mini solar chimney for micro power generation

September 27, 2009 2:17 PM

i'm fascinated with the solar chimney concept for electric power generation. i'd like to build a small, fabric-based model, see if it might be able to provide enough power to charge a couple of car batteries. does the mefi hivemind want to help me brainstorm?

there's some pretty decent video here of a large scale project in australia (not yet constructed).

i'd like to design something which could be built from locally available materials, nothing too complex.

a smallish tethered hot air balloon might serve to support the structure.

a long tapered cylinder made from fabric (fireproof at the bottom) would be suspended from the balloon and anchored to the ground.

at the top of the cylinder we need some kind of turbine. hot air exiting might also feed the balloon. at the base we'd have a small wood fire.

i'd love to hear from any and all mefites, with or without experience in power generation/off grid/ballooning.

if it works, i think we should call it a montgo.

posted by kimyo to technology (18 answers total)

can you explain what the wood fire is for?

is the wikipedia entry the type of project you were thinking of? where the heated air would rise up through the funnel top and spin a wind turbine?

I'm not an engineer, but from my perspective I don't qute understand why the fabric and balloon system is desirable. Why not use permanent and stable materials that could conduct heat much better than a fabric?

sorry for so many questions and so few answers from me. others will be much more helpful I'm sure
posted by Think_Long at 2:55 PM on September 27, 2009

I believe the idea is to not have to burn fuel... In other words, if you're going to burn fuel, better off creating the electricity more directly.

The solar chimneys you get on a house do two things: add a layer of insulation between the hot sun and your interior spaces. Where it becomes a chimney is that the sun heats up the dense material in the chimney, and that in turn heats the air inside the chimney. As the air heats, it wants to rise, which creates a current. If you plumb some ductwork, you can get that current to pull cool air into the building.

The one in Australia I think creates a greenhouse under the fabric and then directs the air up the chimney to generate electricity.
posted by gjc at 3:22 PM on September 27, 2009

The one in Australia is still on the drawing board isn't it?
posted by biffa at 3:30 PM on September 27, 2009

thanks all, here are some replies in no particular order:

the wood fire would serve to start the convection.

yes, the idea is to have heated air (also, possibly heated by the black fabric) draw cool air in from below and drive a turbine.

the concept is properly known as a solar updraft tower, sorry my error for calling it a solar chimney.

the fabric/balloon option might be desirable because the materials will be cheap and readily available.

a fabric tower might also be easier to raise/elongate. to construct a fixed tower of sufficient height would require much more raw materials.

yes, unfortunately the one in australia seems to have been in the same planning stage for the last year and a half.
posted by kimyo at 3:58 PM on September 27, 2009

Thing is, if you're also using the heated air to maintain a balloon, you're going to have to balance how close the balloon is to the outdraft (and therefore, how hot it is and how much lift it generates) versus it 'stoppering' the vent at the top by creating a turbulence bubble of 'hot' air at the top of the vent and killing off the airflow (And cutting energy generation).

Personally, I'd think the only way this is going to be feasible is if you really cut material weights - Skip fabric and go for very thin heat-sealed plastics for your column and balloon. The generator is going to have to be something VERY efficient to see any notable energy recovery, cause there's just not that much energy left after the whole sun->heat surface ->radiate to air ->Convect up process, given that you're losing big chunks at each step.

Going to bookmark this - If nobody more competent crunches the numbers, I'll come back and give it a whack.
posted by Orb2069 at 4:16 PM on September 27, 2009

perhaps the balloon would get fed by a smaller tube, and not be close enough to the output of the top of the cylinder/tower to block the output. it probably wouldn't need a constant stream of hot air.

agreed on the material weights, as long as whichever plastic is used is also cheap and readily available.

for the mock up, i was thinking of using an old washing machine motor.

haven't decided on the fan arrangement.

i wonder if a pelton wheel would work with air?
posted by kimyo at 4:24 PM on September 27, 2009

not a pelton wheel; you want a lift device, not a drag device. Maybe even a Wells Turbine, but they need to be spun up to generate.
posted by scruss at 4:39 PM on September 27, 2009

the wood fire would serve to start the convection.

I don't think this is necessary.

at the top of the cylinder we need some kind of turbine.

No reason to put the turbine at the top. Put it at the bottom, on the intake side.
posted by jon1270 at 5:04 PM on September 27, 2009

Looking at the solar tower diagram, there is no fire, you're using the sun to heat up air inside the base of the tower which pulls in cold air from the bottom/sides.

You could build a small scale model, but I doubt you could get much power out of it. You'd need a pretty large surface area and live in a climate with enough sun to do it. There are a lot of solar air heaters which rely on this concept.
posted by wongcorgi at 5:49 PM on September 27, 2009

What little I know about this technology is that it really needs scale for feasibility. You could probably build one small as a sort of experiment, but the value of the energy you'd get out of it over the lifetime of it's component parts would likely be less than the cost of materials.

Any heated air that you are capturing to 'hold up' the tower is going to be doing that work instead of spinning the generator. I'd actually think that just creating a self supporting solar powered hot air column with balloon at the top would be a good challenge.

I simply don't get the desire for the wood fire. There are biomass burning cogen (heat and electricity) units available in Europe that use the fire heat directly to make electricity. When you burn wood outside it is hard to capture more than a small fraction of the energy it contains, and doing it in an open air tower sounds like a way to spend a lot of energy to get a little back.

I'd definitely think you want the turbine at the bottom. The less weight you are trying to lift a long distance into the air the better, and the pressure delta at the bottom of a closed heated air stack matches that at the top.
posted by meinvt at 8:32 PM on September 27, 2009

i admit i'm fixated on the wood fire, partly cause i expect wood to be plentiful.

but, as i'm not planning to include the skirt, there is a need to start the convection which would 'normally' have come from the sun heating the ground around the tower.

i will look at wood to electricity options as mentioned, certainly it is possible that this is a complete waste of time. i was expecting that the amount of wood required to maintain convection would be relatively small.

the success of the concept almost certainly depends on the difference in air temperature from ground level to say 200', (200' being a number i picked out of my head). i know a pilot friend had an air temp/low altitude difference chart, i'll see if i can get him to dig it up.

turbine at the bottom is better for many reasons as stated, shorter electric cable runs, less weight needing to be lofted.
posted by kimyo at 10:57 PM on September 27, 2009

but, as i'm not planning to include the skirt, there is a need to start the convection which would 'normally' have come from the sun heating the ground around the tower.

By skirt do you mean collector? If you don't have a collector you're not going to get any significant amount of power out, trying to capture hot air to keep the balloon afloat is going to further limit you, and with the wood burning you'll likely operate at a net loss. Fundamentally, you need a large collector area and a tall tower to get a useful amount of power.

I'm all for dreaming big, and even toyed with the solar updraft generation idea at one point, but came to the the conclusion that it just wasn't very workable as a DIY project beyond a test-bed for ideas, especially not unless one livid in a desert or similar area with lots of sun and little or no vegetation.

There's a lot of information on the web now about off-grid generation. Have you considered wind power? People are successfully building their own wind turbines, including making their own alternators, and getting real power out. Otherpower has a lot of good information, including books, plans, and even seminars you can attend. Here are a lot more resources on wind generation.

One thing you'll probably discover is if you're aiming for true off-grid self-sufficiency no single method of power generation is going to cut it. You might end up with wind and (conventional) solar, maybe with a fuel burning generator for emergencies. Also, it's a lot cheaper to use less electricity than to generate more.
posted by 6550 at 7:40 AM on September 28, 2009

Another thing to consider is using the right fuel for the right energy need. You would probably be more energy efficient burning the wood to generate heat for the house or for hot water, than trying to generate electricity. Use wind or some other kind of solar to generate the electricity. Commercial electricity is relatively cheap and clean, especially compared to burning wood at home to generate it; concentrate on reducing your usage of it and fighting other self-sufficiency battles.

Depending on your home's design, you might be able to design a fireplace that heats during the winter, and that is able to provide some cooling during the summer via the solar chimney effect.

Or, build something like this to use your excess heat.
posted by gjc at 8:07 AM on September 28, 2009

Am I wrong in thinking that a significant portion of the solar heating will be accomplished by the skirt? Without it, wouldn’t the loss of heat and surface area make the project much less efficient?

I agree that you should look into wood to electricity options, but I’m also not entirely sure that wood burning is a very efficient form of energy transformation (someone should feel free to aggressively correct me on this), nor is it free from pollutants.
posted by Think_Long at 8:32 AM on September 28, 2009

i've looked pretty hard and long at 'traditional' wind power, have concluded that even at its simplest, it is not a long term solution for a small doomstead (critical parts availability, siting and the difficulty inherent in building a sufficiently tall windmill). also, random violent weather can easily kill your windmill.

if the convection theory works, it's way simple, inexpensive, and unlike a windmill, it will work 24/7.

agreed that i am bastardizing the solar updraft concept by eliminating the skirt and that burning wood is terrible for the environment.

as a first experiment i might try to affix a fabric cylinder to the side of a building/tree/telephone pole, forget the balloon and make some airflow measurements.

using less electricity is best, sure. i'm not going for an all out off the grid solution, just trying to find if a useful amount of power could be generated each day, enough for a bit of lighting, perhaps.
posted by kimyo at 10:04 AM on September 28, 2009

I have absolutely 0 engineering experience, so kudos to you for trying this stuff out, and forgive my skepticism. However, how would the solar chimney be a 24/7 solution? Unless you’re on the extreme rims of the world, will sunlight not be a limiting factor? Not to mention the difficulties of keeping a small fire burning at regular temperatures.

Other green energy solutions may exist. Instructables.com has a bunch of fantastic (looking) how tos, just check out their green section.

Anyway, good luck with the experimentation – keep us posted
posted by Think_Long at 11:01 AM on September 28, 2009

I rather think that random violent weather would destroy your solar tower before it'd destroy a properly chosen wind turbine. Every turbine I have worked with has had a survival wind speed far in excess of 50m/s. Your fabric tower would be all over the neighbouring counties after that.
posted by scruss at 2:24 PM on October 4, 2009

OK - I just cut 10'x12' of this stuff and dropped it on a scale - it comes out to about .5 grams a sqft.

If we assume that we're lofting a balloon 1,000 ft on a tube .07 sq ft in diameter (1' circumfrence, to make the weight calculation a little easier.), that's 500 grams - about a pound of plastic - to carry up. We're leaving out wind side-loading and all the other real-world numbers to make this simpler. We're also using the lightest material possible - Using anything more durable or thicker means adding a LOT to the weight, and makes the result worse. The pipe should probably also be wider, but that also adds to the weight.

Fortunately somebody else has already done some of this math, so we're going to assume his max-height air temp of 28F at the top of the tube, which he claims air that is at 130 degrees has a lift of .28 oz per cubic foot. Which means that, in order to lift 16 oz. of plastic, we need 58 cubic feet of air at 130 degrees. (Which requires at least another 72 sq ft of plastic, but we're ignoring that along with whatever we're using to keep the balloon from collapsing in a stiff crosswind.)

We'll also assume that we've got a solar collection facility at the bottom better than this guy's installed solar trombe wall system - He only gets output air at 120 degrees. We'll also assume that we're not going to lose any heat going up the pipe.

So, if we ignore all the real-world considerations (heat loss, wind load), and assume we can have the best of both worlds (28F is about what the air temp at 1000' would be around 3am, while that guy's output is measured at high noon), then this thing will stay up in the air.

Barely.

TL;DR summary? No way in hell are you going to see any sort of power out of this greater than if you just took the money and bought D cells. Read up on Savonius rotors - You can build yourself one using junked car parts and oil drums. If you're in weather that can knock one of those down, you're going to be rebuilding the doomstead anyways, so power generation is the least of your worries.
posted by Orb2069 at 11:41 PM on October 6, 2009