Depending on the development scenario, Croatia requires between 350 and 1,620 MW of battery storage capacity at strategically favourable locations within its transmission grid. This is a key finding of a study by Miroslav Holjevac from the Institute of High Voltage and Energy at the University of Zagreb (FER) and Dražen Balić from the Hrvoje Požar Energy Institute. The researchers presented their results at this year’s SolarFlex Croatia conference in Zagreb, organised by the Croatian Renewable Energy Association (OIEH) and SolarPower Europe.
Three scenarios examined
Using an optimisation algorithm, the researchers assessed which grid nodes would deliver the greatest benefit to the Croatian power system when equipped with storage and how requirements vary depending on the pace of renewable energy expansion. The analysis was based on three scenarios.
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The first reflects the grid status as of 1 January 2025 and outlines current bottlenecks in the transmission network. The second is based on the National Energy and Climate Plan (NECP), targeting around 9,000 MW of integrated renewable capacity by 2030. A third, more ambitious scenario takes into account all projects that had received an energy permit by mid-2024. If all of these projects were built, the researchers estimate around 12,000 MW of connected solar and wind capacity.
Distributing storage strategically
For the current grid status, the optimisation algorithm identified optimal battery locations. “22 storage systems at strategically favourable locations with 1,340 MW can eliminate all bottlenecks and help with all critical contingencies,” Miroslav Holjevac summarised. “This capacity is sufficient to resolve all identified grid bottlenecks and ensure system security even in the event of individual high-voltage line failures, the so-called N-1 analysis.”
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An alternative approach would be to deploy storage at sites with existing large-scale generation and grid infrastructure. In that case, however, requirements increase, the researchers find, as not every technically optimal node is available. More storage systems with a total capacity of around 1,700 MW would then be required.
Grid reinforcement factored in
In the NECP scenario, based on the government’s expansion targets for 2030, storage requirements are significantly lower. According to the study, ten locations with a combined capacity of around 350 MW would suffice. The reason is that this scenario already accounts for ongoing and funded grid reinforcement projects currently being implemented. “This intermediate step can be managed with reasonable integration levels, grid investment and battery storage,” Miroslav Holjevac explained.
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Even in an alternative decentralised scenario envisaging exclusively the installation of smaller batteries of up to 10 MW, total capacity reaches a comparable 360 MW. Miroslav Holjevac sees this as an indication that grid reinforcement measures through to 2030 are already addressing a large share of current constraints.
Renewables primed for grid integration
If all currently permitted large-scale renewable energy plants are built, storage requirements are similar to the scenario without expansion. For this case, the algorithm identified 17 storage locations with a combined capacity of 1,620 MW. These should predominantly be deployed in the Split transmission area, the region around Zadar, Šibenik and Split, as well as partly in the Rijeka transmission area. Here, storage systems no longer merely compensate for weaknesses in the current grid but also facilitate the integration of large volumes of renewable electricity, absorbing production surpluses at critical moments and balancing power flows towards demand.
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Miroslav Holjevac stressed, however, that these calculations were carried out without the planned 400 kV transmission line from south to north, which is included in the NECP scenario. If this line is incorporated into the modelling, storage requirements are nearly halved, both in terms of the number of locations and installed capacity.
Parallel grid expansion and storage construction
Even then, however, grid constraints remain that must be addressed separately. “The solution does not lie solely in battery storage or exclusively in grid expansion, these are two components that must complement each other,” the researcher emphasised. A particularly noteworthy finding concerns the limitations of small, decentralised storage: in the ambitious scenario, the grid calculations fail to achieve full convergence even when a 10 MW battery is installed at every 110 kV node in the transmission network. Larger storage projects at targeted locations therefore remain indispensable.
Room in the grid for additional storage
According to Dražen Balić, the grid locations identified in the study do not preclude the development of additional market-driven battery sites. In the energy permit scenario, 70 to 90 percent of the currently proposed market-driven locations do not coincide with the optimal grid locations identified in the study. The economic operation of these storage systems will be addressed in the next instalment on the study. The report is available for download, currently in Croatian, on the OIEH website, with an English-language version in preparation. (su)
