AC or DC coupling: Which to use for your next storage project?
Storage can be incorporated into new or existing solar projects through AC or DC coupling, but the guidelines for when to use each architecture are not black and white.
When adding storage to a new or existing solar project, there are two main ways to integrate the two sources—either through AC or DC coupling. Each is representative of where the battery is connected—the AC or DC bus.
Unfortunately, choosing the right method for a project isn’t straightforward. Each has its own pros and cons, but by using a few governing considerations and understanding the benefits of each architecture, contractors can make the best choice possible for each installation.
How DC coupling works
Batteries store energy as DC power, and solar panels only output DC power, so in a DC architecture, the solar array can be directly connected to a DC-to-DC converter (known as a charge controller, DC boost, DC stage, etc.), which is then connected to one inverter to convert the power to usable AC.
How AC coupling works
AC architecture requires two or more inverters. This means both the solar panels and batteries are each connected to AC-to-DC converters, which then feed current into two separate inverters for AC power output.
Benefits of AC coupling
Net metering: The California Public Utilities Commission (CPUC) requirements for net metering generally do not allow for DC-coupled systems greater than 10 kW. However, CALSEIA is petitioning to update these requirements to allow for larger systems.
Flexibility: AC-coupled systems involve two or more inverters, which gives installers more flexibility to adjust the location of batteries and other equipment. AC coupling also works with any type of inverter, even microinverters.
“We see a lot of customers going with AC-coupled systems because there’s a lot of versatility there,” said Eric Every, product manager at Yaskawa – Solectria Solar. “For example, if you’re doing a microgrid on a large college campus, or a community solar project, doing DC cabling all over would be a disaster. AC coupling works well for those projects, as well as commercial rooftops where inverters are on the roof but maybe the batteries are in the basement or elsewhere.”
Resiliency: Having more than one inverter and equipment in various locations helps reduce the risk of the entire system going down if any inverter has a failure.
“AC coupling helps avoid risks of putting all your power electronics eggs in one basket,” said Every. “You get this resiliency through redundancy. In a DC-coupled system, if the inverter has a failure, not only do you lose the energy storage, but you also lose the PV.”
Downtime can have major effects for commercial or utility customers when it comes to demand charge management. Demand charge management requires customers to stay under a certain power consumption to avoid additional charges to their bills. If energy use surges, Every said customers have a short window of about 15 minutes to drop usage down below a certain threshold. This is where storage can step in with backup power to accommodate the increased load without having to draw power from the grid.
“But if the inverter isn’t available to operate during that window, you lose that benefit for the whole month,” he said. “And if the failure is catastrophic enough, some utilities have a clause that says not only do you lose it for the whole month, but a new peak may be set with a higher electricity price for the entire year. This could mean losses of tens of thousands of dollars a year for large commercial and utility customers. That’s where AC electronics really shine; multiple inverters mean you don’t lose everything.”
Benefits of DC coupling
Efficiency: Each time there is a conversion from AC to DC, or vice versa, there is some amount of energy loss. In DC coupling, the power is only converted once, so there is a marginal efficiency benefit.
“DC-coupled systems are great if you’re trying to maximize every watt out of the system,” Every said. “But power electronics are getting more efficient, so that efficiency benefit erodes away.”
Simplicity: Having one inverter means simpler installation with less equipment, so DC-coupled systems can be less expensive.
For example, an AC-coupled large utility project would require separate medium-voltage transformers and switchgear for each inverter. A DC-coupled approach with one inverter eliminates this extra equipment, which increases system efficiency while decreases system cost when compared to AC-coupled systems.
DC coupling is also a good choice for residential installations that don’t use microinverters. In California, residential systems less than 10 kW still qualify for net metering. DC-coupled residential systems don’t experience the financial risk involved with demand charge management that larger projects do because if the power goes out the homeowner may only lose a small amount of money. However, if the home’s system is powering medical equipment or other critical loads, an AC-coupled system would be more appropriate.
Horizon Solar Power director of engineering, Ryan Mcpherson, said it sees advantages of both architectures in residential projects.
“A DC-coupled solution, such as the StorEdge inverter with LG Chem battery, has a slightly lower total cost of entry, and the total system back feed is lower, which helps avoid more expensive tie-in solutions,” he said. “The down side is the limited power output of the DC system, usually only good for self consumption and/or partial backup of small circuits. An AC-coupled system, such as the Powerwall 2, is usually inverter agnostic, making it a great choice for adding onto existing systems, as well as allowing us to utilize a wider variety of PV inverter products. The down side is a higher total cost for the system, as well as additional back feed to add to our total system back feed, which may put us into more expensive/creative tie-in solutions.”
Off-grid projects or microgrids with buildings and loads in close proximity (avoiding the need to run excess DC wiring) can also be a good fit for DC coupling.
DC coupling can also simplify interconnection in locations with certain incentive programs.
“For states that have interconnection limits on the incentive program, or cost of interconnecting, like the Massachusetts SMART program, there’s a lot of interest in DC-coupled systems,” Every said. “You don’t show up with two 500-MW inverters and suddenly they think it’s a 1-MW system, even though you’re only exporting 500 MW. DC coupling eliminates a lot of control overhead of making sure you’re not exporting more than you apply for.”
DC-coupled systems also avoid additional metering and controls needed for AC-coupled systems to verify that batteries are charged from PV energy, not the grid, therefore qualifying for the ITC. Storage can qualify under the solar ITC of 30% if at least 75% of the energy used to charge the battery is coming from solar, not the grid. This can be difficult to prove or track with data from two or more inverters.
“With a DC-coupled design, the inverter will only charge the battery from the PV array, so there is zero risk [of charging from the grid] and tax credits are made available to the owner,” said Chip Palombini, sales and marketing manager at Dynapower.
Value: Palombini explained that while AC-coupled systems can only take advantage of value streams like capacity firming, energy time shifting and ramp rate control, DC-coupled systems allow for those in addition to clipping recapture, curtailment recapture and low-voltage harvest.
“It’s easier to coordinate advanced functions with one inverter, rather than trying to synchronize between two different inverters,” he said. “DC coupling allows the system to capture more energy.”
However, Every cautioned that not all utilities are set up to pay for all value streams.
“There are lots of possible value streams for solar+storage, but not every utility is set up to pay you for those,” he said. “If you are a utility, you can realize these benefits internally, but if you’re a DER or C&I customer, what do you get? You can do peak shaving and shifting if you’re in a place that has time-of- use and demand charge management. But for voltage regulation, frequency regulation and other functions, you need to be an entity that can realize that value internally.”
The decision between AC or DC coupling is not black and white.
“Really it all comes down to the goals of the project. Some are monetary and some are reliability driven,” Palombini said. “Adding storage to a system right off the bat is always easiest. It saves expenses by only mobilizing construction crews once, procurement and permitting can be more efficient and there’s less project overhead than having two separate phases.”
Whether using an AC or DC architecture to add storage, it’s always important to follow the NEC requirements and safety precautions. Site surveys are also an important part of pre-installation, factoring in clearances and communication. Contractors interested in installing storage are also encouraged to attend a training course, whether through a manufacturer or a school. Any doubts or concerns about integrating storage should be addressed with the inverter manufacturer.
“It depends on what the customer’s objectives are, project location, footprint, critical loads, business model, equipment availability and other factors,” said Every. “Every system has to be looked at independently. Eventually we’ll get to a point where we can kind of put things in buckets.”