An integrated spatiotemporal techno-economic assessment of offshore renewable energy complementarity: A case study in China's maritime zones

Offshore wind/solar/wave complementarity can unlock large scale renewable potential, yet spatial potential, temporal stability and techno-economic performance are rarely evaluated together. This paper develops a grid-level spatiotemporal techno-economic framework combining spatial suitability screening, hourly simulations, multi-indicator stability optimization and lifecycle cost evaluation. Under future floating deployment, wind-wave and wind-solar hybrids use about 69% and 31% of wind-suitable waters but deliver 98% and 73% of wind-only electricity, respectively, markedly increasing generation per unit area. Stability-optimized wind-solar reduces offshore wind variability(CV/FR) and intermittency(ZS), whereas wind-wave delivers even stronger FR and ZS reductions but slightly increases CV, with smaller generation losses (12% versus 35% for wind-solar). Techno-economic results show that wind-solar hybrids achieve the lowest LCOE, falling to 43 €/MWh (aggressive case) by 2050, while wind-wave remains about two to three times higher. Relative to wind plus storage reference, wind-solar complementarity reduces lifecycle cost by around 83% across suitable maritime zones, whereas cost reductions for wind-wave occur mainly in southern offshore regions, indicating that temporal complementarity can economically substitute most of the storage required for output smoothing. Technology progress reduces LCOE by only 0.7% for wind-solar but by 40% for wind-wave, lowering the aggressive wind-wave median to about 56 €/MWh and allowing wind-wave to outperform wind-solar in parts of sea. Overall, a cost-effective nearshore belt is dominated by wind-solar hybrids, while wind-wave becomes increasingly attractive in deeper southern offshore waters as technology advances. These findings support wind-based complementary development pathways, clarify spatiotemporal trade-offs and monsoon-driven complementarity mechanisms in China's offshore regions, and offer insights that are relevant to region-specific coastal energy transition strategies, with a framework transferable to other marine regions.