An investor is at install a total of 40 identical wind turbines in a location with an average wind speed of 7.2 m/s. The blade diameter of each turbine is 18 m and the average overall wind turbine efficiency is 33 percent. The turbines are expected to operate under these average conditions 6000 hours a year and the electricity is to be sold to local utility at a price of $0.075/kWh. If the total cost of this installation is $1,200,000, determine how long it will take for these turbines to pay for themselves. Take the density of air to be 1.18 kg/m3.