Seasonal Variability of Biomass Composition and Its Impact on Methane Yield, Kinetics, and Energy–Economic Performance of Helianthus tuberosus

This study evaluated the effects of seasonal variability in the properties of Helianthus tuberosus biomass on the efficiency and kinetics of anaerobic digestion (AD), as well as on energy and economic indicators. Unlike studies focusing on individual biomass properties, this work integrates seasonal changes in biomass yield, biochemical composition, lignocellulosic structure, methane-production kinetics, area-specific CH4 productivity, and energy–economic performance within a single harvest-time optimization framework. As the growing season progressed, the proportions of lignocellulosic fractions increased significantly (p < 0.05), whereas the contents of biodegradable compounds decreased, thereby reducing substrate availability to methanogenic microorganisms. The highest CH4 yields were obtained in summer (July–August), reaching a maximum of 295 ± 14 mL/g VS, with a high production rate of up to 33.0 ± 3.8 mL/g VS·d. In subsequent months, the yield gradually decreased to 163 ± 18 mL/g VS in December, accompanied by a deterioration in kinetic parameters. Area-specific CH4 productivity ranged from 1635 ± 396 m3/ha in June to 3436 ± 778 m3/ha in August and reflected the combined effects of the specific AD yield, biomass yield, and susceptibility to degradation under anaerobic conditions. Despite comparable specific CH4 production in July and August, the economic analysis demonstrated a significant (p < 0.05) advantage for August, when the highest revenue of 3200 ± 725 EUR/ha and a net return of 1829 ± 385 EUR/ha were achieved. Energy and economic performance declined systematically in the subsequent months because of both lower AD efficiency and higher agronomic costs. The results indicate that optimization of AD of Helianthus tuberosus biomass requires the simultaneous consideration of substrate quantity and quality and that August is the most favorable harvest time under the conditions investigated. From a practical perspective, these findings support harvesting at the most favorable maturity stage and subsequent biomass preservation for year-round use in AD systems.