By Tim Stocker, product manager for energy storage at SMA; and Mohamed Mostafa, head of product group, industrial & utility at SMA
Integrating storage into the grid is becoming a more popular and more lucrative way to add value to PV projects as well as the electrical grid. Wood Mackenzie Power & Renewables’ estimates show a year-over-year growth of more than 50% in the non-residential storage market.
Storage, whether integrated into a large-scale PV plant or as a stand-alone asset, can provide a variety of meaningful services to utilities.
Many utility storage-related discussions involve the subject of AC- versus DC-coupled solutions and the merits of each. The fact is, they both have advantages, so it’s best to employ a technologically agnostic approach when designing a solar+storage project. The choice should be specific to the application. There are two main categories of utility storage use cases: renewable integration and grid services.
Renewable integration use cases
Energy shifting and clipped loss capture
As module prices continue to decline, increasing the DC-AC ratio on a PV inverter continues to add benefit by allowing more energy production during the shoulder hours—the left and right sides of the PV production bell curve. The downside is that there is a large amount of energy loss due to inverter clipping since they have maximum AC power limits. These limits are typically defined by the interconnection with the utility. Adding DC-coupled storage can enable the system to capture what would have otherwise been lost due to clipping and export this energy at a later time.
Solar energy is well-known for being an intermittent resource due to variability in weather. When energy storage is paired on the DC side together with a solar inverter, the asset as a whole becomes much more firm and can be controlled in such a way to make it dispatchable. When the asset is dispatchable, it gets treated in the same way a peaker power plant or generation plant would, which further strengthens renewable energy’s position in today’s energy markets.
Ramp rate control
In some renewable energy markets, there are requirements set regarding ramp rates. Controlling ramp-up rates is relatively easy for a PV inverter. However, controlling ramp-down rates can be challenging, even with advance weather forecasting tools. Adding DC-coupled storage to a PV inverter in this scenario can overcome these challenges by using the storage as a buffer, helping to smooth out the PV inverter’s output power without increasing the nameplate rating of the plant.
Frequency is held to a very strict tolerance, so deltas in frequency must be mitigated. Renewable energy can introduce fluctuations in grid frequency. Energy storage, specifically battery storage, is an ideal way to solve this issue due to its nearly instantaneous reaction time. Enhanced inverter controls and rapid response times make for a great combination.
Grid services use cases
Fluctuations in grid voltage, especially at the far ends of distribution feeders, can cause damage to loads and utility equipment. Battery inverters can provide full, four-quadrant support to help maintain voltage levels at points of interconnection by utilizing the volt-var function or a voltage drop.
Transmission and distribution deferral
If demand within a distribution network increases to the point that an asset upgrade is required, it can be extremely expensive for the grid operator. Adding energy storage, at a fraction of the cost of the upgrade, can alleviate the stress on the network caused by the demand increase. Energy arbitrage can be a great tool for grid operators and doesn’t require renewable generation in the network. By charging the energy storage system either at night or during another time when energy rates are low, the grid operators have the ability to discharge later in the day to meet demand and reduce the need for peaker plants. Even if demand growth is not increasing, adding energy storage to an existing distribution network can help extend the life of the assets within the network.
Battery inverters with a DC pre-charge circuit and grid forming features are capable of starting a grid while down and slowly ramping up voltage to successfully start a medium-voltage transformer. Even an inverter that’s meant to be used for different applications can use this feature with minimal effort and cost.
Uninterruptible power supply (UPS)
Battery inverters can be utilized in several ways to replicate UPS-like functionality at a fraction of the cost of a similar-sized industrial UPS. Online UPS-like functionality called double-conversion can be accomplished while also providing ancillary grid services to the grid operator.
Industrial-scale energy storage solutions have become mature technology, incorporated into utility-scale power plants to serve in many different applications. Whether it’s providing ancillary grid services or additional generation capacity or supporting grid stability, utility-scale storage has proven its worth and will continue to experience significant growth for years to come.