Designing partially shaded PV plants
As previously mentioned, output loss due to shade must be minimized in the planning phase. Regardless of technology used, heavily shaded areas will decrease the economic viability of a PV system and should be avoided, as the end goal of a system should always be to optimize cost with performance. To assist the system designer, we have presented the most important planning rules below.
Selection of roof area
Minimizing energy losses in partially shaded module strings is based on setting the inverter to electrically bypass shaded solar cells for optimal use of the unshaded PV modules on the same series-connected string. The power of the shaded solar cells, which is diminished anyway, cannot be used at this time. When selecting the roof area for a PV plant, designers should make sure that permanent shadows do not fall on the PV array. During times of high irradiation (noon, summer months), no shadows should fall on it at all, if possible. Special simulation programs can be used to easily estimate the properties of the shadows, such as their size and how they change over the course of a year.
Selecting the system connection
The connection of the PV array significantly influences the obtainable energy production. To address this, SMA Solar Technology AG has prepared and published the rules of “Shadow Management” . The analysis of the course of shadows is always carried out at the beginning of a system design. The proportion of the shaded PV modules in relation to the array and the course of shadows over time are important characteristics of a PV plant with partial shading. The following recommended actions are important when dealing with partially shaded PV plants:
- When a single PV module or a small number of the PV modules (e.g. < 10 % of the total number) is shaded, the shadow can be distributed evenly on the strings. Since the MPP voltage is always near the nominal MPP voltage in these cases, a special operation control (OptiTrac Global Peak) is not necessary.
- If shading is severe, it makes sense to operate the shaded and unshaded PV modules separately. Take note:
- Group together array parts with similar irradiation.
- No parallel connection of strings with different irradiation; rather, provide a separate MPP tracker for each string. Multiple small inverters or devices with multistring technology can be used for this application.
- OptiTrac Global Peak is necessary to maximize the energy production.
Even with the slight shading described above, the concentration of the shaded PV modules on its own MPP tracker represents an alternative to evenly distributing the shadow over all strings. Even this system design requires OptiTrac Global Peak to minimize production losses.