Control energy is essential for a secure power supply. In electricity grids, supply and demand must always be balanced, otherwise there is a risk of power cuts. Batteries are usually discussed as a primary reserve because of their short-term availability. However, they can also be used for longer-term control needs and are therefore a particularly versatile option for the provision of control energy.

Relevance of this field of application

A secure power supply is the backbone of all industrial nations. Alongside competitiveness and sustainability, security of power supply is one of the three core objectives of European energy policy.¹ The expansion of renewable energies increases the volatility of electricity generation, as the sun does not always shine and the wind does not always blow. Therefore, supply or demand surpluses occur more and more frequently. The task of control energy is to reduce excess demand by reducing electricity consumption or to satisfy it by connecting additional generation plants (positive control energy) and to reduce excess supply by reducing electricity production or to remove it from the grid by connecting technical consumers (negative control energy). The forced disconnection of generation plants in the course of feed-in management is not one of the instruments of negative control energy.²

Electricity producers, electricity consumers and electricity storage facilities are involved in the provision of balancing power. The regulation differentiates according to the time of the first provision: The frequency containment reserves (FCR) must be available within 30 seconds after the occurrence of the balancing event, the automatic frequency restoration reserves (aFRR) within 5 minutes and the manual frequency restoration reserves (mFRR) after 15 minutes. Remuneration is based on performance prices, which remunerate the mere provision of the reserve power, and on working prices, which remunerate the actual delivery of balancing energy. In the special case of FCR, the commodity price does not apply, since in the case of demand, the amount of electricity injected or withdrawn is balanced out again at short notice and there is no price differentiation between injection and withdrawal. Tendering and processing of the balancing power is carried out by the transmission system operators both at national and international interconnection level and only concerns the period of the first 60 minutes after the occurrence of the control event. All balancing measures beyond this ("hourly reserves") are no longer the responsibility of the transmission system operators, but at the subordinate grid levels and must be covered by power plant control, intraday trading or over-the-counter trading by the balancing group managers concerned themselves.²

In Germany alone, a total of 6.8 GW of balancing power was tendered for frequency stabilisation in 2019, of which around 0.6 GW was FCR, 1.8 GW negative aFRR, 1.9 GW positive aFRR and 1.0 GW negative mFRR and 1.4 GW positive mFRR.²

Use of battery storage

Due to their fast reaction times, battery storage systems are predestined for use in the control energy sector. In principle, they can be used in any of the three control reserves (FCR, aFRR and mFRR). Providers of balancing power must have their systems prequalified in order to be allowed to participate in the balancing power markets for balancing power (provision of the reserve) and balancing work (use of the reserve).³

Depending on the control reserve, battery storage systems - like all systems for the provision of control energy - must fulfil specific system requirements: For use as a FCR, it must be possible to provide the full symmetrical (positive and negative) balancing power of at least 1 MW within 30 seconds and for a duration of up to 15 minutes. The power call is frequency-controlled and automatic by the FCR provider. Participation in the aFRR requires the provision of the full control reserve within 5 minutes for a period of up to 15 minutes. The aFRR automatically replaces the FCR. It must be possible to provide the mFRR in full within 15 minutes and for a period of up to the full hour after the control event has occurred. It is called manually by the transmission network operator. In the case of aFRR and mFRR, negative or positive balancing power and balancing work can be offered separately. A minimum capacity of 5 MW applies in each case, which may, however, be undercut to a lower limit of 1 MW, provided that only one bid is submitted per time period. In principle, balancing energy can be offered via auctions per day for 6 time periods of 4 hours each.^2,3

Virtual storage arises from a connection of smaller storage units to an overall storage offer. The individual storage units can also be located at different places. They only have to fulfil the requirements for participation in the balancing energy markets cumulatively, not at the level of the storage units. For virtual battery storage, the special regulation outlined is a particularly attractive opportunity to participate in all balancing markets from a cumulative balancing capacity of 1 MW.²

Performance requirements

Of the three types of control reserve, FCR is the most gentle form from the point of view of classic battery storage. This is why large-scale storage operators place their focus here.⁴ FCR is designed to compensate for minor frequency fluctuations and only has to compensate for deviations over the entire span of guaranteed power provision in rare exceptional cases. Such events are usually associated with extreme weather events, power plant outages or the loss of entire transmission lines. They occur spontaneously and are therefore outside the forecasts. Only then are balance power and balance work fully called up at the FCR level. The more comfortable the FCR is designed, the less critical extreme events are for the security of power supply.⁵ In normal operation, the actual FCR demand is in the lower single-digit percentage range of the control reserve. FCR is thus a kind of insurance power that is rarely called up in full. This reduces the load on battery storage systems, especially in the area of use-dependent ageing behaviour. With aFRR and mFRR, balance power and balance work are much more likely to actually be used. This can also be seen in the distribution of the control power volume. In 2019, FCR accounted for 9 % of balancing power, aFRR for 54 % and mFRR for 37 %.²

With regard to the performance requirements, this means that batteries for FCR must meet fewer restrictions than those for aFRR and mFRR. However, if the same battery storage is to be offered in all balancing markets, the strictest requirements come into play. In any case, batteries for use in balancing energy should have a low ageing effect with a long service life. The lower the ageing effects, the sooner the areas of aFRR and mFRR can be economically developed for battery storage.
Due to the minimum requirements for market participation, battery storage units for balancing energy are very large in size. Explosion hazard and flammability therefore pose an increased risk, regardless of whether the storage units are distributed centrally or decentrally: In the case of centralised storage, fire and explosion risks directly affect large storage units; in the case of decentralised storage, they affect the immediate surroundings of the installation sites.