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How to add storage to an existing solar array

Adding storage to an existing solar array is not always an easy, plug-and-play process. It could be if the solar array was installed storage-ready, but with the rapid advancements of solar-plus-storage in the last few years, it’s unlikely many legacy solar systems can easily adapt to battery connection. But that doesn’t mean it’s impossible; there are just a few important questions one must ask before starting a retrofit solar-plus-storage installation project.

Existing inverters
The inverter used on an existing solar array will dictate the route one must take to add batteries to a solar project. Solar-plus-storage systems are either AC-coupled or DC-coupled. AC-coupled systems require an additional, separate inverter to charge the batteries. DC-coupled systems use a charge controller or other DC-DC converter to feed PV power to the batteries and then through one inverter for grid use.

The inverter already in use on the PV system determines a lot. For example, existing systems with microinverters (which make the DC-AC conversion at the panel-level) will need an AC-coupled system for storage, since the battery cannot be incorporated into the DC lines before AC-conversion. If the existing inverter is in good, storage-ready working condition, AC-coupling storage to an array is as easy as installing a new battery-based inverter along with the batteries. If the existing inverter needs replaced, one can go the DC-coupled route with a new storage-compliant inverter, a DC-DC converter and the batteries.

What is the make, model and age of the existing inverter?
To accommodate energy storage, inverters must have frequency control capability—no matter if the system is AC- or DC-coupled. If older inverters cannot frequency shift, they should be replaced to add batteries, suggested Catherine Von Burg, CEO and president of lithium-ion energy storage manufacturer SimpliPhi Power.

SMA Sunny Island inverters

“If they are not replaced, grid-tied inverters will shut down regularly as the frequency shifts out of range,” she said. Adding new, advanced inverters that can regulate charges is the only way to include storage on an existing system.

The brand of inverter can also lend clues as to how to add storage. For example, if the system uses an SMA Sunny Boy inverter, it will require the addition of a Sunny Island battery-based inverter to AC-couple an array. Likewise, SolarEdge systems require a new StorEdge storage solution to incorporate batteries.

Inverters are usually only warrantied to 10 years. If the inverter is approaching its warrantied lifespan, it’s a good idea to upgrade.

“It may be advantageous to replace the inverter with an inverter designed to accommodate storage, or DC-couple the system with charge controllers and design a more elegant solar-plus-storage solution from the start, allowing options to integrate storage immediately or in the future,” Von Burg said.

Project location
A solar array’s physical climate and political climate will guide how to add storage to an array. Extremely hot locations will require a certain battery chemistry, and not all storage devices can be installed outside in the elements. Likewise, certain states and utilities allow the use of advanced grid functionality—like time-of-use (TOU) arbitrage or backup power. The chosen battery type is dependent on what each jurisdiction allows. The incentives offered in each area could also influence what type of storage is added to an existing solar array.

Where is the solar system installed?
Not all batteries can be used in hot climates or in freezing temperatures. The battery used in Ohio may differ from what’s commonly used in Arizona.

SimpliPhi Power’s AccESS unit featuring three PHI 3.4-kWh, 48-V batteries and a Schneider Electric inverter.

“SimpliPhi’s batteries perform very well in temperatures up to 140ºF,” Von Burg said. “Our LFP (lithium-iron phosphate) battery chemistry does not pose the risk of thermal runaway, fire or require cooling or thermal monitoring that other lithium-cobalt based chemistries do, such as NMC (lithium-nickel-manganese-cobalt-oxide) and NCA (nickel-cobalt-aluminum) used by Tesla, LG Chem and others.”

Because of optimal ambient temperature ranges, some battery chemistries are required to be installed indoors to control their temperatures. Battery warranties could be quickly voided if the storage system exceeds the recommended temperature range. So if the solar system owner only had a spot available outside to install the battery, the owner may have to find a new location or choose an outdoor-approved battery.

Certain pro-storage states have already implemented solar-plus-storage installation regulations. To add storage in California, inverters have to be Rule 21-compliant. This usually just means an inverter has to be “smart” with additional grid functionality, so older inverter models will need an upgrade. Similarly, Hawaiian solar-plus-storage projects must use Rule 14H-compliant inverters.

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