Effects of Solar Irradiance and Temperature on Photovoltaic Module Characteristics using a capacitive load method

— The electrical performance of photovoltaic (PV) cells or arrays is greatly influenced by the ambient temperature and the solar radiation intensity (irradiation) as well. The effect of temperature and solar irradiance on the main characteristics of solar panels and photovoltaic modules is investigated in this paper. The primary parameters are identified and extracted using the capacitive load approach. These parameters are Short Circuit Current (I sc ), Maximum Power Point Current (I mpp ), Open Circuit Voltage (V oc ), Maximum Power Point Voltage (V mpp ), Maximum Power Point (P max ), Fill factor (FF) and Efficiency (η). The PV cell used in this study is poly-crystal silicon. Its commercial name is Kyocera solar KC130GT. MATLAB Simulink is used to assess the capacitive load method in the investigation of I-V and P-V curves. These two curves are derived based on the effects of varying temperatures (30, 35, 40, and 45 o C) at a constant irradiance (1000 W/m 2 ) on the PV cell performance and the effect of varying irradiance (250, 500, 750, and 1000 W/m 2 ) at constant temperature (25 o C) as well. It is concluded that by increasing the irradiance at constant temperature, I sc and V oc are increasing. As a result, η increases from 13.9% at 250 W/m 2 to reach 14.7% at 1000 W/m 2 . In the case of increasing temperature at constant irradiance, η decreases from 13.5% at 30 o C to reach 12.8% at 45 o C. This is due to the large drop in V oc compared to the small increment in I sc .


INTRODUCTION
Recently, the expansion of sustainable and renewable energy has advanced as a supplier of environmentally friendly electricity to solve problems including the shortage of fossil fuels, contamination, global warming, and other difficulties [1][2][3][4][5].Our world obviously needs electrical power.The generation of electricity will progressively depend on alternative energy sources.In this regard, photovoltaic systems are widely used nowadays (solar power plants have steadily increased over the past 10 to 15 years) [6].
The matter of energy is a major one right now.Using renewable energy sources is a potentially beneficial solution to this issue.As a result, there is a significant amount of study and innovation being done in the sector of sustainable energy resources, including solar, thermal, wind, and ocean.However, the fact that solar energy may be employed to operate a variety of equipment draws a significant amount of interest.It is also an environmentally friendly form of energy.For the conversion of sunlight into electricity, solar photovoltaic cells and panels are employed.The most trustworthy information on a solar cell's efficiency at transforming sunlight into electricity is provided by its IV properties [7].The most practical and extensively used way of directly generating electricity from sunlight for solar energy production is photovoltaic (PV) power generation, which may be used in both home and business settings [8][9][10][11].The electrical performance of photovoltaic (PV) cells or arrays is greatly influenced by the ambient temperature and the solar radiation intensity (irradiation) as well.Thus, it is of utmost importance to acquire a quick and accurate measurement method of the voltage-current characteristic curve (I-V curve) of photovoltaic cells or arrays.Thus, from such an I-V curve, it was likely to calculate and evaluate the influence of radiation level and air temperature on the efficiency of PV systems [12][13][14].
It is widely recognized that when temperature rises, the efficiency of solar cells declines, principally because higher carrier concentrations cause the carrier internal recombination rate to increase [15].In a photovoltaic (PV) system, the operating temperature has a linear relationship with both the energy conversion efficiency  and the output power P [16].
The regulation of the current provided by the photovoltaic panels between the zero-current point (Voc) and the shortcircuit point (Isc) is the fundamental idea behind assessing the I-V curve.There are several ways to finish this job [17].
Controlling the current produced by the solar cells or modules between the zero-current point (Voc) and the shortcircuit point (Isc) is the basic notion underlying assessing the I-V curve [18,19].The I-V curve of solar photovoltaic modules can be measured in many ways, such as with variable resistors, electronic loads, capacitive loads, bipolar power configurations, and four-quadrant power supplies [20,22].

A. Single diode model
A Solar module could be simulated as a diode in parallel with a current source.It is widely acknowledged that the circuit in Fig. 1 can serve as a model for a PV cell, having one [23,24], two [25,26] and, uncommonly, three or more diodes [27,28].Using single diode model, Fig. 1 illustrates the corresponding circuit for a solar cell [28].A PV cell's singlediode equivalent circuit is composed of an irradiancedependent current source Iph shunted by Rsh and followed by a series resistor Rs.
Equation (1) could be utilized to represent the currentvoltage characteristics of the photovoltaic cell [29]: where T is the cell temperature, q is the electronic charge, n is the diode ideality factor, and k is the Boltzmann constant.I represents the solar cell's output current, and V represents its output voltage, I0 is the reverse saturation current, and Iph is the photogenerated current.

B. Typical Fundamental characteristics
The usual I-V and P-V profiles of a photovoltaic module are shown in Fig. 2

III. CAPACITIVE LOAD METHOD
The only way to accurately determine the electrical characteristics of a photovoltaic device is by experimental measurement of the I-V curve.The data provided by this measurement is extremely pertinent for the design, installation, and upkeep of PV systems.The regulation of the current supplied by the solar module between the zero-current point (Voc) and the short-circuit point (Isc) is the fundamental idea behind measuring the I-V curve.There are various ways to carry out this task, such as variable resistor, capacitive load, electronic load, bipolar power amplifier, four-quadrant power supply, and DC-DC converter [31].The SPV module's electrical curves are obtained using a capacitive loading approach, which is a common technique for all commercially available outdoor test facilities.Similar to a capacitor, the reactive component provides a low resistance path while it is being charged and an infinite resistance path after the capacitor voltage reaches the DC input voltage generated by the SPV module.The capacitor's auto-sweep capability enables minimal voltage and current ripple when the capacitor is used as a load to automatically and precisely sweep the electrical curve from Isc to Voc [32].
The capacitor simulates the characteristic as a variable resistor in this technique.The capacitor charge grows when the output DC voltage of the PV cell is introduced, the voltage progressively increases, and the current falls [33,34].Fig. 3 depicts the concept of that method [35].
The Photovoltaic module's terminals are connected directly to the capacitive load C via the S1 switch till the current I surpasses the open-circuit voltage Voc and data points are recorded for the characteristic curve because the capacitor's terminal voltage does not shift abruptly but instead starts growing gradually as the charge rises [36].
Throughout the capacitor charging period, an automatic rapid sweep of the I-V output curve from Isc to Voc is possible.After the measurements are finished, the capacitor's charge is discharged using a resistor linked to C via the switch S2 (with S1 open).Using the photovoltaic cell's single-diode similar circuit model from Fig. 2, a suitable capacitor's value C is calculated from [37]: Where tS is the sweep time (scan time), that is the time needed for the current I to charge C starting from Isc until it decreases From (4), When choosing the size of the capacitor, the short circuit current to open circuit voltage ratio matters more than the photovoltaic system power.The "transient I-V curve" could be very well approximated by the I-V curve derived by steady state techniques by determining the minimal scan time (using constant resistive loads).Usually, tS ranges from a few milliseconds to few seconds [38,39].

A. Matlab Simulink Modeling Platform
The most widely available I-V curve tracers use this technique [40,41].This highlights its relevance in practice, particularly for low and high-power PV systems, making this technique adaptable.It is important to stress that the technique gives a high level of adaptability to altering irradiation situations and under partial shading conditions (PSC), thus making the measurements of Isc and Voc with minimum error and ripple [42].The parameters used in the schematic diagram of capacitive load method in Fig. 4 are the basic elements which are the PV array (Kyocera solar KC130GT), temperature sensor, and irradiation sensor.
The main circuit of the capacitive load method as shown in Fig. 5 consists of sensors (current measurement sensor and voltage measurement sensor), ultracapacitor load, and discharge resistor.A portable Digital Multimeter (DMM) with high voltage measurement capacity is typically required for measuring SPV output DC voltage.The DC output current of the SPV module can be measured using a variety of current sensors, including small shunt resistors, the ADS712, and DC current transducers made by LEM [43,44].
When charging, the reactive component, which functions like a capacitor, gives a low resistance path; but, once the capacitor voltage reaches the DC input voltage generated by the SPV module, the reactive component offers an infinite resistance path.Capacitors come in a variety of varieties, but they might vary based on the task that needs to be done [44].I.Each string in the array has one module that is connected in parallel and one that is connected in series [45,46].
For that PV cell, a MATLAB code has been implemented via the available toolboxes in the MATLAB package to get the P-V and I-V curves under various conditions of radiation intensity and temperature.In this paper, MATLAB (R2021b) is used to investigate the results and curves.

B. Irradiance and Temperature Effects on Electrical characteristics of PV module
The influence of solar irradiance on the electrical properties of PV modules is modelled at constant temperatures of 25 o C and solar irradiances of 250 W/m 2 , 500 W/m 2 , 750W/m 2 and 1000 W/m 2 and the modelling outcomes are shown in TABLE II.The P-V and I-V curves for the PV system are illustrated in Fig. 7

C. The Relation of the Electrical Parameters
It is possible to explain these relationships more accurately and graphically by studying the inter-relationships between the PV cell parameters at variable irradiance or temperature [47].The parameters will be illustrated, such as (Voc, Isc, FF, and efficiency η).
First, the relationship at specific temperature between solar irradiation, Isc (short circuit current), and Voc (open circuit voltage).At a temperature of 25 o C, the Irradiance values are 250, 500, 750, and 1000 w/m 2 .As shown in Fig. 9, it has been discovered that the open circuit voltage Voc and the short circuit current Isc both rise with irradiation [48].
The effect of irradiance at constant temperature on the efficiency (η) and the fill factor (FF) is shown in Fig. 10.As the irradiance increases at constant temperature (25°C), the efficiency of the cell will increase to reach 14.7% at 1000 W/m 2 .Regarding the variation with temperature, it is well known that, most of the characteristics of semiconductor materials are impacted by a semiconductor's bandgap, which is reduced as temperature rises [49].Fig. 11 shows that Voc decreases with temperature (at constant Irradiance=1000W/m 2 ).This is attributed to the excess rise in the reverse saturation current compared to the photogenerated current.However, Isc does somewhat rise with temperature because the bandgap energy, EG, drops with temperature, allowing more photons to have sufficient energy to form electron-hole pairs.However, this is a tiny effect.Its relative increase is of the order of 0.06% per °C for silicon [50].
Concerning the effect of temperature on the efficiency (η) and the fill factor (FF), it is revealed in Fig. 12, that η decreases with increasing temperature, at constant Ir (1000 w/m 2 ).The efficiency drops to 12.8% at 45 o C. Its relative decrease is of the order of -0.15% per o C [51].But the fill factor at low temperature less than 35°C increases, and subsequently decreases with increasing temperature.

IV. CONCLUSION
In this paper, the performance of PV cells was examined in relation to temperature and irradiance.Two scenarios were used to conduct this investigation.The first involved investigating how the primary characteristics of a poly-crystal silicon PV module were affected by varying irradiance at a constant temperature.The second focused on the impact of varying temperature while maintaining irradiance.The capacitive load method was used to identify and extract the key parameters.The outcomes showed that irradiance will enhance the performance of the PV cell by raising its efficiency to 14.7% at 1000 W/m 2 at constant temperature (25 o C).When considering how temperature affects PV cell efficiency, it is found that, under constant irradiance (1000 w/m 2 ), decreases with increasing temperature.At 45 o C, the efficiency falls to 12.8%.The researcher recommends an investigation of the effect of series and parallel connection with temperature and irradiance on the PV cell efficiency as a future work.
(a) and (b), respectively.The short-circuit point Isc, open-circuit point Voc, and maximum power point Pmax are marked on the I-V/P-V curves.These three points are then used to determine Vmp, Imp, and Pmax.The solar cell's efficiency (η) and fill factor (FF) are then determined using the equations (2, 3)[30]:  is the input optical power.

Fig. 2 .Fig. 3 .
Fig. 2. (a) A conventional P-V curve showing the Maximum Power Point .(b) Vmp and Imp on the I-V curve are derived from the point Pmax on the P-V curve and also the fundamental parameters Isc and Voc are shown.

Fig. 6 .
Fig. 6.Sub system 2 (output measurment ) in Fig. 4. MATLAB-based Simulink simulations are performed to scan the I-V curve of a polycrystalline solar array with 130W.Its commercial name is Kyocera solar KC130GT, and it has a maximum power of 130.064 watts and 36 cells per module as shown in TABLEI.Each string in the array has one module that is connected in parallel and one that is connected in series[45,46].
(a) and (b) respectively.Under constant solar irradiation of 1000 W/m 2 , the impact of temperature on the electrical properties of PV modules at 30, 35, 40, and 45 o C is modeled and the results are shown in TABLE III.The PV module power-voltage (P-V) and currentvoltage (I-V) curves are shown in Fig. 8 (a) and (b) respectively.

Fig. 7 (
Fig. 7 (a) I-V curves at different radiation and constant temperature (25 o C).(b) The corresponding P-V curves.

Fig. 10 .
Fig.10.The efficiency (η) and the fill factor (FF) for a polycrystalline silicon solar cell as a function of irradiance at constant tempreature (25°C).

Fig. 12 .
Fig. 12.The efficiency (η) and the fill factor (FF) for a polycrystalline silicon solar cell as a function of tempreature at constant irradiance (1000w/m 2 ).

TABLE II .
The PV module characteristics with 250, 500, 750, and 1000 W/m 2 of solar radiation at 25 o C of constant temperature

TABLE III .
The PV module characteristics with 30, 35, 40, and 45 o C of temperature at 1000 W/m 2 of constant radiation.