AC-Coupled BESS Vs DC-Coupled BESS

What is Coupling in BESS?

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In the context of Integrated Battery Energy Storage Systems, coupling refers to how battery energy storage is electrically interfaced or interconnected with the electrical power grid or other sources of energy or distributed energy resources.

 Solar PV array generates DC power that is converted into AC power through an Inverter, which is suitable for domestic or industrial loads. Solar PV batteries store energy in DC form. So, the difference between AC-coupled and DC-coupled batteries lies in whether the electricity generated by your solar panels is inverted before or after being stored in your battery. 

AC-Coupled Integrated BESS

In an AC-coupled Integrated BESS, the Battery system is interconnected to the grid or externally to the solar PV system on the AC side of the PV inverter. The BESS has its own dedicated inverter connected to the battery known as Battery Inverter that converts the DC output of the battery into AC power. AC coupled BESS has two different types of inverters which are PV or interactive Inverter and Battery or multimode Inverter.

In an AC-coupled system, DC power flows from a solar PV array to a, which converts the DC power into AC power. The output AC power from the PV inverter then flows to either the connected AC loads or to a Battery Inverter that converts the electricity back to DC for charging the battery storage and also converts the DC power from the battery storage to AC power to feed the load or export to the grid when there is excess energy from the Solar PV. In other words, the output from the PV array is fed through a PV or interactive inverter before it reaches the battery storage system. This means that the output AC power from the PV inverter must be converted to DC power before charging the battery energy storage system, and the DC power from the battery energy storage system must be converted once again to AC power.  This means that in an AC-coupled Integrated BESS, any energy stored in the battery storage system must be inverted three times (DC &#; AC, AC - DC and DC-AC) before being fed to the connected AC loads. There are energy losses in the system during this inversion process due to conversion inefficiencies.

Since AC-coupled BESS have separate inverters for the solar PV and the battery storage system, the grid, solar PV and the battery storage system can supply power to the connected loads at full power simultaneously or independently, creating flexibility in how the system operates. The solar PV array and battery can both share a common AC bus to the grid as shown in Figure 1.0 or run on separate interconnections. In an AC-coupled system, power from the PV modules is converted to AC before connecting to the ESS. To achieve this, an additional battery storage inverter is required.

 

AC-couple BESS Schematic

 Charging and Discharging of Energy in an AC coupled integrated BESS?

One of the key advantages of AC-coupled BESS is its ability to interact or work in conjunction with the grid. It has the ability for the grid to supply power to the load simultaneously with the Solar PV system and/or Battery inverter.

Below is how typically this works between the grid, Solar PV system and the Battery Energy Storage system:

 Grid power Supply: When no power is available from other sources ( for example, from PV systems and battery energy storage), the grid serves as the primary source of power to charge the battery and at the same time continuously supplies power to the load.

Solar PV System: AC coupled system can receive power both from the grid and solar PV system simultaneously. When there is excess energy produced by the solar PV array, the DC energy is converted to AC power by the PV inverter. This excess power from the solar PV system can exported to the grid or charged the battery energy storage, this effectively reduces the energy consumption from the grid and offsets energy tariffs.

Battery Power Supply: When there is insufficient energy from the solar PV array, and the energy storage has sufficient energy stored in it, and there is a need for additional energy beyond what the Solar PV array is generating, the battery energy storage can discharge to supply power to the load via the battery inverter.

Note: The coordination of power supply to the load between these three power sources is managed by the energy management systems (EMS) and the control strategies, as well as monitoring the charging and discharging of the battery storage system by the Battery Management System (BMS), to ensure that the power is supplied efficiently to meet the load demand and prioritize power supply to the load between the solar PV system and battery energy storage to reduce the reliance on the grid. Also, the dua-inverter set-up in an AC-coupled system allows for the storage of energy and self-consumption optimization.

 The AC-DC conversion has major system design implications. As previously mentioned, solar PV array produces DC power. The DC power must be converted to AC power to be used in AC loads in most residential, commercial, and industrial applications. In contrast, battery energy storage must be charged with DC power and outputs AC power.

 DC-Coupled Integrated BESS

In a DC-coupled system, the battery system and the Solar PV system are connected to the same DC bus with the DC side of an inverter as shown in the figures below. The solar PV array is connected to a DC-DC converter (MPPT solar Charge Controller) at the DC side of a hybrid inverter with an inbuilt MPPT Solar Charge Controller. DC power output from the solar PV array is fed to an MPPT charge controller (DC-DC converter) which converts the DC output from the Solar PV array into a suitable DC power form to charge the battery energy storage.  Since there is no PV inverter in DC coupled system, it means there is no inversion of solar PV array output from DC to AC and back again before the battery stores the energy. The energy produced by the solar PV array will be inverted only once (from DC to AC) as it flows from battery energy storage to the load or the grid. The DC-coupled system typically uses a solar PV charge controller, or solar PV regulator, to charge the battery from the solar PV array along with a battery inverter to convert the DC power to AC Power. DC-coupled systems rely only on a single multimode inverter or battery inverter that is fed by both the PV array and battery energy storage system. With this system architecture, DC output power from the solar PV array can directly charge the battery energy system. Since there is no dc-to-ac conversion required between the solar PV array and battery energy storage system, the energy conversion loss is less as compared to that of an AC-coupled system, which makes a DC-coupled system more efficient than an AC-coupled system. 

In a DC-coupled system, the battery energy storage system can be charged by both grid power and solar PV systems. This is one of the key advantages of a DC-coupled system as it&#;s specifically designed to work seamlessly with solar PV systems. One of the key disadvantages of DC coupled system is that excess energy produced by the solar PV array is not fed or exported to the grid instead it is used to efficiently charge the battery as a backup power.

Note: While an AC-coupled BESS is more efficient when the PV array is feeding loads directly via a solar PV inverter, a dc-coupled system is more efficient when power is routed through the battery energy storage (i.e. when the battery energy storage is charged directly and discharged at a later time) since there is only one conversion from dc to ac via a single inverter, rather than two inverters, to pass through.

 Charging and Discharging of Energy in DC-coupled BESS

Grid Power Supply: Like AC-coupled BESS, the Solar PV system is typically the primary source of energy when other sources are unavailable to charge the battery energy storage system. DC-coupled BESS can also be connected to the grid, allowing power to be supplied to the load from the grid when necessary.

Solar PV Power: The DC-coupled BESS system is designed for solar PV system optimization. When there is excess energy generated by the solar PV array, the excess energy doesn&#;t feed the grid directly, instead, it is used to efficiently charge the battery energy storage.

Battery Power: Since there is no double conversion in DC-coupled BESS, the stored energy in the battery system is primarily supplied to meet the load demand. 

What are the advantages of AC-coupled BESS?

Below are the major benefits of an AC-coupled BESS:

Retrofitting: AC-coupled BESS can be integrated easily with an existing grid or solar PV installation setups, making them a more versatile choice for retrofits, 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: AC-couple BESS has the flexibility to install multiple inverters and batteries in different locations. In the event that an inverter fails, or the battery system is faulty, this does not have any impact on the power generation, and this helps to avoid the risk of a power outage.

Versatility: AC-coupled systems enable batteries to charge from the grid as well as the solar PV array and the grid, so if there is insufficient energy production from the solar PV array, the battery can still charge from the grid.

Grid Support: AC-coupled BESS can provide various grid support services such as voltage regulation, frequency regulation, and power factor control for grid stability and black start.

 

What are the disadvantages of AC-coupled BESS?

Lower Efficiency: Due to the double conversion involved in an AC-coupled BESS (e.g. stored energy is converted three times, from the DC to AC current to supply the load and then back to DC to the battery and again back into AC). Each conversion in the system results in a small amount of energy loss due to conversion inefficiencies.

Higher Cost: The AC-coupled system is more expensive than the DC-coupled system as it uses two inverters.

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.

 What are the advantages of a DC-coupled BESS?

Where AC-coupled system suffers in terms of efficiency and cost, DC-coupled systems have the advantage:

Lower Cost: 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, reducing the overall system costs, and making them cost-effective for certain applications.

Higher efficiency: Unlike AC-coupled BESS which has multiple conversions of energy, DC-coupled BESS only convert the energy once. They directly store and discharge energy, reducing energy losses and making them more efficient.

 Solar PV Optimization: DC-coupled BESS allows solar PV modules to generate more energy than the inverter rating. They are well-suited for solar PV installations as they store DC energy generated by the Solar PV array efficiently thereby reducing energy loss, whereas in an AC-coupled system, the energy is lost.

 What are the disadvantages of a DC-coupled BESS?

• Limited flexibility: DC-coupled BESS may be less flexible and complicated to integrate with an existing Solar PV or grid infrastructure than with an AC-coupled BESS, 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. They are dependent on the Solar PV voltage levels and the characteristics of the solar PV system, which can limit their versatility as compared to AC-coupled BESS.
• Grid Support: As compared to AC-coupled BESS, grid support services may be more limited in DC-coupled BESS.

 Summary:

• The choice between selecting AC/DC coupled BESS depends on the specific application scenarios and system design. And the project requirements. DC-coupled BESS is often more efficient and cost-effective for solar PV-specific installations. At the same time, AC-coupled BESS is more versatile and can be integrated easily into an existing grid or DER infrastructure. The choice between AC-coupled BESS and DC-coupled BESS should be based on the priorities and constraints of the particular BESS project.
• AC-coupled BESS systems are best if it is required to integrate BESS into an existing solar PV system, making them easy to install.
• Energy must be converted three times in AC systems, making them less efficient.
• In DC systems, the energy only needs to be converted once, making them more efficient and less expensive.

High voltage batteries only arrived in the market in the last 12 months. However their arrival has allowed a number of other technologies to changes the way battery storage can be included in the home. AC-coupled storage using these new technology high voltage batteries (i.e. > 120V DC) has a number of advantages for homes wanting to include battery storage with their solar system. In addition to being a lower cost solution compared to alternatives, AC-coupled storage with high voltage batteries can allow your home to increase its independence from the grid and provide you with greater flexibility in the future to change your system to meet the needs of a growing or changing home. AC-coupled storage with high voltage batteries is also typically a more efficient system meaning you&#;ll get more energy and power compared to a similar hybrid inverter system. And perhaps best of all, AC-coupled storage can turn any new or existing solar system into a true battery-ready system.

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Read on to discover the benefits of AC-coupled storage with high voltage batteries and why it can be the best solution to meet the individual energy storage needs of your home.

 

What is AC-Coupled Storage?

AC coupled storage is the connection of a battery energy storage system to a solar system via AC (alternating current) electricity. Energy from a solar system is generated in the form of DC (direct current) electricity which is then turned into AC by the solar inverter. A battery inverter connected to the same electrical network via AC then converts any spare energy not used by the home back into DC so it can be stored in a battery. By contrast DC coupled storage uses what is commonly referred to as a hybrid inverter, where both the solar and battery are connected to the same inverter. However, as we&#;ll see below there are many reasons why AC-coupled storage using high voltage (HV) batteries offers a number of advantages in terms of cost, flexibility and risk compared to DC coupling.

 

 

Turn ANY PV System into a &#;Battery-Ready&#; System

One of biggest advantages of AC-coupled storage is that it turns ANY new or existing solar system, into a true &#;battery ready&#; system. Since batteries and solar cannot be simply connected together, grid-connected battery storage will always be required to connect to an inverter. AC coupled storage means that the inverter to which the battery is connected is separate to the solar inverter. It can be a much simpler installation in many cases by using two separate inverters for battery and solar Even though there are 2 separate inverters, compared to a number of hybrid inverter solutions it can still represent few components overall. The other main advantage is that for existing solar systems, the investment already made does not need to be wasted by removing the system to install a hybrid inverter. With no more than 3 components (shown inside the red dotted line) a new or existing solar system can be &#;battery ready&#; thanks to AC-coupling.

 

Lower Cost Solution

AC-coupled storage with HV batteries offers opportunities for lower cost products and installations As recently as early , HV batteries like those from manufacturers such as LG Chem or BYD were not available in the market. With their introduction however, lower costs are able to be realised with respect to the inverter. Since a HV battery is similar in current and voltage characteristics to a normal solar array, they can utilise similar transformerless inverter technology. In addition to being more efficient than traditional transformer-based inverters, transformerless inverters are also significantly cheaper since they are much smaller and use far fewer components (this also can make them more reliable!). This means that you can add battery storage to your new or existing solar system more cheaply.

One of the realities of a technology industry like solar is each year, the technology gets better and cheaper. So if you are installing a new solar system but want to wait a few years to install batteries, AC-coupling with HV batteries will allow you to do this at a lower cost in the future compared to choosing a DC coupled hybrid inverter now and buying batteries later. This is because in the years between installing your battery ready PV system and buying your batteries, technology will have got cheaper for both the inverter and the battery. It can also be worth considering when installing a solar system, whether you should install a battery or wait some time to determine your self-consumption and then decide on the correct sized battery for your individual needs. If however you originally decided to install a hybrid inverter, you realised a higher cost for the technology rather than being able to purchase AC-coupled storage at a lower future cost. The other advantage about choosing AC-coupled storage rather than a DC coupled hybrid inverter is that you still have the opportunity to take advantage of future technology developments. For example, if you had invested in a hybrid inverter in and decided to buy batteries in a few years, you would not have been able to take advantage of newer and lower cost battery solutions like those from LG Chem and BYD. With AC-coupled storage, you can take advantage of such future developments because the solar and battery systems are independent of each other.

Whether it is a transformerless inverter, future reductions in the cost of products or as-yet undiscovered technology advancements in batteries, AC-coupled storage offers a lower risk and lower cost option for adding battery storage to your solar system.

 

Flexibility of Installation Location

AC-coupled storage provides a greater degree of flexibility, and potentially at lower cost. When adding batteries to a new or existing solar system, one main advantage of AC-coupled storage can be the flexibility of where the battery system is installed. Many solar systems have the inverter installed either on an external wall or perhaps in a sealed services room. These might not be the best or even suitable locations for the installation of the battery system. AC-coupled storage means the battery system can be installed wherever it is best to be installed (e.g. hidden in a garage, etc), and independent of the solar system. This can also be helpful for keeping the cost of installation to a minimum. Different factors affecting each part of the system and its installation, can be addressed independently and potentially avoid additional work required to make an install location suitable for both solar and batteries.

 

Flexibility regarding Upgrades or Changes to the System

Homes change over time. New children arrive or they may finish school and either study at university or move out. Maybe you reach retirement and are spending more time at home during the day. Or perhaps you change your home appliances, buy an electric car or add new home appliance that are still to be invented (after all HV batteries for the home didn&#;t exist only a few years ago!). As homes change, so too does the consumption of energy. This means energy storage (batteries) and energy generation (solar) systems may also need to change. AC-coupled storage means both battery and solar systems can be independently modified without necessarily affecting each. If a home starts to use more energy during the day, the solar system can be upgraded or changed to meet this energy demand without touching the AC-coupled storage. Similarly, if a home starts to use more energy during the evening when electricity prices tend to be higher, the battery system can be upgraded to meet this demand with no changes to the solar system.

AC-coupled storage provides this flexibility to change battery or solar independently which means future potential costs will be lower compared to a similarly sized hybrid inverter solution. This is also important to remember as technology and standards change which, for hybrid inverter systems, may require the both parts of the system be brought up to code if one of them is modified. This can add significantly to modification costs. With AC-coupled storage, system upgrades or changes are more flexible and can be realised at lower future cost and risk.

 

Increased Independence from the Grid

When you add a battery system to your solar system, the main purpose is to better self-consume the energy from your solar system and increase independence from the grid. AC-coupled storage can allow you to have maximum grid independence compared to a hybrid inverter solution since you are able to supply loads in your home from both the solar and battery simultaneously. The example below shows what would happen for a typical single phase home which has solar and storage. Restrictions in the allowed capacity to connect to a network would mean that an AC-coupled system could be supplying 100% of your home&#;s load requirements compared to a DC coupled hybrid inverter which might only supply 66% of your home&#;s load. The flexibility of AC-coupling means that you will increase you independence from the grid and in doing so, reduce your costs related to purchasing grid electricity.

 

 

Simplified and Improved Blackout Operation

Although modern power grids rarely experience blackouts, one of the drivers you may have when purchasing a battery system is to &#;blackout-proof&#; your home (also referred to as backup). This would mean installing and setting up your system so it is able to disconnect from the grid and continue operating. Especially for existing solar systems, AC-coupled storage offers a greatly simplified means of achieving backup. For additional cost and by adding some simple changeover switches which then interface to the AC-coupled battery inverter, any solar system can also provide backup power. And if the solar inverter is able to have a Power-Frequency response programmed into it (SMA or 3rd party solar inverter), the solar system will also be able to add power to your &#;blackout-proof&#; system. If this were compared to a system where a hybrid inverter with additional solar array were added to an existing system, backup power would only be provided from the hybrid inverter and not the other existing Solar system. This type of system simplifies how backup is achieved so that, in the unlikely event of a grid failure, your system can provide power to keep your lights on and your fridge cold.

 

More Energy and Power from Better Operating Efficiency

One of the best advantages which has come from the development of HV battery technology for home energy storage is the ability to connect them to higher efficiency transformerless inverters. Low Voltage batteries need to connect to transformer-based inverters which, apart from being much heavier, are less efficient when they convert between DC and AC. Most hybrid inverters available on the market are designed to connect to Low Voltage batteries which mean they use a less-efficient transformer as part of their system. Even though AC-coupled storage may have more energy conversion steps in the process, since the HV batteries are connected using more efficient transformerless inverters, the energy delivered from the battery to the loads in your home can be at a higher efficiency. This means that your AC-coupled storage system with HV batteries will deliver more energy and power compared to a similar hybrid inverter system. The image below shows what the theoretical maximum conversion efficiencies are for both AC-coupled storage with HV batteries and DC coupled storage with LV batteries.

 

 

Both systems seem reasonably similar, however this does not then take into account the additional losses from the battery. The curve below shows the round trip efficiency between Solar &#; Battery &#; Home loads.

 

This shows a far greater difference in total system efficiency meaning AC-coupled storage with HV batteries could potentially deliver far more energy (hundreds of kWh) per year compared to the DC coupled hybrid inverter system. For a typical family home, this increased efficiency allows greater grid independence which could help you avoid hundreds of $ / &#; / &#; of grid energy supply charges per year.

 

Conclusion

AC-coupled storage can turn any new or existing solar system into a battery-ready system unlike alternate DC coupled / hybrid inverter solutions. With the introduction of new high voltage batteries, AC-coupled storage has become a lower cost option to add battery storage to a solar system compared to hybrid inverters or low voltage battery storage. AC-coupling also offers a number of advantages such as flexibility for installation and also future upgrades or changes to either the solar or storage system. This means the system is better able to meet the individual needs of your home and can allow you to add battery storage at a lower cost. AC-coupled storage also allows you to better increase your independence from the grid saving further on electricity supply charges. This financial benefit is also increased through a more efficient operation. And with a simplified means to blackout-proof your home, AC-coupled storage with high voltage batteries is the smart solution if you want to add battery storage to your solar system.

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