parts of the world, however, the basic solar irradiance data are not always readily available. This paper presents an approach to calculate the solar irradiance on sloped planes by integrating the sky radiance distribution. Sky radiance data recorded from January 1999 to December 2001 in Hong Kong were used to estimate the solar irradiance for the horizontal and four principal vertical surfaces (N, E, S and W). The performance of this approach was assessed against data measured in the same period. Statistical results showed that using sky radiance distributions to predict solar irradiance can give reasonably good agreement with measured data for both horizontal and vertical planes. The prediction approach was also employed to compute the solar irradiance of a 22.3° (latitude angle of Hong Kong) inclined south oriented surface. The findings indicated that the method can provide an accurate alternative to determine the amount of solar irradiance on inclined surfaces facing various orientations when sky radiance data are available.     

Computation of the solar irradiance incident on concentrating collectors based in Turkey

This study aims to provide thorough information on the solar radiation received by the five main design types of concentrating collectors for the conditions of Turkey. These are namely, compound parabolic concentrator with north-south axis and east-west axis alignment, parabolic trough with north-south axis and east-west axis tracking, and concentrator with two-axis tracking. Either daily or hourly means of solar radiation are calculated for different slope, orientation and area concentration ratio (or half-acceptance angle). In this study, a computer program, based on previously developed correlations, is used. Through the graphical display, the results are presented for the six climatologically different stations which are representative of the country's conditions. With some modifications, the results will be valid for almost all known designs of concentrating collectors.     

Stochastic modelling of solar irradiance on horizontal and vertical planes at a northerly location

Sunshine levels incident in the plane of a photovoltaic panel are the overriding influence on electrical output, and modelling solar irradiance is therefore an essential step in the design and performance prediction of solar energy conversion systems. This study aims to assess the efficacy of SARIMA models and their potential for short-term prediction at a northerly latitude. Data was collected from a monitoring site on the roof of a 5-storey building at a city centre location in Newcastle upon Tyne, UK (latitude 55°N). Hourly and ten minute data relating to 13 and 15 day periods in two winters (1993, 1994) and two summers (1994, 1995) were utilised. Univariate stochastic modelling, using SARIMA models, is carried out for horizontal and south facing vertical solar irradiance. Results showed that these models provided a good fit for the ten minute averaged horizontal and vertical irradiance, with, on average, 82% and 85% of total variation being accounted for respectively. Use of hourly averaged data in these models gave a substantial reduction in the fit. Models for the winter data were a poorer fit than for summer for both orientations. It is concluded that the SARIMA approach can be used to develop prediction methods and to study rapid and large changes in PV output from extensive areas of solar cladding.     

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.     

The role of aerosol models of the SMARTS code in predicting the spectral direct-beam irradiance in an urban area

Measurements of the ground-level spectral distribution of the direct-beam solar irradiance between 300 and 1000 nm were made in Athens, Greece, in May 1995. Results obtained using simple model for the atmospheric radiative transfer of sunshine (SMARTS) (version 2.9.2) parametric model for the urban atmosphere of Athens are analyzed and compared to the ground-level experimental spectral solar irradiance measurements obtained by the passive pyrheliometric scanner (PPS) in three discrete bands, UV (320–400 nm), VIS (400–700 nm) and NIR (700–1000 nm). The study uses two different input parameters for the aerosol characterization: the aerosol optical depth at 500 nm, t_α0.5, and the Ångström turbidity coefficient, β. The results clearly show that the nine aerosol models implemented in the SMARTS code lead to quite different predictions of the direct-beam spectrum, strongly depended on the input parameter. In all cases the inadequacies between the measured and the modeled direct-beam spectra are lower and higher when the urban and maritime aerosol models are used, respectively.     

Influence of air conditioners utilization on urban thermal environment

The urban heat island effect is essentially a kind of thermal pollution caused by artificial activity. Air conditioning units used in buildings release a great deal of heat into outdoors while they improve the indoor thermal environment. Therefore, it will cause a rise in the outdoor air temperature and worsen the urban thermal environment. A “box” model is presented in this paper; it is used to quantitatively determine the rise in outdoor air temperature caused by using domestic air conditioners in Wuhan. The results demonstrate the rise degree is 2.56 °C under inversion conditions and 0.2 °C under normal conditions, which indicates that thermal pollution is serious at stable atmosphere. To a certain extent, the rise of atmospheric temperature can be verified by the rise of lowest atmospheric temperature in Wuhan over recent years.     

一种基于经济、社会、环境可持续发展的城市群低碳优化模型

结合"十二五"某城市群区域规划,选取规划中提出的碳强度、能耗强度两项指标为约束条件,构建了一种基于经济、社会、环境可持续发展的城市群低碳优化模型。优化结果表明:在满足碳强度、碳汇面积、能耗强度、GDP等条件下,城市群"十二五"末期碳排放总量为3.70×107t,较2010年区域碳强度下降了35.56%,能耗强度下降了40.49%,城市群"核心区"碳强度下降了36.77%,能耗强度下降了37.99%,不仅满足国家《"十二五"控制温室气体排放工作方案》中提出2015年的碳强度、能耗强度较2010年分别降低17.00%和16.00%的要求,而且满足该城市群区域规划中提出2015年的碳强度、能耗强度较2010年分别降低18.00%和20.00%的目标,该优化方案为城市群的低碳发展提供了一定的决策空间。     

An analytical comparison of four approaches to modelling the daily variability of solar irradiance using meteorological records

Temporal solar variability significantly affects the integration of solar power systems into the grid. It is thus essential to predict temporal solar variability, particularly given the increasing popularity of solar power generation globally. In this paper, the daily variability of solar irradiance at four sites across Australia is quantified using observed time series of global horizontal irradiance for 2003–2012. It is shown that the daily variability strongly depends on sky clearness with generally low values under a clear or overcast condition and high values under an intermittent cloudiness condition. Various statistical techniques are adopted to model the daily variability using meteorological variables selected from the ERA-Interim reanalysis as predictors. The nonlinear regression technique (i.e. random forest) is demonstrated to perform the best while the performance of the simple analog method is only slightly worse. Among the four sites, Alice Springs has the lowest daily variability index on average and Rockhampton has the highest daily variability index on average. The modelling results of the four sites produced by random forest have a correlation coefficient of above 0.7 and a median relative error around 40%. While the approach of statistical downscaling from a large spatial domain has been applied for other problems, it is shown in this study that it generally suffices to use only the predictors at a single near point for the problem of solar variability. The relative importance of the involved meteorological variables and the effects of clearness on the modelling of the daily variability are also explored.     

Prediction of daily global solar irradiance on horizontal surfaces based on neural-network techniques

In this study, a prediction model of global solar irradiance distribution on horizontal surfaces has been developed. The methodology is based on neural-network techniques and has been applied to the meteorological database of NTUA, Zografou Campus, Athens (37°58′26″N, 23°47′16″E). The investigation of the correlation between weather conditions, duration of daylight and the representative peak value of a Gaussian-type function plays an essential role in the development of the model. The weather conditions are categorized into six different states, whereas the daylight duration is obtained by familiar equations. Thereafter, a correction methodology for the Gaussian-type function—which stands for all six different states—is applied. Finally, the reliability of the developed model is investigated through a suitable validation procedure.     

Angstrom turbidity and ozone column estimations from spectral solar irradiance in a semi-desertic environment in Spain

Angstrom parameters are commonly used for modelling the solar radiation extinction by aerosols. Detailed knowledge of these parameters in as many places as possible is desirable since many applications need this kind of information: satellite products assessment, clear sky models benchmarking, spectral modelling, etc. Experimental determinations of the Angstrom turbidity and wavelength exponent have been made for a semi-desertic area in the south-east of Spain using a double monochromator spectroradiometer. Experimental measurements of direct-beam spectral irradiance have been made about every 20 min during one year (from october 2005 to september 2006). Angstrom turbidity has a noticeable variability during the year, showing a seasonal characteristic behaviour. Low turbidity values were found during autumn and winter and quite large values can be observed in spring and summer days. The eventual values of high turbidity with low wavelength exponent found could indicate Saharan dust outbreaks events. In addition, total ozone column have been experimentally estimated from spectral measurements in the Huggins absorption band. The results showed a general agreement with the values extracted from the TOMS web site.     

Solar irradiance & daylight illuminance data for the United Kingdom and Japan

The Commission International de'Eclairage (CIE) declared the year 1991 as the International Daylight Measurement Year (IDMY). This activity was later named the International Daylight Measurement Programme (IDMP). In the UK as a result of the CIE call, solar horizontal and vertical illuminance and irradiance measurements have been carried out for four sites, i.e. - Garston, Manchester, Sheffield and Edinburgh. Furthermore, data for sky luminance distribution have been measured, via scanners, at Garston and Sheffield. These data are available on a CD-ROM. Information related to the recording frequencies, volume and format are described in the present report.     

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.     

Some data on the reliability of pressure equipment in the chemical plant environment

The results of a survey of the failure rates of pressure equipment in a single chemical works are reported. Pressure vessels included in the survey are process pressure vessels, pressure storage vessels and heat exchangers. For these items the overall failure rate for failure as defined in the paper was $\text{4·3 × 10}{}^{\mathrm{?3}}$ failures a year with an upper bound at the 99% confidence level of $\text{6·3 × 10}{}^{\mathrm{?3}}$ failures a year. No disruptive failures were recorded and for disruptive failures the upper bound at the 99% confidence level was $\text{2·8 × 10}{}^{\mathrm{?4}}$ failures a year. Other items covered in the survey include non-pressure storage vessels and fired heaters. The data are also analysed to determine the effect of operating conditions such as high and low temperature and corrosive environments.     

Assessment and recalibration of the Heliosat-2 method in global horizontal irradiance modeling over the desert environment of the UAE

Remote estimation of global horizontal irradiance (GHI) by Heliosat-2 model has been benchmarked against ground-based measurements in different locations. The obtained results have shown that the level of agreement between ground-based and model-based GHI values are location dependent. To our knowledge no similar studies have been carried out over the Arabian Peninsula. The unique climatic condition in the Peninsula, characterized by high concentrations of airborne dust particles and high humidity makes the region a particularly interesting case.In this study Heliosat-2 method was used to derive the ground surface GHI in the United Arab Emirates. Inputs to the model were monthly Linke turbidity factor normalized to an air mass of 2 and a cloud index derived from SEVIRI sensor onboard the European satellite Meteosat. The Linke turbidity factor was obtained from a ground network of seven stations distributed across the UAE. A SEVIRI-based technique was developed and used to derive cloud index from high-resolution visible channels. Ground surface GHI measurements were collected from four inland stations for a period ranging from mid-2007 to mid-2010. The obtained results show that the Heliosat-2 model underestimates the GHI. The obtained root mean square error (RMSE) and mean bias error (MBE) values ranged from 16.3% to 18.5% and ?13.6% to ?15.8%, respectively. A constant bias was observed between modeled and measured GHI throughout the four stations. To correct this bias, the empirical equation used in Heliosat-2 to estimate the clear sky diffuse horizontal irradiance (DHI_clear) was recalibrated. With the new DHI_clear empirical equation, the modified Heliosat-2 model becomes more adapted to desert and dusty environments such as that of the UAE. By applying the modified DHI_clear equation, the RMSE and MBE values dropped to 9.5–10.3% and ?1.2 to +0.8%, respectively.     

Using reduced data sets ISCCP-B2 from the Meteosat satellites to assess surface solar irradiance

This paper explores the capabilities of a combination of the reduced data set ISCCP-B2 from the Meteosat satellites and the recently developed method Heliosat-2 to assess the daily mean of the surface solar irradiance at any geographical site in Europe and Africa. Firstly, we discuss the implementation of the method Heliosat-2. Secondly, B2-derived irradiances are compared to coincident measurements made in meteorological networks for 90 stations from 1994 to 1997. Bias is less than 1 W m⁻² for the whole set. Larger bias may be observed at individual sites, ranging from −15 to +32 W m⁻². For the whole set, the root mean square difference is 35 W m⁻² (17%) for daily mean irradiance and 25 W m⁻² (12%) for monthly mean irradiance. These accuracies are close to those of similar data sets of irradiance, such as Medias and NASA Surface Radiation Budget. It is concluded that B2 data can be used in a reliable way to produce long-term time-series of irradiance for Europe, Africa and the Atlantic Ocean.