The integration of RESs into the power system is changing electricity production profiles. This challenges power-system balancing, since production and consumption must always remain balanced, while traditional production-following-consumption is not feasible with stochastic RESs. Flexibility is one proposed solution. However, the literature lacks a comprehensive, technologically neutral definition of flexibility, which increases barriers for different stakeholders. Existing definitions often emphasize selected viewpoints, such as resources, power grids or services. Therefore, this paper reviews flexibility from resource, service and market perspectives and proposes a definition broadly applicable in the power sector.

The proposed definition applies to many use cases and can support flexibility audits for determining flexibility volume and type. Flexibility describes a process’ or physical device’s controllability with an external signal for a limited duration without disturbing its output. In this paper, flexibility is categorized into resources, services and products. Flexibility resources are sources of flexibility, such as manufacturing processes. Flexibility services are abstract definitions of flexibility use for a specific purpose, such as cost minimization. The same service can be provided in several ways, and the resource should fit the service requirements. Therefore, understanding service requirements is key for identifying suitable resources. Flexibility products represent the contractual or market-based means of trading flexibility services, such as pricing schemes.

The proposed definition is applied in a flexibility audit of a wastewater treatment plant in Greece. The plant’s aeration process is analysed qualitatively. The work is based on Trineflex, a Horizon project studying feedstock, energy and process flexibility. Relevant flexibility resources, potential services and products are identified and discussed. Wastewater in the aeration tank and aerators are identified as intermittent storage and demand response flexibility resources, respectively. Energy can be stored in the tank as oxygen, while aerators produce oxygen by blowing air into the wastewater. By controlling the aerators within process constraints, electricity consumption can be shifted or reduced. The aeration process can provide several flexibility services, including energy cost minimization and peak power clipping. Potential products include mFRR, DA and ID market products.

The proposed definition provides a harmonized and comprehensive framework for understanding and applying flexibility across resources, services and products. By demonstrating its applicability in the use case, the paper shows that the definition can lower stakeholder barriers, support flexibility audits and facilitate integration into energy markets. It contributes to a more resilient, efficient and technology-neutral power system. The work is part of a doctoral research project.