Modeling Microinverters and DC Power Optimizers in PVWatts

Solar photovoltaic (PV) module-level power electronics have become increasingly popular for residential and commercial installations. These systems have several benefits over traditional central inverters, such as:

  • Increased design flexibility
  • Monitoring of system performance as detailed as real time energy production of each panel
  • Improvement of energy capture from single panels not producing electricity rather than entire strings

One of the widest used PV modeling programs is NREL’s PVWatts calculator. This program performs simplified PV performance models based on several factors, including the array’s geographic location, orientation, and several loss factors that are user adjustable. The default losses in the software consider the case of a central inverter system and do not account for the use of microinverters or DC power optimizers. In order to model these systems, NREL has provided a set of guidelines for users to utilize to generate accurate results.

NREL identifies a set of subfactors in Version 5 of PVWatts that are affected by the use of distributed power electronics. These systems include:

  • Inverter:  Microinverters should use the California Energy Commission (CEC) weighted efficiency; DC power optimizers should include the combined (multiplied) efficiency of the central inverter and optimizer devices
  • Mismatch:  Module level power electronics are expected to eliminate all mismatch losses
  • Wiring:  Wiring losses remain relatively unchanged because microinverters will have decreased DC losses but increased AC losses
  • System Availability:  Though system availability could increase due to decreased system downtime; NREL recommends not changing this category because of potential increases in repairs for point failures in the system
  • Shading:  Some of the shading losses will be recovered and is referred to as the Shading Mitigation Factor (SMF), which represents annual percentage of shading losses recovered from distributed electronics
  • Soiling:  Soiling is generally uniform, so no changes are anticipated unless arrays have different orientations or tilts

Modeling Microinverters and DC

Website: http://www.nrel.gov/docs/fy15osti/63463.pdf