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.4.2. Power output at different wind speedAccording to the power curve and the mounting height of thewind turbines, the power of the eight 50 W turbines and the400 W turbine can be got. The curve in Fig. 8a showed the poweroutput of the two wind energy systems at different wind speeds.On one hand, when the wind speed is lower than 7.2 m/s, theeight 50 W wind turbines can produce more power than the 400 W wind turbine. The reason may be that the small windturbine has lower starting wind speed and produces more powerin low wind speed range. Seen from the power curve of the twokinds of wind turbines, the 50 W wind turbine can generateelectricity at 2.5 m/s, while the 400w wind turbine generateselectricity only when wind speed is higher than 3 m/s. On theother hand, when the wind speed is higher than 7.2 m/s, the resultis reverse. The 400 W wind turbine is a conventional product,whose blade is designed by optimization. The 50 W wind turbineis unconventional and its blade is made from the 100 W wind turbine by cutting part of the blade apex off. So the aerodynamicloss of the 50 W wind turbine is bigger than that of the 400 Wwind turbine at the higher wind speed, which leads to more en-ergy loss for the 50 W wind turbine.Fig. 8b shows the power output of the eight 50 W turbines andthe 400Wturbine under naturalwind. The general tendency is thatthe power output of the eight 50 W turbines is higher than that of400 W turbine. During the two testing days, the power output ofthe eight 50 W turbines and the 400 W wind turbine is 3100 Whand 2850 Wh, respectively. The power output of multi-turbinewind system is 8.7% higher than that of the reference system. Butthe gap is slight, which is due to the lower natural wind speed inthe test and the power coefficient difference between the two typesof wind turbine.4.3. The state of charge of the two hybrid systemsThe state of charge (SOC) of the battery is an importantparameter to reveal the stability of the hybrid system. In thesimulation process, the initial SOC value of both systems are set as30% at 1 o'clock (the simulation time), and the charge efficiency isset as 0.7. SOC of the two hybrid systems are shown in Fig. 9. Seenfrom the figure, SOC of the new hybrid system is slightly higherthan that of the reference system.At the simulation time of 48 h, the SOC of the reference systemislower than 30%, and no power will be supplied to the load in orderto protect the battery fromover discharge. However, the SOC of thenew hybrid system is slightly higher than 30%, which means thenewsystemcan improve the stability. At the simulation time of 18 hand 66 h, the SOC of the new system is higher than 100%, whichmeans the battery is fully charged, and the energy produced bywind turbines and PV modules will be unloaded.
4.4. Performance under different weather conditionsIn order to investigate the performance of the new system atdifferent areas, three different cities, Guangzhou(south), Shang-hai(east) and Shenyang (northeast),which represent three differentweather conditions, were chosen. The irradiation and wind speeddata of the three cities are from the weather library of the TRNSYSsoftware.Fig. 10a and b shows the total irradiation and the monthlyaverage wind speed of the three cities in different month. It can beseen that Shenyang has the highest average wind speed and thepoorest irradiation, while Guangzhou has the highest irradiationand the lowest average wind speed among the three cities.The simulated total power output of the two systems in thethree cities is shown in Fig. 11aec, respectively. The new systemharvests more power output than the reference system and theincrement is attributed to the wind energy system. Among themonths with low power output, the increment is very large, whichwill be helpful for enhancing the stability of the system.The yearly wind power output in the two systems is shown inTable 3. When the two systems used in the cities of Shenyang,Shanghai and Guangzhou in China, the wind power output of thenew system has an increase of 18.69%, 31.24% and 53.79%, respec-tively, compared with the reference system. The new system ismore suited for southern areas of China. 5.