The supply side of the energy system is changing, partly due to the rapid increase in solar and wind energy. In order to keep the energy supply safe, reliable and affordable, new knowledge and technology are needed in the short term for the transport and storage of energy. In addition, the energy system is becoming more and more interlinked; for example, electricity can be converted into heat and hydrogen for the production of raw materials. It is very important that system choices are future-proof. ECN part of TNO assists in weighing up choices in the integration of sustainable energy systems, researches the flexibilisation of energy generation and use, and develops technology in the field of conversion and storage of energy.