In solar power systems, a “string” is a series-connected group of solar modules that together form a circuit . Strings play a key role in the efficiency of the system, as the output voltage of each module is added together while the current remains constant.
How does a string work?
Series connection: The solar panels in a string are connected in series. The voltage of each module is added together, while the current is the same as the current of the lowest-power module.
- Example: If a string consists of 10 modules of 40 V and 10 A, the total voltage of the string will be 400 V and the current will be 10 A.
Connecting an inverter : The direct current (DC) produced by the string is converted by an inverter into alternating current (AC) , which can be used directly in the building's electrical network or fed back into the electrical network. For larger systems, one inverter can receive multiple strings.
Advantages and disadvantages of string
Advantages:
- Simple design: Series connection makes it easier to install and electrically connect solar panels.
- Cost-effectiveness: Fewer cables are required than in parallel connection, which reduces system installation costs.
- Compact design: Due to the easy connection with inverters, strings are ideal for small and medium-sized systems.
Disadvantages:
- Effect of shading: If a module is partially shaded , it can reduce the power of the entire string, as the current is adjusted to the power of the weakest module.
- Power loss: If there is a difference in power between the modules in the string (e.g. manufacturing tolerance, different orientation), the lower power modules may limit the overall power of the string.
How to optimize string performance?
MPPT (Maximum Power Point Tracking) : The inverter or a string management algorithm ensures that the system always operates at its optimal power point.
Power optimizers: Optimizers mounted on individual modules ensure that a single shielded module does not affect the performance of the entire string.
Use modules of the same power: Modules of the same type and power should be used within a string to minimize power losses.
Correct orientation and tilt angle : Optimal orientation ensures that all modules receive the same amount of sunlight.
Minimize cabling length: Shorter cables reduce resistance loss.
Connection of strings and inverters
One inverter, multiple strings: In larger systems, the inverter can manage multiple strings simultaneously, increasing energy efficiency and redundancy. In these systems, the inverter can use separate MPPT channels to independently optimize the performance of each string.
One string, one inverter: In small systems or stand-alone systems, one string with one inverter is often used.
Strings and modern systems
In addition to traditional string-based systems, newer technologies such as microinverters and power optimizers offer new possibilities:
Microinverters: Each module has its own inverter, allowing each module to operate independently. This eliminates the effect of shading.
Power optimizers: Devices connected to strings that improve the operation of modules that are shielded or have different power ratings.
Summary
Strings are the basic building blocks of solar systems, allowing modules to be connected in series and operating efficiently. However, the performance of the system depends largely on the design and management of shading effects. For optimal operation, it is important to install the system correctly, use modules of the same power and select the right inverter.