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AC Coupling An Existing Grid-Tied Solar PV System
A Battery Based Inverter System

Almost all residential PV/solar systems fall into three types, the first being the most common:

  •  grid-tied where solar PV modules feed their generated power directly back to the main power grid for a small feedback tariff

  •  off-grid where all solar PV power is stored in batteries and supplies electricity through a DC to AC inverter/charger where a grid is non-existent

  •  grid-interactive where the presence of a battery bank enables the user to achieve both grid-tied benefits with off-grid independence

The last type is especially useful when the main grid power goes down for any reason, when grid power is inadequate or problematic, or when grid power is costly making it advantageous to “offset” the grid using renewably-generated stored energy (solar PV/wind). Concerns about grid stability and even availability are more valid than ever.

As mentioned previously, by far the most common configuration of a solar PV generation system is an array of solar modules feeding a grid-tied inverter that converts the DC power to AC power, which is then fed through the house's main  service panel and on out to the electrical grid. The grid in effect acts like a battery and the renewable energy is placed on the grid for collective consumption, which, in turn, reduces generation by other sources. The grid-as-a-battery is a great concept, until it’s no longer there – the grid-tied inverter requires the grid to stay powered for safety reasons. Without grid power to keep the GT inverter operating, the available PV power just sits on the roof unused. And during an outage, a home or business with solar electricity potentially available, is in the dark just like everyone else.

A battery-based inverter system
does not require the electrical grid to stay active, and is the mainstay of off-grid homes. Using battery-based off-grid technology, a smarter type of “grid/hybrid” inverter technology is able to use solar, wind, hydro-electric and other renewable DC energy sources to keep the batteries charged and sell the excess to the grid, just like the grid-tied unit using the grid as its battery. For those that have not yet invested in a solar energy system and would like to have backup power, a grid/ hybrid inverter/charger with battery back-up is the best choice. Solar energy can still be sold back to the grid, while having the security of knowing your solar based renewable energy system will still provide power during an outage until the electrical grid is back up and running.

Why not purchase a grid/hybrid inverter from the start?
For those staring at their roof full of now totally useless solar modules when the grid goes down, and then the hours without power turn into days, and sometimes even weeks, that very same question is probably being asked many times over. Some may not have realised they would be without  power from the solar modules on their roofs when they bought their grid-tied system. Maybe they thought it would never happen to them. Plus the relatively modest extra cost of a smarter grid-interactive inverter/charger and batteries may have initially been off-putting.

Adding energy storage through AC-coupling:

For the owners of these more common grid-tied, solar power grid-dependent inverter systems, there is a way to tie in a battery-backup inverter system using a method called AC Coupling. It typically requires adding a separate sub-panel with circuit breakers and electrical connections for the home's critical loads. This allows a point at which the grid-tied inverter and the battery-based  inverter “couple” and share their energy to the loads. In the normal mode of operation with grid power available, the energy from the PV array flows through the grid-tied inverter to the critical loads sub-panel, with any excess energy flowing through the load panel to the battery based inverter, and on out to the grid.

When grid power is lost, the battery based inverter activates an internal transfer switch which opens its connection to the grid. This keeps the inverter from trying to feed power to the main grid, therefore keeping energy off the power lines so utility workers don’t get shocked. The battery based inverter also provides a power source to the grid-tied inverter that keeps it online and “inverting” the DC solar power to AC power for the critical loads.

When the sun goes down and the solar power is no longer flowing through the grid-tied inverter to the loads, the power stored in the batteries will now provide power to the critical loads panel until morning.

When the sun comes out the next day, the system uses any extra energy for recharging the batteries. If no excess is available then some manual load-shedding may be necessary through the turning-off of the critical load panel connected devices until the batteries are charged. Turning off less critical loads such as lighting and refrigeration to prioritise the most critical, such as  medical equipment. If the critical loads are absolutely essential and load shedding is not an option, then adding a generator to the backup system can satisfy the critical load demand while charging the batteries.  

Typical AC-Coupling solutions:

AC Coupling a grid-tied inverter and a battery based inverter together is not a straight-forward procedure, especially if you want a “one size fits all” solution for all applications. That “one size fits all” package usually includes one or more diversion loads, possibly a blackout relay or some other method to take the grid-tied inverter offline to prevent it from overcharging the batteries on the battery based inverter. And it requires a lot more design experience to be sure all devices in the system can handle all possible conditions.

It should also be noted that the “one size fits all” AC Coupling solution really needs diversion loads to divert any excess energy in systems that either have too much solar PV power and/or too small of a battery bank which could put dangerous charge levels into the batteries. While it can be argued this energy can be used to heat water or run pumps, that “benefit” often comes when not necessarily demanded, and if the diversion load can no longer accept the available energy, the grid-tied inverter must still be shut down. In addition to the complexity, diversion load options can also be expensive, requiring extensive redesign of the existing installation which can make this design a poor choice for those who want simplicity and cost control in their AC Coupling system. The extra expense of all the associated control and coupling hardware plus adding a more expensive inverter/charger often means skimping on energy-storage with lower-grade batteries and enclosures, lowering overall system performance.

OutBack’s AC-Coupling solution:

Another simple approach is to use a battery based inverter, a battery bank, and a remote-operated relay which can be added to an existing grid-tied inverter system to tie in the house's available solar power to the critical loads during grid loss.  This cleaner, more compact component design allows the safe lock-out of the grid-tied inverter when the batteries are full, and also keeps the grid-tied inverter locked out if an optional generator is started and running in the system. Integrated with a more advanced, smarter inverter/charger such as OutBack’s FXR/VFRX series with dual AC inputs and advanced generator features, the result is a system that achieves higher performance at lower effective cost.

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