Turning summer heat into winter warmth
Heating and cooling account for around 40 % of Europe’s total energy use, making them one of the toughest challenges in the transition to climate neutrality. While renewable electricity has expanded rapidly, replacing fossil fuels in heating and cooling networks has proven far more difficult. The EU-funded RESTORE(opens in new window) project is tackling this challenge by developing a way to store renewable energy seasonally – capturing excess energy when it is abundant and releasing it months later when demand peaks. The aim is to make clean heating available even in winter, when renewable energy production is lower but demand is highest. “The energy used for heating and cooling indoor spaces is a significant part of the overall energy needs of mankind,” said Dr Fritz Zaversky from Spain’s National Renewable Energy Centre (CENER)(opens in new window), which coordinates the research. “To achieve a sustainable energy system, renewable energy must also be used for heating and cooling.”
Why heating lags behind electricity
Progress in renewable heating has been slowed by cost and complexity. Gas boilers remain cheap, familiar and easy to install, making it harder for alternative solutions to compete. While electrification and heat pumps are becoming more common, they rely on a clean electricity supply and strong grid infrastructure, which is not always available everywhere or at all times. RESTORE addresses this gap by focusing on district heating and cooling(opens in new window) (DHC) systems, which serve entire neighbourhoods or cities. Its core idea is deceptively simple: store renewable energy during summer and use it during winter. “If we were able to store the excess solar energy during summer months in an efficient and cost-effective manner, we would not need to burn natural gas during winter,” said Zaversky.
Two technologies, one integrated system
To make seasonal storage possible, RESTORE combines two advanced technologies. The first is thermochemical energy storage(opens in new window) (TCES), which stores heat using reversible chemical reactions. This allows energy to be stored for long periods with minimal losses. “A reversible endothermic reaction stores thermal energy without losses for a long period of time,” Zaversky said. When heat is needed, the reaction reverses, releasing energy into the district heating network. The second technology links heat storage with the electricity system. A reversible heat pump can use surplus renewable electricity to upgrade low-temperature waste heat, for example from industry, to a higher temperature suitable for storage. When electricity is needed, the system can run in reverse using an Organic Rankine Cycle (ORC)(opens in new window), converting stored heat back into electricity and feeding it into the grid.
Benefits for cities and energy systems
This flexibility could be crucial for future energy systems dominated by renewables, where both heat and electricity must be carefully balanced. “Efficient storage of electricity is crucial to guarantee power grid stability,” Zaversky said. In practical terms, a mature RESTORE-type system could offer cities lower heating costs, near-zero emissions and greater energy independence. “A correctly designed and cost-effective seasonal energy storage system could reduce heating costs, eliminate emissions, and provide independence from conventional energy sources,” he said. Although the technology is still at an early stage, its long-term potential is significant. “We could provide clean CO2-neutral heating for whole districts, with solutions that could be replicated across Europe and beyond,” said Zaversky. With further upscaling and real-world demonstration, RESTORE could become a cornerstone technology, helping Europe store today’s renewable energy for tomorrow’s heating needs.
