Understanding the Factors That Affect Photovoltaic Performance

Introduction

With growing use of solar PV technology, it is essential that the efficiency and performance of systems are given high priority. In order to do so, it is a must to understand what affects the performance of PV modules and in what way. There are parameters that define the performance of PV modules.

These usually differ from module to module depending on various factors such as the material used to make the cells, the number of cells in a module etc. The parameters are determined upon testing the modules and thus, the conditions under which these tests are conducted are also important in order to standardize and compare the performance of different modules. This article discusses what effect some of the factors have on the performance of PV modules. The article also discusses the testing conditions used to determine the output of PV modules.

PV Output Characteristics and Parameters of PV Systems

There are two output characteristics of a PV system. These are:

The Current-Voltage Characteristic (I-V): This is the output current of a PV module or system as a function of the output voltage. The general I-V characteristic of a PV system is shown in the figure below:

The Power-Voltage Characteristic (P-V): The P-V characteristic of a PV system is its output power as a function of its output voltage. The general P-V characteristic curve is shown in the figure below:

The following parameters affect the performance of a PV module:

Open-Circuit Voltage (V_OC): This is the maximum voltage output of a PV cell. V_OC occurs when the current in the circuit is equal to zero.

Short-Circuit Current (I_SC): The short-circuit current is the maximum current output of a solar cell. This occurs when the solar cell is short-circuited and hence, the voltage is zero.

The following image shows the V_OC and I_SC of a solar cell in the general I-V curve:

As can be seen in the image above, the highest current on the I-V curve is the I_SC which occurs when the voltage is equal to zero. Also, the highest voltage output of the solar cell is when the current equals zero and it is the V_OC. Even on the P-V curve, the highest voltage output is the V_OC. Another point on the graph is the ‘MPP’ which stands for Maximum Power Point. This is the maximum power output of a solar cell. It is known that Power is equal to the product of the Voltage and the Current (P = V x I). Hence, on the I-V curve, the Power is represented by the area under the curve. The Maximum Power Point is the set of coordinates of an I-V curve that give the maximum power output. These coordinates are written as Voltage at Maximum Power Point (V_MPP) and Current at Maximum Power Point (I_MPP). On a P-V curve, the MPP is the highest point attained by the curve on the Y-axis.

Fill-Factor (FF): As we know, the current at V_OC and the voltage atI_SC are both zero. This implies for the power equation that the output power at both V_OC and I_SC is zero. The FF is the ratio of the Maximum Power the cell attains to the product of the V_OC and I_SC as shown below:

What this represents graphically is how rectangular the solar cell’s I-V curve is. The area under the I-V curve will be larger for a larger FF.

Solar Cell Efficiency: Cell efficiency is one of the most commonly-used parameters to compare solar cells. It is defined as the ratio of the output power of the solar cell to the input power from the sun. Numerically, this is shown below:

Here, η stands for efficiency and P_in stands for the input power from the sun. This is the irradiation. For efficiency calculations, the value of P_in is either 1 kW/m² or 100 mW/cm².

We shall now see how the different factors affect the performance of PV cells and modules.

Effects of Resistances

Since PV cells and modules are made up on semiconductor materials, there is bound to be some sort of resistance in the circuit. The resistance at the Maximum Power Point of the Solar Cell is called the Characteristic Resistance (R_CH):

Silicon PV cells used commercially usually have very high current and low voltage. Typically, a 156 mm² solar cell has a current of about 9A and a V_MPP of 0.6V. This gives an R_CH of 0.067 Ohms.

There are also some resistive elements which are not desirable in a circuit. These are called parasitic resistances. These affect the efficiency of solar cells and the most common ones are Series Resistance (R_S) and Shunt Resistance (R_SH). The following circuit diagram includes both series and shunt resistances in the solar cell representation:

Series resistance is caused due to three reasons. First is the passing of current through the p-n junction and base of the solar cell. Second is due to the contact resistance between the metal contact and the silicon. And lastly, it is due to the top and rear metal contacts. The main impact of R_S is the reduction in the Fill Factor. At high values of R_S, the I_SC could also decrease. Thus, lower the R_S is the more ideal the I-V curve is. The effect of R_S is shown in the image below:

The presence of a shunt resistance in a solar cell is usually due to manufacturing defects and causes significant power loss. The power loss occurs because the shunt resistance provides an alternate current path for the light-generated photocurrent. This reduces the amount of current flowing through the solar cell junction and reduces the voltage from the solar cell. At lower levels, the impact of shunt resistance is higher at since there is less light-generated current. At very low values of shunt resistance, the V_OC decrease. The effect of shunt resistance on the I-V curve is shown below:

Effect of Varying Irradiation

Irradiation (G) refers to the amount of sunlight that is incident on an object in the form of electromagnetic radiation. It is the measure of solar energy that is incident on an area over a period of time. It is measured in W/m². Solar modules go through a variation of light intensity due to factors such as sun’s changing position and the presence of clouds. This can affect the performance of the solar module. The following graphs show the effect of changing irradiation on the P-V and I-V characteristics of a module:

As can be seen in the P-V characteristics above, the maximum output power reduces with reducing irradiance. There is also little reduction in the V_OC of the module. Similarly in the I-V characteristics curves, we can see that there is a little increase in the V_OC with increasing irradiance. Also, the I_SC increases significantly with the increasing irradiance levels.

Effect of Varying Temperature

Solar cells are sensitive to the temperature they are operating in because of the semi-conductor materials they are made up of. There is a reduction in the band gap of a semiconductor when there is an increase in the temperature. This affects majorly all of the semiconductor material parameters. In a way, this can be viewed as increased energy of the electrons in the material. Hence, lesser energy is required to break the bond. Therefore, with an increase in temperature, the band gap reduces. In solar modules, the most-affected parameter by an increase in temperature is the V_OC. The way in which increasing the temperature affects a PV cell or module is shown in the figure below:

As can be seen in the figure above, there is a significant decrease in the V_OC with an increase in temperature. There is also an increase in the I_SC however, this is very minor.

Standard Testing Conditions

The Standard Testing Conditions (STC) refers to a set of conditions under which the tests for PV modules are conducted. Using these standardised conditions, the ratings of solar panels become easier to compare. The following are the conditions that come under STCs:

• Irradiation level of 1 kW/m²
• Cell temperature of 298 K (25°C)
• Air Mass of 1.5

Conclusion

This article has introduced to us at a basic level what the output characteristics of a PV cell or module are. The P-V and I-V curves are an effective way to analyse the output characteristics of a PV system. We then looked at the different parameters of a PV system. In order to see how the different factors affect the performance of a PV system, we first considered the effects of resistance where we shed light on characteristic and parasitic resistances, what types of parasitic resistances there are, what cause them, and also the effect each type of resistance has on the performance of PV systems. We then looked at irradiance and how a change in irradiance affects the output characteristics of a PV system. We also looked at how temperature affects the performance of PV systems. We finally looked at what the Standard Testing Conditions are for testing PV systems and how they ease the comparison between different modules.