This thesis is intended at answering the question: What is the feasibility of using synthetic fuels for reaching a 100% renewable transport sector? To answer this question an energy system analysis was carried out and two pathways for producing synthetic fuels were created with a specific focus on solid oxide electrolyser cells (SOEC) which are combined with the recycling of CO2. The first pathway entails co-electrolysis of steam and CO2 and the second one includes hydrogenation of CO2. These scenarios have been compared in terms of primary energy supply, biomass consumption, flexibility of system and socio-economic costs with two pathways that have direct usage of biomass in the production process of liquid fuels - hydrogenation of biomass and conventional biodiesel pathway. The scenarios provide all liquid fuels that cannot be replaced by direct electrification which has the first priority. The reason for this analysis lies in the fact that at present, the transport sector is the only sector in which there have been no significant renewable energy penetrations and it is heavily dependent on oil with rapid growth in the last decades. Moreover, it is challenging to obviate the oil dependence due to the wide variety of modes and needs in the sector. Nowadays, biofuels are proposed as one of the main options for replacing fossil fuels in the transport sector, along with electricity. The main reasons for avoiding the direct usage of biomass in the transport sector, i.e. producing biomass derived fuels, are land use shortage, limited biomass availability, interference with food supplies, and other impacts on environment and biosphere. Hence, it is essential to do a detailed analysis of the transport sector in order to match the demand and to meet the criteria of a 100% renewable energy system in 2050. The analysis was carried out for Danish system, because the most developed 100% renewable energy system is available from the CEESA project, which includes a wide variety of transport and other energy system measures. The analysis showed that the synthetic fuel scenarios increase the system flexibility and this is essential for the conversion of the energy system into a 100% renewable system. The costs of synthetic fuel scenarios are more expensive, but biomass savings associated with this make the additional costs worthwhile due to the scarcity of biomass for the energy system. With feasible technological development and mass production of the Solid Oxide Electrolyser Cells, synthetic fuels could be competitive and have market advantage over biomass derived fuels based on their supply related issues, land use shortage, limited biomass availability, etc.