## 3 Results and Discussion
### End-Use Applications for Biomethane and Hydrogen  
The analysis of biomethane potential (4.85 TWh/year from pig manure and 1.54 TWh/year from WWTPs) reveals significant opportunities for multiple applications. When examining the transport sector specifically, the data indicate that the biomethane  
Figure 8: Potential UHS per province in GWh.  
purity levels achieved in both the LIFE Smart Agromobility project (90-95%) and Smart Met Value project (>95%) exceed the minimum requirements for vehicle fuel applications. These purities are aligned with Spanish regulations, which require that biomethane should have at least 90% methane content [50] and a maximum of 1% oxygen [51] to enable its direct utilisation in heavy and light-duty vehicles.  
The results from pig manure processing demonstrated that a facility managing 1500 tonnes of manure could produce 41.1 MWh annually, sufficient to power six light-duty vehicles travelling approximately 14,627 km each per year. Similarly, the analysis of WWTP showed that a facility serving 93,000 inhabitants could produce 285 kg of biomethane daily, enough for seven collection trucks and two maintenance vehicles. These suggest that biomethane offers a viable technical solution for vehicle fleets with predictable routes and centralised refuelling infrastructure.  
While the technical analysis confirms biomethane's suitability for transport applications, the economic assessment indicates challenges for grid injection applications. This explains why, despite the substantial technical potential this study has identified across Spain, direct vehicle applications currently represent more economically viable pathways than grid injection.  
When comparing biomethane and hydrogen applications based on our findings, several patterns emerged. The identified hydrogen storage capacity (2865 TWh) significantly exceeds the annual biomethane production potential (6.39 TWh/year), suggesting different but complementary roles in the energy transition. While biomethane production shows relatively consistent year-round availability, as confirmed in the project measurements, hydrogen storage offers seasonal flexibility that addresses intermittency challenges.  
The analysis of hydrogen storage distribution across Spain reveals interesting geographical patterns that influence potential applications. The concentration of depleted hydrocarbon reservoirs in three provinces (Bizkaia, Guadalajara, and Huesca) suggests that large-scale industrial applications would be most practical in these regions. When compared with industrial hydrogen demand patterns, these locations align with existing chemical and refining sectors that require hydrogen for ammonia and methanol production.  
Conversely, the widespread distribution of aquifer storage capacity across 29 provinces matches more closely with distributed power generation needs. The data support the technical feasibility of hydrogen blending with natural gas [52, 53] to serve these geographically dispersed demands. The relatively small but technologically advantageous salt cavern capacity (756 GWh) offers rapid cycling capabilities that, according to the analysis, would be particularly valuable for power generation applications through gas turbines [54, 55] and microturbines [56], especially in the Navarra region, where 44.4% of this capacity is located.