Go big, go DC: an in-depth look at DC-coupled solar-plus-storage
New technologies and designs aimed at driving down the cost of energy storage facilities are currently the FOCUS of intense industry RD. Sara Verbruggen reports on DC coupling, an emerging system architecture that many believe will soon become the industry standard, in a paper which first appeared in PV Tech Power’s Energy Storage Special Report 2019.
As the costs of solar PV modules continue to reduce, and those of batteries follow a similar downward trajectory, solar-plus-storage is in growing demand among utilities and solar developers.
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The US is leading the trend, where these types of clean energy power stations are starting to produce electricity competitively with gas peaking plants, especially when other revenue streams from grid services are factored in.
To further push down the levelised cost of energy (LCoE) of solar-plus-storage and maximise the amount of megawatt hours (MWh) of solar-generated electricity that can be fed into the grid, energy suppliers and developers are turning to direct current (DC) coupling these installations.
Compared with alternating current (AC) coupling, DC coupling the PV array and the battery storage system in front-of-meter installations, such as utility-scale plants, is a much newer, less standardised approach. This had led some US utilities to begin piloting these configurations to see how the technology performs. On the supply chain side, balance of plant (BoP) equipment manufacturers are delivering more standardised and simpler to use power electronics equipment for enabling DC-coupled plants.
DC- versus AC-coupled solar-plus-storage
A DC-coupled battery system at Duke Energy’s Mount Holly test site using Dynapower equipment. Expectations are high that DC coupling will help drive down solar-plus-storage costs. Image: Dynapower.
In AC-coupled solar-plus-storage installations there are two inverters, one for the PV array and another for the battery energy storage system.
With this system configuration, both the battery and solar array can be discharged at maximum power and they can be dispatched independently or together, providing the operator with more flexibility in terms of how they operate and dispatch the asset. Located at the same site the solar array and energy storage facility can either share a single point of interconnection to the grid or have two separate interconnections.
In DC coupling, the co-located solar and energy storage assets share the same interconnection, are connected on the same DC bus and use the same inverter. They are dispatched together as a single facility. DC coupling reduces efficiency losses, which occur when electricity current is converted, such as from DC to AC (Figure 1).
According to Wood Mackenzie analyst Mitalee Gupta: “Hybrid approaches emerged in the past where you would see both PV and batteries connected to the grid via one multiport inverter, a configuration more common in behind-the-meter DC-coupled systems. But one of the disadvantages for the front-of-meter market has been cost of multiport inverters.”
The newer variation of DC architecture that has emerged for front-of-meter solar-storage, which Gupta is referring to, is a DC-DC converter. This piece of hardware is tied to the batteries and connects to the PV inverter along with the PV array, allowing for a single interconnection only.
Figure 1. Co-located solar and storage systems, in AC-coupled (left) and DC-coupled (right) configurations.
Ac coupled pv system
Renewable energy systems, such as your photovoltaic system, produce direct current (DC). The storage battery in your basement also needs direct current. Your computer, your TV, your e-vehicle, LED lamps or electric motors of machines all work with direct current. But the electricity grid in your house or flat uses alternating current (AC) – even the public grid. And everywhere there are converters or inverters that bring voltages and current to the same level. Unfortunately, there are always losses in the process.
It’s as if our appliances speak different languages: English, German, Spanish or Italian. Some languages are similar, others are completely different. What they all have in common is that they need a translator: a voltage converter or an inverter. But what are the consequences?
Fundamental discussion
Well, actually we don’t really want to have this discussion, but we would like to raise a fundamental question that has its origins in the history of electricity. In a battle for market share between Edison (DC) and Westinghouse (AC) at the end of the 19th century, alternating current prevailed in our electricity systems because of its better range. With renewable energies becoming more and more decentralised today, it could make sense in the future to also convert our households with all their appliances to direct current. That would be a big step towards energy efficiency, but also a big step towards system-wide change.
Whether you want to convert from direct current to alternating current or from a high voltage to a lower voltage, every conversion involves losses. As a rule, these are heat losses in which energy is lost. You can always feel this on a power supply unit (e.g. your laptop), because power supplies in operation are always a little warm. Since these conversions take place everywhere in our households, a lot of tiny amounts of electricity are often wasted.
If you google energy loss household, you will almost exclusively come across pages that help you save energy when heating. It is true that this is where the greatest losses occur and this topic also contains the greatest savings potential. But one finds little about the fact that a household constantly wastes electricity by converting energy in the end devices.
Whether it’s the LED bedside lamp or your television – depending on the end device, power supply units or rectifiers are connected between the socket and the device. Most of the time, they are already built into the appliance. Because with every device, you have to bring everything down to the right denominator, speak the right language, or convert it to the right current, bring it to the right voltage. Everywhere, this creates noisy little losses. This also applies to the conversion processes from photovoltaic system to battery storage.
AC coupled storage system
It is summer, the sun is out and not a Cloud in sight. A perfect day for your photovoltaic system, isn’t it? It is, in principle. But in most cases your photovoltaic system produces more energy than your household consumes during the day. While you have to feed surpluses into the grid without electricity storage and buy them back at high cost at night, the RCT Power storage system helps you to make better use of the yield of your photovoltaic system. The AC coupled storage system can be easily integrated into your existing photovoltaic system.
RCT Power Storage ACBattery Inverter
RCT Power Battery
RCT Power Sensor
RCT Power App
AC coupled storage system for your solar power
The feed-in contract for your photovoltaic system is about to expire. Follow-up contracts providing a fair feed-in tariff are increasingly rare. You will receive very little money for the electricity you feed into the grid. There are alternatives, and here you have arrived at the right solution for you. An AC coupled storage system will save you real money. The AC coupled storage system is specifically designed for the scenario when your PV-system requires retrofitting. After the installation, you can store the electricity you generate with your PV-system for later usage. You consume power when you require it. Are you planning to install a new photovoltaic system? Get more information about our DC coupled storage systems for a new installation.
As initially mentioned, an AC coupled storage system is used to retrofit an existing PV-system. The core of the system is a so-called battery inverter that converts the direct current generated by the PV-system into alternating current to be used in the household. The battery inverter simultaneously stores the power gen-erated by the PV-system in the battery storage. You can use your electricity irrespective of whether the sun is shining or not and whenever you want.
AC coupled storage system. build and function
A modern Pv storage system has switching outputs that are easy to program. These switching outputs ensure that any excess electricity from the PV system is not routed to the power grid, but household consumers instead such as heat pumps or electric cars. An AC-coupled storage system has the advantage that it uses high voltages, keeping the charging and discharging currents low. This approach minimises the temperature cycles in the battery and results resulting in a significantly extended battery life.
There are numerous benefits of retrofitting an existing PV-system with an AC-coupled storage system. The battery storage system will provide you with a new level of autonomy, flexibility and self-sufficiency. The battery inverter will distribute the solar power according to demand. This increases the yield of the PV- system significantly and also helps to preserve the battery.The switching outputs of the system are freely programmable. Heat pumps, electric cars and other domestic electricity consumers are controlled individually and reliably supplied with power. We have also equipped our AC coupled storage system with a very efficient heat sink that provides noiseless and maintenance-free cooling. With our AC coupled storage system, you will optimise the way you use the electricity generated by your PV-system.
AC coupled storage system. connect it in just a few steps
You can connect the AC coupled storage system without tools via Plug Play. Use the RCT Power App for commissioning. The AC coupled storage system has a compact and lightweight housing. It is easy to handle, but at the same time, durable and robust. It is connected directly to the existing PV system. The AC coupled battery inverter receives rectified direct current from the PV inverter. It then converts this current into al-ternating current for use in household consumers. The battery inverter can also convert AC power back into DC power. This is a prerequisite for the ability of the system to store the generated solar power in the battery.
Please inform yourself about our AC coupled storage system RCT Power Storage AC.
AC DC Coupled Batteries and Systems
So as to maximise self-consumption and/ or to provide backup power during outages, many people are now adding energy storage systems (batteries) to their PV solar systems.
There are two architectures for the “solar plus battery” systems – DC coupled and AC coupled. Coupling simply describes how the battery system is linked or connected to the PV system. Thus, an AC coupled system or a DC-coupled system describes where the battery system is in the PV solar system.
What is a DC Coupled Solar System?
A DC-coupled system is one with a connection that allows the DC power produced by the solar array to go directly to the battery storage without needing to go through an inverter.
DC coupling refers to a solar plus battery architecture where the solar panels produce DC which outputs directly into the energy storage (solar batteries) and the inverter. The solar inverter converts the DC electricity to usable AC which is then fed into the house to be used by AC loads. Excess power is then exported to the grid.
This means that a DC-coupled solar system requires only one DC to AC conversion.
What Batteries are DC Coupled?
DC-coupled batteries are those that can be charged by DC electricity produced and flowing directly from the solar PV array. Examples of DC batteries include:
- LG Chem RESU. LG is one of the biggest manufacturers of consumer electronics in the world. The company has a range of Residential Energy Storage Unit (RESU), which are available in two voltage options – 48V and 400V. The 40 volts range include four batteries with sizes ranging from 3.3kWh to 13.1kWh, while the 400V range includes two batteries of sizes 7kWh and 9.8kWh.
- BYD B-Box battery system. BYD is the largest manufacturer of rechargeable lithium batteries in China. The BYD B-Box battery system has a modular design that gives the flexibility of having up to 4 battery units in one pack. Each unit has a storage capacity of 2.5kWh, giving the pack a total capacity of 10kWh.
- Generac PWRcell. The PWRcell from Generac is a very powerful and scalable battery. You can configure your storage system with as few as 3 of the 3kWh batteries for a 9kWh capacity. You can use as many as 6 of the units to get 18kWh capacity.
Which Solar Inverter Work DC Coupled Batteries?
- Fronius.
- Sungrow hybrid inverters
- SMA
- SolarEdge StorEdge inverters
- Huawei hybrid inverter
- Solax X-hybrid inverter
Pros and cons of DC Coupling Batteries
DC coupling offers the following advantages:
- It is more efficient. Since there is only one energy conversion, efficiency losses are reduced, making the system more efficient.
- It is more affordable. The one-time DC to AC energy conversion also means that a DC coupled system requires only one inverter. This reduces hardware cost, making the systems more economical.
- It allows for oversizing. DC coupling allows you to oversize your inverter so that you can harvest more power. Oversizing means installing a solar array with a capacity greater than the rated capacity of your inverter.
Generally, the capacity of a solar array (group of solar panels) matches that of the inverter. But with DC coupling, you can get more solar panels on your roof with the same inverter. The power generated over and above your inverter capacity can then be redirected to charge the battery system.
That is, you can install an 8kW PV array for a 5kW inverter. While 5kW goes into the inverter, the excess 3kW will go into charging the battery.
However, DC coupling is not without its disadvantages. These include:
- Relatively difficult to install. Initially, the DC coupled batteries were quite difficult to install because they require a charge controller that makes it possible for the energy coming from the solar panels to charge the battery. However, recent DC-linked batteries are available as hybrid batteries with the charge controller as an in-built feature.
- Not flexible. A DC coupled battery is usually not compatible with an existing PV system. They’ll require you to modify the existing connections before the battery can be added. This increases labour time and cost.
What is an AC coupled system?
An AC coupled solar system is one connected in such a way that the DC power from the solar panels is converted to AC power prior to being fed into the energy storage system (battery).
All power from the module goes through the inverter and is converted to AC power which is fed into the house. The AC power that is not used by the household is then directed into the battery system.
However, since batteries store DC energy, the AC coming to the battery goes through a battery inverter for conversion to DC power. When stored energy in the battery is needed, it comes in the DC form and is converted to AC before being fed into the household.
So, there are three inversions in AC coupled systems:
- DC from solar panels is converted to AC by inverter
- AC from inverter is converted to DC before charging battery
- DC from battery is converted to AC before going into the house
What Batteries are AC coupled?
Some of the top AC coupled batteries (or AC batteries) include:
- Tesla Powerwall. The Powerwall is from powerhouse Tesla. It is available as a single unit with 13.5kW energy capacity.
- Enphase IQ Battery. Enphase currently has four models of the IQ Battery models – 3, 3T, 10, and 10T. All have multiple bi-directional microinverters for safe and reliable operation. These batteries have energy capacity ranging from 3.36kW to 10.08kW.
- SonnenBatterie Eco. The Eco comes from the German company Sonnen, which is the biggest company in Europe when it comes to residential lithium battery systems. The SonnenBatterie Eco does not only store energy, it is a Smart storage system that automatically adjusts your energy usage. It comes with a modular design, allowing you to buy a single unit of 2kW and to later upgrade up to 16kW.
Which Solar Inverter Work DC Coupled Batteries?
The AC coupled battery systems are compatible with inverters from:
- SMA
- SolarEdge
- Fronius
- Enphase micro inverter
- Delta
- ABB
Pros and Cons of DC Coupling Batteries
AC coupling has a number of benefits which includes:
Retrofitting. Batteries that are AC coupled are easier to fit into an already installed PV system. They can simply be added to the existing system without having to do a complete rewiring.
Reliability. Because you have separate inverters for the panels and the battery, both systems are independent. So, a battery fault will not have a direct impact on the power generation of your solar PV system.
Flexibility in Charging. The AC coupled battery can be charged by either the solar panels or the grid power. This means that if your PV system is not producing enough energy, you can rely on grid power for charging your battery system.
However, AC coupled solar systems have a few drawbacks. These include:
Lower efficiency. Every time power is converted from one form to the other, there are losses. So, the multiple conversions required to add up, leading to slightly lower efficiency for the system.
Relatively expensive. AC coupling requires you to have a battery inverter in addition to the solar inverter. This means more components, making AC coupled systems more expensive.
Which is Best for My home?
The “solar plus storage” architecture that is best for you depends on whether or not you already have solar.
If you have already installed solar power, and simply want to add battery storage, an AC coupling system is the best for you. This is because of the flexibility of AC coupling systems. They are very easy to install and are compatible with existing PV systems.
However, if you want to install a “solar plus battery” setup at the same time, a DC coupling system may be best for you. This is because they offer slightly higher efficiency.
Configuring
2.2.1 Setup Local Area Network (LAN) Configuration
In order to establish a data communication between the Fronius and the GX Device, you will have to setup a valid LAN IP address on the Fronius, using one of the following methods. If you are in doubt, which method is the best for your network, please ask your system administrator or IT support.
2.2.1.1 DHCP addressing method
If you have a DHCP server running on the LAN the Fronius and GX Device are connected to, you don’t need to manually configure your LAN IP addresses as the DHCP server does this for you. Make sure, that the “IP Switch” is in “B” (default) position on the datamanager, to allow the Fronius to obtain it’s IP address. It can be reviewed in the Address Lease Table of the DHCP server.
2.2.1.2 Link-local addressing method
When no DHCP server is present on the LAN and you only have one Fronius unit to connect, Link-local addressing can be used to establish data communication between the GX Device and the Fronius. This could be the case, if you use a Wi-Fi Network for the GX Device to gain access to the Internet and have a Fronius connected to the wired LAN Interface of that GX Device.
Note, that no direct access to the internet will be possible for the Fronius in that configuration and therefore only communication to the GX Device is possible.
This method can only be used to configure a single Fronius unit within a LAN. Multiple Fronius devices in the same LAN must be configured with DHCP, or manual addressing method.
Make sure the “IP Switch” on the Fronius Datamanager is on position “A”
Connect both, the Fronius and the GX Device, on the same LAN network, either directly with a LAN cable or using a network switch.
2.2.1.3 manual addressing method
You will have to configure the IP address of the Fronius to be in the same subnet as the IP address of the GX Device. To do this, first have a look at the IP configuration your GX Device is using on that network.
The IP configuration can be reviewed in Settings – Ethernet. The important settings are the IP address and Subnet Mask.
If the LAN IP address and Subnet Mask of the GX Device is not configured yet, you will have to set it first.
As an example, the IP address on the GX Device could be: 192.168.10.1 and the Subnet Mask: 255.255.255.0.
Configuring the Fronius to the same subnet means setting a manual IP address to: 192.168.10.2 and the same Subnet Mask of 255.255.255.0 on the Fronius.
Every device on your LAN has to have a unique (different) IP address and so the last segment (.1/.2) in this example has to differ on every device on the same LAN. The highest value, that can be set for this segment is 254.
The subnet is described by the first 3 segments in this case and so they have to be the same. Configuring multiple devices with exactly the same IP addresses will lead to network errors. Use different IP subnets for the Wi-Fi and wired LAN interface. The IP address ranges used on each interface should differ in the third segment.
As an example: 192.168.10.1-254 for LAN IP’s and 192.168.11.1-254 for Wi-Fi IP’s.
Configure the Fronius Datamanager to use an IP address on the same subnet, as described above. You may need to refer to the Fronius operating manual for how this is done for your Fronius Inverter model,
If you would like to connect the Fronius to be able to use the internet connection of your Local network, you have to set Gateway and DNS IP addresses on the Fronius as well. They should be exactly the same as in the Settings – Ethernet of the GX Device.
2.2.1.4 special procedure for GEN24 model
In Fronius GEN24 devices with software version 1.14 or higher, the Solar API interface is not activated by default and must be activated if required (e.g. integration of a GX device).
The setting for this can be found on the user interface of the Fronius inverter under “Communication”. “Solar API ”.
For GEN24, there is no physical toggle switch required (as there is with other models).
Victron and Fronius have made no tests, nor make any claims about the performance of this integration. If light flickering issues occur, please contact your Fronius dealer.
2.2.2 Adding Fronius to the GX Device
In the GX Device, navigate to Settings and then the PV Inverters section. You will see this menu:
Select Scan in the GX Device menu, and after completion go into the Inverters submenu to see the results. If scanning does not find the inverter, manually add the IP address of the Fronius Datamanager from its card, or box.
Monitoring GX Device
When the Fronius is detected and configured on the GX Device all PV inverters will be visible in the devicelist.
To see the device details, navigate to the menu. Here you can see the current state, current power generated, total power generated.
For more details on each device navigate to the Device menu. Here you will find: IP address, Product name, product ID, and Serial number, etc.
Inverter status
The following status values are supported:
Zero Feed-in
It is possible to make a Zero feed-in system with Victron Fronius. The Venus device will the control the Fronius output power. See the ESS manual, chapter 4.3.11 Fronius Zero feed-in for details.
Q1 Why doesn’t the frequency settings in the Multi and the Fronius need to be the same?
The first two frequencies do not need to be the same since the Multi actively regulates.
Q2 I have an Fronius IG inverter (not Plus), can that work?
No, the IG inverters cannot be made to work in a MicroGrid system. The appropriate substitute for an IG 30 is the GALVO 2.5-1 or an IG Plus 25V-1 (depending on the desired DC voltage range).
Q4 Can I use the Fronius Smart Meter?
That depends on the type of system and software configuration.
A Fronius Smart Meter can be used in these two situations:
when used for limiting export, in a system where the Victron system is not configured as an ESS system.
The Fronius Smart-Meter can not be used for the Fronius built-in export limiting feature when part of a Victron ESS system. Instead, use the Victron ESS Fronius Zero-feed in feature, see chapter 4.3.11, Fronius Zero Feed-in, in the ESS manual.
Q5 How can I get the Fronius firmware that improves flickering?
Fronius has a special firmware available, on request, that fixes light flickering issues that occur on certain installations under certain circumstances.
For Fronius PV inverters produced after 2018-week 16, contain the flicker-fix already straight from production.
To update earlier and/or already installed PV Inverters, contact Fronius Tech Support for the file. The required file is fro29130.upd. Which works for all snap-inverter models (Primo, Symo and Eco). There is no, and will be no, fix available for Fronius Galvos.
Q6 What do I do if my Fronius PV-inverter is detected as Unknown PV-inverter and cannot be configured as 3-phase?
Later Fronius PV-inverters support both the Fronius SolarAPI- as well as the industry standard SunSpec protocol. When SunSpec is not available the GX device cannot properly detect the properties of the PV-inverter unless it is one of a fixed list of known older models. Newer PV-inverters will then show up as Unknown, and you will not be able to configure them as 3-phase.
The solution is to configure SunSpec support on the PV-inverter.