Characteristics of photovoltaic cells
Each photovoltaic cell is unique and has unique characteristics. Since the battery characteristics of a module will affect the power output of the connected modules, it is very important to understand the battery characteristics when designing a photovoltaic array.
Graphical representation of photovoltaic cell performance
It is a very common way to graphically express the performance of photovoltaic cells with a current-voltage (I-V) curve. The I-V curve plots the trajectory of photovoltaic cell characteristics and highlights key features such as Voc, Isc, and Pmax. Photovoltaic cells will always work along this curve, that is, under a given voltage, the output current will always maintain the same value, and vice versa.
The power curve is used to find the maximum power point. The horizontal axis of the power curve is voltage and the vertical axis is power (current multiplied by voltage). When the power curve of a battery and the I-V curve are stacked, you can clearly see where the maximum power point is.
The I-V curve and power curve are very important, because the characteristics of each battery need to be known when designing components. Connecting cells with very different characteristics together has a great influence on the output power of the photovoltaic module (usually a negative influence).
Connect photovoltaic cells into modules
Photovoltaic cells with the same characteristics will be joined together to form modules. In the series connection mode, the battery voltages add up while the current remains the same, that is, the component current is equal to the current of a single battery. Power loss occurs when a battery has poor performance. The battery may be damaged, and the more common situation is obstruction.
There is a free-of-charge data manual on the manufacturer’s website, which gives the battery characteristics. This data can be used to design photovoltaic arrays. The designer should always contact the manufacturer to ensure that the data manual provided on the website is up to date.
Purchasing photovoltaic modules from reputable manufacturers should provide a specification sheet (also called a data sheet). The data sheet includes important technical information required for the design and installation of photovoltaic arrays. When buyers compare different photovoltaic modules, the data sheet is also very useful, because the data sheet provides basic information such as efficiency, rated power and physical dimensions.
PV module string
A large number of photovoltaic modules are connected in series to form a module string. The electrical characteristics of photovoltaic modules connected in series into a module string are similar to that of photovoltaic cells connected in series: the output voltage of the module string is the sum of the output voltages of all modules, and the output current of the module string is the smallest output current among the modules.
Components can also be connected in parallel. In this case, the output currents of the components add up, not voltages. The output voltage is the voltage of a single component.
Designers can use a combination of series and parallel to connect photovoltaic modules into an array, and the output current and voltage are suitable for market applications. In a typical case, the components are first connected in series to form a string, and then connected in parallel to form an array.
The photovoltaic array outputs direct current, while the grid needs alternating current. Therefore, an inverter is required to convert photovoltaic direct current to alternating current, and the wiring of the photovoltaic array should make the maximum power point voltage of the array within the input voltage range of the grid-connected inverter