## 6 Conclusions  
The designed and modeled system for smart urban areas is based on wind, solar, and hydrogen, where fuel cell electric vehicles provide year-round 100% renewable, reliable, and affordable energy for power, heat, and transportation in two different European climates. The two locations in different climate zones--namely Murcia in Spain and Hamburg in Germany--were selected based on several criteria. Both are close to or in a large European urban area in one of the five most populated countries. Located in a different climate zone according to the Koppen-Geiger classification, Hamburg has a temperate oceanic climate (Cfb), and Murcia a hot semi-arid climate (BSh). Both locations have salt formations suitable for underground hydrogen gas storage. One location has a high level of solar radiation (Murcia), while the other has a low level (Hamburg). The two designed smart city areas have the climate characteristics of Hamburg and Murcia; the dimensions are based on, respectively, German and Spanish statistical data. The smart city areas consist of 185,000 m\({}^{2}\) floor area of residential sector buildings, and for Hamburg and Murcia, respectively, 93,000 m\({}^{2}\) and 38,000 m\({}^{2}\) floor area services sector buildings. Hamburg and Murcia have, respectively, a total of 2500 and 2250 fuel cell electric vehicles (FCEVs), of which 2360 and 1850 are passenger cars that can be used for producing electricity via vehicle-to-grid (V2G), so-called fuel cell electric vehicle to grid (FCEV2G). Two thousand households with a total of approximately 4300-5000 inhabitants are the minimum viable economic size for dimensioning the smart city area, as statistically, there is one gas station and one retail food shop per 2000 households. Smaller capacity fueling stations are relatively costlier. The designed smart city area system is 100% renewable. All electricity and hydrogen can be supplied by solar and wind to fulfill the energy demand for power, heat, and transportation. The transportation and energy sectors are fully integrated, and their final energy use is all-electric. Electricity is generated by solar modules on all roofs. Surplus solar electricity is converted via water-electrophysis with rainwater into pure hydrogen. The hydrogen is compressed and transported by tube trailer modules to the nearby hydrogen fueling station (HFS) or underground seasonal hydrogen storage (SHS). At the HFS, the hydrogen is further compressed to fuel all types of FCEVs, from passenger cars, vans, buses to trucks, and tractor-trailers. In the case of a temporary shortage of solar electricity, the fuel cells in the parked and grid-connected passenger cars provide the necessary electricity (FCEV2G) by converting hydrogen from the on-board hydrogen storage tanks. At parking places at home, the office, or the local shopping area, vehicle-to-grid points connect the cars to the smart city electrical grid. The SHS is filled with hydrogen from surplus solar electricity and via a very short pipeline with hydrogen produced from wind electricity from an onshore wind turbine park. The stored hydrogen in the SHS is transported via tube trailers to the hydrogen fueling station. The designed smart city area system is reliable at all times and independent of other energy systems and grid connections. The energy balance is simulated on an hourly basis for an entire year for a Near Future and a Mid Century scenario. Five years are simulated using climate data input from the years 2012-2016, although no significant differences in the energy balance are observed. Reliable electricity supply can be realized at all times, as extreme peaks in the FCEV2G electricity supply never exceed 50% of the car fleet. Maximums of 32% and 42% of all cars (760 and 772 cars) in Hamburg and Murcia in the Near Future scenario drop to 17% and 15% of all cars (391 and 275 cars) in the Mid Century scenario. These maximums are extreme outliers, and values close to these only occur for a few hours per year.