## 4 Results and Discussion  
Calculations were performed using, at first, the base case, where one wind turbine operates, and the PV arrays generate the remainder of the needed energy for the cluster of buildings. The efficiencies for the electrolysis system and fuel cells are 70% and 75%, respectively [24; 25]. The parameters for the solar cell efficiency and the temperature dependence coefficient are \(\eta_{25}=0.22\) (22%) and \(k_{SC}=0.002\)[22]. For the hydrogen storage, the maximum pressure is 50 MPa. Under these conditions, the stored energy density of hydrogen is 1.03 MWh/m\({}^{3}\)[17; 18]. It must be noted that this is not an excessive pressure for hydrogen storage, since several automobile corporations already manufacture hydrogen driven vehicles with the pressure in the fuel tanks in the range of 30-70 MPa. The calculations show that the needed area for the PV arrays to fully meet the demands of the cluster in the base case is \(A=44.410\) m\({}^{2}\) (10.2 MW rating); and the storage capacity of the microgrid is 5345 MWh. Figure 3 shows the monthly electric energy generation from the PV array and the wind turbine, the monthly energy demand of the cluster, and the maximum energy storage level for each month of the year. For the determination of the monthly values, the hourly values in the entire month were added. It was observed that the electric energy generation is significantly higher than the demand during the period January to July, and the storage level increases. The monthly energy demand in the cluster increases significantly from June through September, when the region uses a great deal of electric power for air-conditioning. The energy storage level from June to mid-October significantly drops to meet this additional demand. Based on the computational results for the case when one wind turbine is used, the hourly variation of stored energy--the storage level at every hour of the year--was plotted in Figure 4. It is observed in this figure that the stored energy level increases starting in the month of 16 October and continues increasing--albeit at different rates--throughout the months of autumn, winter, and spring. During the summer months, the storage level decreases at a fast rate, indicating the high electricity demand in the region. The minimum, non-zero energy storage level--approximately 230 MWh--is also evident in this figure. In an emergency, this minimum energy level would supply the entire demand of the microgrid for an entire week. The maximum level of storage is 5345 MWh and occurs on 28 June. This storage level corresponds to 5189 m\({}^{3}\) of hydrogen at 500 bar, or approximately 5.2 m\({}^{3}\) per household. The accumulation of stored energy from mid-October to early June is one of the reasons for the choice of hydrogen as the storage medium. After considering the base case, a parametric study was performed to determine the effects of the several parameters that influence the energy storage needed, the solar power to be produced, and the energy dissipation in the microgrid. The overall efficiency of the energy storage system, e.g., the round-trip efficiency, defined as the product of the electrolysis efficiency and the fuel cell efficiency, is one of the important parameters in the study. The latter two efficiency components vary in the ranges \(0.55<\eta_{el}<0.78\) and \(0.60<\eta_{fc}<0.85\), and this implies that the round-trip efficiency variation is in the range of 0.33 to 0.67. For the base-case with one wind turbine, Figure 5 shows the effect of the round-trip efficiency of the storage system on the area needed for the solar cells and the annually dissipated energy due to the storage-recovery processes. Figure 3: Total monthly energy production, consumption, and maximum storage level during each month of the year. It is observed in Figure 5 that a well-designed storage-recovery system with high roundtrip efficiency has a significant and beneficial effect on both the area needed for the solar cells and on the necessary storage capacity of the cluster of buildings.