For low velocity you can go for vibro wind concept its best method. but its max power rating is 50 watts.

Hey Juan: let's be generous and give you a steady 2 m/s.  To get 500W, you'll need an effective area equivalent to a square 14 m on a side roughly, just to imagine and to estimate from the Betz limit, am I right? That's hoping it's steady. So it would be a device unlike common turbines on the market- and there's therefore a good reason they don't exist. There's little point chasing a wind resource that weak, so be careful.  

Check out the old work of Mertens, from TU Delft: and the new work from Dhiradj Djairam. They provide some thoughtful insight in the limits of urban wind and alternate non-rotating means of extracting energy from the wind, respectively.

It also depends on the condition of your intended wind site. Vertical axis turbines work better in urban area where the turbulent wind condition is often experienced.

Dear colleges, thanks for your comments and suggestions. You're right is not easy to produce high amount of electricity at low velocities. My intention is use the most efficient urban turbine. The reference of Mertens and Djairam are very helpful.

Best regards

You can also explore Tesla turbines (boundary layer wind turbines ---- bladeless turbines).

cheers