OverviewIn order to comply with ambitious climate goals, e.g. resulting from the Paris Agreement, a considerabledecarbonisation of today’s energy system is inevitable. As the heat and transport sectors are at present still stronglyrelying on fossil fuels, this long-term transition cannot be limited to the traditional power sector (Mancarella et al.,2016). Based on current deployments and projections, the future multi-energy system will be primarily based onwind and solar generation to supply and decarbonise the energy demand of all relevant energy sectors throughelectricity-based technologies and fuels. While the additional cross-sectoral electricity demand will entail notablyhigher renewable generation capacities for future power systems, it might also bring, through coupled operation ofbi- and multi-valent or storage-connected consumers, substantial flexibility contributions in its wake.Given the vast potential of offshore wind, it can play a crucial role in scenarios with considerable sector interaction,helping to meet the high electricity demand and offering an alternative for onshore wind which is being increasinglysubjected to social acceptance issues. With that in mind, it is important to acknowledge the fact that offshore windfarms require transmission systems with high investment costs to connect their generation to the onshore marketareas. Of similar importance is the growing challenge of balancing fluctuations between market areas with higherrenewable penetration, both on- and offshore. Making sound investment decisions for offshore grids with theirtwofold connection function of integrating offshore wind generation and facilitating power trade between onshoremarket areas is therefore of great relevance. At the same time, competing onshore flexibility options (e.g. flexibleCHP, heat pumps, electric vehicles) have to be put in the balance when assessing offshore grid infrastructureoptions in long-term scenarios.MethodsTo investigate offshore grid investment decisions for the Northern Seas region in the proposed case study, a twostageapproach is applied.In a first step, a European energy scenario for 2050 is determined by running the SCOPE model which has beendeveloped at Fraunhofer IWES (Trost, 2017). Minimising system operation and investment costs, while at the sametime complying with a given carbon emission target covering all relevant sectors, this deterministic cross-sectoralgeneration expansion planning model is formulated as a linear program (LP) for a full year discretised into8760 consecutive hours. Capturing the flexibility introduced by the coupled operation of power, heat, and transportsectors in a multi-energy system with its various technology combinations is one of the main purposes of this model(Härtel and Sandau, 2017). All time series data related to meteorological information, e.g. wind and solarproduction, thermal and cooling loads, heat pump’s coefficient of performance, is based on COSMO-EU weathermodel data (Deutscher Wetterdienst, 2016). To ensure consistency along both grid planning stages, the same database for offshore wind generation data is used. This means that structural and spatial information of single offshorewind farms, based on (4C Offshore, 2017), is combined with site-specific wind generation profiles.In a second step, the resulting energy system scenario is used in a deterministic large-scale offshore grid expansionplanning model with a particular focus on capturing future onshore flexibility to compute the offshore gridinvestments. For computational reasons, this market-based grid planning model needs to reduce the spatialresolution by clustering the single offshore wind farms to so-called offshore wind hubs. Moreover, it covers allmajor European market areas while keeping the same optimisation horizon in hourly resolution as is used by theSCOPE model. Hence, the model requires aggregated modelling approaches for hydropower systems (Härtel andKorpås, 2017) and thermal power plants (integer clustering), as well as bi- or multi-valent sector couplingtechnologies, see (Härtel and Sandau, 2017). Based on the linear cost model presented in (Härtel et al., 2017), themodel also accounts for the important fixed cost components of offshore grid infrastructure, i.e. fixed converter andplatform costs. It is therefore formulated as a mixed-integer linear program (MILP). What is more, a newinvestment cost parameter set for VSC HVDC technology will be used which has been validated against realisedback-to-back, interconnector, and offshore wind connection projects. Despite the aggregation efforts, a noveldecentralised solution approach based on a proximal bundle method had to be developed to efficiently handle the hourly interactions of the key flexibility providers and the renewable generation when making investment decisionsin offshore grid infrastructure.ResultsFollowing the two-stage methodology, the obtained results will be twofold. First, a possible decarbonisationscenario for the European region in 2050 will be presented as a result of the SCOPE model. To be able to meet theadditional electricity demand for heat and transport sectors, this scenario will show a generation mix withconsiderable renewable capacities, i.e. on- and offshore wind as well as rooftop and utility-scale solar power. Leastcostcompliance with cross-sectoral carbon emission reductions will favour highly efficient and flexible technologycombinations such as electric vehicles, decentralised heat pumps, and multi-valent CHP systems. It is important tomention that these key flexibilities and interactions are taken into account by the second modelling stage. Moreover,the model will also give a carbon price as the emission budget is modelled as a hard constraint.Second, the large-scale offshore grid expansion planning model will be used to assess three offshore grid topologyparadigms facilitating the connection of offshore energy to the onshore market areas and the exchange betweenthem:1. Status quo, allowing radial offshore hub connections and no expansion on existing interconnector corridors,2. Business as usual, allowing radial offshore hub connections and expansion on existing interconnectorcorridors, and3. Meshed grid, allowing meshed offshore hub connections and expansion on existing interconnector corridors.For the highly decarbonised 2050 scenario, these paradigms will show different offshore grid investments and theircorresponding system operation cost. Because the geographical scope of the market-based offshore grid expansionplanning model covers all major European countries, the model results will indicate cost impacts for directly andindirectly connected market areas to an offshore grid in the Northern Seas. To exhibit the competing role of onshoreflexibility for integrated offshore transmission infrastructure, a sensitivity analysis with increased flexibility ofelectric vehicles is going to be presented.ConclusionsThe contribution will provide insights into a long-term energy scenario complying with cross-sectoraldecarbonisation goals and its consequences for offshore wind energy as well as (integrated) offshore gridinfrastructure. As the main implication, the results will show that achieving ambitious carbon emission reductionsacross all energy sectors can enable offshore wind and offshore grid infrastructure to significantly contribute to afuture energy system. However, the role of and necessity for increased power trade facilitated by integrated offshoregrids in the Northern Seas exhibit a large dependency on onshore flexibility developments introduced by increasedenergy sector interaction.
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