Solar-plus-storage projects are proliferating across the globe and necessitate a choice between DC and AC coupling. In this piece we look at the differences and pros and cons of each.
Co-locating solar PV installations with battery energy storage system (BESS) technology is a critical way of either supporting the grid (in front-of-meter applications) or being energy independent (in behind-the-meter applications). The two assets are shared behind a single grid connection; this is a useful way to reduce curtailment in renewable energy installations. Instead of losing energy or causing grid congestion, co-locating solar installations with BESS can store the energy generated at peak times for later, instead of it being wasted.
Co-locating solar and BESS is beneficial to the grid as it can provide stored energy that can be later used for load shifting to reduce the strain on the grid during peak times. For asset owners, the assets can share grid infrastructure (transformers, grid connections, cabling etc) and to save on both capex and opex costs and can also make more money on the energy generated by storing it and discharging it to the grid when prices are high.
Co-location of BESS and renewable assets requires coupling, and there are two main ways to couple the assets: AC coupling and DC coupling.
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AC coupling is the most common, but DC coupling has emerged as an alternative for solar and BESS co-location projects. DC-coupling has also been a prerequisite for some solar-and-storage project subsidies, for example in Germany and the US. Here, we look at the differences in these two coupling approaches.
AC coupling: the simpler and flexible, but less efficient and more expensive, option
AC coupling is the most common approach for co-location projects. AC coupling connects the BESS to the AC side of the solar power system, so both the solar and BESS use their own inverters to convert DC electricity to AC electricity before any electricity reaches the grid.
Solar PV generates DC power, and this is converted to AC by the solar inverter. This power either goes to the grid, or it moves through the BESS inverter and is converted back into DC power and stored in batteries for later use. When the stored energy needs to be used, the BESS inverter (or power conversion system, PCS) converts the electricity back to AC for on-site use or for export to the grid.
This dual-inverter setup is different from DC-coupled systems and is a simpler installation. Because the AC bus connects both inverters, it enables a much higher degree of flexibility in the system than DC coupling. However, AC coupling tends to cost more than DC coupling because multiple inverters need to be bought and installed.
Because the energy designated for battery storage goes through multiple conversions via the PCS (DC–AC to AC–DC back to DC–AC) before it is used or sent to the grid, AC coupling means a lower round-trip efficiency (RTE) than DC coupling. This is because there are always energy losses during an inversion process, so the more inversion processes there are, the more energy is lost and the less efficient the system is.
Here are the advantages and disadvantages of AC coupling when co-locating BESS and solar PV:
| Advantages | Disadvantages |
| AC coupling is highly flexible. That means it can be used for retrofitting existing solar plants with BESS because they both operate independently on the AC side of the installation | Multiple power conversions reduce the efficiency compared to DC coupling |
| Flexible and simpler setup allows for easier inverter integration and multiple types of inverters from various manufacturers can be installed | Higher equipment costs/CAPEX because of the separate inverters for the solar and BESS assets |
| A modular approach that can add more solar or BESS capacity as needs change | AC coupled BESS are not designed to be used off-grid because they are transformer-less, so cannot handle surge loads (e.g. from multiple household appliances in residential setups) in the case of an outage |
| Metering of the solar and storage is clearer because both assets have their own inverter | Export limiting hardware is needed to ensure that the total export doesn’t exceed the grid connection capacity set out by the DNO. If limits are exceeded, grid connection agreements often state that the DNO will disconnect the installation from the grid (as it poses a grid risk if excessive voltage is pushed from the solar installation into the local grid cables) |
| The flexibility of AC coupling means that batteries and inverters can be installed in different locations (no restrictions on their location) to improve the resilience of the system | |
| If an inverter fails or battery system is faulty, there is a lower risk of power outage as these systems are independent of each other | |
| The BESS can be charged from the grid if solar generation is low | |
| Can be used to support the grid through various support services, including voltage regulation, frequency regulation, and power factor control to improve grid stability | |
| Because the BESS and PV inverters are independent from each other, they can discharge simultaneously at full-rated power |
Because AC coupling, like DC coupling, has its own distinct advantages and disadvantages, there are various situations where AC coupling is the better option over DC coupling. Some of the main scenarios where AC coupling is more beneficial are:
- Retrofitting existing solar plants with BESS (the majority of retrofits are AC coupled)
- For installations that plan to take part in the 24/7 ancillary markets for providing frequency regulation and other grid balancing services―such as peak shaving, reactive power compensation, primary reserve control and energy shifting
- Small-scale systems where the flexibility and simpler installation of using modular components is more beneficial than a higher efficiency
- When the system needs to operate under variable conditions
- For large-scale systems that will have expanding capacity in the future as the batteries can be located away from inverters and it’s easier to add more batteries over time without disruption
DC coupling: greater efficiency and cheaper installation but less flexible and more complex
DC coupling integrates the BESS into the DC side and shares a common DC bus with the solar array.
As both assets are connected to the DC side before the inverter, only one inverter is needed (typically a multi-modal inverter), with a combined (not independent) DC power from the BESS and solar panels passing through the inverter, where this combined power is converted into AC for powering either the local loads or being sent to the grid.
This makes DC coupling cheaper than AC coupling, as there is less hardware to buy, but makes it hard to meter, since the energy is shared through the inverter, unlike in AC coupling.
DC coupling is more efficient (up to 98%) than AC coupling because there is only one power conversion process instead of multiple power conversions, as the battery can be charged directly with harvested DC electricity without needing to be converted and converted again.
Solar energy can be directly stored in the battery from the solar panels. DC-coupled systems can also easily charge from the grid in times of low solar energy generation.
Unlike AC coupling, there is no need for export-limiting hardware because the inverter will clip any energy above the grid export capacity, making it physically impossible to output more than the grid capacity in DC-coupled systems. DC-coupled systems are best utilised when the aim is not to use solar power immediately, and one of the main aims of the co-location is to charge the batteries regularly.
Below are the main advantages and disadvantages of DC-coupled BESS and solar systems:
| Advantages | Disadvantages |
| Higher efficiency and lower round-trip efficiency (RTE) losses than AC coupling because of fewer power conversions | Not well suited for solar plant retrofits because there is less flexibility in the system, and requires significant wiring and inverter change to retrofit |
| Lower equipment/CAPEX costs | The installation and wiring are more complex due to higher voltages on the shared DC bus |
| More efficient at charging batteries in low-light conditions due to direct DC connection | Difficult to integrate different types of DC sources on to the shared DC bus, and augmenting existing BESS can cause voltage issues |
| Simpler control systems because there are fewer inverters and power conversions to manage | Much harder to scale as the system is limited by the voltage of the DC bus. Any new assets add to this bus instead of being an independent system |
| Physically impossible to output more than the grid export capacity, so no issues of being cut off by the transmission or distribution operator | Harder to use a range of components from different manufacturers as they need to be compatible with the existing DC bus |
| The BESS can be easily charged by both solar assets and from the grid | The power output from the inverter is a combination of battery and solar energy, which makes it difficult to provide ancillary services, such as frequency response, as these are metered on the AC side of the inverter |
| DC-coupled setups allow the solar array to generate more energy than the inverter rating because any excess gets stored in the battery before the energy is clipped/lost | Integrating both solar and BESS assets on the DC side makes it difficult to commercially separate them, which can make it more difficult to finance |
| The BESS and inverters need to be located close together which limits where the BESS can be located | |
| Resiliency of the system is lower than AC coupled systems, because if the inverter fails, both the solar power and battery capacity are lost | |
| Installations may require extra DC-DC converters to be installed which reduces some of the cost benefits |
While AC coupling is the preferred option for retrofits, DC coupling is preferred for new solar-BESS co-location projects, such as those on greenfield sites. It is also a preferred option for projects that are prioritising RTE and lower costs. DC coupling is also favourable for off-grid and microgrids where efficiency is more critical.
DC coupling is beneficial for large-scale energy storage systems because energy losses from repeated AC-DC power conversions can cause significant energy losses over time. Using DC coupling means that larger systems retain a lot more energy and provides a better ROI over time.
In commercial and industrial (C&I) applications, DC coupling is beneficial because the PV-BESS system can capture excess energy generated throughout the day and store it for later use. C&I applications have a high energy demand, so the higher efficiencies means that more energy harvested by solar panels reaches the loads. DC coupling is particularly beneficial for warehouses, factories, and large office complexes that use a lot of energy.
Here we look at two different DC coupling approaches: the older DC-DC converter systems and newer integrated hybrid inverter systems.
DC-DC converter-based systems
In older DC coupled systems, a bidirectional DC-DC converter with a maximum power point tracking (MPPT) charge controller is often installed between the solar array and the BESS to ensure that the correct voltage is being used to charge the battery. The MPPT voltage from a solar array is typically 300-600V so the converter will increase or decrease the voltage to match the nominal voltage of the installed BESS. This approach is commonly used in C&I and utility-scale installations.
In these systems, the solar panels connect directly to the shared DC bus while the BESS connects to the same bus via the DC-DC converter. A central or string inverter is located at the end of the bus and connects to the AC side of the installation. The DC-DC converter is a bridge between the bus and the BESS, instead of the solar array and the bus. This energy can then be sent to the grid if needed without having to undergo a conversion process, so it is more efficient, but can still be stored in the battery as well (via the converter).
Integrated hybrid inverter systems
The latest DC coupled co-location systems use integrated hybrid inverter systems with integrated MPPT charge controllers. Using integrated hybrid inverter systems is now the most common approach because the hybrid inverter can handle both the solar and BESS inputs. Both the solar and BESS inputs are connected directly to the inverter via separate input ports.
Energy created from the solar panels enters the inverter via the MPPT inputs. The MPPT inside the inverter adjusts the voltage to match the battery voltage, allowing it to be converted in the same component that converts DC to AC. Once energy is needed for loads or the grid, energy from either the battery, solar panels, or both is transferred to the inverter/PCS via their own inputs, and the energy is converted to AC and distributed.
AC coupling or DC coupling: which one to choose?
The choice of AC coupling or DC entirely depends on the type of installation as each approach comes with its own advantages and disadvantages. Some of these are situation specific and others are technical considerations, but understanding the potential pros and cons of both should help to work out which approach is best for your specific co-location project.
For example:
If it is a retrofit project, then AC coupling is the best approach, but if it is a new installation, then the benefits of DC coupling could outweigh the cons.
- Are you wanting to take part in grid supporting services, such as frequency regulation, then AC coupling is the approach that offers the greatest flexibility for supporting the grid.
- Is the installation going to be off-grid? If so, it’s likely that DC coupling will be the best approach as AC coupling is best suited to grid-tied installations.
- Does efficiency and cost matter more than having more flexibility to scale as your energy needs grow? If it is the former, then DC coupling is the best approach, but if it’s the latter, then AC coupling is best option.
These are just a few examples of the situations where AC coupling might be favourable to DC coupling, and vice versa. The detailed advantages and disadvantages of each approach and what systems they are best applied to should provide a good basis for understanding which type of coupling would benefit your solar-BESS co-location projects against you anticipated budgets, CAPEX, available space, operational needs, and efficiency requirements.
