If you're installing a solar-plus-storage system or adding a battery to an existing solar photovoltaic (PV) system, you've probably come across the terms AC- or DC-coupled. In the context of solar, this isn't a classic rock band; it's a bit of industry jargon that's important to your solar-plus-stor Contact online >>
If you''re installing a solar-plus-storage system or adding a battery to an existing solar photovoltaic (PV) system, you''ve probably come across the terms AC- or DC-coupled. In the context of solar, this isn''t a classic rock band; it''s a bit of industry jargon that''s important to your solar-plus-storage system.
AC- and DC-coupled both refer to the electrical connection between your solar panels and your home battery system. The main difference between them is how the electricity from your solar panels reaches your battery.
If you have a solar-plus-storage system, the terms AC-coupled and DC-coupled specifically refer to whether the electricity from your solar panels is inverted before or after it''s stored in your battery.
String inverters are the most common and affordable type of inverter. A string inverter connects multiple panels, transforming the DC electricity they produce into AC power. Multiple strings of panels connect to a single inverter that is usually located in an electronics box on the side of your home or in your garage. They work with AC-coupled systems.
Because string inverters connect multiple panels, underperformance from one or more of your panels (e.g., if one panel is shaded) will reduce the power output from the rest of the panels on that string. It''s possible that your solar panels won''t always generate as much electricity as they could be.
Microinverters are installed on the back of each individual solar panel and convert the DC electricity from your panels into AC power. You''ll have the same number of microinverters as you do panels, with each inverter operating independently. Microinverters are compatible with AC-coupled systems.
While microinverters tend to be more expensive and harder to maintain, they do ensure that your system performs optimally because the power output of one panel is not affected by the power output of another. Microinverters also allow you to monitor performance at the panel level, which means it''s easier to spot a single panel experiencing power losses.
Your roof''s orientation and shading: If your roof doesn''t face south, your panels face multiple directions, or your roof gets shade throughout the day, microinverters or power optimizers may be the better choice. They''ll ensure that lower performance from one solar panel doesn''t affect the overall power output from your system.
At Mayfield Renewables, we routinely design and consult on complex solar-plus-storage projects. In this article, we outline the relative advantages and disadvantages of two common solar-plus-storage system architectures: ac-coupled and dc-coupled energy storage systems (ESS).
Before jumping into each solar-plus-storage system, let''s first define what exactly a typical grid-tied interactive PV system and an "energy storage system" are.
Looking at the diagram below, a simplified interactive PV system is composed of a dc power source (PV modules), a power converter to convert from dc to ac (interactive inverter), and ac loads (main service panel).
When the sun is shining, the PV modules produce dc power which is fed through the interactive inverter which then feeds the main service panel. The interactive inverter "interacts" with the grid to send excess power to the utility and also will shut down during a power outage. This prevents the PV modules from producing power which could energize downed power lines.
Now that we have a simple grid-tied system, let''s build onto it by adding energy storage. Article 706.2 of the 2017 National Electrical Code (NEC) defines an energy storage system as: "One or more components assembled together capable of storing energy for use at a future time. ESS(s) can include but is not limited to batteries, capacitors, and kinetic energy devices (e.g., flywheels and compressed air). These systems can have ac or dc output for utilization and can include inverters and converters to change stored energy into electrical energy."
For the purposes of our analysis, we loosely define ESS as a component(s) of our circuit designed to store energy for later use (e.g., a lead-acid or lithium-ion battery bank).
As mentioned above, PV modules will produce dc power. That power must be converted to ac to be used in most commercial and residential applications. In contrast, battery cells must be charged with dc and will output dc power. The ac-dc distinction has major system design implications.
In an ac-coupled system, power from the PV modules is converted to ac prior to connecting to the ESS. In other words, the output from the PV modules is fed through an interactive inverter before it reaches the ESS. This means that the power must be converted to dc before charging the ESS, and any power output from the ESS must be converted once again to ac. To achieve this, an additional multimode inverter is required.
Moving from left to right in the diagram above: The PV array produces dc power, which is immediately converted to ac by the interactive inverter. That power feeds a backup loads panel containing select loads formerly located in the main service panel. The backup panel is also connected to the ESS, with a multimode inverter acting as a middleman to convert ac to dc when the ESS is charging, and dc to ac when discharging.
In this setup, it''s possible for the PV array, backup loads panel, and ESS to operate independently from the grid. During a power outage, the multimode inverter—using power from the ESS—will mimic signals from the grid, allowing the interactive inverter to stay online and the PV array to continue producing power to feed the backup loads panel and charge the ESS with any excess power.
DC-coupled systems rely only on a single multimode inverter that is fed by both the PV array and ESS. With this system architecture, dc output power from the PV modules can directly charge the ESS. No dc-to-ac conversion is required between the PV array and ESS.
The backup loads panel and main service panel—both of which require ac power—are placed downstream from the multimode inverter. In the case of a power outage, the microgrid interconnect device (which is commonly integrated into the multimode inverter) will cut off the multimode inverter''s output to the main service panel but the inverter will continue to supply ac power to the backup loads panel.
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