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Old 02-24-09, 01:25 PM   #1
Xringer
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Default Backyard Heliostat for space heating?



Is it possible to use a backyard Heliostat for space heating?

I think it's possible, but not really easy to understand (at least for me).

I found some Boston Mass solar data at http://rredc.nrel.gov/solar/pubs/redbook/PDFs/MA.PDF

It says that an area of 1 sq meter in my area gets an average of
5.9 KWh per day. About 4 during the winter and 6 to 8 in summer.
(With 2-axis tracking). The general rule of thumb is about 1 KW per hour.


The array that I'm thinking about would be a 10'x10' rack using 25 2'x'2 mirrors.
100 sq feet. Or, 9.3 Sq meters Mounted on my old C-band 10.5' dish mount.

If 5.9 KWh x 9.3 sq meters ( or 100 sq feet)= 54.87 KWh per day, then
that's a whopping 1646 KWh a month!

-----

Winter is when this would be used. So 4 KWh x 9.3 = 37.2 KWh per day.
37.2 over a 6 hour time frame is 6.2 KWh (per hour).

6.2 KWh is about 21,155.28 BTUh..

Did I goof up somewhere here? Is it possible to get about 20,000 BTU of
solar heat (reflected into a rear window) from a 10x10 foot mirror array??

If this is really possible, it would be a pretty dang cheap BTUs per dollar..

Comments please:

Cheers,
Rich

PS:
If it works, I would build a larger one like this

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Old 03-03-09, 11:48 AM   #2
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I'm sure its possible. What exactly were you thinking about doing with it? Wouldn't it be easier to use a solar collector that passes air through it? That way you don't have to bounce light around and worry about tracking the sun's movement.
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Old 03-03-09, 02:30 PM   #3
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Quote:
Originally Posted by Daox View Post
I'm sure its possible. What exactly were you thinking about doing with it? Wouldn't it be easier to use a solar collector that passes air through it? That way you don't have to bounce light around and worry about tracking the sun's movement.
I've had some experience with a solar (domestic) hotwater system,
and I have a little understanding of what can be done with flat panel collectors.

IMHO, in order to get around 20,000 BTUs of space heating, using air or water as the transfer medium,
Panels are going to cost more and will have to be roof mounted.
(My wife isn't interested in putting big panels back on our roof).

Passive solar is the best way to go. Least amount of moving parts.
And, reflecting solar into a window is almost as cheap.
(when compared to systems with pumps or fans etc).

The cost of the system is low, the payback would be quick.
I wish that I had 20,000 BTUs flowing in the back sliding door right now!

I did some closer reading on the topic and found out that the vertical aiming
problem is a bit more complex than I figured on..
If I used my old dish and added a extra actuator jack, I would need to use
a more sophisticated control system. Most likely a little computer with a string of three sensors around the target.
That would add another $50 to $100 in hardware cost..

Here's the old 10.5' dish..

Not doing much these days.. Right now, the existing actuator jack is configured for vertical motion. I would need to rotate the dish back to it's standard setup and add something like you see in the pic..

-----

This is just brainstorming right now. I'm also thinking about restoring the
dish back to polar tracking and use it to track the sun dead-on.

Maybe I could mount a rack of four 175 watt PV panels.
I need to study what can be done after the leaves come back and the shadows fall.
The dish may not get the sun long enough to be worth the effort.
At first glance, it doesn't look too bad.
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Old 05-19-09, 11:40 AM   #4
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For a lot less money and work, why not make one of those inflatable reflectors? Sorta like an inflatable helium balloon made of Mylar, but has the reflecting material on the back side and clear Mylar on the front side. When inflated, the shape becomes practically parabolic to focus the concentrated sunrays. These cost a fraction of what's been depicted above.

Probably, you could make your own, provided you learn heat welding of the Mylar, or find suitable adhesive. Very light weight. So cheap you could make a bunch of them to capture much more sunlight for the same or less cost as those shown above.
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Old 05-19-09, 01:00 PM   #5
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Or, if you just want to heat up some air, look into using flat panels made of Coroplast. That's the stuff realtors and politicians use for signs, a plastic equivalent of corrugated cardboard, with hollow channels called flutes. ~1/2" thickness probably best, for ease of air circulation. Comes in various colors, including black, or you could paint a white one flat black. After an election, get 4 X 8 panels free from defeated politicians. Make a manifold out of PVC pipe and attach at each end. Point it at the sun and tilt one end up, so natural circulation takes place as the air inside heats up and rises, to be circulated into your living space. Cheap. Light. Efficient. Easy.

Even cheaper would be a large solar oven made of black plastic sheeting, like the 6 mil stuff used to seal building foundations before they pour the concrete on, cheap at Home Depot. Comes in very large sheets, which could be laid out and held down on a south-facing yard or slope. Have a sheet of clear plastic on top, to act as a simple greenhouse effect. Sun heats it up, and you draw the air into your house and circulate via simple ducts. For heat storage, add some rocks which are then heated by the hot internal air, to release their heat later, acting as thermal ballast.
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Old 05-19-09, 05:31 PM   #6
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I've been thinking about the coroplas collector idea lately. I've made some RC airplanes out of the thin stuff and it's pretty easy to work with.


I think it might be sold in 4x8 foot sheets too.

If they make 1/2 thick coroplas, it might be pretty good for low pressure
(drain-down) hot water panels.

But, the thin stuff might be usable for hot-air collection, if placed under a couple of layers of clear Mylar.

I have some coroplas I could use to make a small test collector.
I even have some old glass storm windows that I could use for glazing.

Now that we are getting some good sunshine, it might be fun to play around
with some cheap collectors.
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Old 09-03-09, 12:28 AM   #7
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Hey there, I too have a big ol' dish in my back yard that I thought would make a great heliostat... I think it would collect alot of sunlight, but do you really want to direct all that into a window on the back of your house? Yer gonna burn yer house down man! Not to mention that's ALOT of sunlight... you'd blind yourself everytime you walk in the room.

That said, there are other pitfalls as well. Like you said, accurately tracking the sun is going to require another axis, and unless your a decent hand at building circuit boards, a controller is going to set you back about a pretty penny. On top of all that, the biggest problem is going to be keeping the mirrors clean all winter... dirt and snow will accumulate fast.

While I think heliostats are by far the coolest and biggest heat payoff, from what I've seen so far, passive solar collection seems to be the easiest and cheapest source of heat. Just my buckand a quarters worth...
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Old 09-03-09, 12:48 AM   #8
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Quote:
Originally Posted by cdig View Post
Hey there, I too have a big ol' dish in my back yard that I thought would make a great heliostat... I think it would collect alot of sunlight, but do you really want to direct all that into a window on the back of your house? Yer gonna burn yer house down man! Not to mention that's ALOT of sunlight... you'd blind yourself everytime you walk in the room.

That said, there are other pitfalls as well. Like you said, accurately tracking the sun is going to require another axis, and unless your a decent hand at building circuit boards, a controller is going to set you back about a pretty penny. On top of all that, the biggest problem is going to be keeping the mirrors clean all winter... dirt and snow will accumulate fast.

While I think heliostats are by far the coolest and biggest heat payoff, from what I've seen so far, passive solar collection seems to be the easiest and cheapest source of heat. Just my buckand a quarters worth...
I can see dirt accumulating, but would snow really affect the mirror that is reflecting heat from the sun?
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Old 09-03-09, 09:25 AM   #9
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Dirt wouldn't be anymore of a problem than it is on hotwater panels.
Adding the extra axis would be the main sticking point with me.

Once you have the elevation actuator installed, you can shake the snow off.
I've done it many times.

But passive is really the way to go. No moving parts!
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Old 12-21-09, 03:40 PM   #10
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I don't think you would want to use Colorplas. I think that stuff is toxic when it off gases as it is heated.

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