This article will discuss the advantages of using microinverters with AC coupling for residential, single-phase applications. One of the leading microinverter manufacturers in the U.S. market is Enphase Energy. This microinverter manufacturer is headquartered in California and has pushed the envelope with AC coupling compatibility. For more information regarding the basics of AC coupling, see HERE for design considerations. If you&#;re interested in learning more broadly about retrofitting solar PV with battery backup, see HERE. This article covers the basics of adding energy storage with microinverters; moreover, it discusses the methods Enphase uses to communicate with battery inverters. Further, battery adoption will likely continue to accelerate as utilities impose more time-of-use rates, cap net metering interconnections, and add demand charges.
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For homeowners interested in maximizing their production on a module-level and or those who would also like an energy storage system to take advantage of a TOU rate and/or backup power solution, AC-coupled storage systems with microinverters are a great option. In this type of system, microinverters are attached to the modules and the branch circuits or strings are combined at a gateway combiner box. They are then fed to a critical loads panel which is also connected to a battery inverter. This battery inverter is responsible for managing the flow of energy to the batteries while also mimicking the frequency of the grid in case of an outage to allow for continued PV production. Hence, when the grid goes down, the battery inverter uses an internal contactor to separate from the grid input and to isolate the critical loads panel from the grid. You can also configure the system to have an external ATS or contactor on the grid side of the main service panel (MSP) to power the main service panel directly. This could eliminate the need for a separate critical loads panel if the inverter(s) and battery are big enough to carry all the loads and surge loads on the MSP. An external ATS would be required when a customer is looking to power the main service panel; however, it does add extra cost and complexity.
One advantage of using Enphase in an AC coupled system is that microinverters allow for the flow of power to be divided between the backup system and the grid. Thus, the use of microinverters allows for flexibility in the size of storage capacity. For instance, some branch circuits can connect to the sub-panel that the battery inverter feeds while others can exclusively connect to the main panel. Even when some of the microinverters are connected to the battery, the entire array is eligible for net metering credits. However, if the grid goes down, the microinverters that are connected at the main panel will stop producing as they are isolated from the critical loads panel which is being fed by the battery inverter.
It&#;s important to keep in mind that the battery inverter needs to be sized for the max AC output from the PV array that is connected to the critical loads panel. We want to make sure that all the AC output from that array can pass through the battery inverter system. When designing these systems, select the number microinverters that is equal to or less than the battery inverter&#;s kW capacity and land the rest of microinverter array on the main panel. If you are providing whole house back-up, you can achieve this same result with AC relays on the microinverter branch circuits to open when the grid is not present, thus reducing the effective off-grid PV array size.
Currently, Enphase is brand agnostic towards storage inverters, but some work better than others. Installers can use any backup system that fits one&#;s budget, technical constraints and design preferences. With Enphase, the main feature to keep in mind is where the battery inverter can shift its frequency based on the state of charge of the battery bank. This allows the battery inverter to control the PV array output when in off-grid mode. Outback is a popular battery inverter that can&#;t frequency shift, but it can trigger a relay to knock the PV system offline, thus protecting the battery from overcharging. In this case, the PV system would shutter on and off based on the battery bank&#;s state of charge. Put differently, this scenario equates to trying to fill a cup by turning a firehose on and off. However, if a frequency-shifting inverter like Schneider is used, PV would be curtailed to match the loads that are currently on the panel, even if the batteries are at a full state of charge.
Enphase microinverters have the ability to accommodate a wide range of wattages. Most modules, 60-cell or 72-cell, will be compatible with the IQ7 or IQ7+. See the image below for a diagram of the AC Coupled layout:
Enphase power curtailment options include External Relay Control, AC Frequency/Trip, AC Frequency/Watt and Power Export Limiting.
External Relay Control measures voltage and can trigger a full or partial disconnect if need be. Depending on local interconnections, there may be a time delay in reconnection. This operation can lead to a shuttering of the system where the battery system sees all or none of the full power of the PV array depending on the battery&#;s state of charge. This is a relatively crude way to AC couple.
AC Frequency / Trip is an interconnection setting that sets an over-frequency trip threshold to halt microinverter production when the frequency reaches the pre-set threshold. This method should be used with older M215 and M250 models which don&#;t support gradual curtailment.
AC Frequency / Watt can gradually curtail power based on the AC frequency. The microinverters back off the maximum power point voltage thus reducing overall AC output. This feature is available with the IQ lineup of microinverters. Overall, it&#;s easier on the capacitors within the microinverter and the battery system to ramp the production up and down rather than trip it.
Enphase&#;s Power Export Limiting is for interconnected systems that require an export limit. To limit PV export, the system must be outfitted with consumption metering. It automatically sends power curtailment commands to the microinverters via the powerline communications. Data is pulled from the consumption meter every 500ms and makes adjustments to the microinverter output at 1.5-second intervals. The IQ Envoy adjusts every two to four seconds.
Enphase provides options to maximize the end customers&#; return on investment. The right fit depends on the product mix, design preferences, and size of a system. For more information about retrofitting microinverter systems with storage or interest in AC coupled systems using microinverters, contact your Greentech Renewables Account Manager.
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. The Article 706.2 of the 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.
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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.
Moving from left to right in the diagram above: The PV array outputs dc power to the ESS and the multimode inverter. The multimode inverter will convert the dc power to ac and any power in excess of the loads in the backup and main service panels (or that is used to charge the ESS) is exported to the grid.
Retrofits
Adding an ESS to an existing grid-tied interactive PV system is not uncommon. Doing so can cause headaches for system designers, and the easiest solution is often ac coupling the new ESS.
Compare the simple interactive PV system and the ac-coupled system above. Note that in both cases, the PV side of the system is the same. AC coupling will add a backup loads panel and multimode inverter but, crucially, the existing PV system does not need to be redesigned.
Higher Inverter Capacity
A dc-coupled system relies on only a single multimode inverter and is thus limited by its capacity. AC-coupled systems have two inverters (one interactive and one multimode), both of which feed the backup loads panel. So if an outage occurs while the sun is still shining, the backup loads panel can have the kW capacity of both inverters available.
Redundancy
As with any electrical system, PV designers should consider potential points of failure. In an ac-coupled system, if the battery-based multimode inverter is disabled, a simple bypass switch will keep the PV array and interactive inverter online (as long as the grid is up). This is not the case for dc-coupled systems, which are reliant on a single multimode inverter at the heart of the system architecture.
Efficiency
Interactive inverters tend to be more efficient than multimode inverters. If the sun is shining and the power being produced is consumed immediately (i.e., the power output from the PV array is directly feeding the loads rather than passing through the multimode inverter to charge the ESS) an ac-coupled system architecture will be more efficient than its dc counterpart.
Efficiency
While an ac-coupled system is more efficient when the PV array is feeding loads directly, a dc-coupled system is more efficient when power is routed through the ESS (e.g., when the ESS is charged directly and discharged at a later time) since there is only one conversion from dc to ac&#;a single inverter, rather than two, to pass through.
Full PV Array Power
In typical interactive and ac-coupled systems, inverters are downsized under the assumption that the PV array will rarely, if ever, produce at its nominal rating. For example, consider a 5kW PV array that is tied to a 4kW interactive inverter. This hypothetical system is now limited to a nominal 4kW power output.
For dc-coupled systems, the power feeding the ESS is not limited by an interactive inverter. Returning to the hypothetical system above, but without the interactive inverter this time, a theoretical maximum of about 5kW could be used to charge the ESS and/or feed the multimode inverter without any power limiting.
Direct Charging
Low-battery-voltage situations can arise within ac-coupled systems. If too much energy is pulled out of the battery bank during an outage, that energy can cascade throughout the system, shutting down the multimode inverter, then the interactive inverter and the PV array. Since there is no way for the PV array to directly charge the batteries in this case, system owners may have to wait for grid power to return before the system can come back online.
Conversely, a dc-coupled system can continuously send power (during daylight hours) directly from the PV array to the ESS. The unencumbered pathway between the PV array and battery bank allows the battery voltage to rise and thus the multimode inverter can turn back on and supply power to the backed-up loads without having to wait for the grid power to return.
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