There are two competing concepts in the literature for the integration of high shares of renewable energy: the coupling of electricity to other energy sectors, such as transport and heating, and the reinforcement of continent-wide transmission networks. In this paper both cross-sector and cross-border integration are considered in the model PyPSA-Eur-Sec-30, the first open, spatiallyresolved, temporally-resolved and sector-coupled energy model of Europe. Using a simplified network with one node per country, the cost-optimal system is calculated for a 95% reduction in carbon dioxide emissions compared to 1990, incorporating electricity, transport and heat demand. Flexibility from battery electric vehicles (BEV), power-to-gas units (P2G) and long-term thermal energy storage (LTES) make a significant contribution to the smoothing of variability from wind and solar and to the reduction of total system costs. The cost-minimising integration of BEV pairs well with the daily variations of solar power, while P2G and LTES balance the synoptic and seasonal variations of demand and renewables. In all scenarios, an expansion of cross-border transmission reduces system costs, but the more tightly the energy sectors are coupled, the weaker the benefit of transmission reinforcement becomes. (T. Brown) fuels such as hydrogen and methane (so called 'electrofuels'), or thermally [16]. Long-term storage can smooth out both the seasonal variations of renewables and the synoptic variations (∼ 3-10 days in the time dimension).Modelling all energy sectors in high spatial and temporal detail is computationally demanding. In order to maintain computational tractability, previous sector coupling studies have either focused on just a few demand sectors, or sacrificed spatial or temporal resolution.Studies of a few sectors have either considered just electricity and heat, electricity and transport, or electricity and gas. For example, in [17,18] the possibility of using excess renewable electricity in the heating sector was considered, but no requirements were set to defossilise all heating, or to couple to other demand sectors. In another set of studies, a simplified investment and dispatch scheme was used for a one-node-per-country model of Europe to study electricity-heat coupling [19]. Interactions between the electricity sector and transport were studied for electric vehicles in [20][21][22] and including fuel cell electric vehicles in [23,24]. More general coupling of electricity to gas for use in either heating or transport was considered in [25,26].Studies that include multiple sectors, often encompassing all energy usage, but that sacrifice spatial resolution have typically either considered single countries (e.g. Germany [27][28][29][30], , Ireland [34,35]) or considered the whole continent of Europe without any spatial differentiation [36] so that

It is expected, and regionally observed, that energy demand will soon be covered by a widespread deployment of renewable energy sources. However, the weather and climate driven energy sources are characterized by a significant spatial and temporal variability. One of the commonly mentioned solutions to overcome the mismatch between demand and supply provided by renewable generation is a hybridization of two or more energy sources in a single power station (like wind-solar, solar-hydro or solar-wind-hydro). The operation of hybrid energy sources is based on the complementary nature of renewable sources. Considering the growing importance of such systems and increasing number of research activities in this area this paper presents a comprehensive review of studies which investigated, analyzed, quantified and utilized the effect of temporal, spatial and spatiotemporal complementarity between renewable energy sources. The review starts with a brief overview of available research papers, formulates detailed definition of major concepts, summarizes current research directions and ends with prospective future research activities. The review provides a chronological and spatial information with regard to the studies on the complementarity concept.

We use three ensemble members of the EURO-CORDEX project and their data on surface wind speeds, solar irradiation as well as water runoff with a spatial resolution of 12 km and a temporal resolution of 3 hours under representative concentration pathway 8.5 (associated with a strong climate change and a temperature increase of 2.6 to 4.8 • C until the end of the century) until 2100 to investigate the impact of climate change on wind, solar and hydro resources and consequently on a highly renewable and cost-optimal European power system. The weather data is transformed into power, different aspects such as capacity factors and correlation lengths are investigated and the resulting implications for the European power system are discussed. In addition, we compare a 30-node model of Europe with historical and climate change-affected data, where investments in generation, transmission and storage facilities are optimised. Differences in capacity factors among European countries are more strongly emphasized at the end of the century compared to historic data. This results in a significantly increased photovoltaic share in the cost-optimal power system. In addition, annual hydro inflow patterns of major hydro producers change considerably. System costs increase by 5% until the end of the century and the impact of climate change on these costs is of similar magnitude as differences between the ensemble members. The results show that including climate affected-weather data in power system simulations of the future has an observeable effect.