4) Two small horizontal-axis wind turbines have been set up on top of the building at the height of 20m to generate electricity. The diameter of the wind turbine is 4m, and the combined efficiency can reach 30% of the Betz limit. Assuming that Shenzhen has 200 windy days per year that the wind turbines can operate, each day has an average of 5 hours of valuable wind, and the average wind speed is 10m/s at the height of the turbine, calculate the total electrical energy that can be generated per year by harvesting the wind energy. (10 points)