Correlationships between clearness index and relative sunshine duration for Sudan

The clearness index for 16 meteorological stations in Sudan has been expressed in terms of the fraction of bright sunshine duration, applying a variety of regression forms, namely linear, exponential, power and second-order polynomial. The significance and performance characteristics of the relations have been viewed using several statistical procedures. Although all of the four relationships are empirical and exhibit relative discrepancies, it has been found that they can reasonably be used to represent the underlying effect of relative duration of sunshine on clearness index for mean values. The estimates of the relationships are statistically significant at the 99.9% confidence level. For around 85% of the cases, the absolute error is fairly less than 4%. No estimate is found to fall beyond about ±10% accuracy level. Unless otherwise investigated under extreme conditions, especially S/S_o = 0 and 1, the predictive value of the relations remains questionable for values of S/S_o outside the ranges covered herein. From the bulk of the data available, the important finding is that solar radiation is plentiful in this vast country and could be regarded as an essential, renewable source of energy for various applications.     

Diffuse solar radiation estimates from sunshine hours and clearness index for Karachi, Pakistan

Correlations for the estimation of monthly average daily diffuse solar radiation as a function of the sunshine hours and clearness index have been obtained from Karachi. Generally, two types of correlations are used: (a) diffuse radiation as a function of relative sunshine hours and extraterrestrial radiation, and (b) diffuse radiation as a function of global and extraterrestrial radiation. These correlations are mostly first- and second-order polynomials in the sunshine hours and clearness index, indicating the presence of the diffuse solar radiation component. The diffuse solar radiation shows a peak value during the monsoon months of July–August. The diffuse to global ratio is found to be 0.32 from the analysis, and the diffuse to extraterrestrial radiation ratio is nearly 0.19 throughout the year. Among the established relations, Iqbal and Stanhill overestimate the radiation value, while Liu and Jordan underestimate it.     

Making full use of the clearness index for parameterizing hourly insolation conditions

An enhanced parameterization of insolation conditions based only on the knowledge of global irradiance is presented. Two limitations associated with the current approach using the clearness index are pointed out: its dependence on solar elevation and its inability to differentiate between different conditions that produce the same global irradiance. Suggestions are provided which could overcome part of these limitations. Arguments are substantiated with solid experimental evidence. It is further shown that noticeable gains in accuracy for the decomposition of global into direct and diffuse irradiance are possible if one makes optimum use of the information available within a global irradiance time series.     

Fluctuations in instantaneous clearness index: Analysis and statistics

Solar radiation is characterized by short fluctuations introduced by passing clouds. An analysis of these fluctuations with regard to solar energy applications should focus on the instantaneous clearness index. Its probability distribution for a given mean clearness index is, as a first approximation, independent from the season and partly also from the site. This is verified for four annual datasets from three different sites.An analysis of fluctuations in solar radiation must focus on their amplitude, persistence, and frequency of occurrence rather than their location in time. The Fourier analysis cannot satisfactorily provide this information since time series of the instantaneous clearness index exhibit no periodicity. Instead, a localized spectral analysis based on wavelet bases rather than on periodic-ones has been applied. This analysis allows the decomposition of the fluctuating clearness index signal into a set of orthonormal subsignals. Each of them represents one specific scale of persistence of the fluctuation.The annual mean square values of all subsignals have been analysed, permitting the allocation of the signal’s power content to the different scales of persistence of a fluctuation. These annual mean values agree well for the different datasets, indicating the existence of statistically significant mean square values of the fluctuations as a function of their persistence.The analysis offers a valuable tool for the estimation of power flow fluctuations introduced by direct solar energy systems. With further elaboration it may be applied by power system operators for network planning in distribution grids with a high density of embedded generation.     

Sky conditions at Ibadan during 1975–1980

The sky conditions at Ibadan, characterized by the clearness index, relative sunshine, diffuse ratio and diffuse coefficient, are presented for the six years 1975 to 1980. It is shown that based on the monthly average of the daily clearness index , the annual sky conditions have six patterns, namely, the harmattan-haze dry season pattern (November, December, and January) with , the dustfree dry season pattern (February, March, and April) with , and four rainy season patterns of August ( ), July and September ( ), June and October ( ) and May ( ). The variations of the diffuse ratio and relative sunshine are also in tune with these six patterns while the diffuse coefficient has an almost constant value of about 0.24 for all seasons. Following the work of Liu and Jordan, statistical “monthly K_τ curves” are established for Ibadan along the lines of the six patterns of sky conditions. These monthly K_τ curves can be utilized to determine the approximate statistical distribution of the daily total radiation for other cities which are in the same weather zone as Ibadan when the monthly average daily total radiation is known for such cities.     

Correlation between relative sunshine and state of the sky

It is possible to construct a sunshine map referring to a large zone, lacking in a widespread network of recording stations, correlating the relative sunshine with the “state of the sky”, defined by the monthly mean number of the clear, mixed and overcast days, whose data are more largely available. In the correlation here proposed climatological and meteorological factors are introduced in order to express the helioclimatic characteristics of every location. The application to Italy, using the data of the period 1973–1977 provided by the 31 stations of ITAV, has suggested the classification of these stations in seven “helioclimatic groups” characterized by different values of the above factors, mainly representative of the position and typology of the surrounding surfaces and of the atmospheric transparence in a cloudy or cloudless sky. The positive results of the comparison between the experimental and computed relative sunshine values are shown in the tables.     

Relationship between “clearness index” and “cloudiness index” at a tropical station (Ilorin, Nigeria)

Total (global) solar radiation, H, and diffuse solar radiation, H_d were measured at Ilorin (8° 30′N 4° 42′E) Nigeria. From these, the daily values of H/H₀ and H_d/H were computed, where H₀ is the extraterrestrial radiation. The relationship between the two ratios and their variation with the prevailing atmospheric conditions were examined.The ratios were found to be opposite in characteristics. H/H₀ has high values in clear atmospheres and low values in cloudy or turbid atmospheres; and vice-versa for H_d/H. The two ratios are negatively linearly related and with this fact, two simple mathematical models were obtained for estimating H_d/H in terms of H/H₀.     

Developing an improved global solar radiation map for Zimbabwe through correlating long-term ground- and satellite-based monthly clearness index values

Reliable knowledge of the spatio-temporal distribution of solar radiation is required for the informed design and deployment planning of solar energy delivery systems. In this paper an improved global solar radiation map for Zimbabwe is developed by merging ground-measured radiation data from a sparsely distributed station network, with less accurate satellite-measured data which have an almost continuous spatial coverage. Monthly clearness index values derived from ground-measured global radiation are correlated with those derived from satellite data to obtain a model for calibrating satellite-measured data at a specified grid interval. Two multiplicative factors are to then used to further correct the generated data; CF_m to cater for the in-exactness of the regression fit and the other, IBCF to cater for the interpolation error. Contour maps of global solar radiation are then constructed using interpolation by the geo-statistical method of ordinary kriging. The accuracy of the maps in predicting observed (ground-measured) values was tested by evaluating error statistics; relative bias error (rBE), relative mean bias error (rMBE) and normalized root mean square error (NRMSE) in a “leave-one-out” cross-validation analysis. Results indicate that the maximum normalized root mean square error was 0.028 (about 3%), a significant improvement when compared to an earlier map, the H–G map with a normalized root mean square error (NRMSE) of 0.097.     

Conditional probabilities of daily relative sunshine data and the dependence on the weather of the previous day

In this paper, the conditional probabilities p₁₁(m, x) and p₀₁(m, x) of the daily relative sunshine (DRS) are estimated for each month m, using available data for 30 years. p₁₁(m, x) (p₀₁(m, x)) is the probability that the DRS of a day for a certain month is greater than x, (o < x < 1), given that the DRS of the previous day is greater (smaller) than x. The empirical curves for p₁₁(m, x) and p₀₁(m, x) are fitted and the fitting parameters are estimated for each month. The results show a good agreement between the empirical and the calculated values. The vertical distance between the curves p₁₁(m, x) and p₀₁(m, x) shows a strong dependence of the weather on that of the previous day. A method to estimate a sequence of k consecutive days of “good” or “bad” weather is also given.     

Technical note Measurement of global solar global photosynthetically-active and downward infrared radiations at Ilorin, Nigeria

Methods of measuring and results of analyses of data spanning a period of two years on global solar radiation, global photosynthetically-active radiation (PAR) and downward infrared radiation at Ilorin, a tropical location, are presented.The results show that there exist seasonal tendencies in the variation of these radiations, especially for global solar radiation and PAR. The distribution and range of variation of global solar radiation show that Ilorin has a high potential for solar energy utilization while those of PAR indicate the availability of this very important radiation throughout the year, perhaps with the exception of the months of July and August. For downward infrared radiation, the results show that it is almost uniform throughout the year with the exception of the relatively small values in the Harmattan months (December and January) of the dry season.     

Correlations between monthly average global solar radiation on horizontal surfaces and relative duration of sunshine in Yunnan Province, China

With measured data of global solar radiation and the relevant data of geographic and meteorological parameters at 7 meteorological stations in Yunnan Province, the correlations between monthly average global solar radiation on horizontal surfaces and relative duration of sunshine are developed in this special region which has significantly varying climates. It is believed that the two correlations developed in this work are applicable for estimating monthly global solar radiation on horizontal surfaces at any site in Yunnan Province, China.     

Estimation of ambient temperature bin data from monthly average temperatures and solar clearness index. Validation of the methodology in two Greek cities

Ambient temperature bin data are used for estimating the energy consumption in HVAC systems with air-source heat pumps and cooling equipment. In this paper a methodology for estimating the ambient temperature bin data, based on monthly average outdoor temperatures and solar clearness index, is presented. For the two most populated cities of Greece, namely Athens and Thessaloniki, the estimated data are compared to the bin data produced by statistical analysis of 10 years hourly dry-bulb temperature measurements. Both data sets were also used for estimating the heating and cooling energy requirements of a case study building. The results obtained are similar, with very small differences, suggesting that the proposed methodology can be used for estimating bin data for other cities.     

Methodology for predicting sequences of mean monthly clearness index and daily solar radiation data in remote areas: Application for sizing a stand-alone PV system

In this paper, a suitable adaptive neuro-fuzzy inference system (ANFIS) model is presented for estimating sequences of mean monthly clearness index () and total solar radiation data in isolated sites based on geographical coordinates. The magnitude of solar radiation is the most important parameter for sizing photovoltaic (PV) systems. The ANFIS model is trained by using a multi-layer perceptron (MLP) based on fuzzy logic (FL) rules. The inputs of the ANFIS are the latitude, longitude, and altitude, while the outputs are the 12-values of mean monthly clearness index . These data have been collected from 60 locations in Algeria. The results show that the performance of the proposed approach in the prediction of mean monthly clearness index is favorably compared to the measured values. The root mean square error (RMSE) between measured and estimated values varies between 0.0215 and 0.0235 and the mean absolute percentage error (MAPE) is less than 2.2%. In addition, a comparison between the results obtained by the ANFIS model and artificial neural network (ANN) models, is presented in order to show the advantage of the proposed method. An example for sizing a stand-alone PV system is also presented. This technique has been applied to Algerian locations, but it can be generalized for any geographical position. It can also be used for estimating other meteorological parameters such as temperature, humidity and wind speed.     

Analysis of monthly average daily global radiation and monthly average sunshine duration at two tropical locations

The diffuse radiation falling on a horizontal surface at Madras, India has been analysed and the results presented. Five years of data have been used for the analysis and a correlation has been established between the daily average diffuse radiation with the monthly average clearness index. It has also been shown that there is an appreciable difference in the ratio of monthly average hourly diffuse to the monthly average daily diffuse radiation with respect to day length.     

Analysis of monthly average solar radiation and bright sunshine for different thresholds at Cape Canaveral, Florida

Data for beam, global horizontal, and global normal radiation measured at Cape Canveral were analyzed for the period 1986–1991. Some empirical correlations between these parameters are proposed. A detailed analysis of the beam irradiance data also led to the derivation of sunshine fractions corresponding to different radiation thresholds. It is shown that the current WMO threshold of 120 W/m² corresponds to optimal radiation/sunshine correlations. Finally, improved monthly irradiation empirical fits using sunshine data are proposed.     

Nickel centered metal-organic complex as an electro-catalyst for hydrogen evolution reaction at neutral and acidic conditions

In this study, we report the development of dihydropyrimidines substituted nickel electro-catalyst at mild conditions for hydrogen evolution reaction. The Biginelli type reaction was carried out to produce 4-(4-chlorophenyl)-3,4,5,6-tetrahydrobenzo[h]quinazoline-2(1H)-thione moiety. This was added with NiCl₂.6H₂O to synthesize Ni-quinazoline-2(1H)-thione catalyst. The prepared catalyst was characterized using UV–Vis, FT-IR, Mass, ¹H and ¹³C NMR and SEM-EDX spectroscopy. It showed an excellent hydrogen evolution reaction (HER) activity. LSV polarization showed a low Tafel slope (34 mV/dec) nearly close to Pt/C (28 mV/dec) in neutral medium whereas a Tafel slope of 100 mV/dec was observed with 0.5M H₂SO₄ while Pt/C showed 50 mV/dec. Electrochemical impedance spectroscopy measured by applying −1.18 V in neutral medium showed two depressed semicircles whereas with 0.5M H₂SO₄ and -1.45 V applied voltage single half was observed.     

Comparative evaluation of the mesophilic and thermophilic biohydrogen production at optimized conditions using tequila vinasses as substrate

The objective of this work was to comparatively evaluate the production of biohydrogen (bio-H₂) from tequila vinasses at optimized mesophilic and thermophilic conditions and to elucidate the main metabolic routes involved. Optimal temperatures of 35 °C and 55 °C, and pH of 5.5 maximized the bio-H₂ production rates, 25.5 ± 0.01 NmL h⁻¹ and 169.9 ± 8.9 NmL h⁻¹ in the mesophilic and thermophilic regimens, respectively. During the operation of anaerobic sequencing batch reactors, the thermophilic process allowed a volumetric bio-H₂ production rate of 519 ± 13 NmL-H₂ L⁻¹ d⁻¹ equivalent to 750 ± 19 NmL-H₂ L_vinasse⁻¹, while the mesophilic one 448 ± 23 NmL-H₂ L⁻¹ d⁻¹ and 647 ± 33 NmL-H₂ L_vinasse⁻¹, respectively. Furthermore, the gas produced under thermophilic conditions showed high hydrogen content (86.5%). Finally, formate degradation and glucose fermentation to acetic and butyric acids were the main metabolic routes involved in bio-H₂ production under thermophilic conditions, while at mesophilic conditions, the lactate and formate degradation pathways governed.     

Nitrogen reduction to ammonia at ambient conditions using hydrochar prepared from cigarette filters as catalyst

Ammonia synthesis via the electrochemical method is an environmentally-friendly alternative to the Haber-Bosch process. In this work, carbon materials made from fresh cigarette filters were used as the working electrode for the electrochemical synthesis of ammonia. TiO₂ was found to exist in the hydrochar of cigarette filters with a mass loading of 0.8 wt%, and showed positive impact on the catalytic performance of the hydrochar. A decent NH₃ yield of 6.02 μg mg_cat.⁻¹ h⁻¹ and a FE of 8.96% were achieved at −0.1 V versus reversible hydrogen electrode (RHE) in N₂-saturated 0.05 M H₂SO₄. The design of experiments proved that TiO₂, which was contained in the hydrochar, indeed played a great role in the catalytic reaction of ammonia synthesis.