# Idea I've been kicking around



## Roadking (Oct 8, 2009)

Induldge or bash me, Just want to see if this makes sense to others more adapt in the solar realm than I.
Harbor Freight (I know, been discussed at length before) 45 watt solar kit...charge controller mediocre at best, and currently (pun, sorry) $170-ish.
I have built thermosiphon solar heaters that have worked nicely, but would like more volume. If I were to build a thermosiphon collector large enough to house these 3 panels as the collector and hook directly the panels to a series of 12v computer fans (which I have several)...wouldn't that basically be a self contained solar powered forced air heating unit? (or 3; 1 15 watt panel per heater).
Pros; 1) cost equivalent to a small propane or kero heater, but requires no extra fuel. 2) Won't push the air until there is enough light to make a current, therefore, it will also be generating heat by the time the air flows...kind of like a thermostat, just not as precise. 3) I already use a few that are sized approximately correct (a bit larger than the panels, so fir is not an issue).
4) wouldn't need extra material for a collector plate; surely the panels, being blackish (although reflective) absorb heat as well as light...correct?
Cons 1) No sun, no heat...already have that. 2) $170 out of pocket for a small extra heating source (what is the life span of the panels so I could figure my depreciation, please?)...wouldn't be using for charging batteries, just to power the circulating fans.

Just kicking the idea around...if the useful life of the panels is at least 10 years, it would pay for itself. House has great southern exposure, oil heat which is our preference (wife hates LPG/propane), I hate electric heat. 1860 house...running new radiator lines and/or duct work to off the beaten path or seldom used rooms not happening.
Just looking at it as a mobile supplemental heat source.
How far off base am I?
Again, induldge, bash, or tell me to shut up...all are welcome (as long as you tell me why to shut up, that is...LOL). Thanks in advance.
Matt


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## fordson major (Jul 12, 2003)

I don't know much about the harbour freight panel but I've seen thermo siphons powered from the house supply and they worked fine. Pretty crude construction too.


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## Roadking (Oct 8, 2009)

Thanks about the house current, however, using these as stand alone type units; not using the wall plugs. Simply independent heating sources that could be taken to the garage, campground, spare room... any place that could use a touch of extra heat without wiring or cords.
The thermosiphons I've built do nicely in small areas...just looking to boost the volume (granted, at reduced temperature) for larger areas.
Matt


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## Windy in Kansas (Jun 16, 2002)

Most photovoltaic panels couldn't stand the heat. Reflectors shouldn't even be used with some of them.


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## Roadking (Oct 8, 2009)

That's the rub I was worried about. Figured that if it was viable, someone else would have already done it. Appreciate the info...
Possibly with a vented system? at preset temp, let it "blow off some steam", like a thermostat in a car?
Sorry, need to either kill the idea completely or keep pondering on it.
Matt


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## Harry Chickpea (Dec 19, 2008)

You explore like I do.

Kicking it around further...

What happens if you use clear polyethylene sheeting And fold and lay it on top of the PV panels at an angle, then fill the space between the layers of poly with water, and use the natural thermosyphon of the water as your heat source, while allowing the panels to generate power to move the hot water into the house?


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## mightybooboo (Feb 10, 2004)

Bet Solar Gary can explain some good options for you re: solar heating.


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## Ross (May 9, 2002)

Why do the solar panels have to be inside? it would make for a more compact unit sure but having a stand alone system is a great idea!


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## TnAndy (Sep 15, 2005)

Ross said:


> Why do the solar panels have to be inside?


Exactly. 

All you are doing is subjecting them to more heat, earlier failure, and reduced voltage ( panel voltage decreases as heat increases ).

Mount them outside and be done with it.

Other than that, it's an excellent concept, and simple system.


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## Roadking (Oct 8, 2009)

Thinking "inside the box" as the collector, thus removingthe need for collector material. Wasn't aware that the heat diminishes output. Figured it would shorten lifespan 'tho. Main reason I needed to ask...not exactly a question I was able to formulate except via my idea. Also in the event there was someting else I had missed.
Appreciate the info...and yes, I had thought about the water layer before, but what overtook my mind was the fear of a leak.
Matt


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## roachhill (Jul 8, 2009)

Can't quite picture the design so maybe this isn't helpful but how about mounting a solar panel on a hinged frame so it folds in like a cover when you put it away or move it. Then fold it out and lock it in place when in use.


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## machinist (Aug 3, 2010)

Our DD and son in law have worked out a design for this, but the solar panel is outside the box. They used a high capacity computer muffin fan mounted directly in the 4" outlet duct. Note that diameters are critical here to get efficient air flow! They ended up with a 5" diameter fan, so they had to build a smooth transition for it to 4" diameter. Just plunking it in front of the 4" outlet gave very little air flow--reduced by half at least. 

The solar heat collector can still thermosiphon, albeit with some restriction from the fan, but they have sized things so that when there is adequate sun for heat, the fan comes on because solar PV panel reaches the power threshold of the motor. 

The 4' x 8' collector gives an outlet temp of up around 140* without the fan, but drops to about 110* with the fan running , both measured in full sun. No worries about overheating anything--the fan comes on before it gets that hot. 


This thing is SLICK!


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## SolarGary (Sep 8, 2005)

Hi Matt,
You can use the PV panels to power a DC fan, and, as you say, it sort acts as its own controller by only running the fan when the sun is out. I agree with the others that putting the PV panels inside the collector is not a good idea as the output will be reduced by the higher temperatures.

From the point of view of getting the most bang (or heat) from your buck, I'd think about doing one of these two things instead:

- If you can build a thermosyphon air collector with a good flow path and good sized vents, it really won't need a fan to drive it. The key is getting a low resistance flow path and keeping the entry and exit vents large enough to not restrict the flow. This simple collector I use to heat my shop works really well -- plenty of flow rate, and is very cheap to build:
http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm
Here is a similar one set in a living room:
http://www.builditsolar.com/Projects/SpaceHeating/HouseTS/RobTS.htm
These collectors cost about $2.50 per sqft to build -- the $170 would give you about 70 sqft of extra collector area -- that's worth about 10,000 BTU per hour, or in watts about 3000 watts -- quite a bit more than the 45 watt PV panel!

- If you don't have room for the large vents that a thermosyohon collector requires to work well (that is if you don't want to put big holes in the wall), then I'd have a look at a fan forced solar air heating collector. Lots of them here:
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#ActiveAir
I like the ones with the flow through absorbers (eg screen), but they all work.
And, again, if it were me, I'd use the $170 to just build more collector area and use a regular AC fan to drive the collector. The $170 invested in more collector area will give you a lot of extra heat.

You can control one of these fan forced collector with a simple thermal disk switch that turns the fan on when the collector temperature hits about 100F, and turns it off it drops to about 85F. These are only about 10 bucks -- www.PEXSupply.com has them, or you can buy a replacement temperature switch for an attic vent fan at Home Depot, which is the same device.

Solar air heating collectors are about as good as it gets for investing money in solar -- the shop collector mentioned above has a one heating season payback! (but it is pretty cold here)

Gary


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## Roadking (Oct 8, 2009)

Thanks for the advice Gary. It was actually your cited site where I started getting interested and hands on with solar projects. 
I have used aluminum cans, crushed aluminum cans (restricted flow too much), sheet steel painted flat black, and one a friend gave me a 4'x8' sheet of copper...I built 2 with that!
I have also attached 3 1.5 battery maintainer solar panels, in paralell, to mini computer fans. they powered them, but the volume change was minimal.
These are used inside the house, hence mobile...i.e. from attic to craft room when needed; not outside and piped through the walls. Hence the reason for the sizing...about 3'x5' high. They work well enough...I just wanted more. Don't we all.
Thanks again all!
Matt


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