Variable renewable energy (VRE) resources increasingly add fluctuations to power systems. The required types and capacities of balancing measures, amounts of curtailment, and costs associated with system integration need to be assessed for advising policy makers and economic actors. Previous studies mostly exclude storage from model-endogenous capacity expansion and omit concentrated solar power (CSP) completely. In this study, we stress the need for grid and backup capacity by investigating an integrated market in Europe, allowing for additional short-term as well as long-term storage and considering CSP as a dispatchable backup option. The Renewable Energy Mix (REMix) energy system model is introduced and applied to assess the capacity expansion and hourly dispatch at various levels of photovoltaic and wind power penetration. The model results demonstrate combinations of spatial and temporal balancing measures that enable net photovoltaic and wind supply shares of 60% and 70% of the annual demand, respectively. The usage of storage and grid can keep curtailments below 20% of the demand for theoretical VRE shares of up to 100%. Furthermore, we determine that the VRE supply structure has a strong impact on the least-cost allocation of power plants across Europe but only a limited effect on supply costs

Inverse modeling techniques used to quantify surface carbon fluxes commonly assume that the uncertainty of fossil fuel CO₂ (FFCO₂) emissions is negligible and that intra-annual variations can be neglected. To investigate these assumptions, we analyzed the differences between four fossil fuel emission inventories with spatial and temporal differences over Europe and their impact on the model simulated CO₂ concentration. Large temporal flux variations characterize the hourly fields (~40 % and ~80 % for the seasonal and diurnal cycles, peak-to-peak) and annual country totals differ by 10 % on average and up to 40 % for some countries (i.e., the Netherlands). These emissions have been prescribed to seven different transport models, resulting in 28 different FFCO₂ concentrations fields. The modeled FFCO₂ concentration time series at surface sites using time-varying emissions show larger seasonal cycles (+2 ppm at the Hungarian tall tower (HUN)) and smaller diurnal cycles in summer (−1 ppm at HUN) than when using constant emissions. The concentration range spanned by all simulations varies between stations, and is generally larger in winter (up to ~10 ppm peak-to-peak at HUN) than in summer (~5 ppm). The contribution of transport model differences to the simulated concentration std-dev is 2–3 times larger than the contribution of emission differences only, at typical European sites used in global inversions. These contributions to the hourly (monthly) std-dev's amount to ~1.2 (0.8) ppm and ~0.4 (0.3) ppm for transport and emissions, respectively. First comparisons of the modeled concentrations with ¹⁴C-based fossil fuel CO₂ observations show that the large transport differences still hamper a quantitative evaluation/validation of the emission inventories. Changes in the estimated monthly biosphere flux (Fbio) over Europe, using two inverse modeling approaches, are relatively small (less that 5 %) while changes in annual Fbio (up to ~0.15 % GtC yr⁻¹) are only slightly smaller than the differences in annual emission totals and around 30 % of the mean European ecosystem carbon sink. These results point to an urgent need to improve not only the transport models but also the assumed spatial and temporal distribution of fossil fuel emission inventories.