CFD calculations in solar updraft towers

[Newbie Tinker]

Hi all,

I have been asked to contribute material for a presentation at my daughters school. The subject is alternative energy via solar methods and several other parents have already selected the obvious platforms (photovoltaics, CSP, etc.) and I thought solar updraft towers would be kind of cool and different. I don't know much about the technology other than it's not very popular.

One of the issues I have run into is being able to explain to young kids (and let's be honest, myself included) why the towers have to be so large, both in height and collector area. The calculations involved (computational fluid dynamics) are quite beyond me at the moment and I was hoping someone on this forum might be able to explain it to me in laymans terms.

So: why do solar updraft towers have to be so large both in height and collector area? Would recirculating the air cause it to rise in temperature over time and cause an increase in power output, kind of like burning more fuel in a fireplace causes a faster updraft? If not, why not?

Hopefully someone with a firmer grasp of the physics/engineering challenges involved will be able to help.

Regards,
Carl

 

[Greg Bernhardt]

Thanks for the post! Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?

 

[russ_watters]

Welcome to PF!

I saw this question and must say I was thrown by the title a bit. I don't think CFD is a very significant part of what you need or would play much of a role in the conceptualization of such a device. Beyond that, CFD is highly application specific and expensive, so by citing it in the title, you vastly cut your target audience to...well...almost no one. But to discuss how solar updraft towers work... (to schoolchildren):

Solar updraft towers capture the primary feature of summer weather: thermal updrafts. In the summer(more than in the other seasons), the sun heats the earth, which heats the air near the earth. This air expands and becomes lighter (less dense) than the air just above it, making it rise in bubbles of warm air, like bubbles in a boiling pot of water. (Add some moisture and these updrafts become thunderstorms...and you can watch the bubble rise and grow until they do.) By directing this rising air through a properly-shaped funnel, the effect is maximized and its energy can be extracted with wind turbines.

If you want to do calculations on the process, you can start with a parcel of air near the surface and calculate how warm it gets, how much it expands and based on the height of the tower, how much energy it can provide.

It's a neat idea, but the major (and it is major) drawback is being able to construct a tower tall enough to be efficient without being too expensive to build. Ordinary wind turbines capture the same energy, but though they capture less of it, they are much cheaper to build.

Hope this is helpful and not too late...

 

[Mech_Engineer]

Keep in mind also they have to be large because there just isn't a lot of stored potential energy in a cubic foot of hot air, so you have to flow a lot of it to extract an appreciable amount of power.