# Performance Analysis of a Zero-Energy Building Using Photovoltaics and Hydrogen Storage
## 3 Results and Discussion
### Performance Analysis of the Total Configuration
It is important to state that the charging effect at the beginning and at the end of the year is approximately the same; the fact indicates that the final solution is a converged one. The minimum stored quantity is observed on 27 March and it corresponds to 2.6% of the maximum stored quantity. This small percentage creates safety in the system operation, and it is important that it exists. On the other hand, the maximum charge (100%) was found at 21 of October. In the last part of the present investigation, the summary of the obtained results is included in Table 3. This table includes the load energy quantities, the electrical demands, the electrical production, as well as the electrical energy quantities associated with the electrolyzer and the fuel cell. The specific heating thermal loads of the building per floor area are found at 24.44 kWh/m\({}^{2}\), while for cooling at 18.09 kWh/m\({}^{2}\). The thermal load of the DHW was found at 8.63 kWh/m\({}^{2}\), while the electrical demand for appliances & lighting was at 39.42 kWh/m\({}^{2}\). The global electrical demand for covering all the needs was found at 59.14 kWh/m\({}^{2}\). At this point, it is critical to highlight that the distribution of electrical needs is found at 10.3% for heating, 7.6% for cooling, 15.4% for DHW, and 66.7% for appliances and lighting. It is important to state that the appliances and lighting demand are the major contributors to the total electricity demand of the examined building. The PV performance is found at 18.03% which is a satisfying value. It is remarkable to state that the PV produces 63,637 kWh and only 11,062 kWh are directly absorbed by the building, which is about 17.4% of the produced energy. The other remaining quantity (82.6%) feeds the electrolyzer. The fuel cell produces 12,594 kWh and practically the global efficiency of the electrolyzer-fuel cell is about 24%, which is a relatively low value. This fact indicates the need to enhance the performance of these devices for creating a system with a higher conversion ratio and lower size. Regarding the building loads, the PV directly covers 46.8% of the building loads, while 53.2% is covered by the fuel cell. Figure 13: Variation in the stored hydrogen quantity during the year. At this point, it is important to make a preliminary economic evaluation of the present system. Assuming typical specific costs for the devices and electricity cost at 0.3 \(\upxi\)/kWh [33], the simple payback period is close to 20 years which is a relatively high value. More specifically, this value has been calculated by assuming a specific cost for PV at 1000 \(\upxi\)/kW [39], for electrolyzer at 1500 \(\upxi\)/kW [39], for fuel cell at 2000 \(\upxi\)/kW [39], and for hydrogen storage at 250 \(\upxi\)/kg (mean value of recent trends) [40, 41]. These costs will be reduced in the future and a lot of research is being conducted for reducing the costs of electrolyzers, fuel cells, and storage tanks. In addition, there is the possibility of storing the hydrogen in alternative ways (e.g., cavern, if it is possible) and in this case significantly reducing the cost, especially in great-scale applications. Moreover, the increasing trend of the electricity will lead to reduced payback periods and an optimistic scenario with a 20% reduction in the cost and an increase in the electricity price at 0.4 \(\upxi\)/kWh would lead to a simple payback period of around 12 years. Moreover, it is useful to compare the present system with hydrogen storage with the alternative choice of using batteries. Hydrogen presents some advantages as high energy storage density and the lack of energy loss during the long storage period. Moreover, the batteries present degradation issues and they need replacement after some years. So, there are advantages to the suggested storage technology with hydrogen. In the future, it would be very interesting to investigate a system with both hydrogen and battery storage with proper economic optimization for minimizing the overall system cost.