[Andrew_S]

Thanks for those - nice videos. If I understand correctly the turbines are all close to ground level. Why does the chimney need to be so high?

[TheDude]

We've been over this a few times:

[Solar 8] Solar Tower in Austrailia to provide 200kw

Australia's solar tower plan ?

Solar Tower

Bit of a monstrosity. Outback has recently discovered geothermal potential, too.

$this->bbcode_second_pass_quote('Cynus', 'S')till, 20,000 acres to generate only 200MW seems like a waste of space. Maybe In the Australian outback it's OK for one or two but it could never be widely adopted. They should just take the $750 million it will cost and put the solar panels on people's roofs.

[kokoda]

I must admit that when I look at the concept I find myself wondering whether or not it wouldn't be simpler to just stick a few wind turbines on top of that thing.

The whole point of the project is to generate artificial wind to drive smaller turbines mounted at ground level. Why not just use real wind to drive larger turbines mounted outside the structure.

[mos6507]

$this->bbcode_second_pass_quote('kokoda', 'I') must admit that when I look at the concept I find myself wondering whether or not it wouldn't be simpler to just stick a few wind turbines on top of that thing.

The whole point of the project is to generate artificial wind to drive smaller turbines mounted at ground level. Why not just use real wind to drive larger turbines mounted outside the structure.

[asdar]

If you didn't have any turbines, just the tower and glass, it would draw air up through the chimney because hot air rises. Anywhere along the entire path that air travels the structure could be closed off and have turbines added. The only requirement is that the path for air has to go through the turbines.

Putting them at the base of the tower makes the most sense because it's a small area so easier to force air through the turbines, turbines and electric wires are heavy so having them on the ground puts no stress on the tower, maintenance is easier to do at ground level and wind and there's plenty of room for support and structure at ground level.

There are quite a few positive aspects that won't get viewed because they're not financially important enough to the project to matter. One that excites me is the production of water. With salt water pools under the glass to store energy for night use it's going to create a lot of pure water that will condense on the glass and could be extracted.

I think even though cost wise it's marginal that we should construct a full size version and test it. I know about the prototype, but the concept works better the higher the tower due to the physics.

[Graeme]

New Life for Solar-Updraft Technology?

$this->bbcode_second_pass_quote('', 'T')he solar updraft tower, which uses the greenhouse effect and thermal convection to drive wind turbines and produce electricity, has been hailed as a novel — and promising — approach to renewable energy generation.

The technology relies on an elementary principle of physics: heat rises. To generate power, a massive greenhouse creates hot air and funnels it into a tall chimneylike structure. This hot wind propels a wind turbine within the tower. According to some estimates, such towers could, if sufficiently large and in the proper environment, generate emissions-free power at a considerable discount over traditional renewable sources.

Nevertheless, solar updraft has been a nonstarter in the world of utility-scale power. A 50-kilowatt research prototype was built and successfully operated in Spain for several years in the mid-1980s, but the technology has not been proven commercially viable.

Now, an Australian company, EnviroMission Ltd., hopes to convince investors to support its plans to build two utility-scale solar updraft towers in the desert of Arizona. The company has submitted applications to use several hundred acres of public land in La Paz County for the plants. Last November, the Southern California Public Power Authority approved the company as a potential power provider.

[Tanada]

I love this as a concept, but sadly I think it is too late to build it in the USA, especially close to our southern border. Last I had heard they were going to build the prototype in Australia, where borders are not an issue and the economy is in much better shape than the USA.

[Jonathan_Hoag]

Those solar towers are a pipe dream. You cover the area of a medium size city to generate some 200 MW of power which gives you efficiency of 2% at best. And solar tower backers significantly underestimate the recurring maintenance costs and manpower that the city-size structure would require.

Enviromission has been trying to built the solar tower project in Australia for the last 10 years, with no success. Last I heard, they downgraded the project from 1 km tall to 400 m, which would reduce efficiency further.

[Tanada]

IIRC this project should either be completed or nearing completion by now, whatever happened to it?

[prajeshbhat]

I hit upon this idea about solar updraft tower.
http://en.wikipedia.org/wiki/Solar_updraft_tower
http://www.youtube.com/watch?v=yuOXgTC0SRc

Now suppose I want to build something like this on a much smaller scale, say, just big enough to provide electricity to a small rural household, maybe 150-200 Watts on a sunny day. I want to improvise the design since i think that the maintenance of a glass canopy would be too much hassle. Glass is expensive and breaks easily.

So I have some ideas. Suppose we lay down some metal sheets very close to the ground(maybe half an inch above the ground). We paint the top surface of this metal canopy black, so it absorbs sunlight and gets really hot(I think 60 degree Celsius is easily attainable in tropical regions like southern India). We spread these black painted metal sheets over an area of a few square meters(maybe 100). At the center of this canopy we make a hole and place a 10 feet tall chimney and inside the chimney we place a turbine. So I am hoping that heat absorbed by the metal sheets will be transferred to the thin layer of air underneath the sheets. The hot air will rise through the chimney and rotate the turbine.

Will this work? I welcome advice/criticism/suggestions/curiosity.

[sparky]

.
Good idea , there is plenty of variation on the convection tower
the biggest I know is\was? in Spain .
.....it work , with one problem , there is no torque !
the air velocity is barely a breeze ,
with the same effort and material cost ,
do some windmill powering a car dynamo to charge a battery

[radon]

To provide for continuous convection, the structure obviously needs constant inflow of air coming from the edges of the canopy to inside underneath it for heating, then going to chimney and out. Your chimney is 10 feet tall, suppose this is 3 meters. Lets say its radius is 0.1m to provide for the turbine. Then its volume is pi*3*(0.1)^2=~3*3*0.01=0.09 m3.

Suppose the canopy is held 5cm (0.05m) over the ground surface. Then the volume of the air under the canopy is 100m2*0.05m=5 m3.

Suppose the velocity of the air convection is 3m per second - this is quite a weak wind. This means that the air in your chimney, which is 3m tall, is fully replaced every second. So it will take 5 m3/0.09 m3 =~ 55 seconds, or less then one minute to fully replace the air beneath your canopy. Will one minute be enough to heat the air incoming underneath the canopy to 60C to maintain the constant rate (velocity) of convection? Hardly so.

To be operational, the structure obviously needs to have the volume underneath the canopy be orders of magnitude greater than the volume of the chimney.

[prajeshbhat]

Hmmm..So how long does it take for air to heat up? I thought air heats up rapidly since air is in gaseous state.

Here is a video of a tiny model. It looks genuine. This guy seems to be able to maintain contstant flow.

http://www.youtube.com/watch?v=qikKsxJ3eus

[radon]

Ok, his chimney looks much smaller than 0.1m in radius, probably eighth of that. Volume declines in a square proportion to the radius, so eighth of the radius means 64 times less chimney volume and 64 times more time for heating . But smaller radius reduces the momentum for the turbine. Besides, the convection rate is probably lower than 3m per second, and this also gives more time for heating, but reduces the momentum too.

So overall you either have to sit down and do all the formulas, or build a pilot model.

[radon]

Another interesting thing: they planted trees at the edges of the collector, probably, in order to protect the collector from the natural winds, as these will bring in colder air inside and impede convection. A small structure may be particularly vulnerable to these, since one wind blow can easily wipe out an hour's work of heated air. This is one of the reasons, probably, why the small model has a dome shape - this way it is better protected from the natural wind blows. Therefore some sort of physical barrier would be needed around the structure if the place is windy.

[Shaved Monkey]

They are going to build one in Arizona,they tried to get the money to build it in Australia for many years and couldn't ,its an Australian invention.

http://www.gizmag.com/enviromission-sol ... ble/19287/

[Tanada]

For small scale like you are talking about I read about an alternative plan that might work better for you. The idea was to build a gently sloping metal structure like you are talking about with rocks and gravel under the roof to act as heat mass. They also put the small wind turbine on an intake tube going into the side of the well sealed structure, not in the chimney, so that it was easy to wire and maintain. Other than that the chimney was just there to provide an escape route for the heated air and the structure was very well sealed so that all the incoming air had to get in through the intake tube/turbine.

[prajeshbhat]

Where did you read that idea? Are there any working protoypes of that device?

[Tanada]

$this->bbcode_second_pass_quote('prajeshbhat', 'W')here did you read that idea? Are there any working protoypes of that device?

[Tanada]

$this->bbcode_second_pass_quote('', 'T')he Arizona Project

While this is not the first solar tower that has been built (a small-scale test rig in Spain proved the technology more than a decade ago) EnviroMission has chosen to build its first full-scale power plant in the deserts of Arizona, USA.

The Arizona tower will be a staggering 800 metres or so tall - just 30 meters shorter than the colossal Burj Khalifa in Dubai, the world's tallest man-made structure. To put that in context - it will stand more than double the height of the Empire State building in New York City, and it'll be as much as 130 meters in diameter at the top. Truly a gigantic structure.

Currently undergoing site-specific engineering and land acquisition, EnviroMission estimates the tower will cost around US$750 million to build. It will generate a peak of 200 megawatts, and run at an efficiency of around 60% - vastly more efficient and reliable than other renewable energy sources.

The output has already been pre-sold - the Southern California Public Power Authority recently signed a 30-year power purchase agreement with EnviroMission that will effectively allow the tower to provide enough energy for an estimated 150,000 US homes. Financial modelling projects that the tower will pay off its purchase price in just 11 years - and the engineering team are shooting for a structure that will stand for 80 years or more.