Beyond solar and wind power: Assessing energy transport, conversion and storage technologies for a cost-effective transition towards net zero

Human activities have an unprecedented impact on Earth's environmental systems. As of 2023, six of the nine planetary boundaries identified by the Stockholm Resilience Center have been transgressed, increasing the risk of large-scale abrupt and irreversible environmental changes. This dissertation focuses on the integration of variable renewable energy sources on the path to net zero and addresses specifically climate change as one of the planetary boundaries. The dissertation is divided into two parts: Part I analyses three distinct technologies using a bottom-up approach: Direct air capture, an offshore pumped hydro system and hydrogen combustion in existing combined-cycle power plants. Part II takes a system perspective, exploring the interactions between technologies, energy carriers, and sectors, with a specific emphasis on electricity transmission, hydrogen, and electricity storage. Part II has a particular geographical focus on the North Sea region. Both parts rely heavily on energy system modeling and are carried out using AdOpT-NET0, a multi energy system modeling tool developed as part of this dissertation. The results suggest that technologies required for a successful integration of variable renewable energy sources are not only technically viable but also economically competitive. Challenges lie primarily in regulation, market design and in the political realm. Specifically, grid expansions across the North Sea towards 2030 and 2040 are a no-regret option, contributing to both cost and emission reductions. Similarly, hydrogen production via electrolysis is identified as a no-regret pathway, especially in systems with high shares of variable renewable energy sources. For full economic and environmental benefits of hydrogen production to unfold, its end-use application, however, needs to be carefully considered. Using hydrogen as a storage medium for the power sector, for example, is neither economically nor environmentally beneficial if it can also contribute to emission reductions in other sectors. In contrast, direct electricity storage, while costly, is crucial for deep decarbonization. As such, it is important to also unlock other sources of flexibility, e.g. demand side flexibility or vehicle-to-grid storage. In the context of the North Sea, the siting of energy technologies needs to be carefully considered. While other non-economic or non-technical considerations might push for locating technologies such as storage or hydrogen production offshore, it would also reduce their economic and environmental benefits. Direct air capture can serve as additional demand-side flexibility and should be deployed to offset emissions from sectors and processes where electrification or hydrogen use are not feasible decarbonization options. The dissertation aims to guide policymakers in prioritizing support for technologies for the successful integration of solar and wind power.