I've picked up an unexpected and exciting side gig at EnTech Solutions: tour guide! I enjoy the opportunity to host visitors and events at our microgrid sites; every group has been engaging, with people asking great questions. One topic that we spend ample time on is the difference between an alternate current (AC) coupled microgrid and a direct current (DC) coupled microgrid. Frequent questions include: What is meant by coupled? Why not stay with all one type of current, AC or DC? What are the benefits of switching to a less traditional DC-coupled microgrid?
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To answer these questions, it helps to walk through the design process of microgrids. It begins with constraints that start with the components where we do not have a choice. Solar panels are included on most of our microgrids to provide a renewable generation source. All solar panels start DC-based; physics gets in the way of producing an AC solar cell. When we design a microgrid, we can choose to add an inverter to solar panels, which converts DC to AC, or we can choose to use a DC/DC converter which adjusts the DC voltage from the solar panels to the desired DC voltage.
Batteries are another common item used in microgrids. Solar energy is renewable but not always available; batteries are used to store excess solar energy for future use. All battery cells are DC sources; again, physics is in the way of an AC battery cell being developed. Same as with solar panels, we can choose to convert the DC from the battery to AC or keep it as DC.
Whether it is a DC-coupled or AC-coupled microgrid, there will be a variety of each type of current on the microgrid. The difference is what type of current is used to connect all the components together.
As I lead tours through the DC-coupled off-grid system at our Lakeside Vision Center, I cover the following benefits that led us to choose a DC-coupled design for the facility:
For DC-coupled systems, all components on the system are tied to a common DC bus. This allows for a more refined control system that doesn't rely on devices' communications to each other. Every device on the system sees this directly ' faster than any communication device could measure the voltage and send a signal.
At EnTech Solutions, we use our own AutoSync DC Bus' technology to implement this type of control. Each components' power electronics continuously maintain the DC bus within the appropriate voltage range. For example, in the above solar with battery example, if the load suddenly decreases, the DC bus voltage will increase. The power electronics for the batteries will immediately see the increase in voltage and respond by storing energy. This keeps the DC bus voltage within its specified range and there is no change in the power quality delivered to the loads.
At EnTech Solutions, we're here to support you at every step of your microgrid journey. Please reach out to us to discuss your energy sustainability and resiliency goals.
Solar panels can be coupled, or linked, to a battery either through alternating current (AC) coupling or direct current (DC) coupling. AC current flows rapidly on electricity grids both forward and backward. DC current, on the other hand, flows only in one direction.
DC current is what is generated by solar panels and stored by batteries, but because appliances use AC current (just look at the AC circuit board you have in your home) the current must be converted via inverters.
In the past, AC-coupled BESSs were most often used with residential and commercial solar installations, and DC-coupled systems were used for remote and off-grid installations, but more options for DC-coupled systems have become available. Equipment manufacturers are developing streamlined and standardized power electronics equipment for DC-coupled BESSs.
Over the past decade, inverter technology has advanced and resulted in the development of new AC-coupled and DC-coupled systems. Let's find out more.
In AC-coupled systems, there are separate inverters for the solar panels and the battery. Both the solar panels and the battery module can be discharged at full power and they can either be dispatched together or independently, creating flexibility in how the system operates. The solar panels and battery can either share an interconnection to the grid or run on separate interconnections.
AC-coupled battery energy storage system diagram. Source: RatedPower
AC BESSs comprise a lithium-ion battery module, inverters/chargers, and a battery management system (BMS). These compact units are easy to install and a popular choice for upgrading energy systems and the systems are used for grid-connected sites as the inverters tend not to be powerful enough to run off-grid.
It's worth noting that because both the solar panel and the battery are DC-current compatible, the current will need to be converted three times in an AC-coupled system.
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DC-coupled systems typically use solar charge controllers, or regulators, to charge the battery from the solar panels, along with a battery inverter to convert the electricity flow to AC.
DC-coupled battery energy storage system. Source: RatedPower
The solar panels and battery module use the same inverter and share the grid interconnection, reducing the cost of equipment. This also reduces power losses from inverting the current and running separate interconnection lines to the grid, as the solar array and battery are dispatched as a single facility. But this offers less flexibility than an AC system.
There are several benefits to using an AC-coupled BESS for your solar plant, including:
Retrofitting: AC-coupled batteries are easy to install on an existing solar panel system, and more can be added to expand capacity.
Flexibility: Installers are not restricted in where the inverters and batteries can be located. AC coupling works with any type of inverter.
Resiliency: The flexibility to install multiple inverters and batteries in different locations helps risk of an outage if an inverter fails. Having multiple inverters provides more combined power and battery faults do not have an impact on power generation.
Versatility: AC-coupled systems enable batteries to charge from the grid as well as the solar panels and the grid, so if the solar panels are not generating enough electricity, the battery can still charge from the grid.
Cost: AC-coupled systems cost more than DC-coupled systems as they use multiple inverters.
Lower efficiency: The stored energy is converted three times, from the DC current to AC current to supply the building and then back to DC current to the battery and again back into AC. Each conversion results in a small amount of energy loss.
Supply limitations: AC BESSs are not designed to be used off-grid and as they are transformerless, they cannot manage the surge loads from multiple appliances.
Where AC-coupled systems suffer in terms of efficiency and cost, DC-coupled systems have the advantage:
Affordability: DC-coupled systems tend to be cheaper than AC-coupled systems as the solar panels and battery use a single inverter and less extra equipment such as voltage transformers and switchgear.
Higher efficiency: Unlike AC systems which convert the current multiple times, DC BESSs only convert the current once, reducing energy losses and making them more efficient.
Oversizing: DC-coupled systems allow solar panels to generate more electricity than the inverter rating. The excess energy can be used to charge the battery, an EV charger or a water heating system, whereas in an AC-coupled system the energy is lost.
Limited flexibility: Installers have less flexibility than with an AC system, as the inverter needs to be located close to the battery.
Less resiliency: With a single inverter in a DC-coupled system, if the inverter fails, the solar power as well as the battery capacity is lost.
AC-coupled systems are the preferred option for larger and utility-scale plants. That's because while AC-coupled systems are slightly less efficient at charging batteries (90-94% vs 98% achieved by DC-coupled), they are far easier to install, especially into an existing system.
That said, whether AC-coupled or DC-coupled is the best solution for your PV plant design will be project specific. You can use a PV plant software solution to run several simulations and identify the best option.
Interested in learning how to hybridize your PV systems? Check out this webinar and get all the insights on AC-coupled BESS.
RatedPower platform is designed to optimize and automate the design process, helping you weigh up the options between AC- and DC-coupled BESS. Contact us to find out how RatedPower can increase your project's efficiency and profit margins.
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