Modeling and optimization of urban energy systems for large-scale integration of variable renewable energy generation

To meet the future emissions goals, the energy systems need to be decarbonized. As much of the energy use originates from urban areas, their role will be of key importance in this context. One strategy for decarbonization is to use large-scale variable renewable electricity schemes, but these include several challenges, notably the issue of supply and demand mismatch. Therefore, a mix of technologies may be needed to achieve ambitious decarbonization targets in cities. The aim of this doctoral thesis is to develop solutions for city-level energy system transition. For this purpose, a dynamic energy system model for Helsinki city is used to ana-lyze a range of scenarios for a low-carbon future. Renewable energy, in particular wind power, was chosen here as the key supply technology. As northern cities are heat-dominated, the heat-ing sector was included in the analysis by using power-to-heat and heat pump schemes in par-allel to power production. To meet short peak heat demand conditions, separate bio-boilers were also considered. Such schemes provided deep decarbonization possibilities. In the Hel-sinki case, the use of fossil fuels could be reduced even up to 70% through the coupling of wind power with curtailment and heat pumps. Though the above type of sectoral coupling to heating helps to integrate large amounts of intermittent renewable power, the role of the exog-enous power market proved to be important for wind power integration. For Helsinki, for ex-ample, with a wind power capacity of 1500 MW corresponding to 62% of the annual electricity demand, 89% of the wind electricity could be used locally in the different sectors, but the rest needs to be coupled to the exogenous market due to the mismatch and plant limitations. In-corporating demand-side measures, e.g., building energy efficiency, could save 6%-13% in the annual system costs. Other alternatives such as sustainable gas were also investigated. The results of this thesis indicate that there are several decarbonization pathways of the urban energy systems of which some could even yield a zero-emission energy system.