IN Brief:
- Acciona Energía is testing combined lithium-ion battery and ultracapacitor systems at wind and solar sites in Navarra.
- Ultracapacitors handle rapid power fluctuations while batteries provide longer-duration response and energy shifting.
- The original 250kW pilot has expanded to 1.25MW, with a second 250kW system operating at a solar facility.
Acciona Energía is testing a hybrid energy storage architecture that combines lithium-ion batteries with ultracapacitors to provide grid-forming and stability services at renewable generation sites in Navarra, northern Spain.
Initially installed as a 250kW pilot at the Barásoain wind farm, the system was expanded to 1.25MW during 2025, while the original equipment was transferred to a photovoltaic facility in Tudela. Operating the technology alongside both wind and solar generation allows the control platform to be assessed against markedly different production profiles.
Rather than asking a single battery system to perform every duty, the hybrid arrangement allocates work according to the electrical characteristics of each storage medium. Lithium-ion cells provide sustained power and energy over longer periods, while ultracapacitors absorb or deliver the rapid, high-power pulses produced by abrupt changes in generation or network conditions.
Because ultracapacitors store energy electrostatically rather than through the chemical reactions used in batteries, they can charge and discharge extremely quickly and tolerate very high cycle counts. Their energy capacity is comparatively limited, however, which makes them better suited to transient control than prolonged discharge.
Separating power and energy duties
By directing sharp fluctuations towards the ultracapacitors, the control system can reduce the number and severity of high-power events imposed on the lithium-ion cells. Battery degradation is influenced by current, temperature, depth of discharge, state of charge, and cycling frequency, so changing the duty profile can improve operating life even where total energy throughput remains substantial.
Effective coordination depends on a controller capable of distinguishing between brief transient events and conditions that require a sustained response. Millisecond-scale variations can be assigned to the ultracapacitor bank, while slower frequency movements or longer periods of energy imbalance are handled by the battery.
Such an arrangement demands close integration between the storage units, renewable plant, inverter controls, and grid connection. Communications latency, state estimation, converter limits, and the transition between operating modes all affect whether the combined installation behaves as one coherent system rather than two independently dispatched assets.
Grid-forming capability broadens the technical role of the project beyond conventional energy shifting. Whereas grid-following inverters synchronise with an existing voltage waveform, grid-forming converters can establish and regulate their own voltage and frequency reference, supporting operation where system strength is reduced.
As synchronous generation is displaced by inverter-connected wind, solar, and battery capacity, networks lose some of the physical inertia, fault current, and voltage support historically supplied by large rotating machines. Grid-forming controls can provide virtual inertia, fast frequency response, voltage regulation, and improved behaviour during disturbances, although their performance depends on converter ratings and control settings.
Comparable work is progressing elsewhere in Europe, including a grid-forming programme pairing Huawei technology with Luterra assets, as developers begin specifying storage systems by response quality, overload capability, and system-strength contribution rather than capacity alone.
Renewable plants assume wider network functions
Wind and solar sites present different control challenges. Wind output can vary over seconds, minutes, and hours, while photovoltaic production experiences predictable daily ramps alongside sudden changes caused by cloud movement. Testing the same hybrid design against both profiles should provide a clearer picture of where ultracapacitors offer the greatest operational value.
Once integrated behind a renewable connection, storage can perform several duties independently of the sale of generated electricity. Depending on market rules and connection agreements, these may include frequency services, voltage support, ramp-rate control, congestion management, reserve provision, and energy arbitrage.
Hybridisation nevertheless adds equipment, interfaces, and maintenance requirements. Protection coordination must cover both storage technologies and the renewable plant, while thermal management, isolation, auxiliary supplies, earthing, communications, and fault behaviour require assessment across the complete installation.
Ultracapacitors also introduce their own switching and monitoring requirements. Although their cycle life is high, the surrounding converters, contactors, cooling systems, and control hardware remain subject to ageing, meaning any reduction in battery degradation must be considered alongside the cost and maintenance burden of the additional equipment.
Commercial deployment will therefore depend on more than response speed. Developers will need evidence that lower battery stress, improved service capability, and potential reductions in oversizing can outweigh the capital cost, conversion losses, and greater system complexity associated with the hybrid architecture.
The Navarra installations remain modest beside utility-scale battery projects measured in hundreds of megawatts, yet their purpose is centred on precision rather than bulk energy. Their operating data should reveal whether separating high-power transients from longer-duration energy delivery produces measurable gains in efficiency, availability, and battery life.
Storage procurement is already moving beyond simple megawatt and megawatt-hour specifications. Fault-ride-through behaviour, reactive-power capability, virtual inertia, response speed, harmonic performance, and stability under weak-grid conditions are becoming integral to project design.
Acciona’s programme brings those requirements together within a single control architecture. If the hybrid system performs consistently across wind and solar sites, it could offer a repeatable route for renewable plants expected to provide both electrical energy and the dynamic services needed to keep increasingly inverter-dominated networks stable.
