# Hillside Solar Chimney/Tower



## AgrarianDr (Mar 25, 2011)

Have always been fascinated with heat/solar convection and its many uses, including solar towers - more specifically in my case, a hillside solar chimney.

http://www.tour-solaire.fr/images/img-page/Bernard-Dubos-1934-Solar-Chimney.jpg

Pretty simple concept (the best things usually are) where you take advantage of the solar powered updraft to create energy

https://wryheat.files.wordpress.com/2011/05/lapaz_img1.jpg?w=620


So - we have a nearly square 40ac tract that is almost perfectly south facing. Basically 1350' x 1350', and I have a 700 long x 10' wide strip between fields I can dedicate to this, with a 65-70' drop from top to bottom.

I want to dig a 4-5' deep swale, a trench if you will, again roughly 10' wide, lined with rocks for thermal mass (we have an ample supply of those!) and then cover with what amounts to a hoop house high tunnel. 

In the end I should end up with the equivalent of a 700' tall, 10' wide tower laying on the ground with a slope of about 1' rise per 10' of run 

Even if this didn't produce any notable amount of electricity I can still think of a l-o-n-g list of other uses as the top of the run would end up near the greenhouses, the livestock barn and the harvest house. (I'm envisioning a solar dehydrator from Hell, for starters)

Despite a GREAT deal of personal experience with convection projects, solar chimneys in the greenhouses, rocket stoves and yes, solar dehydrators etc, my dilemma is that there doesn't seem to be any real clear math on what something like this may or may not produce in the form of energy for a Sunny zone 8 location. 
Even the experts with beanie caps are not much help here - since EVERY single commercial solar tower project they have built so far has produced WAY more than they originally estimated. 
Of course, they were using 10 square kilometers of ground cover to capture the solar gain, whereas I'm doing this on the cheap and dirty just to see what happens.

So, any math/mechanical geeks out there that want to take a shot at telling me what I can expect, or not to expect - other than a 700' hoop house going up the hill?


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## Belfrybat (Feb 21, 2003)

Fascinating - but initial outlay looks expensive. Keep us posted as to how it works.


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## AgrarianDr (Mar 25, 2011)

Belfrybat said:


> Fascinating - but initial outlay looks expensive. Keep us posted as to how it works.


Thanks, but actually isn't all that expensive, aside from labor. 
I'm just taking advantage of the incline - digging a ditch (labor), filling it with thermal mass in the form of rocks (labor and free rocks) and covering it with plastic and PVC or fencing wire frame (haven't decided on that yet). So the only real outlay will be the plastic sheet glazing and PVC. 
Yes, for 700' long that could add up, but I will admit I've spent a lot more money for a lot less productive projects than this!

And yes, will keep you posted. This time of year may not be ideal...then again, may in fact be the most educational


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## Steve_S (Feb 25, 2015)

I'm curious about this project of yours as well, please do keep us informed. Zone 8 put's you in some pretty hot sunshine (I'm in zone 4a).

Just some ponderances - thinkin out loud for discussion:
I'm wondering how a low speed turbine would do with this, as I would think the max airflow would be at the top of the long collector/chimney. That being said, the turbine would be internal to the structure so weather issues and wind movement (direction changes) are resolved leaving only the turbine mass as an issue. More blade weight = more force to move them and that's an issue for an "outdoor" turbine. This turbine being "internal" you could go with much lighter weight materials resolving the blade mass. 

Low Speed generation from all accounts is the tricky bit BUT again this is enclosed and essentially controlled (dampers could be incorporated for airflow management) so the real trick would be to find the appropriate generator / turbine head that would generate power at low RPM.

Just thinking of some of the projects at BuildItSolar and it occurs to me that you could in theory not only use the Solar Chimney to spin a small turbine but you could also heat water (black pipe running on top of the rock base), heated air could be vented after the turbine to a building if needed, small hoop house / greenhouse at the bottom... hmmm and a few other possibilities all in one. Any project that can do multiple tasks is certainly something worth exploring, especially if it's money & labour saving.

Glazing might be a tad tricky. At the bottom side (intake) it will of course be cooler but higher you go the hotter it gets and I would imagine in Zone 8 in mid July HOT may be considered an understatement. _Will be interesting to find out how hot it does get up top, might be a bit of a surprise_ Would PVC & Plastics handle that for very long ? Polycarbonate Sheets most likely would last the longest but that adds up $ really quick going by the listings at Greenhouse Megastore polycarbonate-panels Any greenhouses local to you that are being dismantled ? 

Interesting Reading & Reference:
Wind power authority top-5-low-speed-wind-turbines
 The Truth About Small Wind Turbines


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## OffGridCooker (Jan 29, 2010)

Why would you use a thermal mass? 
Would you not want the heat to flow into the airstream instead of into the thermal mass, which would then flow into the ground?
If you want this thing to have high velocity us a half round bottom painted black and insulate it to the ground, and then use a clear top, on the half round bottom.
This will run at higher temps and with more power.
And the smooth interior will help the velocity of the flow.
Another point to consider is a system this large may have enough pressure, at the top of the run, to cause problems with the top cover.
Another issue is the clear top will cause maintenance problems. 
For a maintanance free system use a solid duct.


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## AgrarianDr (Mar 25, 2011)

OffGridCooker said:


> Why would you use a thermal mass?
> Would you not want the heat to flow into the airstream instead of into the thermal mass, which would then flow into the ground?
> If you want this thing to have high velocity us a half round bottom painted black and insulate it to the ground, and then use a clear top, on the half round bottom.
> This will run at higher temps and with more power.
> ...


Thermal mass is something I have played with for years. In this case heat does not really "go into" the thermal mass a away from the airstream - on the contrary, once heated it would actually help, a lot. 
But that is not my main reason for thermal mass. Thermal mass is kind of like a flywheel, meaning it runs long after the input shuts off. Like steam that continues to rise from boiling water long after the fire is out. Might have very little benefit for power production, but for winter heat long after the sun goes down, to a greenhouse, livestock barn or? Yeah, that is a serious consideration. 
We are after all, talking about 220,000 cubic feet of interior volume. With rock lining the bottom half of the cylinder, that's like heating up the concrete floor of a 700' x 14' room at 8' tall.
No matter how you calculate it, it's a LOT of heat retention 

Point taken about the smoothness (or lack of) effecting the velocity of the interior. But I'm not sure that in a 10' wide cylinder, that is nearly horizontal, where only the bottom half round is lined (not filled) with thermal mass will make any notable difference at all. 
Heat rises of course - from start to elevated end - as well as from ground to ceiling within the cylinder itself. Much like one of my rocket stove systems with a long horizontal flue, the true "velocity" is in the upper 2/3 of the cylinder.
"IF" that is true (and it does from a logical sense AND because I have witnessed), then the smoothness of the bottom layer is probably a non issue. Worst case scenario I could cover the rock with a sheet of plastic. May not remove any problem, but I'm guessing would greatly reduce it. Might even help with heat retention

As to the pressure and temps at either end, I am at a complete and total loss as to how in the world I could even guesstimate that. Might just be a pure case of trial and error. 
Again, point taken and will certainly go in with eyes wide open on that potential.

Am sitting here thinking now of several other "issues" I might face. At least I'm not the one trying to figure out whether a shark suit actually works or not


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## TnAndy (Sep 15, 2005)

AgrarianDr said:


> In the end I should end up with the equivalent of a 700' tall, 10' wide tower laying on the ground with a slope of about 1' rise per 10' of run


What jumped out at me is the above. 

I don't think you'll come anyway close to the equivalent. A 700' tall tower would probably get some real velocity going....my guess is a long hoop house isn't going to come close.


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## AgrarianDr (Mar 25, 2011)

TnAndy said:


> What jumped out at me is the above.
> 
> I don't think you'll come anyway close to the equivalent. A 700' tall tower would probably get some real velocity going....my guess is a long hoop house isn't going to come close.


Very true, and don't think I don't recognize this. The big ? I presented was WHAT (if any) power I might get.

Honestly, with a 70' rise I know I will get some power out of it, certainly nothing like a vertical 700' tower, but then again, the cost of a vertical 700' tower, with guide wires and land requirements, etc, etc - is so absurd it's not even worth the time to figure out.
Whatever power I get out of this mess, is just a bennie. I obviously would like to get as much power as I could (intermittent as it might be) But what really got me started down this whole path was having that much rising and latent heat - especially since it ends up near so many buildings and 2 greenhouses.

So, to be fair, no, I have no illusions about powering up from this, just getting whatever else I can out of it
(still surprised no one picked up on my comment about maybe having a dehydrator from Hell )


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## Bellyman (Jul 6, 2013)

I can't do the engineering calculations but it's a really cool project! And the science works. 

Kinda makes a person wonder if an old farm silo might be put to use for something small scale.


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## Robotron (Mar 25, 2012)

Use this as a guide as to results. I will be applying this to my 72 foot south facing wall for heat in the shop. Really like the concept. 
http://www.motherearthnews.com/renewable-energy/solar-air-heater-zmaz06djzraw.aspx


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