How does ZCell interface with existing or new solar panels (AC coupling vs DC coupling)?


First, lets explain what a AC battery inverter/charger is.

This is a device that converts electrical energy between :

MultiPlus Inverter

  • "AC power" - alternating current - as supplied by the electricity grid and as used in most homes; and
  • "DC power" - direct current - that is used in batteries like ZCell (ZCell is a 48 Volt DC Battery) 

 

The AC inverter/charger can be connected 'alongside' your existing home wiring, or it can be inserted in between your existing grid connection (and any existing solar panel systems) and the rest of your home electricity wiring.

 

The 'charger' function refers to the process of taking AC power from the grid or your solar panels (or both), converting it to DC power and storing electrical energy into your battery.

 

The 'inverter' function refers to the process of discharging energy from your DC battery and converting it back 'up' (inverting it) into 240 Volt AC energy. This AC energy can then power your home alongside, or instead of, the energy grid. 

 

Either way, most inverter/chargers can then control the charging and discharging of your ZCell in various useful ways - charging the battery when there is surplus renewable energy (and/or when the grid power price is low), and discharging the battery (powering your home, partially or entirely) in periods of time when energy prices are high, when the solar panels aren't working, or when there is a grid blackout.

 

AC Coupling

If you already have an existing solar panel (and solar inverter) installation in your home, the simplest way to add batteries is to install a separate battery inverter/charger along with your ZCell - leaving your existing solar deployment unchanged. This is called 'AC Coupling'. 

 

AC Coupling Diagram

 

In this approach, we start with the 'original' stuff in your house - an existing set of solar panels attached to an existing AC solar inverter. Your installer adds in a battery inverter/charger, your ZCell batteries and the ZCell BMS. These are added to the 'right' of the existing solar setup, and they don't disturb that existing setup.

 

This can be quite advantageous for various reasons, including:

  • No changes to your existing solar panel and solar inverter means no requirement to re-assess it against potentially changed electrical/electronic installation standards
  • Creating the potential to retain your existing solar Feed-in-Tariff because the battery system can be configured so that it never sends battery energy back to the grid (so the only exported energy is from the existing solar).
  • No need for the brand of new battery inverter/charger to be the same as the brand of solar inverter you already own.

 

This construct is called AC Coupling because the energy path between your solar panels and your ZCell is via the AC electrical wiring in your house. 

 

Note: If you use solar panels with 'Micro Inverters' (many small AC inverters connected directly to the panels in a daisy chain), instead of one big 'traditional' AC solar inverter, this makes no real difference - the considerations are the same - and the coupling approach is the same (add in a battery inverter/charger in accordance with the 'AC Coupling' diagram on this page).

 

Victron Energy CCGXThe 'Energy Control System' box you can see in the diagram is  supplied and configured by your installer. It is sometimes made by the same folks who make the battery inverter/charger, and sometimes its a product from another vendor.

The Energy Control System connects to most of the other elements of your system. It measures and controls how energy flows between the grid, the solar array/solar inverter, the battery inverter/battery and your home. This device may also allow you to view and control your system via your smartphone and to upload statistics and display graphs on an associated Internet portal site.

 

Sometimes, retaining an exiting Solar Feed-in-Tariff (if you wish to do so) can be simpler to gain approval for from your grid operator if you also install the optional extra in the diagram, a 'zero export control box' (such as a ComAp or similar product).

This is a device that will disconnect your battery system from the grid if the associated inverter and batteries ever try to send electrical energy into the grid from the batteries.

It can also provide the required grid-connect 'anti-islanding' function for your battery inverter/charger in accordance with AS4777 standards if the inverter/charger you are considering is not fully compliant to AS4777 on its own.

 

 

DC Coupling

If you are starting from scratch, there is a second way to connect solar and batteries to your home, called DC Coupling.

In this approach, your installer will put in a 'DC Bus Bar'. This is just a box with two copper conductive bars in it, where all of the DC equipment (your ZCells and your solar panels) 'come together', instead of this happening via the AC power wiring in your home.

 

DC Coupling Diagram

 

The DC bus operates at 48 volts - the voltage that your ZCell batteries and your inverter/charger expect. While solar arrays make DC electrical energy, the voltage they create is both much higher and also variable over time.

 

The solar array voltage needs to be 'matched up' to the 48 Volt Bus, and this is usually done with another device called a Maxium Power-Point-Tracker (MPPT). This device adapts the varying DC solar voltage to the 48V Bus and also helps to keep your solar array working at peak efficiency.

 

In the DC coupled arrangement, solar energy can flow (concurrently) from your panels both 'up' to your inverter to help power your home, and also to your battery (to charge it). 

 

Hybrid Inverter/Chargers

You might also want to consider the use of a 'Hybrid' inverter/charger unit.  This is a single box that allows the connection of both solar panels and a battery concurrently.

A single box hybrid product can be simpler to install in a 'new' installation. It is essentially one overall box that contains, wtihin in, all of the new 'bits and pieces' on the diagram noted above.  


This sort of product can be smart for new installations because it can be put in more easily, and it is more neat and tidy - with just one box on your wall to which everything (solar, batteries, grid, house) connect.

 

Is AC coupling or DC coupling better?

There is no single right answer - both approaches have their relative upsides and downsides - its a trade-off. Both of them work fine in practice. Your installer will discuss your needs and your current situation with you, and recommend the best approach for your home. 

 

AC and DC coupling in a single installation

This is an arrangement in which you combine an existing AC coupled solar array with a new DC coupled add-on solar array.

 

AC and DC Coupling in the same site

 

When set up with the new solar array and a new ZCell in a 'zero export' arrangement, the benefit here is the potential to add more solar capacity to your house (strictly for self-consumption of energy) in a way that does not modify your existing solar setup.

 

Your old setup keeps exporting surplus energy to the grid just as it always did, while the new solar panel array is strictiy for internal (self-consumption) use. Because it is to the 'right' of the zero export control point in the circuit, the new solar array can not export energy to the grid (and neither can your ZCell's).

 

If you are currently receiving a solar Feed-in-Tariff, this is a potential mechanism to let you to add more solar panels to your home without that additional solar capacity having any impact on your existing (approved) solar panel export setup.

 

You can also build up to this outcome in stages - starting with an AC coupled ZCell battery system and then coming back and adding that additional solar array (with its associated MPPT unit) at a later time.

 

This Victron Energy whitepaper:

 

https://www.victronenergy.com/upload/documents/Whitepaper-Self-Consumption-and-Grid-independence-with-the-Victron-Energy-Storage-Hub-EN.pdf

 

... contains some more great examples of these alternative ways to construct a home battery/solar energy system, with nicer diagrams and also with some sample calculations around home energy usage examples and a look at the efficiency tradeoffs in such systems.