Adding battery storage to solar systems

AC coupled battery storage system
Figure 1

So called “a.c. coupling” is one of the easiest ways to add a battery storage system, with or without additional solar panels to an existing solar installation. In Figure 1 above, the “Battery Inverter” has been added to “couple” the stored energy in a battery to the switchboard (Load Centre) of the installation. The key component though is the “Energy Meter” upstream of the loads and existing solar generation. This meter allows the battery inverter to “see” the flow of energy into or out of the installation and choose whether to export battery energy to match the energy being consumed by the loads (but not meet fully by the existing solar system).

Put simply, the loads are supplied first by the existing solar inverter and any extra is then supplied by the battery inverter. If both of these is insufficient the the difference is sourced from the grid.

This all happens seamlessly, thanks to the magic of Kirchoff’s circuit laws and to the information that the energy meter supplies to the battery inverter.

DC coupling of batteries to solar PV system

DC coupled hybrid with switched backup circuit
Figure 2

If installing a new solar and battery storage system then d.c. coupling is one of the most popular options as the equipment manages both the battery and the solar generation within the one unit. So called “hybrid” inverters are those that can have both solar PV connected and battery storage.

The advantage of d.c. coupling is that the inverter is only used to converter d.c. to a.c. to supply the loads at the installation – internally the solar is directly charging the battery via a d.c. to d.c. path at very high conversion efficiency.

In Australia and New Zealand where the grid-connection standard AS/NZS 4777.1 applies – total Inverter Energy System (IES) capacity for a single phase installation must be <5kVA and thus limits the number of inverters connected to a total of 5kVA (approx. 5kW). This can be augmented by the local electricity network supply authority (typically with export limiting) but does make adding more a.c. coupled battery storage somewhat limited by total IES capacity of the site.

DC coupled system with no backup
Figure 3

In Figure 3 above the d.c. coupled hybrid system has no backup circuit. This is not an uncommon arrangement and best suits those customers who merely want to shift energy between solar, storage and their loads.

Backup functionality adds cost and complexity and is not aways available with all hybrid battery storage products.

Domestic electricity load profile with solar and battery storage
Figure 4

When sizing a battery storage system for a hybrid solar system it is important to consider to objective. If supplying all the energy to the installation by a combination of solar PV and stored battery energy then the customer’s load profile needs to be carefully considered.

In Figure 4 above you will see that the battery’s State of Charge (SOC) reaches 100% just after midday. This would indicate that the battery capacity is too small to avoid “spilling” solar energy out to the grid and thus loosing the potential savings it might offer.

Also, the energy supplied from the battery to the loads in the evening is capped at 3kW due to the inverter’s limited battery power and thus considerable grid sourced energy is being drawn in during the peak early evening period to make up the difference.

So both battery capacity and inverter power ratings need to be matched to the customer’s load profile.

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