The interest in and adoption of residential storage in the US has grown at a healthy pace in the last few years. Guidance on how to size a system, particularly for AC coupled storage solutions, is a common request from installers with customers interested in adding storage. Few areas in the U.S. justify purchasing a storage system for grid interactivity only. Many areas in the U.S. do however have reasons such as natural disaster or utility safety shutoffs that motivate homeowners to purchase a storage system (often AC coupled with an existing PV system) that allows backup operation should utility power become unavailable. The sizing of the storage system is thus commonly based on the customer’s desired operation during this backup operation.
If the utility outage period were reliable and well known (and the budget was unlimited) then system sizing would be straightforward. Loads can be measured, so both the peak power needs in Watts and the amount of energy needed to run those loads during the outage in Watt-hours (Wh) or kiloWatt-hours (kWh) would be known. Unfortunately, most situations that result in loss of utility power do not have a fixed duration, so the days of autonomy the backup system needs to provide is unknown.
To provide system owners the most optimal operation during backup mode…
… it is important to understand how to maximize power production when operating independently from utility power. In an AC-coupled system with PV available the storage inverter will be responsible for generating the AC voltage waveform at the correct frequency when operating in backup mode. This allows the PV inverter to resume normal operation, providing power to run loads and possibly recharge batteries.
It is important to understand that ‘standard’ UL 1741 listed PV inverters in grid interactive mode will be tracking the maximum power point (MPP) of their array to produce maximum AC power, without regard to load demand. In backup mode, there is no utility grid to push excess power to if that PV power is more than local load demand (including battery charging).
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The PV – load demand dance
If the PV power is greater than total load demand, the storage inverter will increase AC frequency. For standard PV inverters, when the frequency goes above 60.5Hz the PV inverter will stop outputting power completely as this is the upper limit for a ‘good grid’ that can accept power. That means that all load demand must be met by power from the storage inverter. As the battery inverter provides power and the state of charge of the batteries goes down, the frequency will be reduced, going below 60.5Hz.
After the required five minute wait, the PV inverter will go back into MPP tracking operation. If the PV power is still above load demand, the frequency will go back up to remove the excess power. This ‘dance’ will continue until the PV system output drops below the load demand, either from increased loads or lower solar production. This is wasting valuable PV production that could extend the autonomy of the backup system.’
Fortunately, there is a way to have the PV system change behavior and provide the maximum useful power in backup mode. Commonly referred to as Freq-Watt control and activated in grid interactive inverters complying with California Rule 21 requirements, this function allows the PV inverter to respond to increasing AC frequency with a linear reduction in output power.
With this activated in the PV inverter, the storage inverter begins to raise frequency when PV power exceeds load demand. As frequency goes up, PV power will linearly reduce (all the way to 0W if needed) to exactly match the load demand. This reduces the need for battery power to be used to satisfy load demand and will extend the autonomy of the backup system, maximizing the value of the homeowner’s investment.
The manufacturer of the specific AC coupled storage inverter utilized should be able to provide guidance on the appropriate settings for the PV inverter for optimal operation. Please note that since activating the Freq-Watt function does technically change how the inverter interacts with the utility during grid tied operation, approval should be obtained for activating this grid support function pro-actively.
Michael Mahon is a Technical Trainer, Solar Academy, with SMA America
— Solar Builder magazine