German solar plant cleared to provide secondary control reserve
Image: Enerparc
From pv magazine Germany
Enerparc subsidiary Sunnic Lighthouse, flexibility provider Entelios, and transmission system operator 50Hertz completed prequalification for the 37.4 MW Schkölen solar park in Thuringia, Germany, to participate in the secondary control reserve market. The plant has provided up to 25 MW of positive and negative automatic frequency restoration reserve over the past week.
Photovoltaic assets have not previously participated in Germany’s secondary reserve segment. The companies said the Schkölen project establishes “new standards in the integration of renewable energy,” with three additional parks in preparation. Managing Director Arved von Harpe said any large plant could take part and smaller ones could qualify “if we can bundle them together.” He added that networking multiple assets provides redundancy, and that Schkölen still retains a significant buffer even though the maximum reserve capacity is 25 MW.
aFRR sits within one of four reserve segments and must be fully delivered for 15 minutes within a five-minute activation window. Tenders are now issued the day before in four-hour blocks, with the Federal Network Agency planning to shift to 15-minute blocks next spring. That change favors photovoltaic and wind generation. The primary reserve segment must respond within 30 seconds, while the tertiary reserve allows 12.5 minutes for delivery.
Photovoltaic systems can supply positive and negative aFRR, and the Schkölen plant is qualified for both. A rise in grid frequency enables rapid curtailment, while a fall allows a previously curtailed plant to boost output. Under strong irradiation, the response can exceed the ramping capability of fossil-fuel units or large industrial loads. Market rules already anticipate such participation.
Operators said the technical challenge lies in ensuring real-time visibility into available balancing energy, which varies by time, season, and weather. Limited data transparency has constrained wind participation and previously excluded solar entirely. The consortium described Schkölen as an “innovative flagship” that marks a “paradigm shift,” said von Harpe, “not only for Sunnic and our industry, but also for the entire energy system.”
Entelios adapted the park’s control system for aFRR compatibility. The project is “a true milestone – and proof of what is possible when technological innovation meets collaborative partnership,” said Entelios CEO Fabian Becker. Sunnic Lighthouse, Entelios, and 50Hertz jointly developed the metering setup, data architecture, and AI-supported forecasting model. Both companies praised 50Hertz, which initiated the effort. Von Harpe said “a phone call came two years ago” after earlier aFRR-solar attempts stalled, with 50Hertz committing to support solar-based balancing power.
From a policy standpoint, operators said the economics are compelling. Surplus photovoltaic periods allow a solar park to provide secondary reserve at market premium or PPA prices, often at lower cost than gas-fired capacity. While balancing demand remains limited, the service offers a new revenue stream for solar asset owners.