To start developing a solar inverter system, you must first understand the different characteristics of solar cells (PV cells). PV cells are semiconductor devices whose electrical characteristics are similar to diodes. However, PV cells are a source of electricity, and when they are exposed to light (such as sunlight), they become current sources. The most common technologies at present are monocrystalline silicon modules and polycrystalline silicon modules. The model of the PV cell is shown in the figure. Rp and Rs are parasitic resistances, ideally infinity and zero, respectively.
The performance of a PV cell will vary depending on its size or the type of load connected to it, and the intensity (illuminance) of sunlight. The characteristics of PV cells are described by different operating currents and voltages in different environments.
When the battery is exposed to sunlight but no load is connected, no current flows through the battery, and the voltage of the PV battery reaches the maximum value. This is called the open circuit voltage (VOC). When the battery has a load, current begins to flow through the circuit, causing the voltage across the battery to start to drop. When the two terminals are directly connected and the voltage is zero, the maximum current flowing through the battery can be determined. This is called short-circuit current (ISC).
Light intensity and temperature can greatly affect the operating characteristics of PV cells. The current is proportional to the light intensity, but the change in light has little effect on the operating voltage. However, the operating voltage is affected by temperature. An increase in battery temperature will lower the operating voltage, but will have little effect on the generated current. The figure below illustrates the effect of temperature and light on the PV module.
Influence of temperature and light on PV modules
The impact of light intensity changes on battery output power is greater than the effect of temperature changes. This applies to all commonly used PV materials. The important result of the combination of these two effects is that the power of the PV cell will decrease as the light intensity decreases and/or the temperature increases.
Maximum power point (MPP)
Solar cells can work in a wide range of voltage and current. By continuously increasing the resistive load on the irradiated battery from zero (short circuit event) to a very high value (open circuit event), MPP can be determined. MPP is the operating point at which VxI reaches the maximum value, and can be achieved at this irradiation intensity Maximum power. The output power is zero when a short circuit (PV voltage equals zero) or open circuit (PV current equals zero) event occurs.
High-quality monocrystalline silicon solar cells can generate an open circuit voltage of 0.60 volts at a temperature of 25 ° C. With sufficient light and an air temperature of 25 ° C, the temperature of a given battery may be close to 45 ° C, which reduces the open circuit voltage to approximately 0.55V. As the temperature increases, the open circuit voltage continues to decrease until PV The module is short-circuited.
The maximum power when the battery temperature is 45 ° C is usually generated under the conditions of 80% open circuit voltage and 90% short circuit current. The short-circuit current of the battery is almost proportional to the illuminance, and when the illuminance is reduced by 80%, the open circuit voltage may only be reduced by 10%. When the current is increased, the voltage of the lower-quality battery will be reduced faster, thereby reducing the available power The output dropped from 70% to 50%, and even only 25%.
Post time: 2020-04-25