# Micro Wind Turbines – Without The Hot Air

Following up on our series of myth-busting posts we come to the second most asked question: Why don’t you have a wind turbine?

Oh, let me count the reasons why…

Part of the issue is the physics of how a wind turbine works. While they work great at the Megawatt (that’s 1 million watts) utility-scale (and even then require diligent engineering and site selection) they aren’t great for the microscale situations (short blades, low wind speeds) that most off the shelf consumer turbines are used in.

We can find out why, by looking at the equation for the theoretical power output of a turbine, which is equal to:

`Pturbine = 1/2 x rho x Aswept x 0.4 x V^3`
Where rho is the density of air at sea level, or 1.225 kg/m3, Aswept is the effective area the turbine blades present, 0.4 is a standard efficiency factor for a typical blade design (might be less without good aerodynamics). Finally, our key variable is the wind velocity (in m/s), which gets cubed. This equation leaves out any inefficiencies of the motor (85-90%) or the gear train (75-80%) as well as other parasitic losses.

## Issue #1 – Speed

If we run the numbers on a ~2m^2 swept area (about the size of the 2kW unit from MW&S), we see that we need at least 30mph to get 1kW (on a 2kW rated turbine), this needs to be constant and smooth wind velocity, not gusts (turbulent air increases aerodynamic losses).

### Example Turbine Power

0.5×1.225×2×0.4×13.4^3 = 1,178W

If we look at the graph on the MW&S product page, we see almost exactly the same thing graphically. Further, this turbine has a cut-in speed of 6mph (2.7m/s), meaning it needs winds of at least 6mph before it starts producing power. If we have a gander at some wind resource maps of the US (courtesy of NREL) we can see that, save for parts of Wyoming and the Midwest (where most utility installs are!), much of the US does not have the consistent wind speeds of the kind needed to generate significant amounts of power.