AC coupling has been a commonplace way to solve an important problem: Adding batteries to solar PV. But now, there’s a much better solution.
Solar-plus-storage gains steam
With the arrival of lithium ion smart batteries comes an increase in residential buyers for solar energy systems with battery backup capabilities. The benefits of adding storage to a new residential PV system – from clean backup power during grid failures, to managing costs based on time of use and enabling self-consumption – are now widely appreciated.
As a result, residential solar-plus-storage is now a market in growth. GTM Research projects the market to grow from $246M in 2015 to $643M in 2016, reaching $3.1B by 2020. The solar industry is ready to provide the panels and the installations, but until now, the best way to connect batteries to solar has been AC coupling, which is, at best, a great solution for retrofits. But for new residential PV, AC coupling no longer makes sense.
The drawbacks of AC Coupling
AC coupling was an important stepping stone in the evolution of adding storage to solar PV installations. But the low efficiency, high costs, and in some cases, warranty implications, have made it hard for AC coupling to scale as a desirable solution.
As a Home Power article by Justine Sanchez noted, AC coupling is complex and runs into problems with surplus energy, low battery voltage, and tying together equipment from different manufacturers.
Aside from the high installation complexity (especially with retrofitting battery backup to an existing solar system), the AC coupling option loses appeal with decreased efficiency when energy demand isn’t in sync with PV production.
So, while it’s nice for retrofitting an existing solar PV system with batteries, AC coupling suffers from:
- Low efficiency
- High cost
- Installation complexity
- Warranty implications
The limitations of traditional DC coupling
Conventional DC-coupled systems aren’t feasible for most PV systems. In order to achieve a clean and safe installation, conventional DC-coupled systems need to have been designed from the start to accommodate the addition of storage, making retrofits difficult, if not impossible.
Probably the most problematic reality of DC coupling is the need for the PV array to match the input voltage of the battery charge controller to which it is directly connected. As Sanchez notes, this can require a reconfiguration of the existing PV array, and can also introduce the need for a combiner box and overcurrent protection. To avoid these problems, the system can use a charge controller with higher input voltage, but this option inflates the cost of the project out of the comfortable range for many customers.
An additional drawback of DC coupling is that consumers lose out on the possibility of self-consumption benefits.
The Pika Energy Island
For solar-plus-storage, the Pika Energy Island™ uses a single islanding inverter to connect PV and batteries. Solar panels are connected using the Pika PV Link™ optimizers, which feed directly into the Pika Islanding Inverter™.
The Pika Energy Island system is powered by the REbus DC Nanogrid™, a 380VDC bus that carries power, control, and communication on the same wire. With all the system components using this bus, the Pika Energy Island is a perfect match for smart batteries, many of which operate within the same nominal voltage range.
This means you can connect smart batteries directly to the Pika Islanding Inverter to create a home nanogrid with very few installed parts.
The Pika Energy Island provides:
- Quiet, affordable backup power that never runs out of gas
- 120/240V islanding with no external autotransformer or external transfer switch
- Programmable operational modes including self-supply and clean backup power
- Direct integration with high voltage lithium ion batteries
- PV Link DC optimizers install 85% faster than module-level electronics
Installers can now offer this simple, affordable solution to solar PV customers who want to add backup power to turn their home into a smart DC nanogrid.
For more information about the Pika Energy Island, click here.