Battery storage systems are driving the energy transition. As large-scale storage systems at grid level, they can reduce grid congestion, stabilize the frequency, help restart the power grid after an outage – or even optimize the yield of solar farms. Storage experts provide an overview of the latest developments.
There are several reasons why the deployment of fluctuating renewable sources of energy requires the simultaneous expansion of storage capacity. Much of public discussion has focused on the use of storage systems to bridge “dark doldrums”, but large-scale storage systems that provide system services to the power grid are just as important.
Leading international grid expert Sankara Subramanian, from the company Saft Batteries, points out that the grid requires physical inertia to act as a buffer against frequency fluctuations during rapid load changes. Previously, the electricity sector generated this inertia using spinning reserves: that is, the turbines and fly wheels in fossil fuel power plants, often weighing several tons. Photovoltaics do not have inertia. “If the fossil fuel power plants are switched off, we need synthetic
inertia,” explains Subramanian. This is also sometimes known as “virtual inertia” and can be supplied by intelligent batteries thanks to their short reaction time. Furthermore, in purely physical terms, the relevant capacity must be provided immediately when a consumer is switched on in the grid. If that does not happen, the power line frequency will drop.
Adjusting power plant capacity or switching on pumped-storage power plants takes too long to offset this effect. Maintaining a frequency stability of exactly 50 hertz – with a tolerable range of 49.8 to 50.2 hertz – is therefore a vital task carried out by batteries in the grid. An example of this is transmission system operator TransnetBW’s “grid booster” which is currently under construction in Kupferzell in southern Germany. The battery storage system will be exclusively used for system services, primarily to stabilize the voltage and power line frequency and to mitigate power grid disruptions.
Storage systems play another role in their use as “virtual transmission lines,” as Subramanian goes on to explain. This refers to the use of two storage systems at different grid locations, which can clear a congestion in the course of the redispatch. When there is a surplus of power at point A and a shortage at point B, but not enough lines connecting A and B to transport the power, a large-scale battery can store electricity at point A while another large-scale battery at point B can discharge its power. The effect on the grid is the same as if there were enough transmission lines between A and B.
The positioning of the storage systems in the grid is a key consideration in concepts of this kind. That is why Lithuania has carried out one of the largest battery storage system projects in Europe. “This project forms part of the synchronization of the Baltic grid with the continental European grid, planned for 2025,” reports Vitalijus Baranskas, Chief Technical Officer at the company Energy Cells in Vilnius. This involves four battery storage systems in four Lithuanian cities (Alytus, Vilnius, Šiauliai and Utena) each with an output of 50 MW and a capacity of 50 MWh. “We use hydropower plants to compensate grid fluctuations, but they are too slow for system stabilization,” explains Baranskas. Batteries could provide full power within seconds and immediately react to changes in frequency.
Large-scale batteries have yet more applications. During a blackout, they can help restart the grid and, in principle, they can also react to the different balancing markets, such as the secondary control reserve, which have to be available within five minutes based on the way the electricity market is organized.
Marcel Schepers, Product Manager for flexibility marketing at Energie Baden-Württemberg (EnBW) highlighted the fact that there are also various options to the way storage systems operate. While the other applications presented primarily focus on system stabilization and are therefore used in regulated areas of the grid and paid for by grid charges, Schepers describes a market-driven mode of operating battery storage systems.
This involves the use of storage systems for arbitrage transactions on various spot markets, both day-ahead and intraday, as well as for maximizing the yield of solar farms.
When solar farms and battery storage systems are physically separated, they are also managed independently of each other. However, EnBW is primarily focusing on co-location, where battery storage systems are directly connected to the solar installations and share the same grid connection point.
This can have various benefits. Battery storage systems can enable a higher PV output by helping to overcome technical limitations: “For example, even though the grid connection point can only handle five megawatts, you can still install seven to eight megawatts of modules, because the power generated during peak hours can be stored temporarily,” says Schepers.
Another reason for choosing PV-plus-storage is the volatility of electricity prices. There is an increasing tendency for spot prices to be negative during periods of high sunshine, so it makes more and more sense to store electricity until the evening, when demand is greater and prices are higher.
However, this requires a sophisticated measurement concept, which is partly due to the subsidy conditions in Germany, explains Schepers. It is important to prevent green electricity from mixing with gray electricity – each kilowatt hour fed into the grid must be clearly defined in terms of its origin.
Many questions surrounding large-scale storage systems relate to regulatory requirements. Grid operators, whose work is controlled by regulators because it takes place in a largely monopolistic grid, face strict limitations on the use of storage systems. They often buy flexibility services from storage system operators, who, for their part, supply various different markets.
Storage will continue to be very dynamic – the technology will continue to evolve, as will the markets for flexibility and, perhaps in some areas, regulatory requirements. Another question that continues to be important is which kilowatt hours are subject to grid charges and which are not – a hotly debated issue.
This article is based on the session “Large, Grid-Scale Storage for More Supply- and Demand-Flexibility” which was held at the EM-Power Europe Conference 2024. Want to learn more? Check out the recordings of individual presentations on our The smarter E Digital platform.