Stability of Cu(In,Ga)Se2 solar cells and evaluation by C–V characteristics

To confirm the long-term reliability of Cu(In,Ga)Se₂, (CIGS) solar cells, we investigated the I–V and C–V characteristics during tests under irradiation or dark condition. Under irradiation, the test samples showed a little increase in efficiency (η) and open-circuit voltage (V_oc) which showed their electrical durability to light irradiation. But the diode factor (n) and series resistance (R_s) showed large changes in value. Also, the built-in voltage (V_b) and density gradient (dN_A/d_x) in the CIGS layer calculated from the C–V characteristics showed distinct changes during the test. After 4 SUN irradiation, two samples in the same fabrication-lot showed new light absorption in the lower-energy range than sun the energy gap of CIGS. We explain the change of C–V characteristics for the samples under strong irradiation with a new model named “Junction retrograde” which can treat defect generation by irradiation to reduce the acceptor density in graded p-n junction. This model for C–V analysis can be used to investigate the long-term reliability of CIGS solar cells under irradiation.     

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.     

Development of synthetic hemispheric projections suitable for assessing the sky view factor on vertical planes

The solar radiation balance in buildings has a significant impact on their energy needs, as well as on their potential BIPV energy production. It also influences the potentials of daylight, its healthiness and sustainability. Solar radiation models for urban environments require the characterization of the obstruction degree to which each point is subjected due to other buildings, topography, vegetation, etc. This characterization is carried out with the parameter known as sky view factor (SVF). In this paper, we deepen and extend the study of SVF on vertical surfaces that have a high level of obstruction. This paper aims to present a general mathematical method to obtain projection equations in vertical planes, which allow the study of SVF as a surface ratio. It is also proposed an adequate projection for vertical planes under the hypothesis of angular distribution of diffuse radiance based on Moon-Spencer’s model.     

Solar radiation for Shiraz, Iran: a comparative study for two periods

Instantaneous solar radiation data read from the chart of global solar radiation on a horizontal surface as well as sunshine hours in Shiraz during two periods (1980–1983 and 1988–1990) are analysed and compared with solar conditions in the early 1970s. It is apparent from the recorded data and subsequent analysis that while this area still possesses a relatively high abundance of sunshine, there has been a gradual decay in clearness index. Frequency of clear sky days, the average daily irradiation and the mean cumulative annual irradiation for the two periods under study are presented and a decrease in solar radiation received on a horizontal surface due to down grading of Shiraz from the clear sky conditions enjoyed in the 1960s and 1970s is demonstrated.     

基于DEM的坡地太阳总辐射估算

借鉴国内外计算坡地太阳辐射的方法,利用数字高程模型(DEM)生成了计算山地太阳辐射的地形因子(坡度、坡向、遮蔽度、各向同性可见因子),比现有的模型更准确地计算了晴空下山地太阳总辐射、直接辐射、散射辐射和周围地形的反射辐射,在此基础上分析了坡度、坡向对太阳辐射的影响。     

Analysis of a new relationship between monthly global irradiation and sunshine hours from a database of Brazil

Tests of the Suerhcke relationship have been made with a properly selected database of global irradiation and sunshine hours. A simple clear sky model, based on a generalisation of Beer's law using a unique dispersion parameter for the entire wavelength spectrum, has been used. It is shown that the Suerhcke relationship forecasts the solar radiation with a mean quadratic error of (7.5%), slightly better than the precision given by the Angstrom–Prescott model. Moreover, the Suerhcke model offers a great potential for improving the estimation process since, being based on the physics of the underlying fundamental phenomena, it can be used with more elaborated and/or more precise clear sky models. It should be remarked that this procedure, by converting the large database of sunshine hours, existing in the world, into global irradiation, might expand considerably the information about the earth solar resource at a negligible cost, when compared to the use of satellite-based procedures or earth-based pyranometers networks.     

Interfacial oxide layer mechanisms in the generation of electricity and hydrogen by solar photoelectrochemical cells

The effect of an oxide layer of tunneling dimensions, together with localized interface states at the optically-illuminated semiconductor-electrolyte interface, on the transport of electrons and holes from the semiconductor bands into the redox levels in solution is calculated analytically. The interface states are considered to affect the charge transport primarily in an electrostatic manner, by adjusting their charge with changes in the semiconductor surface potential.

A photovoltage, V, is developed in the oxidized semiconductor electrode, which divides between the space-charge region of the semiconductor V_s and the oxide layer V_ox. The majority-carrier current for a given total photovoltage depends primarily on V_s, and is therefore reduced from its value without an oxide layer. This has the effect of substantially increasing the open-circuit voltage and the photovoltaic conversion efficiency of a photo-electrochemical solar cell. The oxide layer is also expected to reduce, or in some cases to eliminate, the requirement for a supplementary external bias voltage in the production of hydrogen or other fuels from solar energy, using photosynthetic devices of this type.     

Using a shading matrix to estimate the shading factor and the irradiation in a three-dimensional model of a receiving surface in an urban environment

In an urban environment, grid-connected building integrated photovoltaic (PV) systems can be subject to complex shading patterns. The study of the shadows projected by nearby buildings and other elements around a PV surface permits cutting down energy losses due to the module’s partial shading and improving the system’s performance ratio, so that the energy production costs can be lower. This paper presents a methodology that estimates the shading factor and irradiation on a three-dimensional model of a receiving surface in an urban environment. The main innovations introduced by this methodology are the building of a shading matrix composed by direct shading factor values around the whole sky dome and the analysis of the shading impacts on direct beam, isotropic diffuse, circumsolar diffuse and horizon brightening diffuse solar radiation components. The shading matrix improves the time spent on long simulation periods and permits an easy numerical integration over the sky to obtain the diffuse shading factors. Using this feature, a plug-in to the Google SketchUp three-dimensional modeling software was built to test this methodology. A series of similar results were obtained between actual measurements and estimates conducted by the plug-in.     

Impact of street design on urban microclimate for semi arid climate (Constantine)

Urban Planning has an immense impact on local microclimate which in turn affects the comfort and space quality within a city. The urban open spaces play an important role in creating the urban climate. The urban streets vary in geometry as defined by height/width ratio, sky view factor (SVF) and the orientation that is defined by its long axis. This directly influences the absorption and emission of incoming solar and outgoing long wave radiation which has a significant impact on the temperature variations within the street as well as the surrounding environment (Urban Heat Island).The objective of this research is to evaluate and to compare how the microclimate variation of urban street canyon can affect the built environment. Therefore the main aim of this paper is to discuss and assess the impact of the geometry on the street climate, in down town of Constantine-Algeria (semi arid climate).In order to achieve this goal, a series of site measurements, are utilised. The preliminary results strongly indicate an air temperature difference of about 3–6 ^°C between the urban street and its surrounding rural environment.     

High efficiency solar energy water splitting to generate hydrogen fuel: Probing RuS2 enhancement of multiple band electrolysis

The conditions under which an oxygen photocatalyst can improve multiple band gap (semiconductor) solar energy water splitting are probed. Recently, we provided evidence that previous models significantly underestimated the magnitude of H₂ fuel which may be generated by solar energy, and demonstrated a bipolar band gap solar system electrolyzing water at V_H2O

H₂O→H₂+1/2O₂; V_H2O>E°_H2O=E°_O2−E°_H2;E°_H2O(25°C)=1.229 V

at an unprecedented 18.3% solar energy conversion efficiency. Three conditions are shown in which oxygen photocatalyst addition can further improve this process; (i) a reduction in V_H2O; (ii) at V_H2O, capability to sustain electrolysis currentsgenerated photocurrents, and (iii) catalyst activation at hν_photo-O2>hν_{photo-bipolar}. We show that RuS₂ with 1% Fe is capable of meeting these conditions.     

Solar radiation on tilted south oriented surfaces: validation of transfer-models

Three transfer-models in use for estimating solar radiation on tilted surfaces are tested. A 12 year series of hourly global, diffuse, and reflected solar irradiation measured with horizontal pyranometers as well as hourly global solar irradiation measured with tilted south oriented pyranometers is available. One model uses daily irradiation, the other two use hourly irradiation. The models converting hourly solar irradiation on a horizontal surface to a tilted surface yield better results than that using daily irradiation. The best results are gained if pairs of hourly global and diffuse solar irradiation are available. The root mean square errors exceed 10% only if the sky is covered by more than 85% with clouds or if the solar elevation angle is less than 10°.     

POTSOL: Model to predict extraterrestrial and clear sky solar radiation

A model was developed to predict potential and clear sky solar radiation for any latitude. The model (POTSOL) uses the fundamental geometric relationships between the earth and sun to predict the theoretical solar radiation outside the earth's atmosphere, clear sky solar radiation received at the earth's surface after accounting for atmospheric interference, and clear sky solar radiation on a panel with any tilt angle between 0° and 90° from the horizontal. The only model input parameters are latitude (PHI), clearness number (CN), and panel tilt angle (PT). The model was verified using weather data obtained from the National Climatic Center, Asheville, North Carolina for Ely, Nevada.     

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.     

A methodology for sunlight urban planning: a computer-based solar and sky vault obstruction analysis

The main purpose of the present study is to describe a planning methodology to improve the quality of the built environment based on the rational control of solar radiation and the view of the sky vault. The main criterion used to control the access and obstruction of solar radiation was the concept of desirability and undesirability of solar radiation. A case study for implementing the proposed methodology is developed. Although needing further developments to find its way into regulations and practical applications, the methodology has shown a strong potential to deal with an aspect that otherwise would be almost impossible.     

A quasi-solid-state dye-sensitized solar cell based on the stable polymer-grafted nanoparticle composite electrolyte

A novel gel electrolyte was prepared by dispersing the polymer-grafted ZnO nanoparticle into liquid electrolyte. This gel electrolyte behaves long-term stability as the poly(ethylene glycol methyl ether) molecules are strongly connected to ZnO nanoparticles with covalent bond in polymer-grafted ZnO nanoparticle. A quasi-solid-state dye-sensitized solar cell (DSC) based on this gel electrolyte yields the energy transfer efficiency of 3.1% at AM 1.5 direct irradiation of 75 mW cm⁻² light intensity. Addition of 4-tert-butylpyridine into the electrolyte results in dramatically improved short circuit current density I_sc, and the overall efficiency is also improved to 5.0%, while the open circuit voltage (V_oc) and fill factor (ff) are insensitive to the presence of 4-tert-butylpyridine. DSC fabricated with this novel gel electrolyte displays better thermal stability than those solidified with the conventional nanoparticle ZnO(Ac).     

Solar radiation patterns during the harmattan season at Enugu, Nigeria

The total solar radiation during the harmattan months of 1982 to the early part of 1984 at Enugu, Nigeria, which lies on latitude 6.47°N and longitude 7.55°E and 140 m above sea level, has been measured and analysed. The effect of the dust haze prevalent at this time is evaluated. Results reveal that the effect of the harmattan is high during the months of November and January but is highest in December. The fairly high dust haze during this period gave a sky condition similar to overcast weather with a clearness index, K_T, of 0.36.

The use of Page's relation for prediction of the diffuse component of radiation is recommended. The regression coefficients a and b in the Angstrom equation are obtained. The predicted solar radiation, using the above relationship, compares well with measured values with an error of 0.07.

These predicted patterns are suggested for use in areas of similar climatological variables and will serve well in evaluating the performance of any solar device during the harmattan period.     

Performance study on the east–west oriented single-axis tracked panel

A theoretical study on the performance of an east–west oriented single-axis tracked panel was originally proposed in this paper. Mathematic expressions applicable for calculating the angle that the tracked panel should rotate by to follow the Sun are derived. The incident angle of sunlight upon the panel as well as the instantaneous increments of solar energy captured by the panel relative to a fixed horizontal surface are then demonstrated graphically. To simulate different operation environments, three kinds of radiation sources will be considered, i.e. the extraterrestrial radiation, global radiation predicted by empirical models under clear sky situation and global radiation observed in Taiwan. Simulation results show that the yearly gains correlate positively with the radiation level, i.e. 21.2%, 13.5% and 7.4% for the extraterrestrial, predicted and observed radiations, respectively, which are far less than those obtained from a north–south oriented single-axis tracked panel. The irradiation increases with the maximum rotation angle of the panel, the benefit of increasing the rotation in overcast environment is not as good as in clear sky, for annual energy collection 45° is recommended. The irradiation received decreases with latitude, but it has a greater gain in higher latitude zone.     

THE HIGH-RISE OPPOSING FACADE IN CLEAR SKY CONDITIONS—NOT ALWAYS AN “OBSTRUCTION” TO DAYLIGHT

The opposing building facade is an omnipresent feature for buildings in the urban commercial setting. In clear sky conditions, there are large surfaces of building facades that have the potential of reflecting sunlight incident on them. When the sun is behind a building (so that the solar facade azimuth is greater than 90°), the building opposite potentially acts as a passive daylighting device to it. The work discussed in this paper shows that the performance of this device varies for each floor and for different building densities. Using the ADELINE simulation software, a street with high rise buildings opposite each other is modelled in clear skies, and daylighting is calculated in rooms at different floor levels. Comparisons are made with daylighting of the same rooms without an obstructing building in order to observe the effect of the “obstructing building” to daylighting. Preliminary results indicate that the opposite facade of high rise buildings has the potential to be considered a passive daylighting device in particular contexts. As a passive device, the opposing facade could be given attention by urban designers and planners, as well as by those drawing up building legislation for the urban built environment. It would be desirable and important for users of the built environment, to determine future developments from the point of view of how opposing facades affect daylighting. This approach could have implications for energy efficiency and conservation and would utilise the free resource of solar radiation more effectively.     

Estimating surface solar radiation from upper-air humidity

A numerical model is developed to estimate global solar irradiance from upper-air humidity. In this model, solar radiation under clear skies is calculated through a simple model with radiation-damping processes under consideration. A sky clearness indicator is parameterized from relative humidity profiles within three atmospheric sublayers, and the indicator is used to connect global solar radiation under clear skies and that under cloudy skies. Model inter-comparisons at 18 sites in Japan suggest (1) global solar radiation strongly depends on the sky clearness indicator, (2) the new model generally gives better estimation to hourly-mean solar irradiance than the other three methods used in numerical weather predictions, and (3) the new model may be applied to estimate long-term solar radiation. In addition, a study at one site in the Tibetan Plateau shows vigorous convective activities in the region may cause some uncertainties to radiation estimations due to the small-scale and short life of convective systems.     

北京地区太阳紫外辐射的长期变化趋势及分析

对实际天气条件下北京地区1990年1月至1992年8月太阳辐射预测资料进行了详细分析。得到实际天气条件下到达地面的太阳紫外辐射的计算公式。结果表明,计算值与预测值吻合得比较好,利用该式计算了北京地区1979年1月-1996年6月的太阳紫外辐射,讨论了1979-1996年北京地区太阳紫外辐射的变化趋势及不同因子（臭氧、水汽、气溶胶等）对太阳紫外辐射的影响。