Author Topic: Air heater sizing and design help  (Read 6558 times)

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isee

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Air heater sizing and design help
« on: January 16, 2015, 02:51:41 AM »
I have a detached workshop in my south facing garden in South of London.
It's well insulated and is about 2.4 meters by 4.9 meters in size. (8ft*16ft).
I am toying with the idea of installing a solar heater on the south wall (it's the 8ft one) which also has a window installed.

I initially priced up some b&q materials based on 1.2m by 0.6m Mainly due to this being a standard size for many many materials and although I need the heater as a proof of concept more than an actual heater, I do wonder if it would be better to make it an appropriate size if I am going to put any effort into it in the first place?

The second question would be with regard to the top lip (sill/bezel) seeing as I would need to make a little sloping roof on top of this wall mounted collector  to keep the rain water off. would it be a good idea to make the actual top part of the frame sloped and transparent, to both keep the water off AND allow that little bit sunlight in? As an added bonus it might actually look neater too.

P.S. the collector will have a snaking 10cm diameter aluminium foil duct (painted black of course) with cold air entering the lower right corner and assisted by a 80CFM PC fan, (which is driven by a solar panel) and exiting in the opposing top corner.

thoughts?

isee

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Re: Air heater sizing and design help
« Reply #1 on: January 16, 2015, 03:00:35 AM »


Quick and dirty drawing of the planned collector, with the sloping and transparent top and the rough duct layout.

I should add that I am planning to use a twin wall window rather than the corrugated one to make the joints that little bit easier to seal.
« Last Edit: January 16, 2015, 03:05:14 AM by isee »

gww

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Re: Air heater sizing and design help
« Reply #2 on: January 16, 2015, 07:20:44 AM »
I.....
I can't answer your questions but wonder if you have given thought to summer time operation and how to handle that? 
gww

isee

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Re: Air heater sizing and design help
« Reply #3 on: January 16, 2015, 07:28:35 AM »
I have, I could put a drape over it, no problem. Also it will have a good amount of shade once the grapevine in front of it gets its leaves.
I started ordering bits and pieces for it already. I really shouldn't, I got soooo much more important stuff to do, but I cannot resist heh heh.

gww

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Re: Air heater sizing and design help
« Reply #4 on: January 16, 2015, 08:26:24 AM »
I
I had seen solar desing houses that used the fact that the sun was higher in summer then in winter and so they used roof overhang to shade the collectors during summer and during winter the sun was low enough to hit the collector.  Sort of sounds like grape vines doesn't it.  I actually trimed my grape vines yesterday after letting them go for 5 years.
Good luck
gww

isee

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Re: Air heater sizing and design help
« Reply #5 on: January 16, 2015, 09:08:16 AM »
Thanks, I am sort of accounting for it by mounting it on the wall, rather than on the roof too. So that the winter sun hits it more directly, and the summer sun hits it at an angle, reducing summer efficiency.

Mary B

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Re: Air heater sizing and design help
« Reply #6 on: January 16, 2015, 04:16:01 PM »
4x8 foot collector using 3" aluminum flex duct. When I rebuild it next summer I am modifying it to use charcoal colored aluminum window screen. 3 layers set in at slant with the top out further than the bottom. Window screen allows a smaller blower or even a large computer fan.





will raise the temperature in my 1500 square foot house by 2 degrees in 3 hours even when it is -10f as long as wind is not blowing directly at it sucking heat off.

SparWeb

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Re: Air heater sizing and design help
« Reply #7 on: January 17, 2015, 06:09:30 PM »
isee,

There are many well-documented projects of this sort on Gary Reysa's website www.builditsolar.com
I've built a few things with solar collectors myself.  You look to be on the right track.
Putting a clear top sill probably won't do much - the angle of the sun incident on the top sill is very shallow.

If you are willing to take measurements and be "scientific" about the project, then there is a lot of benefit to building a small test and scaling up from there.  Using flow and temperature measurements you can work out the amount of heat being absorbed per square foot of collector at any given time.  Then figure out if the full-size collector will have a significant effect.  Again, Gary Reysa does this in detail on his site, so you can learn a lot about this before you start.  Also, knowing in advance if you will benefit from forced air flow (a fan) will help you design an even more effective unit (though I doubt it, England is soooo sunny).

Your shop space sounds ideal for this kind of heating project.  Unlike photovoltaic cells, the shadow of a branch here or there does not hamper the entire panel's output.

Looking forward to seeing the result!
No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

System spec: 135w BP multicrystalline panels, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

isee

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Re: Air heater sizing and design help
« Reply #8 on: January 19, 2015, 12:51:06 AM »
4x8 foot collector using 3" aluminum flex duct. When I rebuild it next summer I am modifying it to use charcoal colored aluminum window screen. 3 layers set in at slant with the top out further than the bottom. Window screen allows a smaller blower or even a large computer fan.

will raise the temperature in my 1500 square foot house by 2 degrees in 3 hours even when it is -10f as long as wind is not blowing directly at it sucking heat off.

Thanks for the pics Mary.
I actually built a quick alu screen test collector in my farm in July, it was a completely overcast week though and I needed to re-use the materials for a building project so couldn't keep the collector for more extensive testing, it worked but I wasn't too impressed. I know that according to build it solar the isofit screen design is perhaps the mist efficient, but I do like a duct design, for the ability to place the entry and exit holes at points other than middle bottom and top respectively. Also, if the ducted design does a good enough job, I will be fitting one on my house and the space there is very wide, but not very big height wise.

Out of interest, do you know if forcing the warm air down requires noticeably more power or a stronger fan than forcing the air sideways as per my drawing?

isee

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Re: Air heater sizing and design help
« Reply #9 on: January 19, 2015, 01:12:32 AM »


Looking forward to seeing the result!

Thanks for the info, builditsolar was my inspiration, but the clear top was my own idea. I realise it won't make a significant difference, but was just checking here if there is a reason NOT to do it :) If there isn't then I might just go for the clear top, simply to squeeze out the extra 0.1% of heat, every bit counts in renewable energy.


gww

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Re: Air heater sizing and design help
« Reply #10 on: January 19, 2015, 08:54:11 AM »
My belief (probly wrong) would be that you would be better served with an insulated top sides and bottom and that a glass top might lose you more heat then any posible gain you get by the few sunrays that get through it.  I have no sience behind my tought just my gut feelling.
gww

Mary B

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Re: Air heater sizing and design help
« Reply #11 on: January 19, 2015, 07:12:27 PM »
X feet of duct takes the same air pressure no matter the direction of air flow.

Back and sides of mine are insulated with 1" foil faced foam board, the front is an insulated polycarbonate panel with dead air space in the tubes that make the panel up.

« Last Edit: January 19, 2015, 07:18:33 PM by Mary B »

MattM

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Re: Air heater sizing and design help
« Reply #12 on: January 19, 2015, 11:07:37 PM »
Mary B's solution is a decent wind brake.  However, they are UV treated, so you will lose some of the heat.

If you cannot find another pane of UV transparent glass then test with some clear plant nursery plastic sheeting.  It will wear out over the course of a winter, but it's a fine (cheap) near term solution.

Mary B

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Re: Air heater sizing and design help
« Reply #13 on: January 20, 2015, 12:03:04 AM »
Heat loss must be pretty minimal, my heater works down to -10f with light winds(-20f if no wind at all which is common with deep cold). So it isn't something I would worry about all that much. I think using the flattest black paint you can get your hands on is more important.