Without the intelligent coupling of electricity, heat and gas networks, the further expansion of wind and solar will hit a ceiling, and not only in Croatia. That is the central argument of Goran Krajačić, Professor at the Department of Energy, Power Engineering and Environment at the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, whose analysis examines the growing role of storage systems and flexibility options in renewable energy integration.
Presenting his findings at SolarFlex Croatia, Krajačić focused primarily on which technologies look most promising for the country. But technology choice alone is not enough: he considers operational management just as decisive as the hardware itself.
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Building intelligent energy systems
At the heart of the analysis is the concept of "smart energy systems," built on the integration of multiple energy infrastructures. Krajačić notes that flexibility in the electricity sector is not a new concept, but that the growing share of variable generation from wind and solar is taking it into an entirely new dimension.
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What matters is which sources can provide it. Four options are available: storage systems capable of absorbing and returning electricity to the grid; demand-side management and sector coupling; transmission capacity to neighbouring countries; and dispatchable generation such as hydro or gas turbines.
Using electric cars as storage
Electric mobility, in Krajačić's view, could play a particular role in future flexibility provision. More than 80 million electric vehicles are already on the road globally, and bidirectional charging alone could integrate around 1,000 TWh of additional storage capacity into the electricity system. He puts the point bluntly: "What is easier to implement, feeding electricity from electric vehicle batteries back into the grid or building a small modular nuclear reactor?"
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Pumped storage takes time to build
Among stationary storage technologies, Krajačić weighs the respective strengths and weaknesses of each. Pumped hydro remains particularly attractive, capable of providing virtually all system services and delivering double the flexibility when switching between turbine and pumping mode.
Their strength increasingly lies in combining energy flexibility with water supply, irrigation, flood protection and desalination. With floating PV installed on their reservoirs, they could double as generation assets. However, the environmental impact, lengthy permitting procedures and high investment costs remain real constraints.
Battery storage becomes competitive
Battery storage faces fewer such hurdles, benefiting from modularity and dramatically reduced costs. On cost alone, batteries can now compete with pumped hydro, and large battery parks are increasingly reaching the storage durations once associated with pumped storage. "We are now seeing battery parks that already rival classic pumped storage plants in scale and storage duration, and I expect batteries not only to catch up on cost but to overtake them," Krajačić predicts.
Making use of existing infrastructure
Among other options, Krajačić points to compressed air storage, with several large plants now built and the technology proven at scale. Thermal storage also remains in the running, covering a wide temperature range from minus 18°C to industrial high-temperature systems above 90°C.
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Particularly attractive, in Krajačić's view, is the conversion of decommissioned coal and thermal power plants. "A decommissioned coal plant does not have to be demolished. We can couple its turbine section with high-temperature thermal storage and turn existing infrastructure into a valuable building block of the future energy system," he says. Vanadium-based redox flow batteries and liquid air storage are also promising, though currently more expensive than established alternatives.
Natural gas is a discontinued model
Krajačić is equally dismissive of blending hydrogen into existing gas networks and of expanding LNG imports. Instead, he advocates using hydrogen in the form of synthetic methane and biomethane, reconvertible to electricity via gas turbines or fuel cells. "This would give us an energy carrier that can be used in existing systems while also serving as seasonal storage for periods with little wind and sun," he explains. Existing gas infrastructure, currently running on fossil fuels, could in this way be carried over into the renewable energy system, serving as both long-term storage and a flexibility resource.
The legal framework is decisive
Krajačić is equally clear that the success of smart energy systems does not depend on technology alone. "Without adapted legislation, clear political frameworks and an electricity market design that genuinely rewards flexibility, even the best technical solutions will not be able to realise their potential," he warns, with Croatia's repeated permitting and regulatory delays firmly in view. (su)
