## 3 Energy Balance Results and Discussion  
### Annual Energy Balance Results  
Key energy balance parameters for FCEV2G, solar electrolyzer, and SHS usage for Hamburg and Murcia in the Near Future and Mid Century scenarios are summarized in Table 3. Detailed background figures that serve as input to Table 3 can be found in Appendix D (Figure A1, load duration curves, Figure A2, hourly electricity balance for an entire year, Figure A3, SHS storage level, and monthly hydrogen flows). The annual energy balances of Hamburg and Murcia in the Near Future and Mid Century scenarios are shown in Figures 4 and 5. The key energy balance parameters and annual energy balances of the years 2012-2015 show similar outcomes. Several major trends can be seen when looking at the FCEV2G, wind and solar electricity production, direct consumption of solar electricity, and seasonal hydrogen storage. * _Reliable electricity supply can be realized at all times, as extreme FCEV2G peaks never exceed 50% of the car fleet_. Maximums of 760 and 772 cars, 32% and 42% of the car fleet in Hamburg and Murcia in the Near Future scenario, are reduced to 391 and 275 cars, 17% and 15% of the car fleet in the MidCentury scenario. The above maximums are extreme outliers, and values close to these occur for only a few hours per year (Figure A1). * _In the Mid Century scenario, FCEV2G usage is comparable to driving_. In the Near Future scenario, the fleet average FCEV2G hours are 880 h/year compared to 440 h in Mid Century scenario at 10 kW/car output for Hamburg. For Murcia, this is 670 h and 330 h. The Mid Century scenarios' FCEV2G hours are similar to the average driving hours for passenger cars: 310 and 280 h/year for, respectively, Hamburg and Murcia. * _The 87% higher solar electricity output in the Mid Century scenario_ in both locations results in less required external wind-to-hydrogen production to close the energy balance. This, together with more than a 30% reduction in building and road transportation energy consumption, and improvements in energy conversion processes, results in reductions of 70% and 90% of wind electricity production for, respectively, Hamburg and Murcia. * _The 40% higher solar hydrogen production_ in the Mid Century scenario in both locations compared to the Near Future scenario. Due to lower building electricity consumption and higher solar electricity production, there is more solar surplus electricity for hydrogen production. In Hamburg, solar electrolyzer power consumption always peaks in the summer's time, whereas, in Murcia, solar electrolyzer power consumption peaks in winter (Figure A2). * _The 40% and 56% higher coverage of electricity consumption with direct solar electricity production_ in the Mid Century scenario in, respectively, Hamburg and Murcia compared to the Near Future scenario. Due to higher solar radiation and lower building and system electricity consumption, a higher percentage can be met directly with solar electricity. Nighttime electricity consumption has to be met with FCEV2G electricity production. * _The 15%-25% lower seasonal hydrogen storage requirements in the Mid Century scenario_ due to a better match of higher solar electricity production and lower building electricity demand compared to the Near Future scenario. For Hamburg, the maximum storage content of hydrogen occurs in the fall for both scenarios, whereas, in Murcia, this period shifts from spring to fall. The minimum storage content occurs in winter for both locations and scenarios. In the Mid Century scenario, a typical salt cavern [104] (Table A3) could serve approximately 23 similarly operating smart city areas in Hamburg and 40 Murcia smart city areas. * _The 40% lower seasonal hydrogen storage and FCEV2G requirements in Murcia_ compared to Hamburg, in all scenarios. In the Mid Century scenario, solar electricity alone is almost able to supply all of Murcia's energy needs for buildings and road transportation (despite its 21% higher consumption of road transportation hydrogen; Appendix B.2).