Correlation between global ultraviolet (290–385nm) and global irradiation in Valencia and Cordoba (Spain)

This paper analyses hourly data of global ultraviolet (290–385nm) and global irradiation over a horizontal surface. The data was taken in Valencia (39.5°N, 0.341W) and Córdoba (37.51°N, 4°48W), (Spain) during the years 1996–1999. In order to estimate the ultraviolet solar radiation using global solar radiation values over a horizontal surface, the method of Foyo-Moreno et al. (Foyo-Moreno, I., J. Vida and L. Alados-Arboleda. J. Appl. Meteor. 1998, 38, 1020–1026) has been used. The results obtained with this method are satisfactory with a determination ratio r²=0.98, RMSE=9.3%, MBE=0.5% for Valencia and r²=0.99, RMSE=6.2%, MBE=0.3% for Córdoba.     

The relationship between global solar radiation and sunshine duration in Vietnam

Two approaches of the well-known modified Angstrom formula were developed from long term records of measured monthly mean daily global solar radiation and sunshine hour values obtained from 12 meteorological stations across Vietnam. These formulae were then used to estimate solar radiation for stations where only sunshine records were available. Three other commonly used correlations between solar irradiation and sunshine duration were also used and their results compared with those of two developed models. The procedure of measurement in Vietnamese weather stations was also indicated.     

Fuzzy inference systems applied to the daily ultraviolet radiation evaluation (295–385 nm) from daily global radiation

This work presents the results of applying different automatic learning techniques to the calculation of daily ultraviolet radiation from daily global radiation on a horizontal surface. Using the data from three Spanish locations, a zonal study was made, which was finally combined in models for general application. Using the corresponding atmospheric transparency index, three models based on multivariate linear regression, non-linear regression and generation of fuzzy inference systems, respectively, were designed. The results obtained verify the good behavior of the fuzzy inference system calculated.     

Highly efficient 1 μm thick CdTe solar cells with textured TCOs

Thickness reduction of CdTe absorption layer down to 1 μm has been achieved by controlling the temperature profile used during the close-spaced sublimation (CSS) growth. Transparent conducting oxides, such as indium tin oxide (ITO) and textured fluorine doped tin oxide (SnO₂:F) films have been investigated as transparent electrodes for such 1-μm-thick CdTe absorption layers to increase the incident light confinement and thus to achieve higher conversion efficiency. The contribution in solar cell performance has been found in the case of textured TCOs with optimum haze ratio (roughness). Conversion efficiencies of 10.6% (V_oc: 0.75 V, J_sc: 22.02 mA/cm², FF: 0.64, area: 1 cm²) and 11.2% (V_oc: 0.78 V, J_sc: 22.6 mA/cm², FF: 0.63) have been achieved for only 0.6-μm-thick CdTe absorption layers with SnO₂:F-TCO of 11% and 3% of haze ratios, respectively.     

Solar global UV (280–380 nm) radiation and its relationship with solar global radiation measured on the island of Cyprus

We report the first observations of solar global UV (280–380 nm) radiation on the island of Cyprus obtained during an ongoing joint research project between the University of Athens and the Meteorological Service of Cyprus. Hourly global UV (G_uv) and global (G_h) solar irradiances are measured and the relationship between the two radiant fluxes is investigated at Athalassa, Cyprus (35°15′N, 33°40′E, 165 m above MSL). These data are used to determine the temporal variability of the percentage ratio of solar global UV to solar global irradiation (G_uv/G_h) and its dependence on various atmospheric conditions. Analyzing the data set, an inverse correlation between ozone column amount and the precise UV/Global ratio was found, which can be rather attributed to the change of various atmospheric parameters than the ozone column. The analysis of hourly percentage ratio values reveals a definite daily pattern with lower values during sunrise/sunset and higher values around noon. The variation of the percentage ratios (G_uv/G_h) ranges from 3.95±0.29% in September to 2.92±0.42% in August for hourly values, while for daily values, the variation is between 2.85±0.32% in August and 3.68±0.1% in September, with annual mean values of 3.33±0.21% for hourly and 3.19±0.17% for daily data. Finally, the present data reveals a seasonal contribution of the aerosol extinction on the precise (G_uv/G_h) ratio values.     

High-efficiency μc-Si/c-Si heterojunction solar cells

P-type microcrystalline silicon (μc-Si (p)) on n-type crystalline silicon (c-Si(n)) heterojunction solar cells is investigated. Thin boron-doped μc-Si layers are deposited by plasma-enhanced chemical vapor deposition on CZ-Si and the V_oc of μc-Si/c-Si heterojunction solar cells is higher than that produced by a conventional thermal diffusion process. Under the appropriate conditions, the structure of thin μc-Si films on (1 0 0), (1 1 0), and (1 1 1) CZ-Si is ordered, so high V_oc of 0.579 V is achieved for 2×2 cm² μc-Si/multi-crystalline silicon (mc-Si) solar cells. The epitaxial-like growth is important in the fabrication of high-efficiency μc-Si/mc-Si heterojunction solar cells.     

Enhancement of optical absorption for below 5 μm thin-film poly-Si solar cell on glass substrate

Optical confinement effect of thin-film polycrystalline-Si (poly-Si) solar cell on glass substrate fabricated at low-temperature has been investigated as a function of cell thickness of less than 5 μm. We found that it is possible to fabricate the textured Si thin film in situ on a glass substrate and that the reflectance at long-wavelength light is reduced by surface texturing. Thin-film poly-Si solar cell and a-Si:H/(0.45 μm)/poly-Si (5 μm) tandem solar cell exhibit the efficiency of 8.6% and 12.8%, respectively. The numerical study in terms of the light trapping explains the excellent high short-circuit current density (_sc above 27 mA/cm² at the 4.7 μm thin-film poly-Si solar cell.     

Modeling global solar radiation using Particle Swarm Optimization (PSO)

The quantity of solar radiation received by the earth’s surface is very important to numerous renewable energy applications. However, direct measurement of solar data is not widely available, especially in developing countries. This paper uses Particle Swarm Optimization (PSO) to train an artificial neural network (PSO–ANN) using data from available measurement stations to estimate monthly mean daily Global Solar Radiation (GSR) at locations where no measurement stations are available. The inputs to the networks are: month of the year, latitude, longitude, altitude, and sunshine duration, and the output is the monthly mean daily GSR at the specified location. Using training data from 31 stations and testing data from 10 locations, the PSO–ANN outperforms a neural network trained using the standard backpropagation (BP) algorithm (BP–ANN) with an average Mean Absolute Percentage Error (MAPE) of 8.85% for the PSO–ANN and 12.61% for the BP–ANN. The performance is improved significantly, when we use the leave-one-out method, where data from 40 locations is used for training and data from the 41st station is used for assessing the performance. In this case the average of MAPE on data from the 10 testing stations is about 7%. We used the same method to assess the performance of the PSO–ANN on testing data from each of the 41 stations with an overall average MAPE of about 10.3%. Comparison with BP–ANN and an empirical model showed the superiority of the PSO–ANN.     

Thermal radiative properties of glass from 0.32 to 206 μm

The thermal radiative properties of glass are calculated using the optical constants data that are recently published in the literature. The directional-spectral reflectivity, absorptivity and transmissivity of glass are computed from 0.32 to 206 μm. These properties are also integrated to obtain the directional-total and hemispherical-total properties. The systems under investigation include a glass surface, a bulk glass plate and multilayer glass systems. Prediction equations are given to show that in the far IR various property equations can be reduced to a function of surface reflectivity. According to the calculated results, only under the normal incidence condition inside the far IR, the glass has a total reflectivity of 0.107 which is very close to the present design value (0.1). However, if the incident energy is coming from all directions as what is normally encountered in practice, the present design value makes, at least, a 50 per cent underestimation of reflectivity. To facilitate application of data presented in the far IR, an equation is also correlated that relates the glass hemispherical-total properties with temperature. While the glass properties are also evaluated inside the solar spectrum in this paper, these properties have been reported elsewhere and therefore, are not elaborated.     

A test of Suehrcke’s sunshine–radiation relationship using a global data set

In this paper we investigate a new relationship between duration of sunshine and solar radiation on the earth’s surface that was derived recently by Suehrcke (Solar Energy, 68(5) (2000) 417). We test the relationship using over 70,000 measured monthly sunshine and radiation data from nearly 700 sites compiled by the World Radiation Data Center. We show that Suehrcke’s equation accounts adequately for the sunshine–radiation relationship on an average sense. There is a large dispersion (12% on average) in the values of solar radiation calculated by the new equation, however it is unclear how much of this dispersion could be accounted for by a better model. The predictive capabilities of the new model are actually roughly equivalent to those of older models such as Ångström–Prescott when the peculiarities of local climatic conditions are not taken into account.     

On the relationship between global radiation and cloudiness in southern Alaska

Radiation data for Alaska are sparse. Hence, empirical relationships between global radiation and the amount of cloudiness, using cloudiness as a predictor, serve a useful function. In southern Alaska there have been two series of observations: an historical one in Annette; and our present one of 28-month length in Kodiak. We developed a regression relationship for Kodiak and tested it against Annette. A satisfactory relationship was established. A correlation factor of 0.995 was found for monthly mean values between the observed data from Annette and the calculated one using the relationship developed for Kodiak. This gives the opportunity to calculate the global radiation for other communities in South Alsaka, where no radiation data exist.     

Monthly average solar radiation in panama—Daily and hourly relations between direct and global insolation

Regressions are developed to estimate daily global and direct radiation and the hourly distribution of direct radiation for Barro Colorado Island, Panama from monthly mean values observed 35 km away at Chiva-Chiva. The ratio model of Liu and Jordan and the logarithmic model of Anderson for estimating direct from global radiation are compared. Both gave satisfactory results after accounting for “seasonal” variation, but the ratio model was preferred in this case for the smaller number of separate regressions required. The ratio model fitted for diffuse radiation at Chiva-Chiva agreed closely with regressions for stations at similar latitude. For a given value of the clearness index, the direct component of solar radiation was relatively (but not absolutely) reduced during the dry season compared with the wet season. A likely explanation for this unexpected result is increased marine and terrestrial aerosol during the dry season when offshore winds are stronger and burning of crop and wasteland occurs. The models of Whillier and of Garnier and Ohmura, which assume constant atmospheric transmittance throughout the day, gave unsatisfactory fits to the hourly distribution of direct radiation. They were also unable to mimic an observed morning/afternoon asymmetry that was strongest in the wet season. Hourly direct radiation was accurately estimated from hourly global radiation by quadratic polynomials fitted separately to the morning and afternoon data. The resulting regressions will enable estimation of radiation in forest understory from measurements of insolation in the open by computerized image analysis of hemispherical canopy photos.     

Absorption coefficient spectra of μc-Si in the low-energy region 0.4–1.2 eV

Optical absorption spectra in the low-energy region 0.4–1.2 eV is reported for μc-Si:H using a photothermal deflection spectroscopy technique. Absorption coefficient spectra in the low-energy region contain important information related to defects and hydrogen. It is demonstrated that there is a good correlation between electron spin densities and integrated absorption coefficient spectra from 0.7 to 1.2 eV. The amount of the hydrogen molecules in microvoids is much larger in μc-Si:H than that in a-Si:H. Light illumination effects in PDS spectra has also been studied from a view point of photo degradation of the μc-Si:H.     

The effect of model generated solar radiation data usage in hybrid (wind–PV) sizing studies

Wind speed and solar radiation characteristics belonging to past years of a region are the main input parameters in wind–photovoltaic hybrid system (WPHS) sizing studies. Classically, these data are fed to several scenarios with different solar panel, wind turbine, and storage battery number combinations. The solutions with minimal cost which also satisfy the desired maximum loss of energy probability are selected. Since the utilized data have random fluctuations because of atmospheric phenomenon, past years’ data are unlikely to appear in a similar manner in future years. Hence, using a robust model that characterizes the general behavior of the data instead of directly using past data should yield more accurate sizing solutions. In order to compare the sizing accuracy obtained by directly using the data to the accuracy obtained by indirect modeling from data, an analytical solar radiation model is first explained. Using this model, 3-year solar radiation data of three geographical sites are analyzed. It was observed that the differences between sample-by-sample hourly recordings corresponding to different years are significantly larger than the difference between these recordings and the data model obtained from an arbitrary year. This provides a hint that a sizing approach carried out using the data of a previous year would not be accurate in producing the same Loss of Load Probability (LLP) for a future year. On the contrary, the accuracy would improve if a generic analytical model of the solar radiation is used in the sizing process. This foresight is tested by comparing the LLPs obtained in the two ways mentioned above. Results obtained using available data are in accordance with the aforementioned propositions.     

Extension of the a-Si:H electronic transport model to μc-Si:H: use of the μτ product to correlate electronic transport properties and solar cell performances

The aim of this communication is to show that it is possible to extend the model of the electronic transport developed for amorphous silicon (a-Si:H) to microcrystalline silicon (μc-Si:H). By describing the electronic transport with the μ⁰τ^R products (mobility×recombination time) as a function of the Fermi level, we observed the same behaviour for both materials, indicating a similar type of recombination. Moreover, applying the normalised μ⁰τ⁰ product (mobility×life-time) obtained by combining the photoconductivity (σ_photo) and the ambipolar diffusion length (L_amb) measured in individual layers, we are able, as in the case of a-Si:H, to predict the quality of the solar cells incorporating these layers as the active i layer.     

The dependence on solar elevation of the correlation between monthly average hourly diffuse and global radiation

In the present work, the dependence on of the correlation between and is studied, , , and respectively being the monthly average hourly values of the global, diffuse, and extraterrestrial radiation, all of them on a horizontal surface, and the solar elevation at midhour. The dependence is studied for Uccle for the following sky conditions. Condition A: clear skies (fraction of possible sunshine = 1) and the maximum values of direct radiation measured during the period considered (each of the hours before or after the solar noon for which radiation is received); Condition B: corresponding to all the values of radiation measured when the sunshine fraction is 1 during the period considered; Condition C: corresponding to all the data collected, independently of the state of the sky; Condition D: corresponding to overcast skies ( ).From the available values of and (monthly average hourly direct radiation on a horizontal surface), values of and for 5° ≤ ≤ 45° and Δ = 5° are calculated using Newton's divided difference interpolation formula. The interpolated values occupy three clearly different regions in the plot, one for each of the conditions A,B, and C. For Condition A and each value of best linear fits with high correlation coefficients are obtained for the correlation. The influence of the Linke turbidity factor on the correlation for Condition A is studied for 5° ≤ ≤ 35°, Δ = 5°.