CTO Technical Q&A: Pairing Energy Storage with Microinverters
As solar microinverters have grown in popularity, we’ve noticed a few common questions popping up from our customer base about microinverters and energy storage. So we took a minute with our founder and CTO Stuart Lennox to get the download on how microinverters and SimpliPhi’s energy storage solution play well with each other.
How do energy storage solutions and microinverters pair together?
The benefit of residential microinverters is that power conversion from direct current (DC) to alternating current (AC) used by the home happens at the individual solar panel level. This panel-level conversion allows for the solar system to compensate more readily for variances in the level of sun the panel is exposed to, creating a more resilient system where each panel’s performance does not affect another’s as it would with a non-optimized string inverter-based system.
Regardless of if you are using microinverters or not, you will always need a device to charge the batteries. For DC power, this would be a charge controller that then feeds DC power to a traditional inverter, which converts the DC to AC. For AC power coming from the microinverters, an inverter charger can be used instead. An inverter charger converts the AC current back to DC for charging, then feeds the power to a traditional inverter to convert the (now) DC power back to AC for use in the building.
What advantages are there for AC coupling microinverters and batteries?
Basically, when you are connecting microinverters to a battery-based inverter, you create a micro-grid. This is generally referred to as AC coupling because the microinverters and the battery-based inverters are “coupled” on their AC outputs. Such systems have advantages over traditional grid tied systems, as they can provide power
during outages, in emergency situations and to remote off-grid locations.
AC coupling allows you to take advantage of all the benefits of microinverters, such as greater design flexibility, increased safety and increased performance that come with module-level maximum power point tracking (MPPT) and monitoring.
What safety considerations should people keep in mind when AC coupling microinverters and battery solutions?
Any energy storage solution is going to be more complicated than a solar solution that feeds energy directly into the utility grid. Installers and designers must be qualified experts before working with battery-based systems and AC coupled systems. While we do not have time to go into all the qualifications required, we can provide some guidelines.
Be careful not to overcharge the batteries. Even with SimpliPhi’s combination of lithium iron phosphate chemistry, a highly innovative battery architecture and battery management system that does not generate heat or require cooling, overcharging is not recommended. If you are using other batteries besides those from SimpliPhi, the dangers of overheating and thermal runaway is even more risky from overcharging.
Thus, with any battery and microinverter solution, you want to provide redundant methods to regulate the charge to the battery. You can use a voltage-controlled relay to shut off the microinverter system when the battery reaches a voltage within the manufacturer’s limit, or a series of relays to taper the charge to the batteries. For additional battery protection, use a dump load to drain excess power out of the batteries in the case of overcharging. Do this by using some of the stored power to heat water or air with a listed and approved heating element.
So in short, using microinverters and energy storage together is very similar to using string inverters with energy storage. You simply swap out some electronics for others. You can do this with new and old microinverter systems. As is the case with any system, you want to make sure everything is matched and configured properly.