RCDs – to ‘B’ or not to ‘B’?

AC Coupled Battery Storage

The UK Government recently released some good news in relation to battery storage.

From 1st February 2024, the UK Government will implement a new policy that reduces the Value Added Tax (VAT) on battery storage systems to 0%. This policy benefits individuals and businesses planning to install stand-alone energy storage systems or add storage to existing solar PV systems, facilitating greater accessibility to green energy solutions”.

For those with an existing PV array seeking to enhance their system with battery storage, the optimal choice could be integrating an AC coupled solution which typically incorporates an AC Coupled Charger/Inverter, and battery storage modules.

AC-coupled inverters are a specific type of power electronic device designed to convert direct current (DC) to alternating current (AC) for charging and discharging connected batteries. Unlike a regular grid tied PV inverter or DC-coupled inverters, which have a DC input from the PV array, AC-coupled inverters are not connected directly to PV – they connect and synchronize with the electrical grid as well as connect to the battery storage modules.

An AC coupled solution allows you to add battery storage without modifying the existing solar PV array or changing the PV inverter configuration.

Like the Emergency Power Supply (EPS) in a hybrid inverter, many AC-coupled solutions incorporate an EPS output designed to sustain essential loads during a grid outage. It is essential to recognise that you might need to segregate circuits within your customer’s consumer unit. In doing so, you would install a secondary consumer unit to specifically cater to the “critical loads” — referring to those circuits you intend to keep powered in case of a power outage. Be mindful of the power constraints associated with the EPS concerning the load requirements of the critical circuits you aim to support.

AC coupled solutions

      Here’s how it works when paired with existing PV:
  • Under normal grid operation: Solar panels typically prioritise supplying daytime loads first, then charge batteries with surplus power. Once charged, any excess PV is exported to the grid
  • During a grid outage: The mains power panel loses supply, and energy flow to the grid stops. Only the appliances connected to your critical loads panel remain operational. Please be aware that not all AC-coupled inverters provide EPS functionality. It is advisable to thoroughly review the data sheet and user manual for detailed information
  • After a grid outage: Batteries can power essential appliances at night, using stored energy generated during the day, even if the grid remains offline

AC Coupled System Diagram

AC-coupled inverters provide a flexible platform for integrating a diverse array of power sources, allowing users to create resilient and sustainable energy systems tailored to their specific needs and environmental conditions.

Inverters play a crucial role in our energy ecosystem by converting AC power to DC, acting as a bridge between various technologies. Regardless of the energy source—be it solar panels, wind turbines, or other renewables—these inverters are not directly tied to specific technologies. Instead, they opportunistically harness excess AC power, ensuring that unused energy finds purpose in charging AC-coupled batteries. This versatility highlights their adaptability, creating a seamless integration within diverse energy systems.

A few of the sources which can be used are listed below.
• Solar photovoltaic (PV) modules
• Wind turbines
• Hydroelectric systems
• Energy storage systems (batteries)

AC-coupled systems offer users significant flexibility and adaptability, especially when retrofitting with existing PV systems. This flexibility allows users to choose components based on factors such as performance and cost, without being restricted to a specific brand or type. Even when integrating AC-coupled inverters with an existing PV system, users can independently select the brand of the inverter. Additionally, AC-coupled systems are modular and scalable, enabling users to expand their energy infrastructure with additional solar panels, wind turbines, or energy storage. However, users should exercise caution and thoroughly check system limitations, including parallel capability and storage capacity, to ensure a clear understanding of the system’s capabilities and constraints.


Advantages of AC-Coupled Battery Solutions

1. Modularity and scalability:
AC-coupled systems can be easily expanded by adding more batteries to the existing configuration. This flexibility allows users to start with a smaller battery bank and incrementally increase its capacity as their energy storage needs grow. It is crucial to exercise vigilance in adhering to system limitations to ensure optimal functionality and prevent potential issues.

2. Compatibility with existing solar systems:
Homeowners or businesses that already have a solar PV installation can seamlessly integrate an AC-coupled battery solution. There is no need to rewire the DC circuit or replace the existing solar inverter.

3. Backup power capability:
During grid outages or emergencies, AC-coupled batteries can provide backup to essential loads by switching to an off-grid mode. This feature provides peace of mind and ensures that critical appliances continue to operate during power disruptions.

4. Time-of-Use optimisation/Arbitrage:
AC-coupled battery solutions excel in optimizing energy usage and reducing utility bills by charging battery storage from the grid when electricity tariffs are cheaper and discharging the batteries to your loads when energy prices are higher. The AC coupled charger must be optimised to ensure there is available capacity for storing the excess solar energy the following day. Typically, though, during the day, the batteries can supply power during evening hours when electricity rates are typically higher, maximizing self-consumption and minimizing reliance on the grid.

5. Greater control and management:
AC-coupled battery systems come equipped with advanced monitoring and control capabilities. Users can track their energy generation, consumption, and battery status remotely through integrated software applications, allowing for better energy management and optimisation. The ability to monitor and control the AC-coupled battery system remotely provides users with a greater level of convenience and flexibility.

Inverters work in harmony with monitoring systems, commonly integrating meters and current transformers (CTs) to monitor surplus solar power. This careful oversight guarantees the effective use of excess energy, enabling inverters to judiciously direct it towards charging AC-coupled batteries, thereby optimizing the entire energy system. Also, AC-coupled systems can function independently as grid-tied battery storage systems entirely on its own without ever installing PV, this provides users with the flexibility to benefit from government VAT reduction even if there is no existing PV system or no plan to install PV in the future.

Hybrid Inverters working as AC coupled.

Some hybrid inverters have the capability to function as AC-coupled devices. This is achieved by adjusting the settings of the hybrid inverters to enable this feature. One advantage of this approach is that for a brand-new installation with no existing PV, it allows for the addition of solar modules connected to the inverter later.

A hybrid inverter can be installed and configured as AC-Coupled until the decision is made to integrate solar modules into the system. Subsequently, the inverter functionality can be switched back from AC-Coupled to hybrid, facilitating the seamless addition of solar modules.

For example, Fox ESS hybrid inverters can function as AC-coupled devices by choosing the “Self-Use without PV” mode in the settings. In this mode, the inverter operates independently of PV modules, generating power exclusively from alternative sources like the grid or other power sources.

Some manufacturers employ different terminology to avail this functionality, but it can typically be located by referring to the user manual or by reaching out to the manufacturer’s Tech Support teams. The manual will usually outline the work modes, providing details on the various operational modes the hybrid inverters can assume. While most manufacturers may label this function as “self-use without PV modules,” there could be variations in terminology.

Don’t forget – we have a team of extremely qualified and experienced Technical Support engineers available to support you for your pre-sale and post-sale Segen queries. We are available from 9am to 5pm by calling 0330 9000 141, then selecting option 2.

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