Need advice on creating power from air shaft

[JP Mikl]

I recently inherited a cabin that is very close to an old mine. There is an air vent that is constantly (24 hrs) blowing a steady, high volume of air from a 3.5 ft hole. I would like to harness that air. I was thinking about taking a 3 ft diam PVC pipe and affixing it into the shaft, with 4 fans (ie car radiator) mounted in the pipe. I want to place the fans on the shaft of electric motors that would generate power. I am not educated in electric or mechanical engineering and would like someone to direct me to some resouces where I can learn the basics of how to accomplish this.

Thanks,

Jeff

 

[AlephZero]

I think the first thing you should do is find out WHY there is a constant airflow from the hole.

It is possible that air is being pumped through the mine even though it is disused, to prevent a dangerous buildup of flammable gas underground or something similar. In that case, whoever is reponsible for the mine safety may not think it's a good idea if you mess around with their ventilation system. They will probably inspect the hole occasionally to make sure everything is as it should be.

It's hard to think of a reason why there would be a constant natural airflow from a "hole in the ground" that would be strong enough to extract any useful power from it, unless it is a VERY deep mine and the airflow is somehow being powered by geothermal energy.

 

[JP Mikl]

Geothermal energy. I just need help with the question.

Jeff

 

[Mordred]

start here

http://www.scoraigwind.com/books/

ther is a lot of factors involved in your design such as needing to know the average wind speed to correlate an efficient propeller design to prevent stall etc. This link will get you started
PS there is tons of materials and articles on the subject simply google " wind generator design " then when you need help with specific parts of the design we will be able to help much more thoroughly than trying to design an overall system for you from scratch.

 

[sophiecentaur]

JP_Miki
You appear to be a lucky man, if things are exactly how you describe.
I would be tempted to use a standard wind turbine/ generator. The only problem could be that the bearings will have been designed for horizontal operation and you would be using vertical orientation.

To get the most power out, you would need to know a bit more detail about the pressure and volume. Electric motors seldom make good generators and, if you seriously want to obtain a domestically useful amount of energy from the system, it would be worth doing things 'properly'. At least use a Car alternator, rather than just any old motor.
I wonder if you could get hold of one of those large fans that they have on top of tall buildings. That might do the job really well and there must be loads of that sort of stuff from demolitions.

Unfortunately, this is a very practical project which requires a fair bit of know-how. What you need is to for an enthusiastic 'Green', living nearby. He'd jump at the chance to get involved, I'm sure, as long as you were buying the equipment.

 

[sophiecentaur]

JP_Miki
You appear to be a lucky man, if things are exactly how you describe.
I would be tempted to use a standard wind turbine/ generator. The only problem could be that the bearings will have been designed for horizontal operation and you would be using vertical orientation.

To get the most power out, you would need to know a bit more detail about the pressure and volume. Electric motors seldom make good generators and, if you seriously want to obtain a domestically useful amount of energy from the system, it would be worth doing things 'properly'. At least use a Car alternator, rather than just any old motor.
I wonder if you could get hold of one of those large fans that they have on top of tall buildings. That might do the job really well and there must be loads of that sort of stuff from demolitions.

Unfortunately, this is a very practical project which requires a fair bit of know-how. What you need is to for an enthusiastic 'Green', living nearby. He'd jump at the chance to get involved, I'm sure, as long as you were buying the equipment.

 

[AlephZero]

The first thing to do is get some idea what power you might be able to generate.

If you have a hole with area about 1 m² there is unlikely to be any significant head of pressure behind the flow, so let's assume it is at atmospheric pressure.

If you assume the air speed is about 10 m/s (a bit more than 20mph), then you have 10 m³ of air flowing per second which has a mass of about 12 kg.

The kinetic energy in the air flowing in 1 second is (1/2)(12)10² so the power in the air stream would be 600 watts.

You will not be able to recover all of that from any sort of turbine. The maximum possible efficiency is limited by something called the Betz limit (which comes from the fact that if your take energy out of the flow, you restrict the flow and reduce its velocity, so if you tried to take out 100% of the energy the flow would be reduced to zero and 100% of nothing is nothing). That means the maximum possible energy you could take out of the flow is about 60% which would give you 360 watts. In practice you would probably get significantly less than that - maybe only half as much unless your turbine was carefully designed for the flow conditions you actually have. Say 200 watts.

If the velocity isn't 10 m/s, these numbers change in proportion the cube of the velocity. So if the flow was only 5 m/s (which you would still feel as quite a strong flow) you would only be looking at 200/8 = 25 watts maximum.

So you might be able to power a few light bulbs, but unless you also spend money on storage batteries etc you won't have enough power for cooking, operating any machinery, etc.

Probably the easiest way to get a reasonably efficient turbine would be to buy a wind turbine designed for use on small boats and suchlike. You should be able to find one with about the right diameter. It is quite easy to make a turbine that "works" in the sense that it extracts SOME power from the air. People have been building windmills for thousands of years, and if you make a windmill big enough (a lot bigger than 3 ft diameter!) it doesn't really matter that your design is only 5% efficient. But designing and maknig a turbine that works efficently is a much harder problem.

 

[sophiecentaur]

@Aleph_Zero

Yes, I agree with all that. Glad you did the Energy sums! for us.
The only problem, as I said earlier, about a boat wind turbine would be that the bearings are not designed for getting power from a vertical air stream. You may need a large pipe, to redirect the air flow to horizontal. Your figure of a few tens of Watts would fit in with what you can get out of typical boat turbines.

btw, wouldn't a suitable tapered 'horn' arrangement be able to get more energy out of the moving air? (i.e. the equivalent of impedance matching)

At least this thing would be working 24-7, unlike your average wind turbine which seems to spend a lot of its time doing nothing. The one on my boat is a good example which works best when it's too windy to go sailing!

 

[AlephZero]

You may need a large pipe, to redirect the air flow to horizontal.

I was assuming the OP was going to make some sort of duct from the "hole in the rock" to the turbine. Near where I live, there are quite a few disused lead mines and natural cave systems, some open to the public and others regularly visited by cavers, that that have 24-hour forced ventilation systems installed to keep the radon gas diffusing out of the rock to safe levels. There are quite a few "holes" in the side of hills, covered by a metal grill to keep out inquisitive sheep, with a gentle stream of air blowing out of them 24/7, if you know where to look for them. That's why I was asking about the cause of the airflow.

btw, wouldn't a suitable tapered 'horn' arrangement be able to get more energy out of the moving air? (i.e. the equivalent of impedance matching)

Hmm... if you do that, then by Bernouilli's theorem there will be a pressure change across the tapered section. I would guess you have a HUGE plenum chamber on the inlet side (i.e. the complete mine!) so you are not going to be able to change the inlet pressure. The outlet is presumably venting to the atmosphere, so that pressure is also fixed.

So I would guess that trying to reduce the area to increase the flow velocity would fail - it would just reduce the area and block off part of the flow.

 

[sophiecentaur]

OK, I'm fine with that.
A duct would do it! But, whilst on the ducting idea, perhaps this air would be warmer than ambient so you could have warm air central heating at the same time. CHP!

 

[Curl]

I'll make a bet with anyone that, if this is truly "geothermal" energy, it wouldn't be enough to power a toothbrush.
If there was really some good airflow coming out, I'd rather recommend you use it for ventilating your house/barn/whatever, if the air coming out is clean.

 

[cjl]

You will not be able to recover all of that from any sort of turbine. The maximum possible efficiency is limited by something called the Betz limit (which comes from the fact that if your take energy out of the flow, you restrict the flow and reduce its velocity, so if you tried to take out 100% of the energy the flow would be reduced to zero and 100% of nothing is nothing). That means the maximum possible energy you could take out of the flow is about 60% which would give you 360 watts. In practice you would probably get significantly less than that - maybe only half as much unless your turbine was carefully designed for the flow conditions you actually have. Say 200 watts.

Interestingly enough, I don't believe that this limit applies for a ducted turbine. There shouldn't be any theoretical limit to the energy extraction, aside from normal considerations such as viscous/frictional losses and mechanical inefficiency. That having been said, I agree that this is unlikely to be a significant source of energy.

 

[sophiecentaur]

I did wonder about a series of turbines in a conical duct (like a standard gas turbine. A big source of inefficiency with all such systems is actually getting rid of the air at as low velocity as possible. Ideally, the exiting.air would be stationary (nonsense, I know) but that would mean all the KE was used.
For off the shelf / surplus components this is a tall order.

I wonder whether, if the flow could be reduced, the exit air temperature might be usefully higher than ambient. This could possibly be the only exit for air.

 

[Curl]

Interestingly enough, I don't believe that this limit applies for a ducted turbine. There shouldn't be any theoretical limit to the energy extraction, aside from normal considerations such as viscous/frictional losses and mechanical inefficiency. That having been said, I agree that this is unlikely to be a significant source of energy.

The limit IS for a ducted turbine. Otherwise you could make the fluid climb and extract energy from the potential, with 100% theoretical efficiency. The limit is for a ducted turbine where the air must flow in and out axially.

 

[AlephZero]

I did wonder about a series of turbines in a conical duct like a standard gas turbine.

The limit IS for a ducted turbine. Otherwise you could make the fluid climb and extract energy from the potential, with 100% theoretical efficiency. The limit is for a ducted turbine where the air must flow in and out axially.

For a wind turbine, the pressure ratio across it is necessarily close to 1:1.

For a typical industrial gas turbine or aircraft engine the pressure ratio across the (multi-stage) turbine may be of the order of 100:1. That means that most of the mechanical energy comes from the reduction of pressure (and the expansion and cooling of the gas that goes with it) not from the kinetic energy of the inlet gas stream. That is not the situation that the Betz limit is talking about. The aerodynamic efficiency of IGTs can be 95% or higher, compared with the Betz limit of about 60%.

But for a wind turbine, the KE in the airstream is the only thing you have to work with. Converting it into a pressure head in front of the turbine won't create any more energy.

Clearly a duct is essential for a turbine with a high pressure ratio, to constrain the high pressure gas to go through the turbine rather than everywhere else.

A duct also has an effect on the efficiency of a wind turbine, for example by changing the airflow pattern around the blade tips, but it isn't essential to the design (that is obvious from looking at any large wind farm!) and it isn't relevant to the ideas that lead to the Betz limit.

 

[sophiecentaur]

Doesn't the fact that we already have a duct here make any difference to the statement above? We are not dealing with a wind farm situation.

 

[cjl]

Doesn't the fact that we already have a duct here make any difference to the statement above? We are not dealing with a wind farm situation.

I believe it does make a difference, since the flow is already constrained to go through the turbine. That's why I think the Betz limit doesn't apply in this case. Extracting 100% of the energy would not require the flow to be brought to a halt - it would just require an appropriate pressure drop across the turbine. The flow could then be slowed in a diffuser after the turbine, which would raise the pressure back up to ambient while simultaneously decreasing the flow speed to near zero.

 

[sophiecentaur]

I believe it does make a difference, since the flow is already constrained to go through the turbine. That's why I think the Betz limit doesn't apply in this case. Extracting 100% of the energy would not require the flow to be brought to a halt - it would just require an appropriate pressure drop across the turbine. The flow could then be slowed in a diffuser after the turbine, which would raise the pressure back up to ambient while simultaneously decreasing the flow speed to near zero.

What would be the point of slowing down the flow of air further in your proposed "diffuser"? You want the air to shift out of the way as easily as possible to make room for the next lot. The only way to achieve this would be for a flared tube with progressively sized (and pitched) blades. The air velocity would be inversely proportional to the area of the tube.

 

[cjl]

What would be the point of slowing down the flow of air further in your proposed "diffuser"? You want the air to shift out of the way as easily as possible to make room for the next lot. The only way to achieve this would be for a flared tube with progressively sized (and pitched) blades. The air velocity would be inversely proportional to the area of the tube.

The point is quite simple. The turbine will extract energy from the air by means of a pressure drop - the air flowing out of the turbine will be at a lower pressure than the air flowing in, while maintaining the same speed. However, the pressure of this air flowing out of the turbine will be below ambient, so you have to raise the pressure somehow before exhausting it to the atmosphere, otherwise it will not work. Diffusers raise the pressure by decreasing the airspeed, trading dynamic pressure for static pressure effectively. Their principle of operation is quite well understood as well - they do not restrict the flow. They merely expand the duct area so that for the same volumetric flow rate, the linear flow velocity is decreased. Ideally, you'd want the exit of the diffuser to be as large as possible, to give the slowest exit velocity possible.

(It's pretty simple stuff, really).

 

[sophiecentaur]

I see now. I didn't realize what the word diffuser meant. Isn't the effect of a diffuser similar to that of a flared horn on the output? I.e. reducing the exitre speed.

 

[cjl]

Exactly. In fluid mechanics, a diffuser is basically the opposite of a nozzle - a nozzle exchanges static pressure for flow velocity, while a diffuser exchanges flow velocity (and thus dynamic pressure) for static pressure. For subsonic flow, a diffuser is just an area increase in the duct (or a flared horn, as you put it).

 

[sophiecentaur]

Ahh! Brilliant.