Author Topic: Solar hot air start (Read 22448 times)

Mary B · « **on:** September 30, 2011, 07:09:04 PM »

My start on solar hot air panels. Going to build 2 this year and see how they do. Design is sort of from BuilditSolar.com. I will finish up the tubing tomorrow and get it painted. Will paint the frame too and get it ready to mount on the wall. These will be mounted vertical on a straight south facing wall.

artv · « **Reply #1 on:** September 30, 2011, 07:53:53 PM »

Hi MA,...that looks' to be about 4x8',....is the duct work really neccesary?.....or does it replace any fans??.......acting like a siphon??
Looking good so far ,.....waiting for the finished product....thanks....artv

Mary B · « **Reply #2 on:** October 01, 2011, 03:04:06 AM »

Duct gets painted black and air is blown through it via a blower fan. Whole thing will get a sheet of polycarbonate over it.

Dave B · « **Reply #3 on:** October 01, 2011, 09:34:04 AM »

It will be interesting to see how this design works out for you. No doubt you will get heat as I have experienced with 2 panels I have been running for a couple years now. It is the best return on investment of anything I have ever built. Mine are flat plate absorbers, no duct work other than intake and exhaust, simple frame and glass very similar to your layout. Keep us posted, the duct work idea is very similar to the pop can absorbers we see and the pretty version that can be purchased for big bucks. It will be great to hear of your results. Dave B.

xboxman · « **Reply #4 on:** October 01, 2011, 12:45:58 PM »

Hi MA
what size is the duct pipe you have in there ??
thanks

Mary B · « **Reply #5 on:** October 01, 2011, 02:15:31 PM »

3 inch semi rigid dryer vent. Stuff is a pain to work with but the ridges will add turbulence and help get the maximum heat transfer.

Mary B · « **Reply #6 on:** October 01, 2011, 08:19:16 PM »

96 feet of dryer duct and fun with spray paint(high temp BBQ). Needs some touch up here and there but my finger gave out after 4 cans.

BrianSmith · « **Reply #7 on:** October 03, 2011, 11:58:38 PM »

Before you install everything, you might want to connect the blower to the dryer vent and see how much air flow you get out the other end. Going thru that much tubing is going to really reduce the air flow. Also, depending on how the tubing is run, you might have some extra slow down due to conduction flow fighting air flow on up to do runs. I have been there, done that with not so good results with a reasonably good set of fans on intake and exhaust side, but they weren't serious high power blowers fans so here's hoping your works better than mine did..... $:-\$

Mary B · « **Reply #8 on:** October 04, 2011, 03:43:27 AM »

I am going to be using a 110 CFM blower. If that doesn't cut it I will double up 2 of them. Got them cheap from Surplus Center.

TomW · « **Reply #9 on:** October 04, 2011, 09:12:48 AM »

Quote from: MaryAlana on October 04, 2011, 03:43:27 AM

I am going to be using a 110 CFM blower. If that doesn't cut it I will double up 2 of them. Got them cheap from Surplus Center.

I am no expert but I had a similar setup as a test where I stuck a length of that 4" flex aluminum dryer vent painted black zig zagged across a window in my office / workshop.

I used a 4"muffin fan connected to a Y2K era 1 square foot glass solar panel to boost flow when the sun was out.

The difference in that fans work load was obvious between open air and on the tube. It worked fairly well but the tube really hammered the air flow on the setup.

If I had to use a longer run I would run 2 side by side rather than in one line, especially if 2 fans were involved anyway.

Good luck with it.

Tom

TomW · « **Reply #10 on:** October 04, 2011, 09:15:46 AM »

Not to complicate things but ...

Slower air flow might be better at extracting the heat and I don't have any real idea at what point that matters?

Tom

Mary B · « **Reply #11 on:** October 04, 2011, 02:20:08 PM »

Blowers are designed to keep the air flow up under back pressure, muffin fans are generally not. And yes you don't want to fast of air flow or you don't extract all the heat.

Mary B · « **Reply #12 on:** October 13, 2011, 07:03:33 PM »

More progress, shiny spots are wet paint not bare metal. Need some more straps to hold the tube down then the cover can go on and it can be mounted on the wall.

ChrisOlson · « **Reply #13 on:** October 13, 2011, 08:13:16 PM »

Quote from: TomW on October 04, 2011, 09:15:46 AM

Slower air flow might be better at extracting the heat and I don't have any real idea at what point that matters?

Tom,

Faster air flow increases the efficiency of the panel and the amount of heat extracted, in what I found out. When I first built my solar hot air panel I had a bathroom fan on it. The glass ran way too hot. I put dual duct fans on my panel and the air temperature coming out was lower, but I get roughly twice the BTU/hr from it by increasing the CFM thru the panel, and the glass runs cool to the touch.

If you can feel heat on the glass, then you need more CFM because you're losing heat thru your glass. I know for a fact that polycarbonate would not work on my panel - it would melt or warp it if the fans are not running. I used 1/4" low-E high solar gain glass on mine, but that stuff is not cheap.
--
Chris

Mary B · « **Reply #14 on:** October 14, 2011, 02:46:15 PM »

There is a point where to fast is bad also, you are just wasting electricity with no further gain in efficiency. I have enough blower power to make it work so I am not worried on that point.

Bruce S · « **Reply #15 on:** October 14, 2011, 03:45:05 PM »

What about the possibility of a PWM'ed blower setup where you could "dial" in the CFM or speed of air circulation?
Having read a lot if different CFMs push versus pull and push pull, it would be interesting to be able to dail in the airflow.

Cheers;
Bruce S

ChrisOlson · « **Reply #16 on:** October 14, 2011, 04:57:34 PM »

Quote from: MaryAlana on October 14, 2011, 02:46:15 PM

There is a point where to fast is bad also, you are just wasting electricity with no further gain in efficiency. I have enough blower power to make it work so I am not worried on that point.

While it's true that there is a point of no return considering energy consumption of the fans vs BTU output of the panel, one of the basic principles of thermodynamics with air cooled systems is that increasing the airflow always improves cooling (or in this case heat transfer).

The viscous drag caused by long lengths of ductwork can increase energy consumption in the fans too, to get the required CFM to get efficient heat transfer. I suspect with your design that a centrifugal fan will provide better performance with lower energy consumption than an axial fan.
--
Chris

12AX7 · « **Reply #17 on:** October 14, 2011, 11:02:28 PM »

I have a pretty good idea what this is going to sound like.. and that's not my intentions!

Blowing into the pipe will cause a certain amount of pressure build up, which for all intents and purposes is the same as resistance.

Maybe it would be better to suck rather than to blow? Draw the heat out instead of pushing it.

if you feel the need to chuckle.. feel free!

ax7

MattM · « **Reply #18 on:** October 18, 2011, 12:43:56 AM »

Decrease your resistance by using a larger exhaust hole than the intake. The hot air is expanding and will follow the path of least resistance.

Mary B · « **Reply #19 on:** October 23, 2011, 05:39:12 PM »

Installed and running, I need a stronger center bar to control the bulge and may need to up the fan. Don't want to make that decision until I see output temps on a colder day. Right now it is 60 degrees and I am running as high as 140 degrees out. Damage to the roof above was caused by 85mph straight line winds, that is tomorrows project. Repair that if possible or decide what I need for material to replace. Need to finish replacing siding on that little strip too, wind stripped the siding off 2 sides of the house and damaged the other 2.

WindriderNM · « **Reply #20 on:** October 24, 2011, 05:34:28 PM »

I have a few plastic triggers that mount to spray cans they are cheap and save your thumbs.

Mary B · « **Reply #21 on:** October 26, 2011, 01:56:09 PM »

Yeah I need to buy a few of those, stop the sore finger and paint dripping off it syndrome.

ChrisOlson · « **Reply #22 on:** October 26, 2011, 10:20:19 PM »

Quote from: MaryAlana on October 23, 2011, 05:39:12 PM

.....and may need to up the fan. Don't want to make that decision until I see output temps on a colder day. Right now it is 60 degrees and I am running as high as 140 degrees out

The output air temp is not meaningful unless you look at input air temp. To tune your panel for optimum air flow, BTU/hr = CFM x 1.08 x temperature rise.

--
Chris

Mary B · « **Reply #23 on:** October 27, 2011, 01:41:34 PM »

Input around 74 degrees +- the temp drop to the floor where I put the cold air intake. Using a 110 cfm blower but I haven't measured the output flow yet.

ChrisOlson · « **Reply #24 on:** October 27, 2011, 02:15:55 PM »

You'll find that the more CFM you put to the panel, the more BTU you will get. You have to sort of arrive at a compromise between the power required by the blower(s) vs what you get from the panel in BTU output. Mine required 160 CFM to keep the glass cool and it's just got a flat plate collector in it so I was able to use small axial fans. It puts out about 13,000 BTU in direct sun. This is the one I built for the side of my shop:

http://fieldlines.com/board/index.php/topic,145880.0.html

With the long ductwork you have in your panel I think a centrifugal (squirrel cage) blower will probable be more efficient (perhaps that's what you're using already). Just don't skimp on air flow thru it. The first reaction is that moving the air slower gathers more heat. Not true. It heats the air more (more temperature rise) but you also get higher losses thru the collector itself. Only doing the math on BTU output revealed to me that more airflow is better. And then after doing the math it's backed up by the fact that the performance in heating the shop is quite a bit better with the higher airflow thru it.

--
Chris

Mary B · « **Reply #25 on:** October 27, 2011, 11:06:54 PM »

Yup centrifugal blower. Found some for $18 each so I can parallel 2 of the 110CFM blowers if I need to. Blower output is about 3 1/2 inch diameter. I square formed the end of some duct and screwed it on. A little tape and it is up and running for now. I may end up building a manifold to hold 2 of them to feed the 4 inch insulated duct going out to the panel where it gets adapted down to 3 inch. Needs more tinkering but I haven't had time.

ChrisOlson · « **Reply #26 on:** October 27, 2011, 11:20:43 PM »

Quote from: MaryAlana on October 27, 2011, 11:06:54 PM

Yup centrifugal blower. Found some for $18 each so I can parallel 2 of the 110CFM blowers if I need to

Just curious - have you tested the watt draw on your centrifugal blowers? The little 4" duct fans I got draw 24 watts each for 80 CFM (48 watts power draw for 160 CFM total). Typically, centrifugal blowers are more efficient and if I could've found a single one that moves 150-160 CFM at that price I would've bought that instead of the axials. I was at Menards the day I was looking for fans and I bought one of those duct fans first. Then found out it didn't move quite enough air so I bought another one. I think they were $20 each.
--
Chris

ghurd · « **Reply #27 on:** October 28, 2011, 12:13:27 AM »

CFM/W, muffin fans can usually hold their own when properly researched.
Most common surplus and cheap units move high CFM at low pressure.
Others move low CFM at high pressure.
Others are somewhere inbetween.
Comair Rotron had a pdf.
G-

Mary B · « **Reply #28 on:** October 28, 2011, 12:56:07 PM »

64 watts running, can't tell you startup(these are capacitor start). With 100+ feet of corrugated aluminum duct I figure the back pressure has to be fairly high so I went with the blower that is designed to handle that. Good source for odd bits and pieces at sometimes cheap prices http://www.surpluscenter.com/ that is where I bought 4 of these blowers.

ChrisOlson · « **Reply #29 on:** October 28, 2011, 01:30:16 PM »

Cool - thanks! I added that link to my collection of links where you can get Stuff. I seen they had these 24 volt contactors with 25 amp rated contacts @ 240 VAC.
http://www.surpluscenter.com/item.asp?item=11-3121&catname=electric

82 ohm coil so the coil only pulls .3 amps. Those are real handy for stuff, like turning on water heaters with 24 volt controls using a Morningstar RD-1. So I just ordered two of 'em

--
Chris

Mladen · « **Reply #30 on:** November 11, 2011, 03:19:25 PM »

perhaps you should use solar PV panel to power-up DC fan , so this project can be FULLY "SOLAR" :-)

Mary B · « **Reply #31 on:** November 11, 2011, 11:59:52 PM »

Down the road the blower will be inverter powered.

Ungrounded Lightning Rod · « **Reply #32 on:** November 12, 2011, 12:07:03 AM »

I don't understand the point of having the pipe in there at all.

Why not just have a black box with no pipe? Intake at one corner, outlet at the diagonally opposite corner. If you feel you must have a serpentine airflow, add a few horizontal wooden baffles starting from alternate sides and going most of the way toward the opposite side, stopping about the baffle-to-baffle spacing from the side to give the air a good passage to the next horizontal path.

Want better light catching? Fanfold some metal, partially unfold it, paint it black, and mount it on the back fo the container. Light will take multiple bounces going down the fold before it can get back out, improving absorption. If your point of the pipe was to keep the hot air separated from the outer glass for insulation, mount your collector nearer the front and do your air harvesting on the backside of it. Or just use two layers of glass.

If you must put the air in a serpentine pipe, at least have the runs horizontal, rather than vertical, so you don't have to repeatedly pump the hot air downward. Convection should be your friend, not your enemy. Heck: Unless you want to force the air down to the floor level you can let convection pump it for you.

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Author Topic: Solar hot air start (Read 22448 times)