Sometimes a VAWT Can Be Too Good To Be True

Since a VAWT seems to fascinate so many people, we will take a critical look at the claims made by several manufacturers of these turbines. This is being done to illustrate the often misleading claims made by manufacturers and marketers concerning their vertical axis wind turbines.

So how to go about this?

Rotor Loading

There is a simple calculation you can use that is a very good indicator of whether or not any particular vawt is going to perform up to the claims made by the manufacturers. To use that calculation you simply need two pieces of information about the turbine – the swept area and the rated power output.

These two pieces of information can be found on any website promoting a vawt.

There is a term, widely used in the wind industry, known as the “rotor loading”, and it refers to the ratio between the stated power rating and the swept area. Hence:-

rotor loading = stated power rating(watts)/swept area – as we stated – a simple calculation to perform.

The significance of this calculation lies in the fact that in the wind industry there is a widely used term known as the “standard rotor loading”. The figure used is 200 watts/sq.m. In general, horizontal wind turbines have rotor loadings between 150-250 watts/sq.m – there are rare exceptions that go up over 300.

Now let's look at what wind industry guru, Paul Gipe, has to say about rotor loading...

"What does a high rotor loading mean? First, it doesn't mean that the wind turbine is capable of generating more electricity than it's HAWT competitors. It simply means that the wind turbine uses a much larger generator relative to the area swept by the wind turbine. It is the swept area that is the prime determinant of how much energy a wind turbine will capture. However, a high rotor loading does suggest that a VAWT manufacturer may be overstating what the turbine is capable of. Rotor loadings of greater than than 300 W/m² should be viewed with skepticism."

So there you have our simple guideline to test out the claims made by the manufacturers and marketers of a VAWT. If you come across promotional material for a vawt, then check out the specifications and apply this test.

Well, we did just that, on a selection of vertical axis wind turbines and our results are shown in the table below.

ManufacturersTurbineSwept Area – Sq. mRated Power – WattsLoading – Watts/Sq.m
WePowerSea Hawk0.92500460
RopatecBig Star Vertical34.420,000581
Sauer EnergyWind Charger1.681500823
Quiet RevolutionQR517.056500381

Now, as you can see, except for the Windspire Standard Turbine, all of the turbines shown have rotor loadings in excess of 300 – one is almost three times greater.

Such results should immediately set your antennae twitching and push you into doing a little more homework. In other words you start checking out the specifications a bit more closely.

So let's do that for the Wind Charger Vertical Axis Wind Turbine (opens in a new window) the vawt with the highest rotor loading.

Sauer Energy has a site that is devoted to promoting a Savonius style vertical axis wind turbine, known as the Wind Charger, due to go into production in 2012. There are quite a number of claims made on this site that simply don't stand up to serious scrutiny.

So let's work our way through a few of them...

Technical Language

Be suspicious of "techno-babble" - by that, we mean the claims for the VAWT performance expressed in flowery language designed to make you think you are dealing with a technological breakthrough. For example, Sauer Energy writes:

”It is designed with a concave/convex angle curvature for a precision fix to create a vortex for maximum torque. Blade configuration will be used to create higher rpm using a 3-blade model for different wind speeds. Gravitational stresses are exactly even. Based on the Savonius, vertical axis wind turbine (VAWT) design, the lift and drag co-efficiencies are usually normalized by the dynamic pressure of the direct air attack.” 

The most important thing is the area swept by the blade – that is the greatest, single determining factor when it comes to generating power.

Promoted As A Rooftop Turbine

"Sauer Energy develops and produces wind turbine systems that are roof mountable on homes or small buildings... and...the vertical axis design is a compact turbine design that can be sited on location without being intrusive and may be integrated directly into existing building structures."

Be immediately wary of any such promotion as mounting any turbine on a rooftop is not recommended if you want to generate any significant amount of power. For more information on this please visit our Rooftop Wind Power page and find out why.

Accreditation Is Pending

This VAWT is promoted as one which is likely to receive official accreditation from the Small Wind Certification Council in the USA.

"In the first quarter of 2011, we expect to submit our design to the American Wind Energy Association’s (AWEA) Small Wind Certification Council, or SWCC. The SWCC will begin testing to certify that our turbine meets industry standards of safety, durability, vibration, noise, performance, power, longevity and more. SWCC will also test to validate that our turbine meets its power and performance specifications."

At the time of writing in early 2012, a visit to the SWCC Certified Page site showed that no such accreditation had been received for the Wind Charger vawt. Furthermore, the SWCC keeps a current list of all the companies who have applied to start the accreditation process and a visit to the SWCC Applicants page shows that the Sauer Energy vawt is not on that list. In other words, one year after the stated intention above, they have still not applied for accreditation.

Performance Data That Doesn't Measure Up

The following data is taken from the Sauer Energy site, outlining the key specifications for their Wind Charger vawt

Blade size: 1.22m x 1.10m

Cut-in wind speed - 2.2m/sec

Rated power at wind speed 11.1m/sec = 1500 watts

Swept Area 1.68 sq m (this is by no means clear as you can see from the site specifications which refers to a “3 blade radius of 5.5ft (1.68m) Swept Area”. However, 1.68 is what we will use.

The claim for a "cut-in" wind speed of 5mph (2.2m/sec) seems highly unlikely when compared to the usual “cut-in” speeds for other turbines. But let's give them the benefit of the doubt.

When we examine other performance data things begin to look suspect indeed.

Performance at cut-in wind speed

The power curve for the Sauer Energy Wind Charger turbine shows it producing 100 watts of power at 2.2m/sec and 1500 watts at the rated speed of 11.1m/sec. If you apply the formula about how much power is in the wind, then the following data emerges, using a swept area of 1.68m2

Note: if you are uncertain about the formula, please see our Wind Power Facts page for a full explanation.

At 2.2m/sec the available power in the wind is 11watts. Therefore, according to the law of physics, the maximum power you can extract will be 59.3%, leading to 11*0.593 = 6.5 watts. In practice though you will get far less than that and throughout this critique I am going to use an efficiency factor of 0.2. On that basis the power generated at wind speed 2.2m/sec is 2.2 watts. At that point you would be drawing more power from the grid to operate the inverter than the turbine was producing.

Performance at rated wind speed

At 11.1m/sec the available power in the wind is 1407 watts & according to the same law the maximum power that can be extracted is 1407*0.593 = 834.5 watts. And the more likely result (0.20) will be 281 watts.

Sauer Energy rates the turbine at 1500 watts at 11.1m/sec. As you can see 281 watts is a far cry from the rated figure which is higher than the actual power in the wind at that speed for that swept area. In other words the stated output at 11.1m/sec is an absolute impossibility!

In practice the data at an annual average wind speed of 5m/sec is what one should look at, as it represents a likely scenario in large parts of the USA and elsewhere. If we do that then the power in the wind is 129 watts. Applying our efficiency rating then the power generated is about 26 watts - hardly an encouraging result!

Annual Energy Production

This factor is really important for any prospective buyer of any type of wind turbine. After all, you are trying to eliminate or greatly reduce your electricity bill. . So if we take our likely output of 26 watts at 5m/sec we would have an annual output of 228 Kw-hrs.

However, in order to obtain a good handle on annual production we need to factor in what is known a Weibull distribution which calculates output according to how often the wind blows at different wind speeds. The Sauer Energy site offers no data about the cut-out wind speed which is necessary for the calculation but using an approximation, it is thought that the annual production would be in the region of 580 Kw-hrs.

Now let's look at what Sauer Energy has to say about output...

"It is designed to provide at least one quarter of an average home’s electricity, economically... and...we are projecting that, in a windy area, the Sauer Energy Turbine can produce about 25 to 50 percent of the electricity for an average home."

Let's ignore the bit about a “windy area”, whatever that means, and stick with 25%. The average household in North America uses about 12,000 Kw-hrs per year. Therefore in order to generate 25% the turbine would need to produce 3,000 Kw-hrs. As you can see from the above calculation, it churns out only 350 Kw-hrs – a long way removed from 3,000 Kw-hrs. Even if your power usage was one half of the overall average, it would only amount to 580/6,000*100 = 9.7%.

Return On Investment

By now you are probably expecting that the claims made here will be suspect as well. And you wouldn't be wrong.

“We are targeting the Wind Charger to retail for about $7,000, not including options or permits, if needed. Federal incentives equal 30% of the installed price of the kit, in the form of a tax rebate. Each turbine kit you purchase and install is eligible for these discounts and rebates. Incentives and rebates may vary in your area. They can come from your state, city or utility company... and...Depending on configuration and output, our turbines are estimated to be retail priced between $6,000 and $15,000. With average wind usage, our preliminary test results indicate an approximate 24-month payback to the consumer including the various federal and other rebates and energy tax credits.”

So we have a $7,000 turbine plus installation costs plus any options we may need, like a tower, if not putting it on the roof, which would not be advisable. Let's be conservative and say $12,000 all up.

If you are only generating 580 Kw-hrs of electricity at say, 12 cents/Kw-hr, then you are only reducing your energy bill by $70.00 per year. Even allowing for tax credits, you can see that a payback in two years is a nonsense.

Anyone tempted into taking seriously the claims made by Sauer Energy for their vawt has only themselves to blame.

As with all things requiring a substantial investment – do your homework!

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