Vertical Axis Wind Turbines - A case study (Windspire)

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VAWT – The good, the bad, and the Windspire Small Wind Turbine.

I came across this unit recently, and it caught my interest. It would appear that it solves a lot of problems related to small-scale wind power. It is aesthetically “pleasing”, easy to install, self contained, and claims to produce a significant amount of power. Unfortunately, like many of its peers, it does not live up to the promise. I still like the Windspire. I hope they resolve some of the problems. It is a neat concept, and a company that appears to want to make it a success. They have numerous installations already, and I wish them the best of luck. However, I am going to take the opportunity to point out some of the challenges that the Windpsire, and most other VAWT designs, encounter. This is not an indictment of the technology, nor of this particular example. It is, just that, an example to be used for education. I believe strongly in large scale wind power. I do not think small scale is ready for prime time. However, by addressing some of the below issues, it can possibly move closer to reality.

The Windspire  HAWT from Mariah Power.
 


Unfortunately, the Windspire website is devoid of much actual technical information – at least that I can find. Neither can I find much information from their installed base. However, I did find that the NREL (National Renewable Energy Lab) tested the Windspire, and produced some useable data. I want to point out, this data is preliminary, done with an early example. If you look at the report, Windspire answered the NREL findings, and described the ways they are addressing some of the findings. Where that process is right now, I do not know.


So, first, the Windspire description. A VAWT (Vertical Axis Wind Turbine) with a maximum output of 1.2 kilowatts. It is capable of direct connection to the grid, and making use of net metering. The reported minimum wind speed is 8 Mph (Defined as a class 3 area). Best I can tell, the purchase and installation is in the range of $6,000 to $10,000.  This price range puts it on a par with the cost of installed Small scale (residential) Solar PV (PhotoVoltaics), albeit before any of the tax credits or incentives Solar is eligible for. I do not know how those credits would apply to Wind.

Here are the NREL testing results: Be sure to read the response letter from Mariah Power in the first link. It is apparent they took the results seriously, and they were proactive in addressing them.

Overview: http://www.nrel.gov/wind/smallwind/pdfs/mariah_report.pdf
Performance: http://www.nrel.gov/wind/smallwind/pdfs/mariah_power_performance_test_report.pdf
Main Testing page: http://www.nrel.gov/wind/smallwind/independent_testing.html


Wind Speed.

Alas, a problem that seems endemic to all small wind turbines, be they Vertical or Horizontal Axis,  is the need for speed – wind speed. Do not believe output claims. The typical wind turbine, in the typical wind, will put out but a fraction of the amount of electricity it claims to. While the NREL results with this particular model are telling, the story they tell can be applied to most, if not all.

Below is the output data  for the Windspire from their web site. Note that it does not produce any usable power at less than 8 mph – it’s stated cut-in speed. At 12 mph it is still producing less than 100 watts (1/12 rated power). It reaches half it’s rated power output, 600 watts in a twenty MPH wind, and does not reach full specified output (1200 watts) until the wind is “howling” at 24 Mph.

http://www.mariahpower.com/testing.aspx

Now, as I said, this seems to be true of most wind turbines, and especially of the VAWT designs I have seen. They need a lot of wind to produce usable amounts of electricity. By perusing average wind speed charts, it can be seen that this high cut-in speed limits the viable areas to a fraction of the US, mostly in the western plains and coastal areas.  Operating at half it’s output rating (20 MPH average winds) it would need to be in a class 5 area on the below chart. As well, the wind is intermittent. This is the average wind speed. Another thing we have to take into consideration is the capacity factor – what percentage of capacity is actually produced. All power sources have a capacity factor. Below I have provided a link to an explanation as related to wind. For a wind turbine of either design, the capacity factor is about 30%. The machine will produce about 30% of what is the calculated maximum. For the above situation, 20mph average, the annual output would be 5,256KWh per year times 30%, or 1,576 kWh per year . About 13% of the average annual US household electricity usage. (Per EIA Data). That is not bad, if you live where the wind blows! At the average electric cost of 11 cents per kWh, that would save you $174.00 per year. That makes the break-even point (at $6000 installed cost) at about 32 years. Oops, didn’t mean to be negative!



Here is a chart of the average annual windspeeds throughout the US.

http://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap.pdf

If you ever wondered, and you have a flagpole nearby, you can somewhat estimate the strength of the wind. Note that on the Beaufort scale used in the below link, you would need a level 3 wind (Flag flying almost straight out) before any of these small turbines would even start to produce usable power. And, at even half power, the  poor flag would be “stiff as a board”. What is YOUR flag doing?

http://www.redwitch.com/extras/flag_wind_speed.aspx


And, here is that explanation of capacity factor I promised.

http://www.ceere.org/rerl/about_wind/RERL_Fact_Sheet_2a_Capacity_Factor.pdf
http://en.wikipedia.org/wiki/Capacity_factor


This link will take you to the specifications of a popular HAWT – The Skystream. It is twice the “size” of the Windspire. Most residential wind turbines are similar.

http://www.talcoelectronics.com/wind-manuals/skystream-specs.pdf

Here is a classroom project that contains power curves for several popular small wind turbines

http://www.kidwind.org/PDFs/LESSON_windpowercurves.pdf


Finally, here is a very interesting, albeit older, article about small wind turbine outputs. The author undertook to test a number of turbines with some revealing, if not unexpected, results.

http://www.wind-works.org/articles/PowerCurves.html




I also want to address Fatigue

A VAWT endures a lot of stress. The bad kind of stress. Repeating and reversing stress. Much more than a HAWT.  Many of the problems the Windspire encountered at NREL were due to poor stress management – in the turbine design, not necessarily in the creators.

 In a horizontal (propeller) turbine, the force of the wind is always from one direction, and pushes the blades in one direction. This stress is transferred via a thrust bearing directly into the tower structure. Propeller blades are a mature technology, and it is well understood how to make them withstand this type of stress. The direct, compressive nature of the force transfer to the support structure is also well understood, and  easy to implement. HAWT’s themselves do not often suffer failures from imposed wind loading.

In a VAWT, on the other hand, in every revolution of the blade structure the wind load reverses. First the blade is pushed one way, then the other. This creates fatigue. If you have ever bent a coat hanger back and forth until it snaps in half, you understand the process. This is not an easy thing to overcome. Some have tried – unsuccessfully – to address this with complex mechanical linkages. Complex and mechanical are not two words you want associated with something that is supposed to have a long lifetime. I want to touch briefly on the effects of these stresses, and the failure modes. While this is based on the NREL experience with the windspire, the principles apply to all.

As any structural engineer knows, you do not try to resist these forces, you absorb them by moving in a carefully defined way, and by spreading those forces out over as much of an area as you can. Trying to create a structure that will physically, with brute strength, resist these loads results in an incredibly massive structure. The bridge or building will collapse under it’s own weight, and the airplane would never get off the ground, let alone carry anything else. A wind turbine would be too heavy to move, and too massive to support (and cost even more of a fortune). It is much more practical, and in most cases necessary,  to go with the flow, than to fight against it.

If you take that same coat hanger we mentioned above, put your hands at the very, outer, ends of the hanger, and try to break it, you will find the task much more difficult, if not impossible. The stress you are putting on the hanger is no longer concentrated, it is spread out over a large area – the span of the wire.

It would appear the designers of the Windspire forgot this principle. To their credit putting it in practice is a tough job. Their test article at the NREL met with a number of fatigue failures. Interestingly enough, the primary place that these failures were concentrated in was the welded joints.

Now, about, welding. The joints on a VAWT have to be strong, and light. A welded joint does not lend itself well to this. If you look at bridges, large buildings, and Airplanes, you will find they are bolted (or riveted) together. Indeed, all of those structures are designed by people who know a lot about stress and repetitive reversing fatigue– especially airplanes. They learned long ago that you bolt the joints together. Rather than concentrating the stress in one, weak, spot, a bolted joint distributes that stress equally over the entire area. In effect, a bolted joint “gives” a little.

In their answer to the NREL test, it would appear the Windspire folks have learned that lesson, finally. The replaced a number of the failed welded joints with bolted ones. Hopefully this will eradicate some of the problems. I hope they also learned you need more than one engineering discipline to design something like this… Airfoils, electrical, mechanical, AND structural.

The other major source of stress in a VAWT is literally at the bottom. All of the bending loads that are imposed on the rotor are transferred to the bearing at the bottom. A critical, single, point that needs to not only resist all of these forces, but needs to turn freely at the same time. This is one of the major obstacles to engineering a VAWT, and one that especially challenges homebuilders.

Here is an interesting “PowerPoint” presentation on Wind Turbine design from Cornell.

http://cfd.mae.cornell.edu/~caughey/WindPower_09/Presentations/Lyons.pdf


Electronics

Finally, I truly do not understand why the Windspire suffered so many electrical problems, mainly with the inverter. Inverters are a mature technology. Many millions are happily and quietly puttering along converting DC Power to AC.  My guess would be that either they need another electrical engineer on the team, or that they tried to cut corners by making the inverter components just sufficient to do the job – in theory. This is not good engineering or good marketing, practice, especially in something so critical to the success. The cost of properly over-sizing the electrical components is very small compared to the cost of the unit, but the failure of these components exacts a high cost in their reputation. Hopefully a re-design will put these issues to rest.


Well, from the length of this post, It would appear I have overstayed my welcome. These are but a few of the challenges facing wind turbines. I will address others (and perhaps the same ones) from time to time. Eventually we will get there. Perseverance, patience and a commitment to putting one foot in front of the other. For now, I will maintain my belief that our best path leads to Large-Scale Wind power, and small scale Solar. But, who knows. I wish Mariah Power the best of luck, I will be watching.

Comments

[wind generator]

A vertical turbine picks up wind from all directions. Magnetic suspension minimises frictional drag. Large mass maximises the flywheel effect.

[Mark]

I like the way you write, and you have answered some of my (limited experience) technical questions. I have learned not to get too excited about the next new sexy project. I couldn't find any technical testing of this wind = power machine. I couldn't understand the braking element at higher wind speeds, now I know that they don't build them tough enough to take it. Any inverter problem is easily fixed by installing the correct eqipment. I would rather mix and match my green components. Wind, good wind, to generator, to controller, to batteries to inverter, and maybe to the grid. I like the quietness , the opportunity for power when the sun isn't shining. Sounds great if you live on an island or mountain top. But 32 years payback??? I guess it's still about philosophy and freedom trumping economics if you really want one of these things.

[Michael Herr]

I've been waiting 4 months for WIndspire to "fix" my inverter. Their warranty work leaves something to be desired.

[Electrical Components]

I don't think Wind power is mature enough as a sustainable renewable type energy to be used at home yet... I personally think that Solar panel is much more practical. I don't have a full on Solar Panel system yet, but uses it to heat my pool's water. =)

[Allan Wolpowitz]

Windspire did not deliver my 2 turbines for more than 6 months. The testing by NREL caused them to have to redisign the invertes. Now they being made by Mastech Energy in MI. I paid for them nearly a year ago. I have the frames now (finally), but no inverters so they are lying on the ground waiting after a year to be installed. I have no idea when the inverters are going to be delivered. They just give me the run around. Ithink that they are in serious trouble (and so am I). They talk the talk, they take the money, but they do not walk the walk. There is a lot of advertizing hype on their website - I am not sure if I believe any of it anymore.

[Tom Dodge]

I too have had my frustrations with a unit purchased from Mariah Power in July 2009. At a speed of 65 or so RPM the unit apparently enters a period of harmonic vibration causing the top of the turbine to oscillate, describing a radius of a foot or more -- essentially "wobbling". It is so bad I feared the welds at the bottom where the base is welded to the "hinge plate" would break.

Windspire has promised to replace the unit, having extensively reworked the design somewhat. To prevent the unit tearing itself from its base in moderate winds I have removed the airfoils and am still awaiting (after more than a year) the promised replacement.

I understand the company is being reorganized with the acquisition of a new CEO in mind, but is presently being run by a man named James Horn -- head of a management committee. I think the idea is great but the unit still has a way to go, or those of us seeking problem resolution have a time to wait.

I am informed the local distributor in Maine having shipped all his units back for rework last summer is just now getting new inventory. Perhaps I too will eventually get a replacement.

[Mike Muchard]

My Windspire was installed in November 2010. In March 2011 the unit "died".

The behavior is as follows: The windspire rotates at the slow breaking rate, after about 5 minutes the break releases for several seconds and the windspire begins to rev up slightly then the break engages again. This precess repeats continuously during both high wind periods and low wind periods. Also, there is no wifi connection available from the unit - the modem can not connect to the windspire.

I have tried cycling the circuit breaker with no effect.

I have checked the power feed to/from the windspire and it is OK (measured at the outside disconnect switch).

Has anyone else had this problem?

[zey]

has anyone checked out the WEPower Falcon? If so, any comments?

[Rob]

Thanks for the article, and the comments. I have been interested in this technolog as i have a house off the grid, but the high cost, design issues, and warranty problems described here have curbed my enthusiasm greatly. ...r

[Home wind turbine kits]

excellent post!

I agree that in their rush to market, many products have gone to market prematurely.

I disagree though that small wind isn't the ticket long-term. I think we have to make it work - there's just too much roof real estate available not to use it to harvest the wind there.

[Denise Justus]

We had a Windspire installed in September 2009. The inverter has been replaced 3 times and we're scheduled for a 4th. The unit has from the beginning slammed on the brakes in high wind and low wind. Contact with the Windspire was lost early this year. We were told in April that the unit needs an overhaul. It is now late August and the silence from Windspire is deafening. In the meantime, we have a 30 foot lawn ornament that the birds are roosting on.

[yashuen]

Location is critical for maximizing the electricity wind turbines can produce. The amount of kinetic energy available in the wind is a cubic function of wind speed—that is, for every doubling of wind speed, there is a corresponding eight-fold increase in available energy. This exponential relationship between wind speed and wind energy makes location extremely important. A site with high average wind speeds can provide considerably more wind energy than a site with only slightly lower wind speeds.

[Marthy]

I can never understand the logic for VAWTs. Yes a HAWT will stugrgle in this location but that does not make a VAWT a good choice. The wind is lacking, and the result will be disappointment whatever choice of blade configuration. Wind is free in a sense but those who know the wind are aware that you need a tall tower to reach it and that it comes at a price in terms of investment, so the energy you get is never actually free. In this case I would confidently predict that the energy would turn out very expensive indeed.

[wachoon]

Bachelor of Science in Mechanical EngineeringMasters or Dr is even betterI am a reclntey graduated engineer and am doing wind turbine research with cfd (computational fluid dynamics) software.The degree wont guarantee anything (the world doesn't work like that). But it will give you an excellent understanding for all aspects of wind power. A BS in mechanical engineering is a well respected degree nationwide(renewable energy is currently not).I went to Boise State University, but any Abet accredited university will teach you well.Best of luck to you!

[Kevin]

There is no motor in those things to make them spin when the wind isn't that makes no sense and deefats the purpose. Each wind site has a certain wind rating and average power is estimated to determine the energy and financial benefits of putting them up in that location. There are windy seasons and low wind seasons, but the average wind and estimated benefits give each project a justification. When the wind doesn't blow, they don't spin. Would you rather have coal stacks?

[Gerdes]

First of all i was talking about pniayg back the energy to manufacture and operate a wind farm (this video suggests that wind power relies on fossil fuels). You got it all wrong dude. It has nothing to do with them being competitive with coal and gas plants under current market conditions because at the moment they are not. In the long term though fuel prices can only go up (by their very nature) and wind power can only become cheaper from economies of scale (check Enercon E-126).

[Can]

This is a ludicrous amerugnt. Let's not take into account that a turbine's average life span is at least 25 years . Sure the initial construction and installation requires *some* fossil fuels, but are you really trying to argue that negates 25 years of clean energy production? Oh no! They use fossil fuels to design, produce and transport hybrid and electric vehicles. They probably should be scrapped too since there is no net savings in fossil fuel consumption.I call shenanigans.

[Jose]

I don't know any wind power proponents who claim wind is the only socure of power we need. However, I have looked at the numbers pretty carefully and I'm impressed by the potential of wind. I'd give you references but YouTube doesn't accept links in these replies. I don't have a huge problem with more nukes, it's just that a nuke plant takes ten years to build, compared to a few months for a wind farm. Wind farms can expand incrementally and that's important.

[Michael]

This is ludicrous. Those are short term costs, we don't imrpot the same equiptment everyday. Once the turbines are here, they're here to stay obviously we're not moving them back and forth. If they want to make that arguement, the rebuttal's child's play, fossil fuels=fossil fuels (same transport methods..).The wind turbines aren't run on fossil fuels this clip is an insult to our intelligence. How stupid do they think we are? Wind turbines are an investment.

[Sawssen]

It depends on your detinifion of large. The largest of the wind turbines have a capacity of around 5 MW in ideal conditions. The average home consumes about 1000 watts of power (averaged over a 24 hour period), so one of these monsters could, in theory, power 5000 homes.As David has correctly pointed out, the problem is that wind does not always blow at the optimal speed, and residential power consumption is not linear. Peak demand (between 7 am and 9 am, and again at 5 pm to 9 pm) rarely coincides with maximum generating capacity. The next problem is that surplus capacity is almost impossible to store.David even at 1 MW, that's enough to power 1000 homes not 100.

[Mikki Kattilakoski]

Wind turbine now a days is really a great source of electricity and most of the study they are having difficulties by a lot of consequences in Finland country it is also source of electricity and now a days its been successfully manage.

[Mike Hussy]

I don't think Wind power is mature enough as a sustainable renewable type energy to be used at home yet... I personally think that Solar panel is much more practical. I don't have a full on Solar Panel system yet, but uses it to heat my pool's water. =)

emergency warning lights

[Bruce BecVar]

Change Wind Corporation has a new game changing helical wind turbine. Their CWC 35kW VAWT is under 42 ft tall and generates more than 400% more power than the competition. Starts making electricity in just a 6 mph breeze, has friction mitigation, and torque multiplication that drives a proprietary patented low-speed high-energy density generator and can capture inertial inputs that generally go unnoticed by other machines. They have a turbine operating in Pennsylvania that has produced over 101,000 kWh's each year (average 8 mph wind) for the last five years. See prudenciapower.com.