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.

Most of the East and West coast is at or below 8mph sustained

Issue #2 – Height

The other major assumption most wind turbine retailers make is that you put this turbine high in the air (smoother air and more constant wind speed) with no obstructions (65ft in the case of MW&S). I can tell you from my ham radio background that a 65ft tower built to withstand 100mph+ winds is not cheap. You also need a high power rectifier and diversion load (or brake/clutch on the turbine) for when your batteries are full and the wind is blowing, since, without some kind of load at full wind speed, the turbine would experience an unscheduled rapid disassembly (which is to say go…BOOM) from the resulting excessive speeds (fun video of that). These extras, in turn, up the price (still not including the cost of the tower, needed footings, or guy wires).

Issue #3 – Area

The worlds largest turbine ranked by area is the Samsung 7.0-171, with a whopping 171m (561ft) diameter, equating to a 22,966m2 swept area, this thing could theoretically crank out almost half a megawatt in 8mph winds (not sure what it’s cut in speed is though). While having this gargantuan whirly gig in your backyard might make the neighbors blush, it underscores just how much of an impact the catchment area of a turbine has on captured power. The astute amongst you may also realize that you would need to put the turbine on a tower at least 85m (278ft) tall, to prevent the blade tips from chipping away at the dirt underneath, this, of course, brings us right back to issue #2. But why not just get a bigger turbine! I can hear you say. Well, as an exercise, we can calculate that we would need a catchment area of 130m2 or roughly a 12m (40ft) diameter turbine to collect 2kW at 4m/s (8mph), easy enough you say, but think of the cost of building 6m blades and attaching them to a generator at 30m up in the air, the answer is that it is far from trivial for the average DIYer. Even if we assume you live in a very windy part of Texas with an average wind speed of 7.5m/s, you would still need a 5m turbine to collect 2kW, again placed 30m up in the air. At that point, you might be approaching a reasonable power v.s scale level, but the capital costs to get there are likely to be astronomical on a $/Watt basis compared to solar.

As all these extra gotchas start to add up, they make what seemed like a cheap way to generate electricity at first, an expensive construction project with ongoing maintenance costs on second thought. So in short, if you want an expensive hobby project they are great, but for generating power on the single home scale, cheaply, reliably and year-round, my advice still stands, getting a few solar panels is far and away easier and cheaper, there are also no moving parts to worry about.

To learn more about how to design and size your own solar array, the right way, check out my course below, where I cover all the topics you need to know in thorough detail, to ensure you aren’t stuck in the dark when you need power most (like winter or cloudy days).

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