Chimney Power?

The other day I was reading a great thread here about practical alternative energy where they mentioned wing-nut ideas as being only helpful as far as they go towards educating people about practical power generation. I doubt this is practical power generation. But I am no less curious as its an Idea I've been toying with.

What about a turbine in a chimney? The hot air rises rushing through and turns the fan (blades in line, like a water wheel?). Sure its a trickle of electricity but it probably is running for 10 or 12 hours at a time right? I wouldn't expect to get much juice from it - but perhaps enough to run a light or charge a tablet?

Just curious. Total novice. Would love to hear thoughts.

Landon : there are several devices that can capture millivolts worth of electricity which can then power various types of detectors that can measure air flow, humidity
and for the presence of many airborne chemicals, then squirt out that information in a micro burst of reporting! Mostly this is useful where otherwise man hours would
be tied up in changing these units batteries.

One of the better ones the last time I paid attention was a wire that when vibrated moved a metal shaft in and out of a mini stator, this whole system was supposed
to last decades. Being able to generate enough to power a cell phone will probably happen when cell phones catch up with the improvements in broadcast power
reductions ! Hope this helps a little. Big AL.

I did a little looking around and found a few things regarding this idea. There is a company
who wants to create super power producing chimneys that will solve global warming. It is an extreme example of what you are saying.
But I guess on some rotisserie ovens there are turbines you mount in a chimney that turn the meat. Seems like if you could turn meat with one of those you could turn a generator.

Wow. That was a way cooler and more far out answer than I was expecting. Glad other people are thinking thoughts too.

If you go to Google images and look for solar chimney, there are dozens of proposals for enormous power generating chimneys. Some have been built.

I have always wanted to do this on a small scale but haven't put in the effort to see how feasible it is. On a large scale, Solar Updraft Towers can be economical but like any power plant are expensive up front.Solar Updraft Towers, wikipedia.

I wonder what sort of potential there is to couple this with the exhaust of a rocket stove. It would depend on the temperature of the exhaust leaving the chimney.

Alright, so my take on all this is that a generator turbine in chimney not outside the realm of the possible. In fact I learned a few other things too. In theory I could have several turbines, and the ones farther up would be the one which would rotate the fasted due to compression. Hmmmm.....

My neighbor asked me about this the other day. I thought I would do a 'example calculation' to see how much energy could be produced from a residential chimney.

Start with the mechanical energy equation:

pressure_in / density + velocity_in^2 / 2 + gravity / height_in = pressure_out / density + velocity_out^2 / 2 + gravity / height_out + Work_shaft + Work_loss

pressure_in = the pressure at the entrance of the chimney
pressure_out = is the pressure at the exit of the chimney
velocity_in = the velocity of the air at the entrance of the chimney
velcoity_out = the velocity of the air at the exit of the chimney
gravity = you should be familiar with
height_in = the height at the base of the chimney
height_out = the height at the top of the chimney
Work_shaft = is what will turn the turbine
Work_loss = is the loss due to friction in chimney

I'm going to not show all the steps in the calculation...

If you neglect gravity and frictional losses you can simplify the equation.

If you then plug in some approximate values, say 10 pascal pressure difference (which might be typical above a hot fireplace chimney) and velocity difference of 1 meter/sec (which is really moving, say a nice roaring hot fire) you get a value.

You then need to multiply that value by how much mass of air per unit time is going through the chimney to get the work. (Air weighs almost nothing and will make the work very small)

Thus, in this example you get:
0.01 Watt

Keep in mind that I have never seen a turbine on this power scale, and you can lose 50% of the theoretical power in the turbine when producing electrical power.


****

You only really start to get some power if the chimney is really really hot and you move a lot lot lot of air through the chimney.

Ryan Workman wrote: I wonder what sort of potential there is to couple this with the exhaust of a rocket stove.

I would say zero.

In my previous post I neglected friction losses, but they are important. In a rocket mass heater work that could be used to turn of shaft is lost due to work done on the air by friction.


In a rocket mass heater you are really after the thermal energy and storing it by having it lose all its heat energy in a long frictional pipe, not the ability to turn a shaft.

Calculation is volume per second by pressure. Pressure is easy to gather, drop a plate across the chimney, upward pressure will register on a scale.
Volume a litter harder to gather when gas not liquid. Best gas turbine convert 30%.

If you have a chimney good chance you have heat. Steer far far from inefficient steam and use gas/hydro. Cheapest and with more than twice the conversion rate of gas turbine.

Landon Sunrich wrote:Alright, so my take on all this is that a generator turbine in chimney not outside the realm of the possible. In fact I learned a few other things too. In theory I could have several turbines, and the ones farther up would be the one which would rotate the fasted due to compression. Hmmmm.....

Did you also learn that to be "practical" they have to be really big? The first picture I saw of one looked really good till I figured out it was a mile high or so. The base would use a lot of room too. It seems the efficiency goes up with height, so a one mile high can produce a lot more than two 1/2 mile high units... it basically came out that anything I could build would produce less energy in it's whole life than the cost of building the unit by a long shot. The chimney is a useful device for removing air from a building no matter if it is just warm air or smoky air. Probably not so good at making homestead power.

I think the real question is when to use electrical power in the first place. The average home uses it (and wastes it) for almost everything. It is sooo easy to use. However, when homesteading off the grid it is not so easy (or it is very easy to run out). It becomes time to reevaluate how we use energy and what we use what kind of energy for. These are not new thoughts, but I think turning everything into electrical power is probably not the best solution. The tower generator I saw used a large base area that acted as a heat collector to warm the air, what could be done with solar collectors (heat or power) of smaller size? (and less intrusive)

If you think about it, all the wind turbines in use do actually use this system as much of the wind we have is due to systems that work much like this, but are miles high with a base many miles wide... saves me building the tower, just use the one that is there

Good comments by Len Ovens. These towers are quite literally heat engines (so is a conventional wind turbine). The system works by decreasing the density of the air in the tower. This is done by increasing the average temperature of the air column. The effect is to decrease the pressure at the base of the column to lower than the air pressure outside the base of the column. It's this differential pressure that generates air flow. It's clear from this perspective that increasing the height of the tower and/or increasing the temperature of the air that enters the base of the tower will increase the differential pressure, and hence increase the performance of the system (all else equal). Other factors include not having a tower that is too narrow as this would introduce back pressure and reduce differential pressure at the base of the unit, and hence slow air flow. A tower made of a material with low heat transfer will keep the air temperature in the tower higher, and this will also enhance results. The main problem with having a small unit achieve good results is the required height of the tower. Heating the air with the green house effect which has been done in some systems would allow for getting much better results from a smaller tower - but then that increases costs. This system really is a wind machine. It takes the same energy that otherwise generates wind (i.e. solar), and directs the air to enter the base of the tower and across high speed turbines for power generation as opposed to capturing this energy in a diffused manner with scattered wind turbines. A tower that is sufficiently tall could see impressive performance without having to devise a means to heat the air that enters the base - this air would be sufficiently heated by the ground itself (exactly how most wind is generated).

Gizmo the on-line magazine is reporting that at least one of the Australian Solar Towers mentioned here as a '' Chimney'' Is Officially up and making power as of Today!
BIG AL

Solar tower, not solar chimney!

Hi, Guys I am working on the power generation from the chimney by using Horizontal Axis turbine. Mine is simulations study. I have gone through the comments on this forum and in my opinion too the power can be generated from the hot gases coming out of chimney.

Harjeet

I have been having similar thoughts. about chimney power. I am coming at it a bit different. I am in the process of building an outdoor rmh using soapstone slabs. I have a large thermal mass of soapstone. I am also examining additionally trying to heat it with super acute solar energy using a large spot fresnel lens. When the conditions are right. , you can output a 1800 degree focused beam of heat. Between those two, my goal is to super heat the soapstone. With that heat, I am interested in creating a separate output chimney (not the one used to create the initial heat (heat riser and barrel) that concentrates thermal updraft by being shaped like a Da Laval nozzle. By utilizing the Bernoulli principle and the Venturi effect in this chimney, I am wondering if I can create enough force to spin a Pelton wheel attached to a pmg and make some energy. I am not a physicist so I am going to have to do a lot of trial and error but it seems to me that I should be able to create some wind power through these concepts. What if additionally, I utilize some steam at the bottom of this chimney by adding small amounts of water that will instantly be vaporized when it hits the soapstone?

Thoughts?

Carlos

Solar Chimney

Been thinking along the same line, the difference is that I started with VAWT. I thought why not make the support a chimney and add turbine blades inside. In a nut shell VAWT and chimney turbine in one.

The base also looks to me like a perfect solar  dryer. I've been wanting to build a house using the roof as a solar dryer. Adding a swirl vanes at the bottom of the chimney will also increase the efficiency of the system. Similar to that of Viktor Schauberger's water turbine.