Solar Fans vs. Wind turbines

[texan1]

Seems like we need to replace a roof after most recent storm in our (Dallas,TX) area.I'm thinking about putting impact resisting shingles and since it requires more ventilation I can't decide if I should put additional wind turbines or have a solar fan installed instead. Our electric bill is not that high during the summer so I'm not sure if there is any benefit of having a solar vent + we have plenty of wind to keep turbines rolling.

Any advice will be appreciated.

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[LouB]

Aren't wind turbines noisy? What happens to them in high winds?

Lou

[Jim Yanik]

didytch_at_yahoo_dot [e-mail] (texan1) wrote in news:49e74cd4$0$31255$ [e-mail]:

solar panels may not survive a storm;they can be broken,hail-damaged,etc. They also get dirty and output drops until cleaned.

The wind turbine follows the KISS principle.

[olddog]

I looked at solar powered vents too but I came to the conclusion they would have to be replaced periodically. I bought wind turbines and then we got hit by that hail storm in Austin. I had some roofers come out and one said he'd install a ridge vent for free, with a new roof of course. (Lon Smith Roofing, They are in Dallas too) I took that est. and called a local roofer and he said he could match it. So I took the turbines back to Homedepot.

Problem with turbines is you have to put in so many. I have a small 1700 sq ft house and the roofer told me I'd need 6 turbines.

Efficiency wise the ridge vent is supposed to be the best current technology and can be installed for about $450 to $800 for a 36 ft ridge on a single story. These are est. I got from two different roofers.

Let us know what you end up with.

od

[BobR]

Since I live in the Dallas area as well, I have to comment on your statement about having plenty of wind. YES, we have plenty of wind right NOW but not necessarily during the hottest part of the summer. The wind turbines will however work even in light or no wind conditions if you have good soffit venting as well. The heat on the roof will help to create a constant movement of air from the soffits to the peak of the roof. The wind turbines will allow that hot air to escape while pulling in air from the soffits. A solar powered fan will produce a more active vent but only when the sun shines.

I used to have an electric powered vent that came on when the temperature reached a certain point and would cut off when it cooled. The problem with them was always NOISE! They were very noisy and so the last time I had the roof replaced, I opted of the ridge vent system instead. It works on the same principle as the turbine vents and allows the hot air to excape while pulling cooler air from the soffits. If you are replacing the roof anyway, that would be the way I would go.

[trader4]

I don't see that you get any mechanical advantage out of this scheme. If there is enough wind blowing, I can see how a turbine vent can use the wind power moving by the roof to move air out of the attic. It's using the force of the wind as power. But I don't see how naturally rising hot air moving from the soffit vents and up through the turbine is doing any good beyond what just a plain similar size vent opening would give. In fact, it would seem to be worse, because there is some energy loss in friction, etc with the turbine.

In other words, in still air, I think you'd be better off with just a ridge vent.

[SteveBell]

The first time you install a wind turbine, you'll discover that the rotation doesn't actually do anything, except maybe keep rain out. The turbine isn't fastened to a fan of any kind, so the square inches of pipe is the only thing you need to consider. You need to have roughly the same number of square inches of exit for hot air at the top of the roof as you have inlet for cool air under the eaves.

The guy who recommended ridge vents has the best solution.

[BobR]

If you read my last paragraph, I stated that I went with the ridge vent as what I considered a better alternative. The design of the wind turbines is not really all that good at "pulling" air up through the attic and mostly turn in the wind. The real benefit to them is providing an opening for the hot air to escape. I think that a simple covered vent would probably work almost a good. The ridge vent provides a much more effective area across the highest point of the roof for the heat to escape.

[HeyBub]

This assertion can be debunked quite easily.

Simply stand under the turbine with a stick of incense and watch the smoke. Then stop the turbine and watch the smoke again. In the former case, the smoke is sucked out quite rapidly; in the latter case, the smoke goes nowhere.

A ridge vent - or hole in the roof - depends on a temperature difference between the air in the attic and that outside to generate convection currents.

A 12" wind turbine will move about 350 CFM of air in a 5MPH wind, irrespective of the temperature differential.

With no wind, the turbine acts like a ridge vent, dependent entirely on convection currents.

[BillGill]

My understanding is that turbine effect of wind turbines doesn't really do anything. The hot air is actually removed by the thermal siphon effect of hot air rising. That's why mostly they recommend ridge vents these days.

Also as one responder mentioned, it takes a lot of turbines to really be effective. When I had to replace one that had failed I noticed on the box that they recommended 5 turbines for a house my size. My house had 2. When I got a new roof last year I went with ridge vents.

Bill

[SteveBell]

So what's the mechanism? How does the spinning top pull air out?

[RicodJour]

Analogous to a flywheel and utilizes the Venturi effect.

R

[HeyBub]

SteveBell wrote: .

It's not a "spinning top." It's a fan.

A 12" wind turbine moves 350 CFM at a wind speed of 5mph, but it moves 1350 CFM at a wind speed of 15 mph. It should be obvious that, since its effectiveness depends on the wind speed, it's more than a hole in the roof.

Here's the way I understand it works (and I could be wrong - the whole thing may just be magic):

1. There's air in the turbine.
2. The wind spins the turbine.
3. The air in the turbine also spins is slung outward by centrifugal force. The air exits the turbine thru the vanes.
4. This exiting air creates a vacuum in the space enclosed by the turbine.
5. The vacuum, in turn, sucks air from the underside (attic).

Both ridge vents and turbines have their relative advantages and disadvantages. Turbines don't move much air when the wind doesn't blow; ridge vents don't move much air in the winter.

In a light wind, turbines move considerably more air (about as much as a robust bath exhaust fan) than a ridge vent

Both are cheap, both are passive, both trouble-free, and both use no energy.

I have ridge vents AND turbines on my house.

[LouB]

More info:

[BobR]

The same mechanism also works with the ridge vents if there is wind. The wind blowing against the roof will be forced up and over the roof resulting in a low pressure zone on the back side of the roof. This low pressure zone will draw the air up and out of the ridge vent even more effectively than the wind turbine. So the result is that ridge vents are not totally dependent on convection currents.

[SteveBell]

I saw that page in my searching, but it just crows about how it's "better", not explaining the physics.

Still searching since this is interesting.

[HeyBub]

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There are at least two exceptions.

1. If the wind is blowing parallel to the ridge vent, no suction.
2. If the wind is blowing perpendicular to the ridge vent, as much (or more) is blown INTO the ridge vent as is sucked out.

All in all, I suspect it's a push regarding wind and ridge vents.

[texan1]

texan1 had written this in response to

: Thank you all for the pointers here. I think that I will go with wind turbines for the part of the attic that has really steep roof and will do a ridge went for the the game room add-on where there is almost no space between sheet rock and the decking. It seems that pretty much every roofer that I talked to is proposing this solution. Their take was that either ridge vent or turbines will as good of a job but ridge vent had a better chances of being torn up in a storm.

tx1

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[BobR]

Do some research on the subject and you will see that is not the case. Its a rare case the the wind will be blowing directly parallel to the ridge but even if it is, it will reach some point of leaving the roof and will thus draw a low pressure. As for the perpendicular blowing as much in as is pulled out, that is not even close to the case. The low pressure on the back side will far exceed any air blown in and the air blown in will be directed directly out the other side increasing the pull on the air in the attic.

A machinist friend of mine once got a contract for producing a vacuum pump that used compressed air. It operated on what I believe they term the venturi effect. The compressed air was released at a point where it pulled additional air from the vacuum feed line. The resulting low pressure zone would pull a complete vacuum on a 55 gallon drum in a matter of seconds. It was amazing to see how effective it was.

[LouB]

I looked also. If you find a good site please post.

Lou