Influence of Temperature on the Performance of Photovoltaic Polycrystalline Silicon Module in the Bruneian Climate

The influence of working temperature for a polysilicon module has been investigated in Brunei Darussalam for a period of two years. The rise in temperature produces thermal agitation which not only increases the dark current but also enhances the losses of free carriers in a polycrystalline module. The efficiency and the output power decreases with an increase in the working temperature. A maximum decline in the output power of 97% has been measured under a dominated diffused radiation environment. The temperature coefficients have been obtained and equations are developed to evaluate the change in the rating of module at any working temperature with reference to their values at STC.


Introduction
Brunei Darussalam is located in the equatorial region of the globe where day length lasts for 11-12 hours with a bright sunshine of 6-7 hours.The monthly-averaged daily mean temperature is 30-35 o C. In such a climate, the working temperature of photovoltaic (PV) cells/modules was measured as high as 75 o C. It is well documented in literature that an increase in the working temperature of PV devices alters their electrical performance [1][2][3][4][5][6][7][8][9][10][11][12][13].A number of researchers have investigated how an increase in the temperature influences the electrical properties of PV devices in a controlled environment using a sun simulator and/or environmental chamber [14][15][16][17].In such experiments, all other variables for example, irradiance, wind speed, etc are kept constant.These hypothetical and artificial operating conditions cannot be simulated in a natural sunlight where a number of micro-climatic parameters along with the temperature simultaneously drive the module.A temperature in a natural environment is no longer a single valued function as in the case of controlled environment.The objective of the present study is to investigate how the temperature variation influences the rating of a polycrystalline silicon module in a natural environment, where these devices are to be operated.

Methodology
A polycrystalline module (Kyocera LU181C58A, 5.83 W) was placed horizontally on a stand at the roof of Faculty of Science Building, University of Brunei Darussalam for a period of 24 months (December 04 to November 06).The schematic of the experimental set up, shown in Figure 1, consists of one high resolution Keithley 2420 Digital SourceMeter, and a Kipp and Zonen pyranoemter.The experiment was carried out twice a week and each experiment consisted of twelve measurements taken at a half hour interval starting at 1000 hours to investigate temporal variation on the electrical performance of PV devices.

Theoretical Background
The electrical properties of a PV device comprises of seven parameters: open circuit voltage, V oc , short circuit current, , maximum voltage, , maximum current, , maximum power, , conversion efficiency, , and fill factor, FF.These parameters measured at Standard Test Condition (STC) supplied by the manufacturer are tabulated in Table 1.Solar cells only convert a small amount of absorbed solar radiation into electrical energy.The remainder is dissipated as heat in the bulk of the silicon solar cell which increases its temperature compared with the environment.An increase in the working temperature of solar cell reduces the band gap allowing more energy to be absorbed which increases the short circuit current (I sc ) of a solar cell for a given irradiance.The band gap energy E g (T) of the material as a function of temperature can be written as [18][19][20]: (1) Where E g (0) is band gap energy of the material at room temperature; a and b are constants.This effect alone raises the theoretical maximum output power of the solar cell.
At the same time an increase in the temperature increases the population of electrons exponentially.This enhances the dark saturation current (I o ) that is a minority carrier current and its variation with temperature can be written as [18][19][20]: ( Where A o and k B are the area of the device and Boltzmann's constant.The increase in the dark saturated current decreases the open circuit voltage, V oc , of a device that can be expressed as: (3) Theoretically, a decrease in the open circuit voltage would reduce the output power of the device.The short circuit current and open circuit voltage are related to an important property of a PV device known as a fill factor ,FF, which is defined as the ratio of the maximum power of the PV device to the product of I sc V oc : (4) The fill factor diminishes as the temperature of the device increased.The decrease in V oc and FF with the working temperature of the device outweighs the slight increase in the short circuit current.

Results and Discussion
There are 12 data sets for each test.Each data set was used to plot I-V and P-V characteristics of the module.The seven variables in the electrical characteristics of a PV device known as open circuit voltage, V oc , short circuit current, , maximum voltage, , maximum current, , maximum power, , conversion efficiency, , and fill factor, FF were calculated for each data set.These parameters are compared with those measured at STC and given in Table 1.
Out of seven electrical parameters mentioned above three are independent (V oc , and FF) and the rest are interlinked with the independent parameters.The data obtained have been used to compute the temperature coefficients for V oc , , FF, and .The test period of two years can be divided into   2).This may be due to two possible facts.Firstly, a polycrystalline module contains crystal defects and high mobilities of free carriers provide a greater probability for recombination as well as trapping in sinks like grain boundaries etc.Secondly, it is difficult to explore the dependence of only the temperature on V oc and of the PV device installed in an open environment.However, theoretically it is possible by freezing all other dependent variables.In a natural environment all localized climatic parameters act simultaneously on the module and its response represents the collective effect of these parameters on the performance of the module.
There were 27 events in a period of two years when the working temperature of the module was in the range of 27-40 o C with the irradiance of 27-347 Wm -2 .One of these is depicted in Figure 3. Figure 3 represents working of the module in a very low irradiance of 60 Wm -2 with the working temperature of 27 o C representing a diffused dominated radiation environment.The maximum power of the module was 97% less than that at STC with an efficiency of 6% and the FF equal to 0.69 that is reduced by 0.05 with reference to that at STC.The value of the FF at this point of time indicates that under such a low irradiance the quality of the module has not deteriorated.The working temperature of the module is lower due to the diffused/scattered nature of solar radiation.It is notable that even with the presence of crystal defects in the module it can work under such an unfavourable climate.The variation of the maximum power, the efficiency and the fill factor with the working temperature of the module are depicted in Figures 4-5 and values are given in Table 2.  current decreases instead of increasing due to recombination current and other mechanisms through which free carries lose identity.The loss of free carriers is dominant in poly-silicon cells at moderate and high working temperatures and is not common in single crystalline modules [23].An efficiency of 7% with a loss of 4.5% with reference to that at STC and the maximum output power of 3.57 W that is 2.26W less than that at STC was noted Figure 6: I-V and P-V characteristics of the module at high irradiance and moderate working temperature.
There were 82 tests carried out when the working temperature and irradiance lay in the ranges of 61-75 o C and 780-1250Wm -2 respectively.Out of these the results of one test run is shown in Figure 7.A slight increase in the short circuit current of the module was noted that for a working temperature of 68-75 o C with an irradiance greater than or equal to 1250 Wm -2 .This indicates that the influence of a combination of high working temperature and irradiance suppressed the free carrier loss mechanisms in this polysilicon module.Under these operating conditions our module lost a minimum of 35% efficiency, 18% of the maximum output power and 6% of the fill factor compared with that at STC.It is also noted that the loss in these parameters are nonlinear with the working temperature and irradiance.It is very difficult to generalize our findings at this point of time as we would need to contrast our findings with other similar products from various manufacturers.Nevertheless these findings challenge and call for caution when using the electrical ratings of a PV device at STC for the design of a PV generator/system for any locality on the globe without considering how the climate of that locality affect the performance of the PV system.set of low working temperature of 38 o C and irradiance of 242Wm -2 .In both cases an increase in the working temperature decreases the short circuit current in different fractions.The decrease in the short circuit current at a temperature of 60 o C is more than that at 38 o C indicating that losses of free carriers are more in the first case compared with the second.In reality, it is difficult to compare a set of four results presented in Figure 8 because the temperature and irradiance are not the only parameters that describe a natural environment.The wind speed, humidity, visibility and concentration of atmospheric constituents also effect the performance which are not taken into account in the present investigation.
The temperature coefficients given in Table 2 can be used to determine quantitatively the effect of temperature on different electrical parameters.One can use the following equations to find out the effects of working temperature (T W ) on these parameters with reference to their values at STC: In equations (5-7) a temperature of 25 o C is used as a reference temperature corresponding to STC.
The temperature coefficients obtained by other researchers [12,20,[24][25][26] for polycrystalline module(s) are quantitatively different from those obtained in the present investigation.Because the characterization of modules are dependent on the environment in which these are exposed.A few researchers [24][25] used the sun/solar simulators (pulse-light or continuous-light solar simulator) that represent an artificial and controlled environment.The temperature coefficients measured by these research groups do not represent a natural environment and therefore our results should differ from those obtained by these research groups Meyer and Mapuranga [25] carried out rating of a multicrystalline PV modules consisting of 36 cells in the climatic conditions of Alice, South Africa.Our results quantitatively should differ due to two different natural testing environment as well as using two modules of different rating as well as manufactured by different manufacturers.Their results show an increase in the short circuit current as the working temperature increases while our results indicate an increase in the short circuit current when the working temperature and irradiance are either low or high.For intermediate temperatures with either high or low irradiance the short circuit current decreases.These differences might be due to the fabrication processes used to make polycrtslline modules, the concentration of impurities and defects in the module and the operating environment.

Conclusions
It has been observed that the polycrystalline module can work in a very low irradiance as 60 Wm -2 with a loss of out power of 97% with reference to that at STC.The value of fill factor for this event was 0.69 indicating that the quality of the module is not deteriorated.The combinations of low working temperature with low irradiance and high working temperature with high irradiance increase the short circuit current of the module.A combination of intermediate working temperatures with irradiance in the range of one full sun decreases the short circuit current of the module.The maximum power, efficiency and the fill factor of the module degraded as the working temperature increases.A linear relation could not be established to correlate these variables.
The duel behavior of the short circuit current with an increase in the working temperature of the module is due to the following facts:  An increase in working temperature of the module increases the dark current that in turns increases the short circuit current  An increase in working temperature also increases the free carriers losses The net effect of these two terms determines the destiny of the short circuit current in polycrystalline modules.
A set of equations (5-9) can be used to evaluate the rating of a polycrystalline module at any working temperature not only for Brunei Darussalam but also for those localities on the globe with similar climatic conditions.

Figure 1 :
Figure 1: Experimental set up for measuring current voltage (I-V) characteristics of a polycrystalline silicon photovoltaic module (Kyocera LU181C58A, 5.83 W) in a natural environment.
three categories with reference to the working temperature of the module.The first corresponds to the working temperature in the range of 27-40 o C, the second 41-60 o C and the third 61-75 o C. The temperature coefficients for V oc and are shown in Figure 2 and tabulated in Table2.

Figure 2 :
Figure 2: Variation of V oc and I sc with the working temperature of the module with reference to STC.The data presented in Figure2demonstrate that an increase in the temperature decreases both the open circuit voltage as well as the short circuit current of the device.The open circuit voltage decreases more sharply than the short circuit current and can be understood from the Equation (3).A decrease in the short circuit current negates the theoretical expectation as expressed in Equation (2).This may be due to two possible facts.Firstly, a polycrystalline module contains crystal defects and high mobilities of free carriers provide a greater probability for recombination as well as trapping in sinks like grain boundaries etc.Secondly, it is difficult to explore the dependence of only the temperature on V oc and of the PV

Figure 3 :
Figure 3: I-V and P-V characteristics of the module at low irradiance and low working temperature.

Figure 4 :Figure 5 :
Figure 4: Variation of (W) and efficiency with the working temperature of the module with reference to the STC

Figure 7 :
Figure 7: I-V and P-V characteristics of the module at high irradiance and high working temperature.

Figure 8 :
Figure 8: P-V characteristics of polycrystalline module at different working temperature and irradiance level in natural environment.

Figure 8
Figure8depicts the P-V characteristics of a polycrystalline module at different working temperatures and irradiance levels in the Bruneian climate.The results presented in this figure indicate that the power output of the module at high irradiance and working temperature are very similar to that at low working temperature and low irradiance.This is due to the fact that in both cases the working temperature increases the short circuit current.The lowest output power corresponds to an intermediate working temperature of 60 o C and a high insolation of 980 Wm -2 and an intermediate output power is recorded for a