Generating Electricity from Exhaust Fans

No, but why would you want to? Placing a generating turbine (or something similar) in the air exhaust path is going to waste more power in the HVAC stage than will be generated by the device (due to, among other things, increased air resistance).

You were not thinking of powering the HVAC off the exhaust air flow were you. What sort of power levels were you looking at?

What I had in mind was a separate turbine being powered by the exhaust air. The separate turbine would not be physically coupled to the exhaust fan nor would it interfere with the flow of the exhause air. It seems that some electricity could be generated from the exhaust air presently going into the atmosphere.

No, I am not thinking of powering the HVAC from the exhaust air flow. Perhaps a small amount of electricity could be fed back into the Grid.

I don't know what power levels to expect. I am wondering if someone has already done this. I tried finding that answer by searching Google. Windmills were the only results I found.

I still stand by my original post. Placing something in the air exhaust path is going to cause the HVAC stage to use more power than will be generated by the device (due to, among other things, increased air resistance). Remember there will still be an increase in resistance to airflow, it is a necessary component to make the blades of a generating element turn. It is possible to do, but I can only think of small scale applications for powering remote sensors or devices where a local supply is unavailable and other methods of remote power (solar, etc) are not suitable. As for grid connected generation, it wont work (you will use more power than gain). If you think about it, if it did work you would get a perpetual motion machine.

Oh, part of my job is (among other things) commissioning wind farm sites down here.

I agree. If it "doesn't interfere" then it doesn't do anything. It has to get energy from the original motor which will need to be made bigger to produce the same exhaust effect. If the exhaust fan is too large you will save more by simply putting in one that does not produce more airflow than is required.

As stated by jack of all trades, ANYTHING that extracts energy from the exhaust air will tend to impede its flow, in proportion to the energy extracted, and you will need a bigger, more powerful fan in the HVAC to compensate. It is a lose/lose proposition.

This is akin to mounting a small generator on a wheel of your car, or a small wind generator on its roof, to get "free" energy: your motor will only have to work harder to drive the generator, and with inefficiencies at both ends, you will get less power than the additional power expended. It is the Law of Conservation of Energy.

Having said this, it is however possible to extract a small amount of energy from the moving air downstream of the HVAC exhaust, in "free air", without impeding its flow through the heat exchanger, but it would be a negligible amount, and would not likely ever justify its installation, let alone "feed the grid". This does not violate the Law of Conservation of Energy because after the air is accelerated and passed through the heat exchanger of the HVAC, then exhausted into "free air" (unrestricted path) it still retains energy in the form of its overall momentum, and a very small amount of that energy could theoretically be extracted, however this would be very small in proportion to the energy used in the fan or blower, which is mostly expended moving the air through the heat exchanger. When the air exits, it is diffuse and also turbulent on a small scale, and this will degrade its forward movement over a short distance. Any attempt to increase the energy so obtained from it beyond the very small amount, would have to impede the airflow.

The air turbulence mentioned, caused by the movement through the heat exchanger is created by design, to promote maximum heat transfer with minimum airflow, so in a properly designed system, there is little wasted energy available for reclamation. The larger the HVAC unit the more "wasted" energy you could extract, but it will never likely amount to more than a percent or two at best. As stated, any attempt to channel or "concentrate" this airflow will impede it.

As an illustrative example, see how fast the velocity of the exhaust air drops off with distance, in comparison to the same size fan or blower moving air without passing it through a heat exchanger.

As I think Jack of all trades is right - if you have an efficiency, p.example, of 10%, you remove ten times the energy you get from the airstream the exaust fan has to apply.

Your error in reasoning is that "the separate turbine would not be physically coupled" - but in reality it is, the air stream is a physical coupling, otherwise you would get no electrical energy.
Regards Uwe

We are also looking to develope or if anyone has any ideas on this technology it would be great. The unit would need to be able to produce 420 volts to make it really attractive.

If it is placed in the exhaust vent and does not restrict to much air flow I think this would work well.

Please see other comments regarding why it is not a good idea to try and place a generator in series with the exhaust of a process. Remember also that a generator is an active device, the more current you try and draw from it, the more effort is required to turn the blades.

The only real use I can see is if you needed a small remotely-located power supply and you weren't concerned about the increased HVAC load (ie- where it was too difficult or expensive to run a power cable too). I cannot think of any real world applications off the top of my head thou.

Possibly we may think of a turbo generator that works on this air from exhaust fan.

And it produces 'X kw'.

But, since an obstruction on the free discharge of the exhaust fan, the excess power consumption by this fan could be 'X+ kw'.

M.Alphan Namlı

No, I've never seen this, although it isn't a bad idea, especially for large fans that run all the time. Initial cost and payback would be something to look at to see if it is feasible. You might find that it may not be, due to the increased motor load for this process.

Nimrod was a great biblical hunter who tried to kill god from the tower of Babel. Me thinks ye is trying the same! Nothing is for free...an over simplication of a law of physics. As soon as you put anything into the air stream/column of an exhaust fan, you will increase the static pressure of that air column forcing the motor to work harder increasing the electrical power consumption of the exhaust fan to maintain the same performance level "which it wont!"

Hence the electricity to operate the exh. fan will be increased and cost you more to produce then the savings you can generate.

Simply put once again: There ain't no free lunch!

Where's my that was E-Z button!

How is it that only Greg understands where I wanted to install a turbine? I did not suggest installing a turbine in a duct, where it would restrict the flow of exhaust air, nor to the exhaust fan blade, where it would require the exhaust fan motor to work more.

who knows:

"As soon as you put anything into the air stream/column of an exhaust fan, you will increase the static pressure of that air column forcing the motor to work harder"

That is absolutely true if we are talking about a ducted system or a case where the airflow through the heat exchanger is in ANY way obstructed. However, in FREE AIR, a short distance away from the exhaust outlet you can in theory (and fact) extract a very small amount of power from the momentum of the downstream air flow without affecting the static pressure or velocity at the outlet. If the static pressure at the outlet is unaffected, then so too will the motor be unaffected.

"Simply put once again: There ain't no free lunch!"

Generally true for sure, but don't try telling that to some hapless soul picking through a dumpster behind a restaurant.

Greg

However, in FREE AIR, a short distance away from the exhaust outlet you can in theory (and fact) extract a very small amount of power from the momentum of the downstream air flow without affecting the static pressure or velocity at the outlet. If the static pressure at the outlet is unaffected, then so too will the motor be unaffected.

I understand there is no free lunch :)

Nimrod,

"I understand there is no free lunch :)"

In other words, if you are pumping water through a length of garden hose, and out through a nozzle (in free air), and you point that stream of water at a garden pinwheel, causing it to spin, or, point it at a wall for that matter, then the pump sees that added "load"?

I said in free air, and a very small amount of energy obtained.

There is no free lunch here, only extracting a bit of the energy remaining in the moving air after it has performed its useful work. That energy will be expended by mixing with and adding momentum to the ambient air around the outlet in any case. In free air, the deceleration of the air as it passes through the small driven turbine or fan will not affect the upstream velocity of air beyond a short distance, air being a compressible free flowing medium.

I am not in any way arguing that this is a way to capture a useful amount of energy, it clearly is not. I am only pointing out that it is possible to extract a small (trivial in practical terms) amount. So, in other words, the OP's idea is only 99% wrong rather than 100%.

Greg

Has anyone thought of extracting electrical energy from the heat in the air and letting the air go? Use of some circuits I saw a while back that work on temp differential like a thermocouple in large size would take the heat of the discharge air and the cool air from the inlet and create a small amount of power from the temp differential. No moving parts to obstruct the flow of air and only a pick up plate to absorb the heat. ???? Ric

Ric,

Yes, but the plate in the cold air stream gives off heat ... that's what provides the electric current. So you don't gain a thing. Absorbing heat from the discharge is fine, but adding it to the cool air isn't.

You could however have the cool side in ambient air (outside air in the case of an A/C), and have the hot side against the hot refrigerant before it goes into the condenser, and get some small amount of power that way, and it would even improve the efficiency of the A/C very slightly, but the cost wouldn't begin to be justified by the power you got. Thermionic piles (converters) are very expensive for the power they produce, and typically have very much higher temperature gradients than we are talking about. Lower cost versions produce very small amounts of power.

Greg

This certainly looks like a perpetual motion problem except that some surplus energy is available through the chimney effect if it is a tall building with the air intake at the bottom and that the air is heated by the building activity.

All in all, very little energy can be extracted. Just reduce the thermostat setting in the winter and increase it in the summer and you will save much more energy without cost and problems.

The conclusion is that it isn't efficient to use exhaust air to generate electricity. Air to air heat recovery seems to be the only pratical means of saving gas usage, or electricity on a packaged gas fired rooftop unit with multiple compressors. This is as you know, is an industry standard means of using exhaust air that is currently in use. I haven't seen any other means in the field other than heat recovery. Anyone installing this should also look into energy rebates from the gas and electric utilities. A three year payback is usually what I've noticed on installation cost vs. savings.

I think the problem here is that Nimrod is under the impression that by having the recovery turbine some distance away from the exhaust, it will no longer affect the free flow of exhaust air. The garden hose example is close to what he was thinking, but there is a big caveat here. The water from the hose is not compressible, so once released from the hose into free air, the kinetic energy it contains is free from influence back to the origin nor does it affect the surrounding atmosphere, meaning it will no longer have an affect on the pipe or whatever created the original pressure. But with air, since it is compressible, ANY restriction, at ANY distance has a measurable effect on the delivery system. For example, if the atmosphere is truly unrestricted, the kinetic energy in that exhaust air stream is dissipated into the atmosphere evenly. If a recovery turbine is mounted 1 foot away, the exhaust air stream is now compressing against the turbine blades and increasing the HP required to exhaust, so no net gain. If the recovery turbine is moved far enough away so as to not affect the exhaust HP by compressing the air stream, it also will not receive any input energy from the air stream.

JRaef,

I think you have me confused with Nimrod. He is taking your position, I am in disagreement with you.

You seem to be picturing some sort of restrictive "large" turbine or fan in the exhaust air stream, whereas I am talking about something with across sectional area a fraction of the exhaust area. Your reference to the hose example I gave, relying as it does on the incompressibility of water is flawed, as is your belief that in free air you can have a region of air that is compressed that extends back any appreciable distance upstream, and that this would somehow impede the flow of air through the heat exchanger, any more than standing a few feet away, but in front of a fan blowing in free air imposes an added load on the fan motor or effects the velocity of the air at its face. In the garden hose example I gave, if you replace the water exiting the nozzle, and impacting something, with air being blown out of 6" diameter tube and impacting a 3" diameter "micro-windmill" a foot away, there will be no added load on the blower, or any reduced velocity at the tube outlet. Some air will pass through the "micro-windmill", turning it, and the rest will simply move around it, with no effect upstream of the "micro-windmill" beyond a short distance.

"For example, if the atmosphere is truly unrestricted, the kinetic energy in that exhaust air stream is dissipated into the atmosphere evenly."

What is the difference how the kinetic energy is "dissipated" as long as the conditions at the exhaust outlet are unchanged?

Greg

No, in Reply #12 he indicated that he thought he could still extract "free" energy if the recovery turbine were mounted a short distance away. That is not the case. If there was enough pressure to move the turbine blade, that same pressure would represent an additional load on the exhaust fan. There is no way around that.

"Some air will pass through the "micro-windmill", turning it, and the rest will simply move around it, with no effect upstream of the "micro-windmill" beyond a short distance."

Nope, not the case. If there is enough distance so as to not create back pressure on the fan, it is because ALL of the kinetic energy of the moving air is dissipated in the surrounding atmosphere and therefore there is nothing left to turn your "micro wind mill". If even a tiny stream carries enough pressure to turn a tiny turbine blade, that represents a tiny column back to the fan creating it and can be measured as a tiny increase in the HP load. No matter how tiny you want to discuss, it is always there. Energy cannot be consumed or created, it can just be moved.

Think it through: if you could extract even 1 watt from the air stream of an exhaust fan that is consuming 1000kW without that energy having come from, and therefore affecting, the motor on that fan, then you would be over-unity! What you are thinking is that once the air is moving and exhausting into free air, the work is done. Fan engineers would be able to show you that this is absolutely incorrect. ANY obstruction has an effect on air flow.

JRaef,

You are incorrect.

Nimrod agrees with you, and the paragraph you are attributing to him was a copy and paste from my post# 11. If you read back to his previous posts, and mine you will understand.

"No, in Reply #12 he indicated that he thought he could still extract "free" energy if the recovery turbine were mounted a short distance away. That is not the case. If there was enough pressure to move the turbine blade, that same pressure would represent an additional load on the exhaust fan. There is no way around that."

I knew I was asking for trouble when I started this small (as in not worth doing) extraction of energy theme with people that are misapplying the Law of Conservation of Energy:,

"... you would be over-unity!"

Or who don't understand the dynamics of fluid flow (sub sonic air) in an unbounded (free air) condition:

"ANY obstruction has an effect on air flow."

"... that represents a tiny column back to the fan creating it"

Of course it has an effect on (downstream) airflow, but that effect does not propagate very far upstream in an open system. It is not a "column" at all, but more like an extremely flared cone, with the point facing upstream.

You seemed to accept my garden hose analogy with water, because you said that water is incompressible, so in other words you are saying it would be different if you were pumping air through the hose and nozzle instead?

I suggest that it is you who needs to "think it through". Try looking at flow patterns of air moving around obstructions in an unbounded (as in free air) condition and observe how far upstream the flow-lines are affected. The velocity profile does not change until it approaches the "obstruction".

Greg

In response to my earlier post# 22:

This is a correction and apology to JRaef and Nimrod for my confusion over what Nimrod meant. I lost track that he was the OP, and therefore was to some extent at least in agreement with my position on being able to extract some (small) amount of energy.

"JRaef,

You are incorrect.

Nimrod agrees with you, and the paragraph you are attributing to him was a copy and paste from my post# 11. If you read back to his previous posts, and mine you will understand."

The above statement of mine in quotes, was in error. It was I who got confused on Nimrod's position, NOT JRaef!

My remaining post is unaffected.

Greg

Nimrod

Yes, there is current research that is investigating purposefully harnessing the static electricity generated from any liquid or gas flowing through a conduit, such as the cowling around an exhaust fan or vents leading therefrom to air. This might be an additional take-off to some of the other suggestions being debated here.

I am not familiar with any firm applications yet. I googled a website for you, though, re: the research.

http://www.uofaweb.ualberta.ca/newtrail/nav03.cfm?nav03=47269&nav02=47268&nav01=43139

Mark

Thank you Mark Handyman for answering the question. It is encouraging to read about some young men that are doing research on this subject rather than saying, "It can't be done."

actually...

it cant be done

pulling static from the system offers an alternitive to what you were suggesting rather than an example of what you were suggesting (as was clearly stated in the text)

you could also collect solar energy from light reflected off the blades, but this would also not be what you were talking about.

but this is true...

if you collect some energy from air motion caused by a fan you are collecting energy from the fan

look at it this way

if your fan uses 100 units of energy

and your device collected 1 unit without changing how many units of energy your fan needed to do the same job

and you installed 101 of your devices creating 101 units of energy

and used 100 of those units to power the fan

leaving 1 unit to sell back into the grid

big oil would have you killed

so it cant be done

:)

-craig kirby

A fan, by definition, takes mechanical energy put into its shaft and imparts it INTO a gas/fluid. Similarly, a propeller (boat, aeroplane) takes mechanical energy put into its shaft and imparts it INTO a liquid/fluid but to different effect. A mill, as in windmill, (also waterwheel, saw mill, gristmill, etc.) takes fluid energy OUT OF a fluid and imparts it into its shaft. In the instance where a fan, as in an attic fan, rotates due to the rising of hot air in an attic, it would be behaving as a mill. Typically, AC driven fans do not get up to a speed which is synchronus with the motor's "standard" cycles per second for any sustained period to be useable. DC fans could be part of such a passive recovery system if rotational speeds could fall within usable parameters of a "load." An example of the phenomenon's novelty is the candle driven holiday toy which spins and which looks like a "fan" (wide flat blades) but is simply a mill for entertainment generated from the fluid driven part of aestheticlly pleasing candle light - the rising column of heated air. I am not sure what physical arrangement or layout you refer to. If an HVAC input fan on one end of a not quite "closed system" is pushing an out of service Exhaust fan this is simply inefficiency as the conditioned air is probably not arriving at its intended desination.

we done it

just use dynamo. i'm 12 n this is my science project