Production of Synthetic Fuels as a Form of Utilizing Renewable Energy Surpluses—Spain and Poland Case Study

The increasing share of variable renewable energy sources (RES) in power systems leads to growing challenges related to grid balancing and the management of periodic electricity surpluses. One potential pathway for utilizing these surpluses is their conversion into synthetic fuels via Power-to-X technologies. This study analyzes the technical and economic potential of surplus renewable electricity utilization for the production of green hydrogen and synthetic fuels, using Poland and Spain as representative case studies of power systems with low and high RES penetration, respectively. An original methodology based on hourly power system data was developed to identify technically feasible surplus electricity volumes, accounting for changes in renewable and conventional generation, minimum renewable energy share thresholds, and a minimum two-hour continuous operation requirement. The analysis quantifies both instantaneous and usable surplus energy on an annual basis and evaluates the resulting capacity factors of Power-to-X installations. The results show that the annual usable surplus energy amounts to approximately 886 GWh in Poland and 2329 GWh in Spain, corresponding to maximum capacity factors of about 27% and 50%, respectively. Based on these surpluses and assuming low-cost electricity during surplus periods (10 EUR/MWh), the levelized cost of green hydrogen was estimated at 4.1 EUR/kg in Poland and 2.18 EUR/kg in Spain. The resulting production costs of green methanol reach approximately 739 EUR/Mg for Poland and 378 EUR/Mg for Spain after accounting for avoided CO2 emissions. The findings indicate that surplus-based Power-to-X systems can play a meaningful role in integrating high shares of renewable energy, particularly in power systems with high and stable RES penetration. However, their contribution remains strongly constrained by surplus availability, temporal continuity, and system-specific characteristics.