The climate crisis, ever more stringent targets and the simultaneous increasingly unprofitable fossil energies are ensuring the advancing implementation of renewable energies. Storage systems play an indispensable role in the widespread, decentralized utilization of energy and the security of energy supply. Storage based on sustainable sources is one of the pillars to end the fossil age and shape the energy transition in an ecological and decentralized way. Powered by renewable energy, storage systems work against resource-based dependence on producing countries.

The annual International Renewable Energy Storage (IRES) conference brings together the latest technologies and research from experts to drive development momentum. This year, representatives from the political, business and research communities once again came together to exchange ideas in a variety of digital meeting formats. IRES is predestined for its broad exchange opportunities and represents one of the largest conferences in the field of renewable energy storage. At IRES, speakers present exclusively models based on renewable energy sources. More than 80 speakers presented their results in the areas of "storage technologies, networks and systems, system analysis and planning, storage in distributed energy resources, storage in industry and industrial production and processes, storage opportunities specific to emerging countries, and storage cities: municipal energy supply and storages as well as storage stories: worldwide energy storage case studies".

Founded in 2006 by the World Council for Renewable Energy (WCRE) and EUROSOLAR e.V. - the Association for Renewable Energy - IRES has become a central platform for the exchange of knowledge and ideas in the field of renewable energy storage. Over 15 years, IRES thus creates a platform for the important pillar of the energy transition.

IRES 2021 was held in digital format due to the Corona pandemic. Despite limitations, the 2021 event was once again able to present a wide range of new findings in the fields of batteries, thermal energy storage and power-to-gas, as well as solutions for storage in private households and industrial heat supply. In the digital format, participants were able to contribute in different ways. Researchers were able to present their results in the form of oral contributions or through poster contributions followed by online meetings and question forums.

In this issue of the EUROSOLAR IRES Proceedings, we reliably publish the IRES contributions that have been accepted and judged most qualified by the extensive evaluation process of the IRES Scientific Committee. Our thanks go to all supporters and contributors, researchers and conference participants. We extend special thanks to this year's Scientific Committee. Without the cooperation of the committee members, the publication of the IRES 2021 Proceedings would not have been possible.

IRES 2021 Editors
Peter Droege, President EUROSOLAR e.V., Germany
Antonia Riemer, EUROSOLAR e.V., Germany
Peter Schossig, Fraunhofer ISE, Freiburg, Germany