Solar Inverters Are Key in RESSs

One of the primary market drivers of residential energy storage systems (RESSs) is lithium ion (Li-ion) battery technology advancements. Li-ion batteries have a longer lifespan, higher depth of discharge, and higher energy density than lead-acid batteries. Thus, Li-ion batteries have taken most of the market share in RESS deployments because technological benefits allow for designing smaller and lighter batteries with a longer lifespan, and Li-ion battery prices continue to drop rapidly.

ESSs Cannot Operate without Inverters

A battery needs an inverter to become an energy storage system (ESS) because direct current (DC) from the battery must be converted into alternating current (AC) used by most devices. As the market increasingly adopts solar along with RESSs, solar inverters are playing a key role. Several types of solar inverters exist, including central/string inverters, microinverters, and power optimizers.

Types of Solar Inverters

(Source: NRG Clean Energy)

Other terms classify different technology configurations depending on the connection method between the solar installation and the RESS or the transmission method of AC to the power grid. Also, microinverters and power optimizers are categorized as Module-Level Power Electronics (MLPEs).

MLPE dominate the US solar and RESS market due to the National Electrical Code's rapid shutdown protocol. Rapid shutdown was introduced in 2014 to allow firefighters to de-energize solar systems on building roofs during a fire. MLPE meet the shutdown code requirements much better than string inverters because the individual power electronics are connected to the solar module. Additionally, the inverter also helps boost DC voltage and convert DC to AC. Among MLPE inverters, microinverters have a built-in DC volt boost function. Power optimizers separate this function into the optimizer.

A solar plus battery system architecture may involve DC-coupled and AC-coupled configurations. In a DC-coupled configuration, the solar installation and the battery share the same interconnection and are connected to a DC bus with an inverter. DC-coupled configurations charge DC power from the solar installation to the battery directly without AC conversion. In AC-coupled configurations, the solar installation and the battery are connected to separate inverters. DC-coupled configurations are usually more efficient in power conversion. However, AC-coupled configurations are more flexible in installation (e.g., retrofits) because they make it easier to add the RESS (i.e., a battery with an inverter) to existing solar.

RESSs with three-phase or triphasic inverters were introduced in Germany and Central Europe. In these regions, three-phase AC electricity is supplied to households. Industry players prefer three-phase inverters to stabilize the AC power balance. A triphasic inverter integrated with a solar RESS can handle sudden voltage rise on a sunny day.

Careful Market Entry Strategy Is Needed

Inverters are an essential component to RESSs integrated with solar. Manufacturers and distributors should consider the following:

• Monitor new technology advancements: A DC optimizer increases the DC voltage of individual solar module levels as a separate component among MLPE. However, a solar cell optimizer can manage the solar voltage at a more granular level than the DC optimizer by boosting the DC voltage at the string level, not just the module level.
  
• Understand regional differences: As a result of rapid shutdown in the US and Central Europe’s preference for triphasic inverters, regional differences exist in adopting these technologies. Therefore, it is important to understand market characteristics before entering the market. Solar RESSs are also closely tied to monitoring and control.
  

As adoption of solar RESSs increases, manufacturers and distributors that take these actions in adopting inverters will likely be successful.