Cell Mind and suena energy: Optimizing Grey Co-Located Storage Within FCA
The grid connection has become the scarcest factor in the storage rollout. In many distribution grids, connection requests significantly exceed available capacity, commitments are being pushed back, and grid expansion is therefore only slowly keeping pace with project development. This is precisely why concepts that make better use of existing connection capacity are moving into focus.
Grey Co-Located Storage Systems are one such concept. They share a grid connection point with an existing generation asset but — unlike Green Co-Located Storage Systems — are permitted both to charge from the grid and to feed into the grid. This is a decisive difference for optimization: the charging pattern is not tied to the generation of the combined asset, the number of economically usable cycles increases, and the storage system can participate across all markets — from wholesale to balancing energy markets.
In Nägeleried, suena energy trades such a project on behalf of the operator Cell Mind. Beyond the project configuration itself, however, this plant presents a particular operational challenge: in this area, the grid operator only grants grid connections on the condition of a flexible grid connection agreement (FCA).
Project Overview: Grey Co-Located Storage in Nägeleried
At the Nägeleried site, the storage system complements a 5.7 MWp PV plant that suena energy has been trading since 2025. The battery storage system, with 6.9 MW power and 13.8 MWh capacity, has been in trading since 2026. Both assets feed in via the same grid connection point.
Project Information
Customer: Cell Mind GmbH
Asset: existing PV plant with battery storage at a shared grid connection point
PV capacity: 5,7 MWp
BESS: 6,9 MW / 13,8 MWh
Trading: wholesale and balancing energy markets
Grid Connection: flexible grid connection agreement with AllgäuNetz, dynamic power limitation via API
Technology: suena Energy Trading Autopilot
Dynamic Power Limitation Via API
Since 2025, the grid operator AllgäuNetz has only made grid connections available for battery storage systems on the condition of a flexible grid connection agreement. For plant operation, this means: no fixed, full grid access, but rather an obligation to temporarily reduce feed-in and offtake when the local grid situation requires it.
For the technical implementation of this requirement, the grid operator chose a particularly innovative approach and provides an API — that is, an interface that allows different software systems to exchange data directly. This enables direct communication between grid operator and trader, allowing the permissible power at the grid connection point to be adjusted automatically accordingly.
In the case of Nägeleried, this is not a flat, permanently reduced connection capacity but a dynamic variant. This means the restriction is not defined via feed-in or load profiles, but only takes effect when the grid control center forecasts an actual bottleneck. Via the API, the grid operator communicates the maximum value specifications for the following day on the preceding day, which then must be taken into account in the trading strategy for the next day — if no restriction is specified, the full power at the grid connection point is available.
With the battery storage system in Nägeleried, we have realized another co-located project that will help to better integrate renewable energy into the energy system and stabilize the power system going forward. A project of this scale naturally cannot succeed alone — it requires strong, competent, and forward-looking partners. A battery storage system can only reach its full potential when all components — the plant, energy management, and trading — are intelligently interconnected. The API plays a central role here as the digital interface between AllgäuNetz GmbH & Co. KG as grid operator and suena energy as energy trader. With suena energy, we had a reliable trading partner at our side from the very beginning, one that ensured exactly this technical connection. The fact that we successfully implemented this integration together is an important milestone for the energy transition in the Allgäu region and for increasingly flexible energy infrastructure beyond the region. – Roger Köhler, CEO Cell Mind GmbH
What Had to Be Developed To Implement The FCA
A dynamic power limitation is only as good as its integration into the trading systems. Every new specification arriving via API must be automatically incorporated into schedule generation. On suena energy's side, this required additional development effort to connect the interface to the suena Energy Trading Autopilot, so that the grid operator's specifications are reliably reflected in both optimization and trading. This was implemented together with Energiekoppler, whose platform handles the technical plant connection and control.
Nägeleried was the first project for us where we dealt concretely with a fully dynamic FCA in operational practice. Because the grid operator announces restrictions with a clear lead time, we can adjust our trading strategy accordingly in good time and achieve maximum revenues under these conditions. We were able to implement this project requirement so successfully mainly because all parties involved — Energiekoppler, AllgäuNetz, and Cell Mind — worked with us constructively and transparently from the outset. This allowed us to take many new factors into account and gain a great deal of experience for future projects." – Ida Ballmann, Energy Market Operations Manager suena energy
Grid Conenction Becomes A Matter Of Negotiation
What becomes visible on a small scale in Nägeleried is part of a larger development: since 2025, the legislator has explicitly permitted flexible grid connection agreements under Section 17 (2b) EnWG (and Section 8a EEG). The underlying idea: a plant obtains a connection faster and accepts in return that full connection capacity will not be available to it at every hour.
What sounds simple remains open in its specific design. Grid operators may offer FCAs but are not obligated to do so — the Bundesnetzagentur (Federal Network Agency) confirmed this in a ruling from March 2026. To date, there are no uniform requirements: the agreements arise bilaterally, and views on how far restrictions may go, how they are announced, and how they are to be implemented technically vary widely between grid areas. The Bundesnetzagentur does not intend to develop guidelines for FCAs until 2027, as part of the AgNes grid tariff reform.
Conclusion
The Nägeleried project shows that storage systems can be traded economically even under grid restrictions, and can be optimized in line with actual bottlenecks. This is an important finding, since flexible connections are foreseeably becoming the rule rather than the exception.
At the same time, the project makes clear the limits of today's approach: if every grid operator sets its own requirements for implementing FCAs, the implementation effort for operators and traders quickly becomes too extensive. The fact that the specific design of FCAs varies from grid area to grid area has a factual basis, since technical limits differ. What can be reduced independently of this, however, is the implementation effort — through binding guardrails set by the BNetzA, within which each grid operator can continue to adapt its FCA to the local grid situation, as well as through an essentially standardized communication interface between grid operator and trader for exchanging feed-in restrictions and comparable specifications. This preserves local flexibility while significantly reducing the effort for traders across multiple grid areas.
For Cell Mind and suena energy, Nägeleried was thus a first step to build on. For further joint projects already in planning, Nägeleried now provides a proven template and valuable experience.
Über Cell Mind
Cell Mind GmbH is a project development company based in the Allgäu region, specializing in battery storage projects. It manages projects holistically — from site analysis and grid connection through permitting to implementation and long-term operation. With intelligent battery storage solutions, Cell Mind supports the integration of renewable energy, reduces grid bottlenecks, and contributes to a secure and economical energy supply. In doing so, the company works closely with landowners, municipalities, industrial companies, energy producers, and investors.
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