Answering some questions about rooftop wind

My answers to some questions:

(1) We’re looking at some of the design problems of rooftop turbines, and we had an expert installer tell us that in a best-case scenario, a rooftop turbine can generate 18 to 33 percent of a household’s power. Is that an accurate assessment?

It’s a very vague promise. The best case would be a house on a bare hilltop with nothing around it. A realistic case would be dramatically less productive. For real examples see this study
http://www.warwickwindtrials.org.uk/resources/Warwick+Wind+Trials+Final+Report+.pdf

(2) Is it true that a rooftop turbine generally creates 2 to 3 times as much noise as a tower turbine? And, if so, what can be done about this?

All wind turbines produce some noise and vibration. The amount varies, but whatever level of noise and vibration is produced, the associated problems will be much worse if it is connected with a building. Only a very massive concrete building can absorb this noise and most buildings will transmit it. The occupants will be subjected to this noise. The building may be damaged by the vibration in some cases.

(3) An installer told us that a tower turbine generally requires a consistent 10 mph wind to work. Is that true?

Most properly designed turbines are optimised for the range 3 – 12 m/s in which most of the energy will be produced. (that would be 7 – 25 mph in layman’s terms). Below 7 mph (3 m/s) the density of power is extremely low, so one would need a very large turbine working for a very long time to produce useful amounts of energy.

And by “work” do they mean to generate their maximum power capacity, or to generate power at all?

Most small wind turbines will start to produce at 6, 7 or 8 mph (around 3 m/s) but obviously the power has to start from zero. Maximum output is achieved at 20 – 30 mphs (9 -14 m/s) depending on the make but such winds are relatively rare so they do not contribute a lot to the overall energy production.

Are there any tests that indicate that tower turbines ever hit their maximum capacities?

Of course a wind turbine on a good site will hit its maximum. But the more important thing is the average power it can produce, which can also be expressed as the energy produced over the year. This basically depends on the swept area of the rotor and on the windspeed ‘regime’ that it is exposed to. Rooftop turbines are usually rather small and cannot access good winds, so they are incapable of producing the best results in terms of energy per year.

(4) What good do you think will come out of the Small Wind Certification Council’s certification program for power, sound emission and durability? Will the SWCC truly act in the consumer’s behalf, or will they accede to manufacturer demands? Will we see less models, better models?

Independent testing of wind turbines is long overdue. I hope that it will not impose overly onerous burdens on small wind manufacturers, but it can only be a good thing to obtain independent test results for performance, noise and safety. I hope we will see plenty of models and that we will also get past the hype and learn what they can do for us.

(5) I heard that WindTronics will be releasing a turbine this fall that generates power in winds as low as 2 mph. Have you seen it, and does it work?

while it is possible to generate power in such low winds, the amount of energy will be relatively pathetic. The laws of physics determine the maximum power available per unit of size in a turbine and a very small turbine with very little wind can only produce a negligible amount of energy.

Is it a big step forward in rooftop turbine designs, or is there still much to be done to make sure these things work?

I would rather describe such products as a big step backwards (a disaster even) as they will encourage unrealistic expectations in the consumer, who will later decide that wind energy is a waste of time and money, based on one bad experience.

Two follow-up questions:

* Is it accurate to say that unless your home fits the (location, wind and zoning) requirements for successful wind generation, that a residential wind turbine (tower or rooftop) won’t pay for itself completely in its average 20-year lifespan?

Yes, I would say that it is unlikely that a rooftop wind turbine will pay for itself unless the site if unusually windy, and the property is very open to the wind.

* According to National Renewable Energy Laboratory, a 1.5-kW rooftop wind turbine will meet the needs of a home that requires 300 kWh per month in a location that has an annual average wind speed of 14 mph. ————Are any of those numbers reasonable?

The numbers might add up, yes, provided you can find a house with that sort of wind at rooftop level. I have to say that such houses are very unusual indeed! And the turbine would need to have a diameter of at about ten feet.

Is a 1.5-kW turbine an average size,

The power rating doesn’t really tell you anything about the size, so it is not a good description for a wind turbine. A better description would be the rotor diameter.

and a is a 300 kWh monthly requirement typical for a family?

Yes in The UK that would be sufficient for a very small, energy efficient family. It’s a little less than a typical North American family.

Moreover, I found a chart that says that only 5 of 275 American communities averages an annual wind speed of 14 mph. So how can that “14” number be used as a realistic benchmark?

I don’t think that anyone could claim that it’s a realistic benchmark, no.

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See the full article in Consumers Digest

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2 Responses to Answering some questions about rooftop wind

  1. Chris Varrone says:

    Your frustration with house-top turbines is understandable. But let’s separate the “relatively pathetic” efforts of house-perched machines from the potential for “urban wind” generally.

    Anyone who has been on the roof of a tall building will recall being buffeted by winds. Is there really no way to practically take advantage of this?

    If not with conventional HAWTs (because of loads, turbulence, vibration, noise, and safety) — then why not with VAWTs? They should be lower load, vibration, noise than HAWTs and safer. Turbulence is less of an issue for a VAWT. The big issue may be getting them “up into the airflow” — but surely a platform or pole can do that?

    We have a LOT of high rooftops in the world – think of London, New York, China…
    Also many on the edges of towns in the midwest, lower than skyscrapers but with great winds. Can we really not design something that works for these spaces?

    • admin says:

      Hi Chris,

      It’s important to understand that wind turbines are expensive pieces of engineering. a lot of cost and energy goes into their manufacture. To repay this and make a sustainable and profitable small wind industry you need to put the turbine in a place where there is useful wind energy. And it also makes sense to use an efficient design of turbine.

      These is no evidence that VAWTs are efficient. Most are also sited in really poor locations. Tops of buildings in cities will be better than normal houses but there are still a lot of issues. Safety is not the least – parts might well fly off and fall on someone. Noise inside the building has prevented such projects from operating at night.

      Turbulence is a big part of the problem too. Yes a VAWT needs to be able to survive turbulence since it lives in a world of turbulence all the time, but we need evidence that they can even survive normal winds. Historically they usuall seem to fall apart. I think the whole project is misguided waste of resources leading to problems that the small wind industry does not need.

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