## Materials and Methods
### Location Selection, System Design and Dimensioning, System Components, and Scenarios
#### 2.2.2 System Design and Dimensioning  
The smart city area energy and transportation system is designed in such a way that it fulfills the following design requirements:  
* uses only electricity and hydrogen as energy carriers and is all-electric in end-use
* uses only hydrogen as seasonal energy storage and fuel to power all road vehicles
* can be applied to an average European city area and is a scalable design
* can be operated in a network of multiple smart city areas and renewable hydrogen and electric energy hubs or centers [106, 107, 108, 109, 110]
* can be integrated into existing infrastructure and buildings
* is not dependent on an in-urban area underground hydrogen pipeline transportation network
* uses abundant renewable energy sources in Europe: local solar and large-scale wind only
* is independent of high and medium voltage electricity grids, natural gas, and district heating grids or the expansion of these. By applying the design requirements, the integrated system design of the smart city area has the following seven major elements and functional energy performance and conversion steps (Figure 2 and Table 1):  
1. _Local solar electricity and hydrogen production (orange)_: Local rooftop solar electricity and rainwater collection, purification, and storage systems (S1-S3) produce solar electricity (E\({}_{S}\)) and pure  
Figure 1: Location selection steps and criteria resulted in the urban area of Hamburg in Germany and Murcia in Spain. water (H\({}_{2}\)O\({}_{8}\)). A part of the solar electricity is directly consumed (E\({}_{\rm DC}\)) in buildings and other sub-systems. The remaining surplus solar electricity (E\({}_{\rm S}\)) is used with purified water (H\({}_{2}\)O\({}_{\rm S}\)) in the hydrogen production, purification, and compression system (S4-S6) for filling tube trailers (TT1) with hydrogen (H\({}_{\rm S}\)). 2. _Fuel cell electric vehicle-to-grid, building electricity consumption, and smart grid control (yellow)_: The smart electric grid is managed by a controller, which connects all buildings, grid-connected FCEVs (FCEV1 and2), the hydrogen fueling station (HFS1-HFS4), solar electricity and hydrogen production (S1-S6), and the tube trailer filling station (SHS2) at the seasonal hydrogen storage (SHS1). The directly consumed solar electricity (E\({}_{\rm DC}\)) is divided amongst the all-electric residential and services sector buildings (E\({}_{\rm B}\)), HFS (E\({}_{\rm HFS}\)), and SHS (E\({}_{\rm SHS}\)) electricity consumption. Any shortage of electricity is met by the electricity produced from hydrogen (E\({}_{\rm V2G}\)) through parked (at home or in public or commercial spaces) and V2G connected FCEVs (FCEV1 and2). 3. _Hydrogen tube trailer transportation (grey)_: Tube trailers (TT1) towed by tube trailer tractors (TT2) transport hydrogen from either the local solar hydrogen production or the SHS to the HFS, or from the local solar hydrogen production to the SHS. 4. _Hydrogen fueling station (blue)_: Hydrogen from tube trailers is further compressed (HFS1) and stored at high pressure (HFS2). A chiller (HFS3) cools the dispensed hydrogen (H\({}_{\rm HFS}\)), including sufficient dispensers (HFS4) to provide hydrogen for both road transportation (H\({}_{\rm road}\)) and V2G (H\({}_{\rm V2G}\)) use. 5. _Road transportation (purple)_: A fleet of road transportation FCEVs, namely passenger cars, vans, buses, trucks, and tractor-trailers. 6. _Large-scale and shared wind hydrogen production (green)_: A large-scale wind turbine park (W1) that is not located near or in smart city areas is shared with other smart city areas and renewable hydrogen hubs and consumers. All wind electricity (E\({}_{\rm W}\)) is used with purified water (H\({}_{2}\)O\({}_{\rm W}\)) from local surface water or seawater in hydrogen production (W4), purification (W5), and compression system (W6), which includes a water collection and purification system (W2 and W3). The hydrogen produced (H\({}_{\rm W}\)) is stored in a large-scale underground seasonal hydrogen storage (SHS1). 7.