BuildItSolar: Solar energy projects for Do It Yourselfers to save money and reduce pollution This site has more info then a guy would know what to do.
He has been in Mother Earth News on some of his own projects.
solar thermal heating
- Thread starter forgeblast
- Start date
BuildItSolar: Solar energy projects for Do It Yourselfers to save money and reduce pollution This site has more info then a guy would know what to do.
He has been in Mother Earth News on some of his own projects.
Back in the late 70's I built a cheap and quick 8x12 ft. hot air collector as an experiment. 2x6 frame, Thermax foil covered insulation, eave trough and downspouts for collectors, fiberglass roof for glazing, painted everything with flat black Rustoleum. It had a 160 degree fixed temperature thermostat for the fan and it ran a lot.
I didn't cover it during the summer and over 2 summers it baked itself to death. The wood fell apart, the foil on the Thermax began to peel, the fiberglass became opaque. IMHO even with really well built collectors it will extend their life if they're covered through the summer, the heat build up has to be huge.
This fall I decided to kill 2 birds with one stone. My windows are getting old and leaky, I wanted to try a little better built longer lasting collectors as another experiment.
I built what was basically inside storms with small collectors on half the south facing (slider) windows. The collectors are made of .024 aluminum trim coil stock. They are only 3 1/2" deep so they don't hang out past the window sills very much, foam board for insulation and thermosiphon.
The collector is the flat plate that's the back of the inside box with a 1/2" gap and another piece of aluminum behind it, air flows between the two. At the bottom there are 1 or 2 openings (depending on window width) on the back with slide dampers. On top are slots for the air to exit.
They work amazingly well considering the sun light is passing through 2 sheets of glass and a layer of plastic. If I shoot the back of the collector (laser thermometer, all kinds of nifty tools we didn't have when I built the original collector) through the bottom opening the temperature is 125 degrees. The ceiling above is 20 degrees warmer than a few feet away. You can feel the air flow if you put you hand by the opening.
IMHO you do need a damper to close the vent for the night. It will thermosiphon in reverse if the room air is warmer. Right now the room temperature is 70 degrees, the back plate is 49 degrees and there is a cold flow out the vent. If you don't block this somehow I think anything you gain during the day you will loose overnight, maybe more considering cloudy days (which we get a lot).
One caution. I used the pink Dow foam board for insulation. I built a small one of these as another experiment in 2009. The inside aluminum box sat tight against the insulation on the sides. When I tore it apart it had melted the foam. Got in touch with Dow and they said the maximum temperature is 160 degrees. On the latest version I left a 1/4" gap between the insulation and aluminum. These will get hot, anything you use needs to be capable of handling higher temperatures than normal building materials.
I didn't cover it during the summer and over 2 summers it baked itself to death. The wood fell apart, the foil on the Thermax began to peel, the fiberglass became opaque. IMHO even with really well built collectors it will extend their life if they're covered through the summer, the heat build up has to be huge.
This fall I decided to kill 2 birds with one stone. My windows are getting old and leaky, I wanted to try a little better built longer lasting collectors as another experiment.
I built what was basically inside storms with small collectors on half the south facing (slider) windows. The collectors are made of .024 aluminum trim coil stock. They are only 3 1/2" deep so they don't hang out past the window sills very much, foam board for insulation and thermosiphon.
The collector is the flat plate that's the back of the inside box with a 1/2" gap and another piece of aluminum behind it, air flows between the two. At the bottom there are 1 or 2 openings (depending on window width) on the back with slide dampers. On top are slots for the air to exit.
They work amazingly well considering the sun light is passing through 2 sheets of glass and a layer of plastic. If I shoot the back of the collector (laser thermometer, all kinds of nifty tools we didn't have when I built the original collector) through the bottom opening the temperature is 125 degrees. The ceiling above is 20 degrees warmer than a few feet away. You can feel the air flow if you put you hand by the opening.
IMHO you do need a damper to close the vent for the night. It will thermosiphon in reverse if the room air is warmer. Right now the room temperature is 70 degrees, the back plate is 49 degrees and there is a cold flow out the vent. If you don't block this somehow I think anything you gain during the day you will loose overnight, maybe more considering cloudy days (which we get a lot).
One caution. I used the pink Dow foam board for insulation. I built a small one of these as another experiment in 2009. The inside aluminum box sat tight against the insulation on the sides. When I tore it apart it had melted the foam. Got in touch with Dow and they said the maximum temperature is 160 degrees. On the latest version I left a 1/4" gap between the insulation and aluminum. These will get hot, anything you use needs to be capable of handling higher temperatures than normal building materials.
GaryGary
New member
Hi,
I don't cover them in the summer.
I block off the hot air vents going into the shop so they don't overheat it, so the collectors basically run stagnated all summer.
When I built the collectors, I was worried about the collectors overheating in the summer when stagnated, but have found that vertical collectors (especially single glazed ones) don't run all that hot in the summer because the sun is so high. The highest temperature I've seen in the stagnated collector in the summer is 185F, which seems fine to me.
A vertical collector gets about 2.5 times more sun in the winter than in the summer -- kind of nice the way that works
Overhangs just above the collector also help with summer overheating in that (if you get the length right), they shade most of the collector in the summer, but not in the winter.
Google SketchUp is good of working overhangs out -- it has a built in sun.
Edit: missed the part about the night dampers.
I use thin poly sheets on the collector exit vents to stop reverse flow at night. They are basically check valves that blow outward (into the room) to let hot air flow, but when the collector starts to reverse flow at night, the poly gets pulled right up to the hardware cloth screen that goes across the exit vent. They work just amazingly well. Fun to watch them -- as soon as just a bit of sun gets on the collector they open just a little bit, and a little warm air starts flowing -- by noon on a sunny day, they are streaming out at 70 degrees or so -- as soon as the sun goes off the collector they get sucked tight against the hardware cloth backing and seal quite tightly.
The poly film has to be lightweight --I use a double thickness of Costco 30 gallon garbage bags.
It would be a little better to put these dampers on the bottom (inlet) vent, but its harder to do there (they have to swing into the wall), and it does not seem to make a lot of difference.
Have 2nd Argosy's note on pink or blue or white styrofoam -- these collectors run too hot for this. You can use the polyisocyanurate rigid foam board -- most lumber yards carry it. Atlas R Board is one brand.
I've not had the problem with the collector deteriorating -- its 5 years old and looks just like the day it went up including the SunTuf glazing. Part of this may have to do with the vertical orientation.
Gary
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GaryGary
New member
Hi,
On the collector I built for my shop, I use 2 layers of ordinary window insect screen for the absorber. The air path is such that the inlets on the bottom of the collector direct the air to the glazing side of the screen, and the air has to flow through the hot screen to get to the exit vents at the back top of the collector -- a "flow through" absorber.
Screen works especially well for thermosyphon collectors because it has a lot of surface area, but very little flow resistance.
On the tests that Scott and I are doing, we are trying both the same kind of window screen as well as vented aluminum soffit. The soffit is supposed to work the same way as the screen does with the little vent holes allowing the air to flow through it. So far, the screen seems to be outperforming the soffit, but more to do on this.
Furnace filter media (the stuff you can buy in rolls) is also rumored to work well as a flow through absorber, but I've not seen any numbers.
Gary
OP
forgeblast
Elite Member
do you use the aluminum screen and paint it or use the black fiberglass/nylon screen.
How deep do you make your box then, 2x6, or 2x4 with the screen. I would imagine wiht the insulation you use the 2x6.
How deep do you make your box then, 2x6, or 2x4 with the screen. I would imagine wiht the insulation you use the 2x6.
GaryGary
New member
Hi,
I used the aluminum screen that comes in the "charcoal" color. I don't think that its painted -- looks more like it might be anodized.
Scott used the black fiberglass screen in his tests, and also performed well. I'm not sure whether the aluminum screen will have a service life advantage over the FG or not.
For fan forced collectors I think that a depth of about 4 inches is good. The screen should be slopped so that its close to the glazing at the top (exit), and far from the glazing at the bottom (inlet).
This link show how I constructed the aluminum soffit collector -- just substitute the 2 or 3 layers of screen for the soffit.
http://www.builditsolar.com/Experimental/AirColTesting/SoffitCollector/Building.htm
For thermosyphon collectors, the design rule is to make the depth of the collector about 1/15th of the height. You want to slope the screen in the same way as the link above shows.
On thermosyphon collectors, its important to have sufficient inlet and exit vent area. A 4 by 8 collector bay wants to have about 1 sqft of inlet and 1 sqft of outlet -- this should be spread over the width of the collector as evenly as possible. For example, I use two 4 by 18 inch inlet slots on the bottom and two 4 by 18 outlet slots on the top of each 4 ft wide collector bay on mine.
Gary
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OP
forgeblast
Elite Member
Not sure the wife will let me put some long holes in the house, ill post some pics showing our issues.
2manyrocks
Super Member
- Joined
- Jul 28, 2007
- Messages
- 7,319
Pictures of the solar collectors that the other guys have built would be good, too.:thumbsup:
The whole solar heating thing was a hot topic in the late 70's and then cooled off. As expensive as fuel has become, it may heat up again.
The whole solar heating thing was a hot topic in the late 70's and then cooled off. As expensive as fuel has become, it may heat up again.
Gary, would a dual purpose collector work? IE, Hot air and solar water?
Thanks, Tim
PS What do you think of the 2 and 3 pass hot air collectors?
GaryGary
New member
Hi Tim,
I do think that a dual purpose collector will work. Here is one example:
DIY Solar Air Heating Collector with Water Heating
You would, of course, be splitting the heat between the two -- no free lunch
I've not looked in any detail at the multi pass air heating collectors, so I can't really say on that.
Gary