KEY CONSIDERATIONS IN SELECTING PV MODULE
PV modules in case of large-scale power plants are selected based on the integration of modules, voltage and current rating with the inverter voltage and current rating. Typically for a grid connected plant the inverters operate in the voltage range of 450-750 volts and accordingly the modules are selected to match the desired operating voltage level through parallel and series combinations.
While selecting PV modules, it is important to consider the power tolerance limits under the standard temperature conditions. Typically, power tolerance in PV modules range between -5 % to 5%. For example- a 280-watt module with -5% to 5% tolerance will give output of the order of (280* (1-.05) = 266 and similarly (280* (1+.05)) =294.
While selecting PV modules, it is important to negotiate for a minimum tolerance level or at least modules with positive tolerance should be selected. If the modules of different wattage are connected in a single string, this leads to module mismatch losses and output of the string is governed by the weakest module. For example- in our case, the string has module ranging from 266 watt to 294 watts are connected in series, it will lead to higher module mismatch losses. In order to avoid the module mismatch losses, proper sorting of modules should be carried out while installing the modules in plant, it advisable that same wattage modules are connected in a single string through proper module sorting. For example, in our case a single string should have 266 W module only, and the other string should have 294 W module only in order to avoid module mismatch losses.
The power output of PV module is governed by operating voltage and current, hence in above example if the voltage is 36 volt the current of each module will be 7.8 amps. While 294 W module with same voltage level may produce 8.2 amps. And module of 266 W module, will produce 7.38
If the modules with positive and negative tolerance are connected in single string, the output of the string will be governed by minimum performance module (266 W, 7.38 amps). The module characteristic is typically represented by I-V characteristic curve and the maximum power is produced by at a voltage and current rating known as maximum power point (MPP). If different modules are connected in series, they will have various MPP’s and output of string is not decided as average MPP. It is decided based on minimum MPP condition of module which is connected in the string.
While the module ratings are not always identical and it is decided after the flash test analysis at the manufacturing unit and accordingly the rating of module is specified on each module, it is important that during planning phase the overall plant capacity is estimated based on the minimum power capacity of the modules based on their tolerance limit. Module mismatch losses are critical and avoidable by proper sorting of modules while installation in the plant. Also, the project developers can negotiate for minimum power tolerances for the specified modules during their purchase negotiations.
Module mismatch factor = 1- (.25*warranted power tolerance range)
Module mismatch for 280 W module = .975
Following are the specific points which need to be specified to any vendor while sourcing PV Modules.
- Module shall have electrical conversion efficiency of PV module shall be of highest in the class.
- Module shall be made of high transmissivity front surface giving high encapsulation gain and hot butyl rubber edge sealant for module protection and mechanical support. All materials used shall have a proven history of reliable and stable operation in external applications.
- The module shall perform satisfactorily in relative humidity up to 100% with temperatures between -20oC and +85oC and with stand gust up to 170 km/h from back side of the panel.
- Offered modules must be IEC 61215 (Crystalline Modules) or IEC 61646 (Thin Film Modules) and IEC 61730 part I and II certified.
- The module power tolerance should be only in plus side. The module should not have negative power tolerance.
- The module frame (wherever applicable) should be made of corrosion resistant materials, which is electrolytically compatible with the structural material used for mounting the module.
- Protective devices against surges at the PV module shall be provided, if required. Low voltage drop bypass and/or blocking diode(s) may also be provided, if required.
- Module Junction box (weather resistant) shall be designed for long life out door operation in harsh environment.
- A minimum warranty of 25 years shall be given with degradation of power generated not exceeding 10% over 10 years period and not exceeding 20% over the entire 25-year period.
- The complete module assembly should be of a relevant rating to comprehensively cover climate and fire protection.
- The solar modules shall have suitable encapsulation and sealing arrangements to protect the silicon cells from the environment. The arrangement and the material of encapsulation shall be compatible with the thermal expansion properties of the Silicon cells and the module framing arrangement/material. The encapsulation arrangement shall ensure complete moisture proofing for the entire life of the solar modules.
- Each module shall have low iron tempered glass front or equivalent for strength and superior light transmission. It shall also have tough low iron tempered glass back or equivalent for environment protection against moisture and provide high voltage electrical insulation.