Solar and Battery Storage Systems

Solar & Battery Storage System Configurations

Typical Configurations

• Typical solar setup:

  • Photovoltaics → Inverter → Grid.

• Battery setup:

  • Batteries → Inverter → Grid Substation.

Combined Photovoltaics and Batteries

• Two main coupling methods:

  • AC side

  • DC side

AC Coupled Systems

• Characteristics:

  • Photovoltaics have their own inverter.

  • Batteries have their own inverter.

  • Inverters are coupled on the AC side.

• Examples:

  • Neptune, Thunder Wolf, and Sonora sites observed used AC coupled systems.

  • SMA inverters for photovoltaics.

  • PEs inverters for batteries.

DC Coupled Systems

• Configuration:

  • Photovoltaics → Inverter → Batteries → DC to DC Converter.

  • Coupled on the DC side.

DC to DC Converter

• Function:

  • Acts as a charge controller.

  • Takes DC energy to charge batteries.

  • Provides DC energy from batteries to the inverter.

• Purpose:

  • Maintains a consistent DC voltage (e.g., 1500 volts).

  • Compensates for voltage drop in batteries as they discharge.

  • Regulates the varying voltage and current from photovoltaics to charge batteries at a consistent level.

AC Coupled vs. DC Coupled

• DC coupled (theoretically):

  • Extra photovoltaic energy directly charges batteries.

  • Batteries seamlessly take over if something goes down.

• AC coupled (challenges):

  • To charge batteries via photovoltaics, energy must be inverted and then rectified.

  • This results in losses due to heat during the conversion process.

• Efficiency:

  • DC coupled appears more efficient.

  • AC coupled requires using substation switches to charge the batteries.

    • Instead of outputting all energy to the grid, some energy is diverted to charge batteries.

    • Depends on the feeder providing to the Grid Service Unit (GSU).

• Battery Recharging (AC Coupled System):

  • Batteries are recharged from the grid.

  • Power is drawn from the GSU, stepped down via a medium voltage transformer (reverse direction).

  • To discharge, energy flows from batteries straight to the grid.

• NextEra's preference:

  • Currently using AC coupled systems.

  • No megawatt-scale system using a DC to DC converter has been observed.

Small Scale Applications of DC Coupled Systems

• Common in microgrid situations:

  • Campsites, homes, or boats.

  • Off-the-shelf DC to AC converters are readily available.

• Megawatt-scale platforms:

  • Inverters are easily accessible and do not necessarily dictate where the DC comes from.

  • Software determines energy input/output.