Stand-alone system configurations

There are many different possible designs and configurations for stand-alone off-grid PV systems. The focus of this design project is on small-scale systems, which will serve to constrain the number of designs that will be explored here. It is worth noting that the possible designs increases greatly as system size and complexity increases. Small-scale stand-alone systems can be divided into three main categories based upon the loads that they power: DC only, AC and DC, and AC only. The most appropriate out of these designs will be determined by the energy needs and particular loads of the project.

The three-line wiring diagrams presented below and in the rest of this design process omit certain components as they are are not installed with all systems and adding them would add additional complexity to the wiring diagram. The following components should be considered for incorporation into any off-grid system:

• Ground fault protection device (GFPD)
• Residual current device (RCD) or ground fault current interruptor (GFCI)
• Shunt
• Charge controller temperature sensor
• Low voltage disconnect
• DC-DC converter

DC only system

Wiring diagram of a DC-only stand-alone PV system with a charge controller with DC lighting control for lighting and small DC loads.

An ideal system for systems that have limited load requirements like lighting, cell phone charging, a radio, an energy efficient DC TV, and an energy efficient DC fan. DC appliances do not suffer from the efficiency losses of an inverter, the stand-by consumption of an inverter, nor from downtime resulting from issues from an inverter.

Considerations:

• A system voltage of 12 V is typical for these systems in order to ensure compatability with DC appliances, although a DC-DC converter is an option for larger 24V or 48V designs in order to ensure compatability with 12 V appliances. DC appliances are not common in many areas of the world and may also be higher in price - this must be considered when designing a system.
• If the current requirement of the loads exceeds the lighting/load output of the charge controller, then a seperate connection to the battery with a low voltage disconnect should be made.
• The low operating voltage of 12 V DC systems can make them susceptible to voltage drop problems with longer circuit runs.

AC/DC system

Wiring diagram of a stand-alone AC/DC PV system with a charge controller with DC lighting control for lighting/DC loads and an inverter for AC loads.

A system that incorporates an inverter to create a more flexible system that can power both DC and AC loads. Retaining DC lighting allows the user to turn off the inverter when not in use in order to avoid stand-by consumption losses and to still have light in the event of inverter issues.

Considerations;

• This type of system will still suffer from voltage drop issues on longer circuit runs for lighting.
• The inverter may need to be turned on/off regularly based upon the needs of users.

AC only system

three line wiring diagram pv solar system stand alone off-grid direct current alternating current inverter charge controller