## 5 Discussion
### Other System Designs  
* _A national electricity grid connection_ would make it possible to import electricity or export peaks of solar electricity to other cities or electricity consumers in different sectors, such as industry, for example, by importing lower-cost onshore or offshore wind electricity during periods of insufficient solar electricity production (e.g., at night). This would reduce the need for hydrogen storage and FCEV2G electricity. High solar output at midday in the Mid Century scenario results in high surplus peaks to be absorbed by the solar electrolyzer. Exporting these high peaks of solar electricity to, for example, industrial cooling warehouses would reduce solar electrolyzer installed capacity and costs. Using only one electrolyzer connected to the national grid and placed next to the hydrogen station could reduce hydrogen transportation. Smart placement of electrolyzers in the electricity grid could obviate electricity grid congestion and reduce or avoid the need for expensive capacity expansion [144]. * _A hydrogen pipeline network_[145; 146; 147; 148; 149] could reduce hydrogen transportation via tube trailers and fueling station capacity. Multiple electrolyzers and hydrogen fueling stations could be interconnected via a pipeline network [150]. In this way, tube trailer hydrogen transportation could be replaced, and hydrogen transportation costs reduced. Furthermore, the partial re-compression of hydrogen when emptying a tube trailer could also be reduced or avoided altogether. The compressor could even be omitted, provided the electrolyzer hydrogen output pressure is higher than the pipeline pressure. In the case of parked FCEVs delivering V2G electricity, the fuel cell could be connected directly to the hydrogen distribution pipeline network, instead of using hydrogen from the on-board hydrogen tank [151]. Not using hydrogen from the 700 bar tank eliminates the need for refueling for V2G purposes, which in turn reduces the required capacity of hydrogen fueling stations. * _Import of low-cost renewable hydrogen could partially replace, possibly costlier, local hydrogen production and seasonal hydrogen storage, and thus total system costs._ Locally and at certain times of the year, there could be insufficient solar and onshore wind sources available to produce hydrogen. Regions with abundant and low-cost hydro, solar, or wind power [152; 153; 154; 155; 156; 157; 158] could produce low-cost hydrogen for export. This hydrogen could be imported at demand centers instead of being produced and stored on-site. Several ideas already exist, for example, producing hydrogen (far) offshore [159] from fixed or floating wind [160; 161; 162; 163; 164] and solar structures [163; 164], or wave energy [165] and bringing the hydrogen onshore via existing natural gas or newly built pipelines [32] or ships [166; 167]. The onshore pipeline network would then distribute the hydrogen to the consumers. * _Using a lower-cost mix of renewable energy sources._ In this study, the rooftop solar surface area was kept equal in both locations, even though solar electricity is more expensive in Hamburg than in Murcia. Therefore, using the lowest cost renewable energy source locally available could reduce total system costs even further. For example, hydropower, offshore wind, biomass, concentrated solar power, by-product hydrogen, or tidal or wave energy could result in lower-cost electricity than onshore wind or solar Photovoltaic (PV). * _Tailor electricity mix and its supply pattern to local demand._ In Murcia, solar electricity production has a better time match with electricity consumption on both a daily and a seasonal basis. During the day, solar electricity production in summer aligns well with electricity demand in buildings for space cooling. Therefore, a lower total system cost can be achieved by tailoring the renewable energy mix to allow for a better match between the production pattern and the demand pattern [61; 63; 65; 102; 168; 169; 170; 167; 168; 169; 171; 166; 172; 167].