Flexibility of Distributed Energy Resources in Islanded Multi-Energy Systems

In the European Union, 15 million people live on 2,400 inhabited islands that to the largest part rely on relatively small and isolated energy systems. Many of these energy systems are expensive, inefficient, have a high carbon intensity, and depend on external supplies such as fuel imports. The local upscaling of variable renewable energy sources (vRES), such as wind and solar photovoltaics (PV), is a key step in achieving cheaper and cleaner energy, as well as providing benefits to the local economy and society. Yet, the electricity generation of wind and PV is intermittent and variable, making the supply-demand balancing of islanded power systems a challenging task and requiring additional sources of readily available flexibility. Both controllable distributed energy resources (DERs) and the efficient coupling of different energy sectors can contribute to the reliability of islanded systems.

This thesis studies the flexibility of DERs and their joint coordination in islanded multi-energy systems (MES). By developing modelling frameworks and conducting experimental tests at different scales, it discusses both operational and planning aspects. Building upon the findings of six research articles, the thesis addresses two overarching research questions: (i) What is the flexibility potential of key DERs on the generation and consumption side?, and (ii) how do their synergies enable the reliable operation and planning of integrated MES in the context of islands? These questions are discussed in two subsequent chapters.

The first question of the thesis deals with the operational flexibility potential of key DERs, and more specifically with their capability to modify production and consumption schedules. For assessing flexibility on the generation side, this thesis presents a modelling framework for biogas plants. These versatile units can provide dispatchable co-generation of electricity and heat and allow for biogas upgrading and the production of synthetic fuels. Validated against empirical measurements from a 3 MW biogas plant on the island of Bornholm, the developed framework identifies how the internal processes of biogas plants can be exploited for the provision of flexibility towards MES. Furthermore, in the framework of this thesis, the active power control of wind turbines and PV plants, located on Bornholm, was experimentally tested with high-resolution measurements. The tests show that these units can deliver fast and accurate regulation, and thus fulfil flexibility requests from the system operator. On the demand side, the thesis examines the role of stationary battery storage in providing flexibility and ancillary services for the power system. On top of stand-alone solutions, batteries represent the core of hybrid charging stations (HCSs) with integrated renewable generation for facilitating high-power electric vehicle (EV) charging at low-rated grid connection points. This thesis proposes the energy management of a HCS comprising a multi-battery prototype with reconfigurable topology. Based on an electro-thermal model, the performance of the developed control is tested with numerous charging scenarios of EVs. The investigation demonstrates that including renewable energy forecasts in the control decreases the impact on the grid and increases the self-sufficiency of the HCS. Electrolysers are discussed as a further key component to help realise decarbonisation in hard-to-decarbonise sectors. At the same time, they show promising capabilities for following renewable production and thus allow for coordinated control in connection with vRES. This thesis investigates their role as providers of flexibility in MES.

The second research question moves the focus from the single component to their joint operation at the substation and island level. In particular, the thesis analyses the optimal sizing of hydrogen assets in MES, and studies their role and value in coupled supply-demand balances. In the context of islands, seaborne passenger transportation is a prominent use case for hydrogen applications, and often a large contributor to the island’s total carbon emissions. For the case of a renewable-dominated substation on Bornholm, the thesis shows that renewable-powered electrolysers can absorb large parts of local excess generation while contributing to meeting district heating demands. Yet, only a fraction of the ferries’ hydrogen demand can be covered. Zooming out from the substation to the island perspective for Bornholm, the last part of the thesis presents a security-constrained techno-economic analysis of the trade-off between investments in flexibility assets. Incorporating all generating units and the 60 kV network, the analysis shows that electrolysers and battery storage fulfil different roles in highly renewable-based power systems, and can thus be seen as complements rather than substitutes for flexible system operation. In the planning process for reliable sizing and placement of such assets, a realistic and complete depiction of the available flexibility in the remaining system is found to be essential. Neglecting either security constraints that maintain N-1 system reliability or unit commitment constraints that detail the operational flexibility of thermal generators underestimates investments in flexibility assets. The siting is, by contrast, predominantly impacted by the estimated capacity of the network.

In general, DERs can provide significant flexibility to the power system, while contributing to multi-energy demands. As the amount and type of flexibility differs, a mix of technologies and measures will be needed to achieve a low-carbon energy future for islands.