solar air heating collectors

[GaryGary]

Hi,
I know its a long ways from winter, but I just did some testing on two DIY solar air heating collectors and thought I would pass it on.

http://www.builditsolar.com/Experimental/PopCanVsScreen/PopCanVsScreen.htm

One is the pop can collector in which the absorber is made from columns of alum pop cans with the ends cut out, and the other is the screen collector in which the absorber is made from 3 layers of black insect screen.

The screen collector is kind of amazing -- very quick and cheap and easy to build, but performs very well. Can be a couple year payback in a descent sun location.



GAry

 

[Ehouse]

What would happen with a finer mesh screen, like landscaping fabric? More than three layers?

 

[Seasoned Oak]

WHat about just a black painted sheet of metal or a piece of sheet metal roofing,the wavy kind for the heat sink?

 

[jebatty]

In the early '80's I built a 4' x 8' hot air collector mounted on the roof, 3/12 pitch, 45 degree latitude, and used steel corrugated panel as the collector, painted black, fiberglass greenhouse panel for the glazing, foam insulation panel on bottom and sides. Air flow was under the collector and then back over the top of the collector, 4" ducting and a 50 cfm bathroom exhaust fan for the blower. I used a snap thermostat, on at 100F in the collector. Air was blown in from our basement at about 60F and returned to the basement as high as 140F during winter on a calm, -20F outside temp day. Very satisfactory, but the problem was condensation inside the collector and rotting of the wood. The collector lasted about 6 years before I removed it due to deterioration.

 

[Seasoned Oak]

but the problem was condensation inside the collector and rotting of the wood. The collector lasted about 6 years before I removed it due to deterioration.

SO treated wood could solve this problem,RIGHT?

 

[Ehouse]

What about combining functions using a collection material porous enough to allow fan induced air flow but acting as insulation when the fan is off?

 

[GaryGary]

What would happen with a finer mesh screen, like landscaping fabric? More than three layers?

Hi,
Some of the people who have tried landscape fabric have reported a short life, but that may depend on the brand and material.

2 or 3 or 4 layers of screen is an open issue, and I may have a go at testing some combinations. One thing that does happen is that the solar radiation that gets through how ever many layers of screen hits the black painted back of the collector and gets absorbed there, and the air flowing up the box picks of that heat as well. But, it may be more efficient to have it absorbed in the screen as the back probably has to run quite hot in order to get the heat transferred into the air.

All good things to try.

Gary

 

[GaryGary]

WHat about just a black painted sheet of metal or a piece of sheet metal roofing,the wavy kind for the heat sink?

Hi,
This works, and people use it, but I think that the flow through absorbers are better.

Scott and I did some testing a couple years back comparing several types of absorbers, and the screen was the efficiency winner -- I think this has to do with the large surface area and lots of heat transfer area that thousands of strands of the screen wire provide.

This is the stuff on the earlier test: http://www.builditsolar.com/Experimental/AirColTesting/Index.htm there is nothing exactly like what you propose, but some are fairly close. I do think there is a lot left to learn about these collectors and more experimenting would be a good thing.

Gary

 

[GaryGary]

In the early '80's I built a 4' x 8' hot air collector mounted on the roof, 3/12 pitch, 45 degree latitude, and used steel corrugated panel as the collector, painted black, fiberglass greenhouse panel for the glazing, foam insulation panel on bottom and sides. Air flow was under the collector and then back over the top of the collector, 4" ducting and a 50 cfm bathroom exhaust fan for the blower. I used a snap thermostat, on at 100F in the collector. Air was blown in from our basement at about 60F and returned to the basement as high as 140F during winter on a calm, -20F outside temp day. Very satisfactory, but the problem was condensation inside the collector and rotting of the wood. The collector lasted about 6 years before I removed it due to deterioration.

Hi,
Nice.

The 60F to 140F is a bit high on the temperature rise and it probably would have produced a bit more heat with more airflow and a smaller temperature rise, but clearly it worked well.

I think that if you live in a climate where moisture problems can occur, its better to use a metal box for the collector. There is a design that uses galvanized metal sill track for the box. Its not too expensive and pretty easy to build -- should last a long time.


One example from Kennith: http://www.builditsolar.com/Projects/SpaceHeating/KenSoffitCollector/Main.htm
This one has a flow through absorber that uses aluminum vented soffit panels -- it was a performance tie with the screen absorber in our testing.

Gary

 

[jebatty]

As mentioned, winter performance was good, but the heating period was very short in Dec - January, about 2 hours maximum. The low sun angle does not provide heat for very long.

 

[Augie]

I have been thinking of Building Two 4x8 Panels for Shoulder Season Heating. 10-15k BTU's per hour would keep my house nice and warm when temps are in the 40's or 50's. This is my hardest time to heat as well because it is easy to over heat the house with the stove. I have an area on my house that is shaded in the summer but not after the leaves fall, this is when I would be interested in the type of heat that I could get form this kind of setup. Im thinking that If I placed The Units on the Roof I could run them right into the house, with a temp controlled fan I think this could be a great solution. I actually have a bunch of leftover materials and might be able to get away with almost no cost on this one...

 

[GaryGary]

As mentioned, winter performance was good, but the heating period was very short in Dec - January, about 2 hours maximum. The low sun angle does not provide heat for very long.

Hi,
This is not as bad as it first seems when you look at the day length in summer vs day length in winter -- the summer day may be 16 hours long and the winter day 8 hours, but in the summer the sun is behind the panel for much of that 16 hours. The real useful solar heating time is 9 am to 3pm -- if you can get that, then you are good. As long as the collector is not shaded during those times it will do well, winter or summer.

Clear winter days here in MT at 46 deg N lat produce a ton of heat -- the combination of the low sun shining nearly directly on a vertical collector and the strong reflection off snow in front of the collector provides much more radiation on the collector than that same collector in the summer.

Its more the amount of cloud cover in winter than the length of the day that limits solar heating in the winter, and that depends on your local climate.

-----------

People often rate the economic viability of solar PV on a peak watts per dollar (eg PV SYSTEMS are now selling for about $5 per peak watt).

So, what's the cost per peak watt of one of the DIY solar air heating collectors?

You can build a 4 by 8 ft air heating collector for about $200 in materials (this can include collector, ducting, blower, and thermal control switch).

This collector (if its a good design) under sunny conditions (1000 w/sm) and with a 35F ambient temperature and 120F outlet temperature operates at 53% efficiency with 168 BTU/sf-hr output. Its total output 5380 BTU/hr or 1575 watts.
http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm

Since the collector costs $200, the cost in dollars per peak watt is: ($200/1575 watts) = $0.13 per peak watt.
About 38 times less than a PV system at $5 per peak watt.

Of course, this is not fair because the PV system operates all year and solar heating system only during the heating season, but the difference is still pretty staggering (at least to me

I'm not trying knock PV ( I have a PV system and love it), but I think people don't appreciate what a good deal these air heating collectors are.


Gary

 

[Seasoned Oak]

As mentioned, winter performance was good, but the heating period was very short in Dec - January, about 2 hours maximum. The low sun angle does not provide heat for very long.

Wow 2 hours, My passive solar room gets about 7+ in the dead of winter from 9 AM to 4PM and also heats my entire house on sunny days in winter.

 

[woodgeek]

In my case, the PA resource is pretty good, say >1200 hours of peak output per year (for PV). During the 5 mos where I need real heating, I might have 400 hours of peak output resource. For PV, those partly cloudy and hazy days still contribute to those 400 hours. For thermal, a 50% cloud cover will hurt the eff a lot, so I will get less than 50% rated heat out. On the other hand, during a lot of that time, my outdoor temps are higher than Gary's, then my eff might be a tad higher.

So, if I assume I still get 400 hours of peak output per heating season, then that is 1.575 kW * 400 hours * 3414 BTU/kWh = 2 Million BTU/season.

My wind-powered BTUs from my heat pump cost me ~$15 /MMBTU (an estimate, they will be more expensive on colder cloudy days and at night), so a 4'x8' would save me ~$30/yr, for a 7 year payback at a $200 upfront cost (costing my 'hobby' labor at $0). The panel would offset 3% of my ~65 MMBTU annual heating load, or save me 1/10th cord of wood equivalent.

In contrast, a few weekends with $300 of weatherstripping, caulk and foamboard netted me a reduction of ~30 MMBTU/season, 15x what I would get from building a panel, and with a 150% annual ROI (on material cost).

 

[jebatty]

Low hanging fruit often is not glamorous. Glitz is attractive. I am as guilty as many others, but I do a pretty good job of both picking the low hanging fruit as well as picking the shiny apple at the top of tree. In this regard perhaps the lowest hanging "nutritional" fruit I picked was the insulation and plumbing modifications of my electric hot water heater at a one time cost of less than $50 and with a payback of $60+ per year in perpetuity, increasing as electric rates go up. That fruit was so low on the tree that no one even knows I plucked it unless I tell them, and when I do most look on with disinterest because it's not exactly pretty.

 

[GaryGary]

In my case, the PA resource is pretty good, say >1200 hours of peak output per year (for PV). During the 5 mos where I need real heating, I might have 400 hours of peak output resource. For PV, those partly cloudy and hazy days still contribute to those 400 hours. For thermal, a 50% cloud cover will hurt the eff a lot, so I will get less than 50% rated heat out. On the other hand, during a lot of that time, my outdoor temps are higher than Gary's, then my eff might be a tad higher.

So, if I assume I still get 400 hours of peak output per heating season, then that is 1.575 kW * 400 hours * 3414 BTU/kWh = 2 Million BTU/season.

My wind-powered BTUs from my heat pump cost me ~$15 /MMBTU (an estimate, they will be more expensive on colder cloudy days and at night), so a 4'x8' would save me ~$30/yr, for a 7 year payback at a $200 upfront cost (costing my 'hobby' labor at $0). The panel would offset 3% of my ~65 MMBTU annual heating load, or save me 1/10th cord of wood equivalent.

In contrast, a few weekends with $300 of weatherstripping, caulk and foamboard netted me a reduction of ~30 MMBTU/season, 15x what I would get from building a panel, and with a 150% annual ROI (on material cost).

Hi,

There is the common advice to work on the basics like insulation and sealing and you will get the greatest return, and I tell people the same thing all the time.
But, I've kept track of all the projects we have done around the house to save energy and estimated cost, energy saving, carbon saving, and return on investment (ROI) on each one -- they are all listed here:

http://www.builditsolar.com/References/Half/ProjectCharts.htm
The last chart is the ROI one.

Each project and the calculations are described here:
http://www.builditsolar.com/References/Half/Projects.htm
I'll be the first to admit that some of these energy and cost calculations are very hard to do accurately, but I decided I would attempt a calculation for every project.

If you look at the ROIs for the different projects, the insulation ones do generally do well, but some of the DIY solar thermal projects do as well or better. And, some of the oddball ones like the electric mattress pad heaters do even better.

The message for me is that nothing is cast in concrete, you have to try to estimate the cost and saving for each project and let stand on its own merits -- you might be surprised.

Some insulation projects can be quite expensive depending on what the current insulation level is and how hard it is to add more. We are having our roof redone, and part of the house has a cathedral ceiling that is hard to add insulation to except on top of the roof deck before the new shingles go on. The current ceiling is insulted to R38 with FG, and I was just not able to make adding insulation on top the roof deck pay at all.

---------
I'll see if I can find a more accurate way of estimating the seasonal heat output of the air collector -- I think you may be on the low side.
One rule of thumb that is rough, but is used a lot is 1 gallon of oil per sqft of collector. That would give about 4.5 million BTU per year for a 32 sqft collector.
For $2 propane (which is what we use) in an 80% efficient furnace, this would be $130 a year for a 1.5 year payback -- not counting any state rebates. The MT $1000 tax credit would actually pay for 5 collectors.

Agree that one 4 by 8 collector is not going to revolutionize anyone's heating bill in a cold climate. More area is definitely better, and its easy to build bigger -- south walls are made for space heating collectors.

Not sure how low the sun gets under cloudy conditions (probably varies a lot), but at half sun (500 w/sm) with 40F ambient and 100F collector outlet temperature, a well designed collector will still have about 40% efficiency -- still pretty good. But, I suspect that cloudy days are typically well under the 500 w/sm.
http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm

When I look at the power output on my Enpase PV system, the output on cloudy days is pretty bad. It does still produce a little power, but my impression just looking at the graphs is that the bulk of our PV power comes from sunny days. I wish I had saved some of the winter graphs, but did not. The next time we get an honest cloudy day, I'll keep track of it.

-------
My reservation about heat pumps is that for most people with typical fossil fuel generated electricity the carbon emissions including generation are about the same as if you heat with natural gas in an efficient furnace. To me the carbon emissions are as important as reducing costs.
In MT, each KWH comes with a 1.6 lbs of CO2 emissions even with some hydro and wind in our power mix.

There is also the eventual compressor replacement cost.

It looks like you have (or buy?) wind power, so the carbon is not an issue for you.
Also possible to put in enough PV to run the heat pump, but, to me, that seems like a lot of cost and complication compared to a simple solar thermal system.

Lots of trades to consider.

Gary

 

[Where2]

SO treated wood could solve this problem,RIGHT?

ACQ (the wood treatment) generally is intended to reduce fungi growth and decay damage. If you want to avoid water problems, use several coats of a quality paint to keep the moisture out of the wood. Not sure I'd want to have long term exposure to whatever off-gasses from ACQ as it ages. I'd seal it with paint...

What about just a black painted sheet of metal or a piece of sheet metal roofing,the wavy kind for the heat sink?

What I would propose, is to use a corrugated black metal surface elevated with stand offs from the rear insulated back. That way, air can flow over the top and the bottom of the absorber.

The OP's screen collector is a curious thought. Were you using fiberglass, or metal screen mesh? I wonder how much additional surface area is available using multiple layers of screen vs using a corrugated metal plate? The whole conversion efficiency of a thermal system is driven on surface area for thermal transfer.

 

[woodgeek]

Gary,

I love the projects, and especially the ROI calcs you do which are right on. But every case is different. I still think the rockies and west are much better for solar space heating than New England, due to the very different winter resource. I would like to make DIY solar work on my site, but I can never get it to pencil out (even if I weren't shaded by neighbors trees/hills).

Lots of folks seem to be going net zero with super-insulation+grid-tiePV+minisplits. We can say that that is dependent on grid, govt spending, complicated tech, but it is being done. Active solar thermal sometimes gets incorporated into those projects, sometimes not, depending on how it pencils out.

 

[4acrefarm]

I have built 2 solar collectors using black polyester felt and I am very happy with the results.

One is a small 61/2'x 21/2' in my master bathroom . This one is powered by a pv fan. Air enters near floor at 65* and returns at 130*. It runs 4 or more hours per day.

The other one is 3 times larger and suck air out of the basement and into our tv room, this one runs a 120v snap disc setup.This one stars around 10:00 am and runs till at least 3:00.

The bathroom temp is increasing about 15* and the attached bedroom is also heated by it. The large panel helps heat 400sf. Iam very happy with the results.
https://www.hearth.com/talk/posts/53589/

 

[Where2]

I have built 2 solar collectors using black polyester felt and I am very happy with the results.

One is a small 61/2'x 21/2' in my master bathroom . This one is powered by a pv fan.

^^^ Makes plenty of sense to me. I've found some 130CFM server fans that only take 6W@12V to run. Throw a snap switch in the system, and a dryer vent style self actuating flap and it would be automatic.

 

[GaryGary]

ACQ (the wood treatment) generally is intended to reduce fungi growth and decay damage. If you want to avoid water problems, use several coats of a quality paint to keep the moisture out of the wood. Not sure I'd want to have long term exposure to whatever off-gasses from ACQ as it ages. I'd seal it with paint...

What I would propose, is to use a corrugated black metal surface elevated with stand offs from the rear insulated back. That way, air can flow over the top and the bottom of the absorber.

The OP's screen collector is a curious thought. Were you using fiberglass, or metal screen mesh? I wonder how much additional surface area is available using multiple layers of screen vs using a corrugated metal plate? The whole conversion efficiency of a thermal system is driven on surface area for thermal transfer.

Hi,
Scott and I did some side by side testing of various solar air heating collectors a couple years ago Results here:
http://www.builditsolar.com/Experimental/AirColTesting/Index.htm
Also just got done comparing the pop can collector to screen collector -- basically what started this thread.

None of them are exactly what you propose, but it might give some useful info on how the various designs do.

We were both surprised that the screen collector was tied for best performer of the bunch -- happily, its also the easiest one to build.

I've used both fiberglass and alum screen. I don't believe that there is any significant difference in performance between the two.
I did attempt a calculation of absorber area for a three layers of screen collector, and came out with 2.5 times the flat absorber area. In addition to that, any light that gets through the screen hits the back of the collector, so, to some degree this also counts as absorber area.

I agree that absorber area is a big factor in air collector design, but I think there are some issues I'd call airflow quality.
Like:
- is the flow mixed well enough that all of the flow actually contacts the absorber, or is the airflow layered enough that some air goes through the collector without enough contact with the absorber to pick up heat?
- Getting the airflow velocity over the absorber high enough for good heat transfer is important.
- Is there moving air contacting the glazing, and how warm is that air -- both increase heat loss out the glazing.

Collectors like the pop can collector and the downspout collector have the advantage of no airflow against the glazing and a lot of absorber area (about Pi times the flat plate area). Yet they don't test out at the top of the heap.

The picture that people like to think about for the screen collector is that the air resistance of the screen makes the air spread out uniformly over the screen and then flow through the screen (the air path is arranged so that the air must flow through the screen to get out). There is airflow against the glazing, but its the lowest temperature air in the collector (direct from the inlet). Whether this picture is strictly accurate or not I don't know, but the testing does indicate its an efficient collector. The thermal images of the screen collector also seem to indicate pretty uniform flow without much in the way of hot spots.

Gary

 

[Augie]

I think what gives the Screen Collector the advantage is greater surface area of the absorber. You want the absorber to both adsorb as much solar radiation as possible and give it us as quickly as possible. To do this you need surface area. And the screens have a huge surface area, I don't know how much more exactly but Im guessing on the order of 1.5x2 times as much as pop cans. So even if transmission from screen to air is not as good as the pop can to air, you have so much more surface area that is doesnt matter

 

[woodgeek]

---------
I'll see if I can find a more accurate way of estimating the seasonal heat output of the air collector -- I think you may be on the low side.
One rule of thumb that is rough, but is used a lot is 1 gallon of oil per sqft of collector. That would give about 4.5 million BTU per year for a 32 sqft collector.
For $2 propane (which is what we use) in an 80% efficient furnace, this would be $130 a year for a 1.5 year payback -- not counting any state rebates. The MT $1000 tax credit would actually pay for 5 collectors.

Agree that one 4 by 8 collector is not going to revolutionize anyone's heating bill in a cold climate. More area is definitely better, and its easy to build bigger -- south walls are made for space heating collectors.

Not sure how low the sun gets under cloudy conditions (probably varies a lot), but at half sun (500 w/sm) with 40F ambient and 100F collector outlet temperature, a well designed collector will still have about 40% efficiency -- still pretty good. But, I suspect that cloudy days are typically well under the 500 w/sm.
http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm


Gary

Part 1:

Here's a more careful take. I still think the major difference is the Montana vs East Coast resource....

Using PVWatts with a vertical '1 kW collector with 100% inverter efficiency', I get ~500 kWh of total projected output from Oct 1st to May 1st (I gave you a seventh month). I interpret that to mean that my total projected harvest is equivalent to 500 hours of operation of a system that would give 1 kW perpendicular to the sun under clear skies, spread out over about 2000 hours of daylight, i.e. averaging only 25% sun.

It appears that your test panel scaled to 4x8' would give 6400 BTU/h. TImes my 500 hours, it would give 3.2 MBTU/year in my location, mounted vertically, assuming the perpendicular efficiency you got at all insolation angles and cloud cover (i.e. PV assumes similar eff across insolation levels). That is a rather favorable assumption for your case...your test is in the summer, with perpendicular incidence under clear skies. At half sun (or 30° incidence), your stated eff drops by a third. What about 40°F ambient? Let's derate you 25% to 2.4 MBTU/year. I trust your figure at 4.5 MBTU, which is almost twice as high....I figure it is the resource difference.

Part 2:

If I was still burning oil, 2.4 MBTU would require ~22 gallons of oil, maybe $80 and ~550 lbs CO2. My payback on a $200 panel is 2.5 years.

You are a little queasy on my heat pump, but at HSPF = 8.5 (verified), I can make 2.4 MBTU using 2400/8.5 = 282 kWh = $40. My simple payback versus the solar air is now 5 years.

If I used my conventional grid power, which is 1.06 lbs CO2/ kWh, running the heat pump releases 300 lbs CO2. This is 300 lbs more than the solar case, but only 55% that of the oil case. IOW, the HP/conventional grid is ~45% less CO2 intensive than my legacy oil system. This is indeed more than natgas, but like you I have no access to that.

Solar air thermal is effectively carbon free, but IMO it would be hard to get to more than 50% space heating fraction. My HP/conventional power and 50% solar air + oil backup would have **similar** CO2 emissions (while requiring 10 4x8 panels, some good thermal mass and some complex controls to avoid overheating).

But I can and do buy 100% wind power for the same price as conventional kWh, and get effectively carbon free BTUs for 100% of my heating needs, which I think would be effectively impossible with solar air.

And you're right, if I get only 20,000 run hours out of a compressor, and replacement is $3-5k, it increases my cost per BTU 25-40%, still comfortably less than oil, and still very low carbon. I'm at ~13000 hours now, and hoping to get 15-20,000 more. Time will tell.

Still, I think my HP would choke on your climate, the same as I think your collector would disappoint in mine. Different strokes.

 

[georgepds]

Part 1:

....You are a little queasy on my heat pump, but at HSPF = 8.5 (verified), I can make 2.4 MBTU using 2400/8.5 = 282 kWh = $40.......

FWIIW, I just installed a Fujitsu 12RLS2 heat pump, it has a HSPF of 12 (bigger is better here) . As to verified,you can find a good lab tests comparing similar Fujitsu/Mistsubishi 12 k BTU/h units here

www.nrel.gov/docs/fy11osti/52175.pdf


This Fujitsu units can go down to - 5 F .. at this temperature the unit can put out 12 kBTU/H with an COP of ~2 (according toboth Fujitsu and NREL( figs 4 and 5))

 

[isipwater]

Wow 2 hours, My passive solar room gets about 7+ in the dead of winter from 9 AM to 4PM and also heats my entire house on sunny days in winter.

Tell us more about your passive solar room. Do you build it? Will you share some photos? I am interested in doing a passive solar project on my house. Thanks!