As we move to a digitalized, decentralized, and decarbonized energy infrastructure, Battery Energy Storage Systems (BESS) are clearly emerging as a sustainability and energy efficiency game-changer. One obvious use case is as a residential energy storage system, providing the capacity to store excess energy generated from intermittent sources like solar and wind power, and allow it to be used when needed. Alternatively, BESS solutions can provide a portable off-grid power source that doesn’t come with the noise and air pollution often associated with diesel generators for industrial and construction industries.
In transitioning away from carbon fuels, it is important to balance energy supply and demand, provide grid stability, and mitigate the intermittency issues inherent in renewable energy sources. BESS solutions are ideal for this. However, the effectiveness of BESS hinges on the efficiency and reliability of the chosen topology and components used. Just as with Uninterruptible Power Supply (UPS) systems, there are numerous different topology options available both on the input and output side, each coming with their own advantages and disadvantages.
Residential Energy Storage Systems
Residential energy storage systems are typically combined with solar panels and electricity from the grid, to provide a stable power supply that maximizes the use of renewable energy. In terms of topology the first choice to be made is the connection between the photovoltaic (PV) power and the battery power. This determines whether the BESS system and solar panels use separate inverters (AC-coupled) or a single hybrid inverter (DC-coupled).
Where an existing residential solar installation is installed, it is relatively easy to retrofit it and add an independent BESS system that takes the AC voltage either from the grid or from the PV inverter. Having multiple inverters makes the system more scalable and provides redundancy, but at the same time they are less efficient and more costly. To reduce costs, the two systems (BESS and PV) can use a single hybrid inverter, which brings the added bonus of reduced conversion losses. However, if the inverter fails both the solar power and the battery capacity are lost.
A complicating factor is when electric vehicle (EV) charging is included in the mix. Particularly with the trend towards smart home grids or microgrids where the EV battery is used as an additional BESS system in Vehicle-to-Grid (V2G) or Vehicle-to-Home (V2H) setups. This can also influence the choice of whether to use high-voltage or low-voltage batteries for energy storage.
DC-conversion and inverter topologies
Once the broad system configuration is made, then the next level of topologies come into view. When it comes to DC/DC converter stages, the key requirements relate to isolation, bidirectional energy flow, and ranges of input and output voltages. Where isolation is required, engineers can choose from an LLC or a Phase-Shifted Full Bridge (PSFB) topologies. For the battery interface, bidirectional energy is mandatory, and so the topology requires a dual-active bridge, and an LLC would typically become a symmetric CLLC.
In terms of the DC/AC inverter, several power converter topologies can be employed to connect the system to the home grid. For a unidirectional solar inverter, a conventional topology is usually sufficient to deliver an adequate and efficient solution. On the other hand, a bidirectional DC/AC inverter requires a more sophisticated control and additional components. One of the best choices out there is the bidirectional totem-pole PFC, which can take maximum advantage of the efficiency that wide-band gap transistors can deliver and minimize the cost of the overall solution.
Nexperia’s 4 kW analog bridgeless totem-pole PFC evaluation board is a good example of a unidirectional totem-pole PFC, which can be relatively easily modified with changes to the control and sensing circuitry to be bidirectional.
GaN efficiency and CCPAK
Switching frequency and efficiency are central to energy storage solutions. Two areas where the 650 V cascode GaN FETs from Nexperia excel. By harnessing the efficiency, reliability, and performance advantages of GaN-based power electronics, BESS can achieve higher levels of efficiency, reliability, and flexibility. Along with overall efficiency gains, GaN FETs can also help considerably reduce the BOM cost of both BESS and PV systems.
This leads to one final decision for engineers – through-hole or surface mount GaN FETs. It all comes down to preference and capability of the manufacturer creating the energy storage system. Through-hole (TO-247) devices are a simple option, that allows the designers to bolt the devices directly to a heatsink. Providing an effective and efficient way to take the heat away. Although an inherent drawback is the inductance that comes with that package and the manual assembly required for a through-hole package.
To get the best of both worlds (SMD and through-hole), Nexperia created the CCPAK. A state-of-the-art packaging technology that minimizes inductance and maximizes current capability and package reliability. In addition, our top-side cooled 12x12 mm CCPAK gives engineers a way to easily connect a heatsink directly to an SMD device. Delivering 30-40% more power compared to an equivalent RDS(on) in a conventional SMD package, a true game-changer technology.
The synergy between Battery Energy Storage Systems and GaN FETs is a noteworthy milestone in the evolution of energy storage and distribution technologies. In fact, the potential for a transformative impact on the energy landscape is significant.
