# Performance Analysis of a Zero-Energy Building Using Photovoltaics and Hydrogen Storage
## 3 Results and Discussion
### Performance Analysis of the Total Configuration  
This section includes results for the performance of the overall unit including the photovoltaics, the electrolier, and the fuel cell. Figure 9 illustrates the total electricity demand of the building that has to be provided by the energy system. In addition, the cumulative electricity demand is given in this figure. It is interesting to state that the maximum electrical load of the building is 6.42 kW. In addition, it is essential to state that  
Figure 8: Cumulative electricity demands for covering the different needs of the studied building. Figure 7: Cumulative thermal loads for heating, cooling, and DHW of the studied building. the cumulative curve of electricity has a relatively linear character, something that proves the low variation in the electrical demand during the different seasons. This result is also verified by the existence of similar heating and cooling loads in terms of [kW] and [kWh]. Figure 10 exhibits the electricity production of the photovoltaics in terms of [kW] and also the cumulative electrical production curve is given in this figure. The cumulative curve has a greater slope in the summer period, a fact that indicates that in this period, there is greater electricity production. The maximum instantaneous electrical production was found at 41.04 kW. The yearly electrical production was found at 63,637 kWh, while the solar potential was at 352,971 kWh. The mean yearly electrical efficiency was found at 18.03% which is a reasonable value considering that the nominal efficiency of the PV cell is at 19.7%. It is obvious that the produced electricity from the photovoltaic field is significantly higher than the electricity demand. However, this is a reasonable fact because there are important conversion losses in the electrolyzer and in the fuel cell, and thus an electrical quantity is lost during the charging/discharging processes. Figure 10: Variation in the produced electricity from the photovoltaics in [kW] and the cumulative electricity production in [kWh]. Figure 9: Variation in the total electricity demand in [kW] and the cumulative electricity demand in [kWh] of the studied building. The next step is the presentation of daily results for the electricity and the hydrogen flows. More specifically, Figure 11 shows results for a typical winter day and Figure 12 for a typical summer day. Similar profiles can be found with the PV production to be maximized around the summer, with the maximum hydrogen demand being later in the afternoon (20:00). However, the production of the PV is higher in the summer (32.5 kW) compare to winter (20.3 kW). These figures practically prove that the present unit is an appropriate choice for the year period, and so is a sustainable one. However, a crucial issue is the selection of a proper storage capacity for hydrogen. Figure 13 depicts the variation in the hydrogen stored quantity during the year. This figure is the core figure of this analysis because it is directly associated with the sizing of the system. After an iteration process, it was determined that the maximum storage  
Figure 11: Winter Day: Variation in the electricity demand/production and hydrogen demand/production. Figure 12: Summer Day: Variation in the electricity demand/production and hydrogen demand/production. capacity has to be 258 kg of hydrogen which corresponds to a tank of around 34 m\({}^{3}\). The PV field area has to be at 203 m\({}^{2}\) for producing the demanded quantity. More specifically, the nominal power of the photovoltaic field is about 40 kW and in total 116 panels have to be installed. Practically, the PV area was found to be approximately half of the roof area and this is an interesting conclusion that can be used as a preliminary assumption in future studies. The initial charge of the tank is selected at 150 kg of hydrogen which corresponds to a 58% charging percentage.