As discussed previously, a pn junction diode and a light-generated current are required to model a solar cell. Hence, the simplest solar cell consists of a pn junction diode connected in parallel with the currrent source — shown below in Figure 1.
Figure 1 : Real solar cell model
Meanwhile, when there is no light present to generate any current, the PV cell behaves like a simple pn junction diode. However, if the intensity of incident light increases, current is generated by the PV cell. An interactive link is given here http://www.pveducation.org/pvcdrom/solar-cell-operation/iv-curve, where the operation of the PV cell starts from darkened to illuminated condition.
There are 2 major, important parameters to understand the performance characteristic of a PV cell. They are:
- Fill Factor: The short-circuit current and the open-circuit voltage are the maximum current and voltage respectively from a solar cell. The “fill factor,” abbreviated as “FF,” is a parameter which, in conjunction with Voc and Isc, determines the maximum power from a solar cell. The FF is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc. Basically, with higher FF, the performance of a solar cell is better.
- Efficiency: Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun. In addition to reflecting the performance of the solar cell itself, the efficiency depends on the spectrum and intensity of the incident sunlight and the temperature of the solar cell. The efficiency of a solar cell is determined as the fraction of incident power, which is converted to electricity and is defined as:
Some important factors responsible for the performance and efficiency of a solar cell are given blow:
- Characteristic resistance: The characteristic resistance of a solar cell is the output resistance of the solar cell at its maximum power point. If the resistance of the load is equal to the characteristic resistance of the solar cell, then the maximum power is transferred to the load and the solar cell operates at its maximum power point.
- Parasitic Resistances: Resistive effects in solar cells reduce the efficiency of the solar cell by dissipating power in the resistances. The most common parasitic resistances are series resistance and shunt resistance. In most cases and for typical values of shunt and series resistance, the key impact of parasitic resistance is to reduce the fill factor. Both the magnitude and impact of series and shunt resistance depend on the geometry of the solar cell, at the operating point of the solar cell. Impact of both resistances on the solar cell could seriously reduce the fill factor. For an ideal solar cell, series resistance equals to 0 ohms while shunt resistances equals to infinity.
- Temperature: Solar cells are sensitive to temperature. Increases in temperature reduce the band gap of a semiconductor, thereby effecting most of the semiconductor material parameters. In a solar cell, the parameter most affected by an increase in temperature is the open-circuit voltage. As the temperature increases, the open-circuit voltage decreases, thereby decreasing the fill factor and finally decreasing the efficiency of a solar cell. It is recommended to operate at 25 degree celsius. The power output for different operating temperatures is shown in Figure 2.
Figure 2: Solar cell power characteristics temperature dependency
- Light intensity: Changing the light intensity incident on a solar cell changes all solar cell parameters, including the short-circuit current, the open-circuit voltage, the fill factor, the efficiency and the impact of series and shunt resistances. The light intensity on a solar cell is called the number of suns, where 1 sun corresponds to standard illumination at AM1.5, or 1 kW/m2. Solar cells experience daily variations in light intensity, with the incident power from the sun varying between 0 and 1 kW/m2. At low light levels, the effect of the shunt resistance becomes increasingly important. Consequently, under cloudy conditions, a solar cell with a high shunt resistance retains a greater fraction of its original power than a solar cell with a low shunt resistance.
Figure 3: Solar cell power characteristics for different irradiation values
- Ideality factors: The ideality factor of a diode is a measure of how close the diode follows the ideal diode equation. The derivation of the simple diode equation uses certain assumptions about the cell. In practice, there are second-order effects so that the diode does not follow the simple diode equation and the ideality factor provides a way of describing them.
With such basic ideas, even a customer can determine the performance of a solar cell. In the next part of this series, a converter topology used in the PV system will be briefly describe.
