A simplified method for estimating direct normal solar irradiation from global horizontal irradiation useful for CPV applications

The design and analysis of CPV systems require knowledge of direct normal solar irradiation but ground-based measurements of these data are only available for very few locations. Nowadays, meteorological databases that estimate direct normal irradiation from satellite images and other data sources are used. However, values provided by the different existing databases show large dispersion due to different estimation methods, input data and base years. In this paper, a simplified method for calculating direct normal irradiation is presented. It has been obtained from previous models proposed by several authors. One of its advantages is that it only requires latitude and global horizontal irradiation as input data. As global irradiation is easy to find or measure, the procedure becomes a useful tool in renewable energy applications. The accuracy of this method is similar to that of the existing databases and it is able to easily generate a mass of direct normal irradiation data for different areas worldwide.     

A method for estimating direct normal solar irradiation from satellite data for a tropical environment

In order to investigate a potential use of concentrating solar power technologies and select an optimum site for these technologies, it is necessary to obtain information on the geographical distribution of direct normal solar irradiation over an area of interest. In this work, we have developed a method for estimating direct normal irradiation from satellite data for a tropical environment. The method starts with the estimation of global irradiation on a horizontal surface from MTSAT-1R satellite data and other ground-based ancillary data. Then a satellite-based diffuse fraction model was developed and used to estimate the diffuse component of the satellite-derived global irradiation. Based on this estimated global and diffuse irradiation and the solar radiation incident angle, the direct normal irradiation was finally calculated. To evaluate its performance, the method was used to estimate the monthly average hourly direct normal irradiation at seven pyrheliometer stations in Thailand. It was found that values of monthly average hourly direct normal irradiation from the measurements and those estimated from the proposed method are in reasonable agreement, with a root mean square difference of 16% and a mean bias of −1.6%, with respect to mean measured values. After the validation, this method was used to estimate the monthly average hourly direct normal irradiation over Thailand by using MTSAT-1R satellite data for the period from June 2005 to December 2008. Results from the calculation were displayed as hourly and yearly irradiation maps. These maps reveal that the direct normal irradiation in Thailand was strongly affected by the tropical monsoons and local topography of the country.     

Interrelationships between total, direct, and diffuse solar radiation in the tropics

Formulas that directly yield intensities of direct and diffuse radiation on horizontal surfaces from measurements of hourly total radiation only have been developed. Alternatively, atmospheric turbidity and solar altitude can also yield similar results. These formulas result in obtaining relations similar to those given by Parmelee from data collected in the United States. The data for New Delhi have also been shown to agree with these correlations.

Computed values of direct solar radiation on a horizontal surface or at normal incidence, are expressed by a mathematical expression that is shown to agree closely with the computed values obtained by Rao and Seshadri.

Correlation between hourly direct and total transmission factors has been shown to depend on the solar altitude, in addition to the atmospheric turbidity.     

Comparative study of various models in estimating hourly diffuse solar irradiance

This work presents a comparison among seventeen different proposals for estimating the hourly diffuse fraction of irradiance. Twelve of them are polynomial correlations of different orders, two are based on a logistic function and the three last ones consider the diffuse irradiance values in the previous and posterior hour to that of the calculation. In general, the proposals showing the more favourable statistics indexes are those that consider the process dynamics, as they behave better than the rest of the models even when the polynomial correlations and the logistic function are calibrated for the experimental data used in this work.The models Dirint and BRL are the ones recommended for the data of this study, as they exhibit the highest precision and generate a series of hourly diffuse irradiance values of which the distribution functions are very similar to those of the experimental data.     

Evaluation of models for estimating solar irradiation on vertical surfaces at Valencia, Spain

Hourly irradiation data recorded on vertical surfaces at north, east, south, and west orientations during the winter period going from December 1989 to March 1990 in Valencia, Spain, have been compared with estimated solar irradiation from several tilted-surface models. The isotropic-, Temps' and Coulson's-, Klucher's-, Hay's-, Skartveit's and Olseth's-, Gueymard's- and Perez' (simplified) models have been considered for this comparison. Root-mean-square-difference (RMSD), mean-bias-difference (MBD) and mean-absolute-difference (MAD) estimators have been used to measure the departure of models from experimental data. Modeled values are evaluated with the original coefficients proposed by the authors. Results of comparison show that the south orientation is less prone to modeling errors, Perez' 25° circumsolar simplified model being the less affected by errors in north, east and south orientations; for west orientation Klucher's model gives a lesser error. These results are consistent with those reported in previous studies.     

Measurement of direct, diffuse, and total solar radiation with silicon photovoltaic cells

The paper presents the results of tests carried out during the spring of 1962 at the Yellott Solar Energy Laboratory to determine the suitability of commercially available silicon photovoltaic cells for use in solar radiometers. Since the short-circuit current of the silicon cell has been shown to exhibit linear variation with varying intensity of incident radiation, this quantity was measured for six cells, selected at random, under widely varying temperatures, under both tungsten illumination and solar radiation. Over the range of temperatures likely to be encountered in solar radiometry (40 to 160 degrees F), the short-circuit current varied from 10 to 15 percent, under tungsten illumination. Under natural sunlight, the maximum variation was 6.3 percent. By measuring the millivolt drop across a small resistance, instead of determining the short-circuit current directly with a milliammeter, temperature compensation is shown to be adequately accomplished by the use of a thermistor and a Manganin wire shunting resistance, which are maintained at the temperature of the cell.

The effect upon the cell coefficient of solar altitude, in terms of deviation from the cosine response and air mass effect, was then investigated, using a silicon cell radiometer which could serve both as a pyrheliometer and as a pyranometer. It was found that, for solar altitudes above 60 degrees, no correction for cosine deviation or air mass effect is necessary. For lower solar altitudes, these factors must be taken into consideration by determining the coefficient at varying solar altitudes, through calibration against a standard thermopile-type instrument.

A day-long comparison of the output of a silicon cell pyranometer with the output of an Eppley 180-degree horizontal pyrheliometer showed an agreement in total radiation within 1 percent. It is concluded that, with proper calibration and adequate attention to the effects caused by solar altitude, a temperature-compensated silicon-cell pyrheliometer can be used with confidence to measure direct, total, and diffuse solar radiation with an accuracy of at least ±3 percent.     

A model for calculating direct and diffuse solar radiation

A new model is presented for computing both direct and diffuse solar radiation for a cloudy sky, based on the model of King and Buckius for the direct component for a clear sky. The model is employed to calculate the daily global insolation incident on a horizontal surface at Ibadan for the year 1977. The results are presented in the form of both weekly and monthly averages, and compared with the experimental data provided by the International Institute of Tropical Agriculture (I.I.T.A.), Ibadan. Two values of cloudiness coefficient k (= 1.0 and 0.75) are used in the calculations, with the case of k = 0.75 being superior and for which the deviations from the data do not exceed 15 per cent.     

Global, direct and diffuse solar irradiance in Riyadh, Saudi Arabia

The regression coefficient of the well-known Angstrom correlation are determined for Riyadh city (the capital of Saudi Arabia) longitude 46° 14′E 24°55′N The relationship of the daily and monthly variation of the fraction of the diffuse solar irradiation to extrateresstrial and the clearness index are obtained. The variation of the values of the average daily global solar radiation against the month of the year is reported. The value of K_n defined as the ratio of direct normal Insolation, H_bn measured in Riyadh to direct normal solar extraterrestrial radiation with the K_T and K_d were obtained. The daily diffused ratio and the daily clearness index are shown as a function of the month of the year.     

Semi-empirical models for the estimation of clear sky solar global and direct normal irradiances in the tropics

This paper presents semi-empirical models for estimating global and direct normal solar irradiances under clear sky conditions in the tropics. The models are based on a one-year period of clear sky global and direct normal irradiances data collected at three solar radiation monitoring stations in Thailand: Chiang Mai (18.78°N, 98.98°E) located in the North of the country, Nakhon Pathom (13.82°N, 100.04°E) in the Centre and Songkhla (7.20°N, 100.60°E) in the South. The models describe global and direct normal irradiances as functions of the Angstrom turbidity coefficient, the Angstrom wavelength exponent, precipitable water and total column ozone. The data of Angstrom turbidity coefficient, wavelength exponent and precipitable water were obtained from AERONET sunphotometers, and column ozone was retrieved from the OMI/AURA satellite. Model validation was accomplished using data from these three stations for the data periods which were not included in the model formulation. The models were also validated against an independent data set collected at Ubon Ratchathani (15.25°N, 104.87°E) in the Northeast. The global and direct normal irradiances calculated from the models and those obtained from measurements are in good agreement, with the root mean square difference (RMSD) of 7.5% for both global and direct normal irradiances. The performance of the models was also compared with that of other models. The performance of the models compared favorably with that of empirical models. Additionally, the accuracy of irradiances predicted from the proposed model are comparable with that obtained from some rigorous physical models.     

Technical note a multiple linear correlation for diffuse radiation from global solar radiation and sunshine data over Egypt

Measurements of global solar radiation, diffuse radiation and sunshine duration data during the period from 1982 to 1988 at different locations over Egypt were used to establish empirical relationships that would connect the daily monthly average diffuse irradiation with both relative sunshine duration and clearness index separately and in combination. The selected locations were chosen to represent the different weather conditions of North, Middle and South Egypt. Our correlation equations were tested using measured data for the year 1992 at the same locations. The correlation connecting diffuse radiation with both clearness index and percentage possible sunshine is found to be applicable over Egypt.     

Stochastic models for sunshine duration and solar irradiation

Harmonic analysis of sunshine duration and solar irradiation measured at Sebele, Botswana is carried out. The data used consists of the monthly averages and the Julian-days averages of sunshine duration and solar irradiation sequences. This study involves splitting the time series into deterministic and stochastic components, and determining the proportion of the variance explained by each component. The stochastic component is analyzed for persistence using the Box and Jenkins technique. It is found that the stochastic component for monthly averages solar radiation series is best described by the second-order autoregressive Markov process, while that for Julian-days averages series has no memory.     

Evaluation and establishment of diffuse solar radiation models for Bursa,Turkey

The main objective of the present study is to evaluate the applicability of the diffuse solar radiation models from previous studies and establish new models for Bursa. Therefore, 35 regression models from previous studies in the literature are used and categorized as follows: (1) cloudness index is the function of clearness index; (2) cloudness index is the function of relative sunshine duration; (3) the diffuse coefficient is the function of the clearness index; and (4) the diffuse coefficient is the function of the relative sunshine duration. Also, new four diffuse solar radiation models were developed using 1968–2015 long-term global solar radiation and sunshine duration data. The new models are then compared with 35 empirical equations available in previous studies in terms of different statistical tests. Consequently, Models 6 and 8 are found as the most accurate and high-performance empirical models for estimation of the monthly mean diffuse solar radiation on a horizontal surface for Bursa, Turkey.     

An atmospheric model for computing direct and diffuse solar radiation

This paper proposes an atmospheric model, which extends the computation of the direct radiation given by Cole's model to the cloudy sky and shows a method to calculate the diffuse radiation.Therefore the monthly average values of the global radiation incident on a horizontal surface at Palermo are computed by this method. These values are compared with the experimental data provided by the Istituto di Idraulica Agraria, Università di Palermo and generally exhibit a mean deviation not more than 10 per cent. The deviations become lower than 8 per cent taking into account the effect of the underlying surface albedo.In order to better verify the validity of the proposed method, it should be suitable to extend its application to other locations provided with actinometric stations. This should allow to use this method with more realibility to predict the radiation incident on the locations lacking in actinometric data.     

An investigation of the performance of 14 models for estimating hourly diffuse irradiation on inclined surfaces at tropical sites

The performance of 14 models for estimating hourly diffuse irradiation on inclined surfaces was investigated. In order to obtain solar radiation data for this investigation, equipment for measuring solar radiation on inclined surfaces facing to the north, south, east and the west at different tilted angles to the horizontal surface (30°, 60° and 90°) were constructed and installed at two tropical sites in Thailand, namely Nakhon Pathom (13.82 °N, 100.04 °E) and Ubon Ratchathani (15.25 °N, 104.87 °E). Radiation data encompassing different periods of 1–4 years were used in this work. Diffuse irradiance measurements at different tilted angles were compared with various model algorithms. Results show that the Muneer and Gueymard models have comparable performance in terms of root mean square difference (RMSD) and these models give the lowest RMSD, as compared to that of the other models.     

Assessing the potential of concentrating solar photovoltaic generation in Brazil with satellite-derived direct normal irradiation

With the declining costs of flat plate and concentrating photovoltaic (PV) systems, solar PV generation in many sunny regions in Brazil will eventually become cost competitive with conventional and centralized power generation. Detailed knowledge of the local solar radiation resource becomes critical in assisting on the choice of the technology most suited for large-scale solar electricity generation. When assessing the energy generation potential of non-concentrating, fixed flat plate versus concentrating PV, sites with high levels of direct normal irradiation (DNI) can result in cost-competitive electricity generation with the use of high concentrating photovoltaic systems (HCPV). In large countries, where the transmission and distribution infrastructure costs and associated losses typical of centralized generation must be taken into account, the distributed nature of solar radiation should be perceived as a valuable asset. In this work we assess the potential of HCPV energy generation using satellite-derived DNI data for Brazil, a large and sunny country with a continental surface of 8.5 million km². The methodology used in the study involved the analysis of global horizontal, latitude-tilt, and direct normal solar irradiation data resulting from the Solar and Wind Energy Resource Assessment (SWERA) Project, and an estimate of the resulting electricity production potential, based on a review of HCPV generators operating at other sites. The satellite-derived solar irradiation data, with 10 km × 10 km spatial resolution, were analysed over the whole country, in order to identify the regions where HCPV might present a considerable advantage over fixed plate PV on an annual energy generation basis. Our results show that there is a considerable fraction of the national territory where the direct normal solar irradiation resource is up to 20% higher than the latitude-tilt irradiation availability. Furthermore, these sites are located in the most industrially-developed region of the country, and indicate that with the declining costs of this technology, distributed multi-megawatt HCPV can be a good choice of technology for solar energy generation at these sites in the near future.