AC Coupling

An Eccentric Anomaly: Ed Davies's Blog

Most off-grid houses (and small boats) use DC coupling for the electrical system where power sources (PV panels, wind or water turbines, alternators, generators, etc) feed DC electricity to the battery via a charge controller. The DC from the battery is then either used as such or converted via an inverter to AC at normal household voltages for use.

A few, though, use AC coupling where the power sources feed their output as AC into the “mains” wiring to form a sort of mini-grid with the power flowing in and out of the batteries via an appropriate inverter which also acts as a charger.

I'd assumed I'd just use DC coupling for my house but I'm gradually coming round to the idea of using mostly AC coupling. I'm not really clear about some of the details so here's a snapshot of my thinking to help make things a bit more concrete.

Note that I'm writing purely about off-grid systems here. On-grid AC coupling is actually more common but a different matter for both practical and legal/regulatory considerations.

AC vs DC coupling

The general scheme with DC coupling is that harvested power from the PV panels or whatever are fed, via a charge controller, directly to the battery. The charge controller's primary responsibility is to limit the current into the battery so that it is not overcharged. Typically it will also convert the voltage and current from the source to suitable levels for the battery so that the power transfer is maximised.

The battery-based inverter (BBI) then converts the DC from the battery to “mains” levels (typically 230 V, 50 Hz, AC in Europe) to feed normal household wiring.

PV panel(s) or other power source. Charge controller Battery Grid-tie inverter Battery-based inverter Breaker box (“consumer unit”) Household loads AC “mains” supply to household loads DC Coupling PV array output: generally less than 150 (or sometimes 250) V DC. Charge controller output: battery voltage, typically 12, 24 or 48 V DC nominal. Battery output, typically 12, 24 or 48 V DC nominal. Inverter output, typically 230 V AC in Europe.

DC power flows are shown in blue, AC flows in red. Hover over the various boxes and lines for labels if they're not clear.

For AC coupling, however, the power sources are connected to grid-tie inverters (GTIs), of the same sort connected to the supply of on-grid houses, via breakers in the normal household breaker box. Some of this power can be used directly within the house, any excess can flow “backwards” through the battery-based inverter to charge the battery.

The battery-based inverter controls the frequency and voltage of the mini-grid. The grid-tie inverters synchronise to that and are, maybe, controlled via the frequency in the same way that they are in on-grid systems, backing off their production when the frequency gets too high.

AC Coupling PV array output: generally up to about 1000 V DC. Grid-tie inverter output: typically 230 V AC in Europe. Inverter input/output, typically 230 V AC in Europe. Battery input/output, typically 12, 24 or 48 V DC nominal.

AC Coupling Advantages

  • Flexible wiring: higher voltages mean smaller wires and therefore more flexibility as to where devices are placed in the house. The only real constraint with AC coupling is that the BBI be close to the battery which isn't likely to be a problem.

    Grid-tie inverters typically also have higher input voltages (of the order of 1000 V) than charge controllers (often a maximum of 150 or 250 V) so there's less need for wiring from PV panels and less losses if that's not kept short. However, the higher voltages do introduce some safety concerns, e.g., for firefighters.

  • Cheaper inverters: because so many are produced, grid-tie inverters are typically cheaper than charge controllers for a given power handling ability.

  • More power available: at times when the external source is generating (e.g., when the sun is shining on the PV panels) more power is available to handle peak loads as the outputs of the grid-tie inverters can be added to those of the battery-based inverter.

  • AC load shedding: for simple operation of a DC-coupled system, when external power is available once the battery is charged the excess goes to waste or, more awkwardly, gets diverted to water heating or other such uses.

    With AC coupling it's considerably simpler to divert the power to standard AC appliances like immersion heaters. Of course, using AC diversion loads is also possible with DC coupling but that means depending on the battery-based inverter to prevent overcharging which might not be entirely wise.

  • Direct-use efficiency: where power is used directly, e.g., while the PV panels are generating, there are only the losses in the grid-tie inverter to consider rather than the combination of losses in both the charge controller and the battery-based inverter.

  • Metering: power and energy meters for AC are more cheaply and widely available than those for DC. In particular, it's likely easier to get an approved meter for “feed-in-tariff” and similar schemes if it's to measure AC sources.

AC Coupling Issues

  • BBI current limit: the battery-based inverter will have a limit on the current it can absorb and pass on to charge the battery otherwise it'll cut out (or worse, let out the magic smoke) resulting in the whole grid collapsing.

    The current the BBI has to deal with is, of course, the sum of the outputs of the grid-tie inverters minus any household loads. The problem is that this balance can ramp up very quickly, e.g., if an immersion heater thermostat switches off. It's difficult to have a control system which will ensure reduced production from the grid-tie inverters or diversion of the power quickly enough to prevent the BBI tripping out if the grid-tie inverters are producing more power than it can handle.

    A solution is to not allow the output of the GTIs to exceed the rating of the BBI (Victron call this the factor 1 rule). That's a bit limiting but not as terrible as it might seem as it's the maximum output of the GTIs which matters, not the maximum output of the PV panels. It's common to have the PV panels able to produce more power than the GTI can handle, knowing that the amount of time when the PV panels actually do produce their maximum is so short that energy lost is comparatively small. Also, it makes sense to have some of the PV charge the batteries via DC coupling (see blackstart below).

  • Battery charge management: as the battery fills up it's necessary to reduce the charge current, to zero when it's full. This is not quite so urgent (seconds, rather than milliseconds for the BBI current limit) so it's practical to have a control system to manage this.

    Some BBI's specifically designed for this role are able to increase the frequency of the mini-grid as the battery approaches the full state so that the grid-tie inverters can throttle back their output. This, of course, means that some energy is not harvested so other control mechanisms which allow more useful diversion might be attractive.

  • Blackstart: If the battery-based inverter detects that the battery has reached the lowest allowed state of charge it will cut out to prevent the battery being damaged by over discharge. If the BBI is the only route to recharge the battery this presents a conundrum. It's best to have some power source directly connected to the batteries (via a charge controller) to put in some energy to break this deadlock.

Inverter Choice

There are a few other considerations but clearly the key to getting an AC-coupled system working efficiently and safely is to have suitable inverters suitably configured.

GTI choice

As far as I can see there's not much problem choosing appropriate grid-tie inverter(s) to couple the various power sources onto the mini-grid. You just need ones with the appropriate protections so they won't produce too much output voltage and will respond as required to changes in the mini-grid's frequency. Pretty much any of the “proper” ones sold in Europe should be fine.

There might be a need to get the installer codes, etc, to configure them differently for this off-grid application compared with the normal settings for the country in which they're sold.

BBI choice

It seems to me that there are three classes of battery-based inverter to consider. I don't know of any accepted terminology for these so I've made some up:

  • Islanding inverters: Inverters designed specifically for this application which typically control the mini-grid frequency to signal to the GTIs when to throttle their outputs as the battery becomes fully charged. The classic example is the SMA SunnyIsland.

  • Backfeeding inverters: Battery-based inverters which are primarily designed and sold to feed power from the battery to local AC loads but do allow AC power to flow back into the battery when it's available. An example where this is officially supported but not the mainstream intended application would be the Victron Multiplus (see their AC Coupling page). Paul Camilli seems to have success feeding back through Outback and, IIRC, Trace inverters though I'm not sure whether that's officially supported.

  • Smoking Inverters: Ones which block this backflow, cut out if it's detected or just burst into flames if it happens. Absent information to the contrary, I'd think it'd be best to assume any inverter is in this class.

Perihelion

There are a few reasons why I'm thinking more about AC coupling now. In no particular order:

  • Changing the east-gable solar collector from thermal to PV would mean panels a long way from the planned location of the battery at the west end of the main loft.
  • The flexibility of control for the diversion loads is very attractive in a house aiming to get the best possible use out of available energy.
  • The thought that it might be better to put the batteries, etc, in the shipping container (assuming I decide to keep that) if only from a house-insurance point of view as it might only take a few silly accidents for insurers to be freaked out by batteries in houses. Keeping the batteries warm enough in the container needs some thought, though.
  • Future proofing: changes to generation and battery technology would likely be more decoupled. E.g., substituting a “powerwall”-type store for my planned 48v batteries and inverter would not require changes to the electronics for the PV panels or (if I have one) wind turbine. Addition of a flow battery would also likely be simpler with AC coupling. Even if the main batteries were in the house, flow batteries, because of their size and weight, would likely be outside in the porch/greenhouse or in the shipping container.
  • The application for planning permission for a site across the road might, if granted, constrain where I could put any possible wind turbine under permitted-development rules to be a bit further from the house than the position I currently have in mind. While that might be aerodynamically a bit better it would also be easier with AC coupling.

So, overall, AC coupling looks like the main way my system will be set up but with as many PV panels as the Tristar MPPT I already have can handle wired up directly to the battery. Still, as always more research is needed. In particular, I need to look more closely at what information is available from Victron inverters and/or inverter/chargers so that diversion loads can be controlled suitably.