Effect of Junction Cell Temperature and Geographical Coordinates on The Electrical Performances of a Photovoltaic Module

After the absorption of the photons, during the photovoltaic conversion process, one part of the radiation remains unabsorbed causing the cell to overheat and thus a drop in e ciency. The purpose of this study is to explore the e ect of junction temperature, geographic coordinates as well as the season on the electrical performance of the photovoltaic cell. The results obtained show that the junction temperature has an e ect which is not favorable on the electrical e ciency of the module for high temperatures around midday which is of 11% however it reaches 14% for low temperatures in the morning. Geographical coordinates at di erent altitudes, have no e ect on the energy produced from the module, but the e ect of the season on the efciency con rms the previous results, that, the e ciency is good for low temperatures. The results are obtained by simulation, through a computer code in FORTRAN language, designed for this purpose.


INTRODUCTION
Currently the use of renewable energies as an alternative source or even as a main source of energy has spread greatly, particularly in remote areas where the distribution of conventional electricity is not guaranteed. The most common primary source for the supply of renewable energy is solar energy by means of solar panels [16]. Extracting power from these resources requires further research and development to increase reliability, reduce costs (manufacturing, use and recycling) and increase energy eciency [19].
Photovoltaic solar cells are used to convert sunlight energy into electrical energy [5].The photovoltaic eect used in solar cells makes it possible to directly convert the light energy (photons) from the sun rays into electricity. When the photons strike a thin surface of a semiconductor material (silicon), they transfer their energy to the electrons of matter. They then move in a particular direction, creating an electrical current.
According to the literature, the eect of several parameters on the characteristic and electrical behavior of a solar cell is studied, such as the inuence of solar irradiance and ambient temperature on the optimal resistance of a PV generator [17]. [6], taking into account the environmental parameters relating to illumination and ambient temperature on the characteristic behavior of the PV cell, A. Khalifa et al. [7], presents the results of his work on the eect of junction temperature (cell). The increase of the latter, which will cause the drop in the cell's electrical eciency, is due to the part of the radiation not absorbed by the cells [7]. [8] has studied the characterization of the electrical operation of PV panels. According to these results, when the radiation varies between 300 W/m 2 and 900 W/m 2 , the optimum voltage decreases by 10.2%.
The work of [9] on PV modules performance degradation in the Saharan environment showed that the eciency lost 19% of its initial value. The main objective of this work is to study the inuence of the geographic coordinates and the junction temperature on the performances of a photovoltaic module and therefore provide recommendations to overcome the unfavorable effect of the latter.

System description
A photovoltaic cell, consists of two doped silicon layers, one of which has an excess of electrons (layer N) and the other has an electron deciency (layer P). There is production of electrical energy after absorption of incident photons and creation of electron-hole pairs if the energy of the photon is greater than the gap of the material.
A panel is a series assembly of these cells. The power generated depends on the load at the panel output.
The system studied in this paper is a photovoltaic generator, Fig.1. Located at the level of the city of Constantine with the following geographical coordinates (6 o longitude, 36 o latitude and 693 m of altitude) and an inclination equal the latitude of the place, composed of fteen (15) monocrystalline modules type "CNPV-50M", mounted in series. Each module consists of (36 cells), whose characteristics shown in table.1. The study of the solar potential is the starting point of any study concerning the dimensioning of a solar installation or of an energy system. Then, it is necessary to know the meteorological weather conditions of the site of implantation to evaluate and to estimate the solar potential [10].
In Algeria, the number of stations to assess the solar eld is very limited [11]. Certain approaches are used to predict the characteristics of solar radiation [12]. Several models exist for the estimation of the dierent components of the global, direct and diuse solar irradiation. Most existing models require the knowledge of a large number of site data. In Algeria, generally these data are not all available. In our study, we adopted the Kasten model [1], which is valid for heights of more than 10 o , and considers the atmospheric disturbance [1]. Similarly, there are Power temperature coecient %/ o C -0.45 practical algorithms that allow an evaluation of the illumination received by any orientation surface from the astronomical, geographical and geometrical data of the place, Fig. 2.
Direct illumination (I) on a horizontal plane: Direct solar radiation dened as the radiation from the solar disk alone and calculated by the expression: Direct illumination on an inclined plane (Si): The global illumination on a horizontal plane [6]: where M, N : characterizing the state of the atmosphere.
Depending on the state of the sky, the global illumination is determined on a horizontal plane by one of the Following formulas [6]: The diuse illumination (D 0 ) on a horizontal plane [6]: The diuse illumination (D i ) on a plane of inclination (i) The global illumination on an inclined plane (G h ) is given by: With: The following expression, called Gauss's formula, gives the angular height of the sun [2]: Where, (n) is the number of the day in the year. Liu and Jordan proposed to take the 16 th day of each month, as the most representative of the average day of the month considered. As for Klein he showed that it was better to choose that day using

Modeling of junction temperature
where

Daily energy generated
In order to evaluate the energy produced by each module, it should be reminded that the modules are classied under nominal operating conditions, but the working conditions actually recorded in the eld rarely correspond to their nominal values. Thus, the daily energy produced by the photovoltaic generator can be estimated by the following equation [13]: where And: -T ST C : temperature under reference conditions (= 25 o C).

Electrical eciency of the panel
The eciency of a PV module depends on the junction temperature given by the following formula [14]: Calculations are made from an initial time (t 0 = 6 h .00) for the day of August and (t 0 = 8 h .00) for the days of December (n = 344) and April (n = 105)because of the Kesten model that is valid for Sun heights greater than (10 o ), and a time step equal to one hour. The results obtained by simulation and through a computer code in FORTRAN language. All the following results are about the representative day of August (n = 228).

Temporal evolution of solar irradiation
The observation of Fig. 3 shows that the curve representing the global radiation temporal evolution calculated by the Kasten formula is close  radiation component represents the sum of the two components of radiation, the direct and the diuse. The two gures below clearly show the validation of Kasten's model. Figure 4 shows the temporal evolution of the two components, the curve from experience (Fig. 5); except for the direct radiation in the morning, which can be due to more precise factors and coecients (albedo, condensable water height, etc.), which were used in the calculations.

Temporal evolution of ambient and cell temperatures
As shown in Fig. 6, the ambient temperature is constantly changing with time. For the cell temperature, it is higher than the ambient temperature. It has the same outline as reference [7], but we notice that it takes the shape of the component of the solar irradiation, so the latter increases as the irradiation increases, same results of the reference [8].

Temporal evolution of the daily energy produced by the photovoltaic generator
The analysis of the curve of Fig. 7, clearly demonstrates that the curve takes the same form as that of irradiation, which indicates that the daily energy produced by the GPV, being of the order of (8 KWh) at solar noon, is proportional to irradiation. 3.4. Eect of cell temperature on eciency Figure 8 shows the evolution of the electrical eciency. According to this gure, the electrical efciency is inversely proportional to the junction temperature of the PV panel. Therefore, the effect of the latter is not favorable on the electrical eciency. It is of the order of (14%) at the minimum cell temperature (25 o C) to (6.00 a.m.) and of (11%) for the maximum temperature (64 o C) towards (12.00 h), close to the experimental result (10 %) of the reference [7] at solar noon. It has been shown previously that the temperature of the photovoltaic cell will rise with increasing radiation, therefore according to the literature, the more the illumination increases, the temperature of the photovoltaic cell drops and the noload voltage (V co) falls; this induces the degradation of the performance of the photovoltaic module [8].

Eect of the season on the electrical eciency
In order to show the inuence of the season, on the electrical eciency, We have shown the evolution of the electrical eciency as a function of the junction temperature for two months of dierent seasons. April (n= 105) and December (n = 344). The results illustrated in the two gures, Fig. 9, Fig. 10, clearly show that the eciency towards solar noon is slightly higher in December (13%, T j = 41 o C ) than in April (12%, T j = 51 o C ). This conrms the previous results, that the eciency is good for lower junction temperatures.

Eect of geographic coordinates on energy production
We Observed in Fig. 11, that all the curves representing the temporal evolution of the energy produced by the generator for dierent altitudes (Z) of the implantation site of the photovoltaic generator and for the same day in the year (n = 228) are superimposed, indicating that the geographical coordinates have no eect on the production of daily energy.

CONCLUSION
It is important to study the inuence of the interior and exterior parameters of the photovoltaic system on these performances. According to the obtained results, the external parameter (geographical coordinates) has no eect on the electrical production and the internal parameter studied (junction temperature) has an eect that is not favorable for high temperatures on the electrical eciency. However, the increase in irradiation has had a good eect on electrical production. Hence, the importance of lowering this temperature by cooling and adopt a hybrid system PVT, and integrate the MPPT technique into the system, in order to nd the most economical system and even to value the solar electrical production.