7 Reasons Why String Inverters Make Increasing Sense for Energy Storage
As markets and technologies for inverters grow, so does the importance of choosing between central and string inverters for energy storage projects.
Typically, central inverters have been the standard for commercial and utility-scale energy storage applications. But that is shifting as costs drop and developers, EPCs, owners and operators discover more about the performance benefits of string inverters. The solar PV market embraced string inverters first, but energy storage is gaining momentum.
In this post, we’ll take a closer look at string inverters and their benefits for energy storage.
How do central and string inverters differ?
An inverter turns the direct current (DC) output of a battery or solar panel into alternating current (AC) for use in homes and businesses or to feed directly into the electrical grid. Inverters also serve as the brains of an energy storage project, managing, optimizing and driving project performance and financial returns.
The difference between central and string inverters, while simple, has profound implications for long-term project design, performance and safety. As their name implies, central inverters centralize the project’s energy through one conduit. String inverters use a distributed architecture, breaking the system down into smaller, multiple inverters and usually more than one DC bus.
The concept behind string inverters should be familiar. The same idea underpins everything from the Internet to distributed energy resources. Rather than put all your “eggs” in one basket, it makes sense to use more, and smaller, interconnected units. This spreads risk and minimizes the chance of an entire unit being out of service. A decentralized approach offers numerous other benefits, which we’ll detail below.
7 benefits of string inverters
- High system availability. Building on a proven concept from the solar PV industry, creating smaller, independent strings of batteries and inverters on separate DC buses inherently leads to higher overall system availability. If one string inverter goes out of service, the remaining inverters on that DC bus remain in service. The project continues to be available for capacity, energy or other system services. Batteries last longer. And not needing to re-balance strings translates to greater uptime.
- Right-sizing power requirements. String architecture enables right-sizing of inverter systems by paralleling multiple, independent, smaller string inverters. The power requirements of the project can be easily matched by configuring only the needed power modules. Customers benefit because they don’t have to pay for excess inverter capacity. This modularity also allows for future string inverter additions as part of a battery augmentation strategy. That makes scaling a project a simple matter of adding modules.
- Energy and power augmentation. Configuring multiple DC buses opens opportunities for flexibility and expansion. For example, a battery string on one DC bus can operate with a different DC voltage range from a battery string on a neighboring DC bus while the AC side of the inverters are connected in parallel and operate as one larger unit. Thus, the different DC voltage ranges typically encountered with batteries of different generations can be combined behind the same string inverter system. In addition, if room is left in the initial design for more string inverters to be added (and open racks in the battery container for more battery modules), the power and energy augmentation options for a project increase significantly. This provides the potential for longer project life, ease of mixing battery vintages and chemistries, and greater project revenues.
- Combining hybrid configurations. You can combine different generations of batteries, different storage technologies and different distributed resources behind a string inverter. The benefit is optionality. For example, remote microgrids often have diverse power and energy needs across a day, depending on the load and generation resources on the system. Solar PV output variability could create a need for high-cycle, short-duration storage technologies like ultracapacitors, while long-duration technologies are also needed to shift energy through time. A string-based system can include both.
- Serviceability. You may need a forklift or crane and specialized crew to swap out a central inverter. Lighter, smaller and simpler, string inverters are easier to service and maintain. A trained technician can replace a string inverter simply by unplugging the old one and sliding in a new one.
- Increased safety. The higher-capacity bus required for a central inverter poses safety concerns from large potential short-circuits. By breaking the system down into smaller, independent buses with string inverters, you produce more manageable, safer DC circuit currents. That also allows for the use of readily available and less expensive DC protection devices.
- Grid-forming capability. Inverters for solar PV are unidirectional, but string inverters designed for energy storage are bi-directional and some (such as those from LS Energy Solutions) have grid forming ability. This opens up the opportunity to create a microgrid. Were the utility grid to go down for any reason, grid-forming allows the inverter to become the reference frequency and voltage for a microgrid. Large commercial and industrial users with solar and storage can take advantage of this capability in case of a power interruption to form a behind-the-meter microgrid. The company could keep critical loads online and safely shut down systems until grid power is restored.
Productization and customization
As the energy storage market becomes more competitive, so do demands for innovative, cost-effective inverter technologies. One response is to turn to off-the-shelf components, such as placing batteries in smaller enclosures along with associated air-conditioning and fire-protection systems.
Despite this trend toward so-called productization, every project has different needs. By combining these containerized products with right-sized string inverters, you get a more flexible solution that combines the benefits of modularity and customization.
Central inverters do offer lower upfront capital costs. However, that edge is eroding as prices of string inverters fall and their advantages over the lifecycle of an energy storage project add up.
An apt example is the LS Energy Solutions PowerBRiC (Bi-directional, Resilient, intelligent Converter) system, a modular building-block string inverter focused on maximizing reliability and creating value-stacking opportunities for energy storage projects. To learn how a flexible, standalone inverter design offers scalable solutions for a wide range of applications in distributed power generation, visit our solutions page or contact us.