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International Journal of Agriculture and Crop Sciences. Available online at www.ijagcs.com IJACS/2013/5-21/2650-2654.ISSN 2227-670X 2013 IJACS Journal

Estimating global solar radiation from commonmeteorological data in sari station, Iran

GholamrezaJanbaz Ghobadi 1, Bahram Gholizadeh 2, Sadroddin Motavalli 3

1. Islamic Azad University Jouybar Branch, Jouybar, Iran2. Applied Meteorological Research Center of Golestan province

3. Islamic Azad University, Nur Branch, Nur, Iran

Correspon ding Author : GholamrezaJanbazGhobadi

ABSTRACT: This study aimed to calibrate existing model and develop a new model for estimatingglobal solar radiation data using commonly and available measured meteorological records such asprecipitation or temperature. In this study, we were calibrated two solar radiation models including

Angstroms-Prescott in Sari, Mazandaran, Iran by using the least square error. For the study sites,the coefficients of Angstrom-Prescott model were determined as a=0.22 and b=0.52 the statisticalcriteria (R 2, RMSE, MBE, and t-test) showed that these coefficients were acceptable. These criteriawere illustrated that two developed models were more accurate than Angstrom-Prescott model.Keywords : Solar radiation, Angstrom-Prescott model, Meteorology factors, Mazandaran

INTRODUCTION

Knowledge of the local global solar radiation is required by most modals that simulate crop growth, andis also essential for many application, including evapotranspiration estimates, architectural design, and solarenergy system. Design of solar energy conversion system requires precise knowledge regarding the availabilityof global solar radiation at radiation at the location of interest. The Iran has the opportunity to utilize the solarenergy effectively, promoting a clean environment, and developing renewable energy technology in country.

The use of photovoltaic devices, on the other hand, is suitable for rural electrification, pumping water fromwells, telecommunication, solar thermal devices,etc(Almoroxet al. 2005).Given these many possible uses ofsolar energy, it is important to know the global solar radiation distribution throughout the year for the interestedregion.In addition, the values of the average daily global radiation in the solar energy applications are the mostimportant parameter, measurements of which are not available at every location due to cost, maintenance, andcalibration requirements of the measuring equipment. In places where no measured values are available, acommon application has been to determine this parameter by appropriate correlations which are empiricallyestablished using the measured data(Yang and koike 2006). Several empirical models have been used tocalculate solar radiation, utilizing available meteorological, geographical and climatologically parameters suchas sunshine hours, latitude, relative humidity, air temperature, precipitation, cloudiness. The most commonlyused parameter for estimating global solar radiation is sunshine duration. The first attempt at estimating globalsolar radiation was the well-known empirical relation between global solar radiation under clear sky conditionsand bright sunshine duration, given by Angstrom, sees(Sozne 2005). Theoretical and empirical models have

been postulated to compute the components of the insolation. Some of these models are theoretical, dealingwith the solution of the radiative transfer equation, while others are simply regression models.

The solar radiation modeling has shown significant progress in recent decades, reaching at presentintegration in geographic information systems that allow quantification at its spatial distribution(Elagib andBabiker 1998). There is a range of estimation methods, the firsts uses formulations that seek empiricalparameterization of the local physical conditions, using measurements in this field, from which quantitativelydescribes the optical characteristics of the air by simple equations and attenuation of solar radiation on thesurface(Ehnberg and Bollen 2005). This paper presents analysis of the relationship between daily global solarradiation and some geographical and meteorological factors. The reason for this approach comes from the factthat the air temperature and humidity is worldwide measured meteorological parameters, and is used byseveral authors in solar radiation estimation techniques. The objectives of this study were to compare, calibrateand validate existing solar radiation models to predict solar global radiation from available meteorological data.It is a goal of this author to find a model in daily scale based on meteorological variables without sunshinehours will provide a significant and new contribution to the methodology.
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MATERIALS AND METHODS

Stud y area and meteorolog ical dataThe province of Mazandaran covers an area of 23701 km2 in northern Iran. It is constitutes the

northern slopes of the Alborz Mountain Range and coastal lowland along the Caspian Sea. It is dominated bythe green belt of the Hyrcanian vegetation zone, which stretches over the northern slopes of the Alborz

mountain range and covers the southern coast of the Caspian Sea. Sari province extends between latitude 37N and longitude 53 E (Figure 1).

Figure 1. Location Mazandaran province in Iran

Daily data were taken from the synoptic meteorological station Covering the period from 1January 2000to 31 January 2010. The following meteorological variables are currently recorded in the IRIMO1 dailydatabase: actual global solar radiation (MJ m2 day1), maximum relative humidity (%),minimum relativehumidity (%), mean air temperature (C), maximum air temperature ( C), and minimum air temperature.Measurements of global solar irradiance were taken by pyranometer (SKYE SP1110). For quality control, allparameters were checked; the sensors were periodically maintained and calibrated. All data being recordedand hourly averaged on a synoptic station.

METHODOLOGY

The Angstrom - Prescott model is the most commonly used model as given by

N

nba R R a s

(1)

Where Rs is the monthly average daily global radiation, a R is the monthly average daily extraterrestrial

radiation, n is the day length, N is the maximum possible sunshine duration, and a and b are empirical

coefficients(Allenet al. 1998). The values of the monthly average daily extraterrestrial radiation ( a R ) are

calculated for days giving average of each month. a R Was calculated from the following equation,

sinsin360

.2sincoscos

365

360cos033.01

*24 s

s

sc w

wn I

Ra

(2)1 I.R.of IRAN METEOROLOGICAL ORG
http://www.aensionline.com/aeb_March_2011.htmlhttp://www.aensionline.com/aeb_March_2011.html

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Where sc I is the solar constant (=1367 Wm-2), is the altitude of the site, is the sun declination and sW is

the mean sunrise hour angle for the given month.The regression models proposed in the literature based on Angstrom and Prescott and other

Parameters are listed in Table 1(Lopez and Rubio 2000).

Table 1. Regression models proposed in the literatureSourceModel symbolRegression equation

( 1940 ) Angstrom and Prescott M1

N

nba R R a s

Akinoglu and Ecevit (1990)M2

2

N

nc

N

nba R R a s

Alagib and Mansel (2000)M3

N

nba R R a s exp

Allen (1998) M4minmax T T a R R a s

Bristol and Kemble (1984) M5 ca s T T ba R R minmaxexp1 Chen (2004) M6

d N

nbT T a R R

c

a s minmaxln

Bahel (1987) M7

)( minmax T T d RH c N n

ba R R a s

Abdullah (1994) M8

32

N

nd

N

nc

N

nba R R a s

Compar ison TechniquesThere are numerous works which deal with the assessment and comparison of monthly mean daily solarradiation estimation models. The most popular statistical parameters are the mean bias error (MBE) and theroot mean square error (RMSE). In this study, to evaluate the accuracy of the estimated data, from the modelsdescribed above, the following statistical tests were used, MBE, RMSE and coefficient of correlation (r), to testthe linear relationship between predicted and measured values. For better data modeling, these statisticsshould be closer to zero, but coefficient of correlation, r, should approach to 1 as closely as possible.

The Mean Bias ErrorThis test provides information on long-term performance. A low MBE value is desired. A negative value givesthe average amount of underestimation in the calculated value. So, one drawback of these two mentioned testsis that overestimation of an individual observation will cancel underestimation in a separate observation.

n

O P

MBE

n

ii 1

)(

(3)

The Root Mean Square ErrorThe value of RMSE is always positive, representing zero in the ideal case. The normalized root mean squareerror gives information on the short term performance of the correlations by allowing a term by term comparisonof the actual deviation between the predicted and measured values. The smaller the value, the better is themodels performance.

n

O P

RMSE

n

ii 1

2)(

(4)

The Coefficient of Correlat ion

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The coefficient of correlation, r can be used to determine the linear relationship between the measured andestimated values.

T-Statistic Test As defined by Student in one of the tests for mean values, the random variable t with n-1 degrees of freedommay be written here as follows:

22

2)1( MBE RMSE

MBE nt

(5)

The measured global solar radiation data are checked for errors and inconsistencies. The purpose ofdata quality control is to eliminate spurious data and inaccurate measurements. In the database, there aremissing and invalid measurements in the data and they are identified in the data. The missing and invalidmeasurements account for approximately 0.5% of the whole database. To complete the data, missing andatypical data are replaced by estimated values(Almoroxet al. 2005).

Figure2.Correlation of measured and estimated radiation data by Angstrom model,(a): calibration, (b) validation

The objective of this study was to evaluate various models for the estimation of the monthly averagedaily global radiation on a horizontal surface from bright sunshine hours and to select the most appropriatemodel for Sari station. All available empirical models that can be used to estimate monthly average daily globalsolar radiation Sari station in Iran have been collected from literatures to evaluate the applicability of thesemodels. The collected models were compared on the basis of the statistical error tests such as mean bias error(MBE), root mean square error (RMSE), correlation coefficient (r) and the t-test. According to the results,

Angstrom and Prescott model showed the good estimation of the monthly average daily global solar radiationon a horizontal surface for Sari station. Therefore, based on the statistical results a new simple linear model

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R/Ra= 0.22 + 0.52 (n/N) based on modified Angstrom model is extremely recommended to estimate monthlyaverage daily global solar radiation for Terengganu state areas and in elsewhere with similar climatic conditionsareas where the radiation data is missing or unavailable. The present work will help to advance the state ofknowledge of global solar radiation to the point where it has applications in the estimation of daily global solarradiation.

RESULTS AND DISCUSSIONS

In this study, the accuracy among the ten models was determined using the data measured at Sari stationin the periods between 2000 and 2010. The values of monthly mean daily global solar radiation intensity estimated using the above model werecompared with the corresponding measured values. The statistical tests of MBE, RMSE, r and t-test weredetermined for the entire period; the results are summarizes in Table 2.

Table2. Statistical test MBE, RMSE, r and t-test for sari station, in the period of 2000-2010R RMSE MBE t

calibration 0.8649 2.4648 -0.1368 2.061validation 0.8683 5.1495 -4.6284 66.17

ACKNOWLEDGMENTS

This paper is a partial result of the project, supported by the Islamic Azad University, Jouybar branch, Iran

REFERENCES

Allen R, Pereira L, Raes D, 1998.Crop evapotranspiration guidelines for computing crop water requirements, FAO Irrigation andDrainage, Paper No 56, Rome.

Almorox J, Benito M, Hontoria C, 2005. Estimation of monthly Angstrom Prescott equationcoefficients from measured daily datain Toledo,Spain, Renewable Energy 30: 931 936.

Ehnberg J, Bollen M, 2005. Simulation of global solar radiation based on cloud observations. Solar Energ y 78, 157 162.Elagib N, BabikerSh, 1998.New empirical models for global solar radiation over Bahrain, Energy Conversion and Management, Mgmt

39(8), 827-835.Lopez G, Rubio M, 2000. Estimation of hourly direct normal from measured global solar irradiance in Spain, Renewable Energy, 21, 175-

186.Sozne A, 2005. Solar energy potential in Turkey, Applied Energy 80, 367-381.Yang K, Koike T, 2006.Improving estimation of hourly, daily, and monthly solar radiation by importing global data sets, Agricultural and

Forest Meteorology 137, 43 55.

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