collector question

[artv]

Hi all,...I've been testing a big rad I got from a building that used a boiler for heating. The rad is 3'x6', it has two layers of 1/2" copper tubing with aluminum fins on each tube, each layer has 21 tubes in it that are paralleled off the header.(one header for each set of 21 tubes)..... So 21 tubes paralleled hooked in series with the second set of 21 tubes paralleled. The question is , when I set up the collector to face the sun , one layer of tubes will get direct sunlight , whereas the 2nd set will be behind the first set, should I pull the heated water out of the front side(the direct sunlight tubes) or should I pull the water out of the tubes behind the ones in the direct sunlight.
I ran it for ~4 hours the first day with the collector just laying in the back of a 1/2 ton truck, circulating the water every hour for 60 seconds, feeding into a 40gal hotwater tank (it's a closed system). The next day I ran for another four hours.
Then I drained the system,  the water in the tank was nice temp for a shower(~40 gallons) , the water in the collector was scolding hot you couldn't keep your hand in it.
Just wondering if I should pull the water out of the front header or the back header?? Also if I should maybe circulate the water more often than 1 min every hour??.........side note the pump will empty a 5 gallon pail  in about 5-8 seconds.......sorry for the long post explaining myself is not one of  my strong points.......thanks ....artv

[zap]

If you want to keep the system simple... I've never tried this...

Put the rad (I'm assuming you mean radiator?) under some clear or nearly clear plastic, pull the water off the "shade" side, and circulate it constantly if you can.. although the pump you mention is probably over kill.

Then I drained the system,  the water in the tank was nice temp for a shower(~40 gallons) , the water in the collector was scolding hot you couldn't keep your hand in it.

How you have is set up is not the most efficient but if it gets the job done then why change it?
If you pulled water off the bottom header then I would think some of the heat (from the hotter front) would make it's way back into the front?

I had a couple of heat exchangers that I wanted to try the same idea with... but the scrappers that prowl the area made off with them before I had a chance to try them.

[rossw]

Put the rad (I'm assuming you mean radiator?) under some clear or nearly clear plastic, pull the water off the "shade" side, and circulate it constantly if you can.. although the pump you mention is probably over kill.

I'm curious as to the logic for taking it from the back ("cool") side? I'd have thought taking the hottest water from the sunny side would give you the best energy transfer and lowest losses. Pump the coldest water into the "cool" side and let it pre-warm before it gets to the sunny "hot" side.

[zap]

I'm curious as to the logic for taking it from the back ("cool") side?

Yeah... it was mostly a guess
That's why I put the question marks and the disclaimer

Honestly, with it set up the way it is, I doubt it would make much difference where you pulled the water off of?

[rossw]

I'm curious as to the logic for taking it from the back ("cool") side?

Yeah... it was mostly a guess
That's why I put the question marks and the disclaimer

Ahh.

Honestly, with it set up the way it is, I doubt it would make much difference where you pulled the water off of?

My thinking is this (and spelling it out may help people either agree with my views, or disagree - and give a counter explanation!)

If the second part (21 tubes, cooling fins etc) is in the shade and the hot water is drawn off the "hot" side through the second radiator, it will radiate some (or most of) the heat to the air (since the water was "scalding hot" and the air isn't, the radiator is designed to transfer heat to the air!).

The alternative, bringing "cold" water in to the shaded radiator might let it pick up a little heat (to ambient temperature anyway), or at least not radiate heat away as badly (since the greater temperature differential, the greater the heat loss/gain will be).

[ghurd]

Put it in a black box with a glass front!
The heat coming off the front side is trapped in the box, and that will heat the back side.
Plus it will stop convection of the heat into the air, and wind blowing on it, etc.

My new solar hot box is not finished, and has large air leaks with the wind blowing through.
Hazy day, 85F (29C), 15 minutes in the sun had the temperature in the box to 178F (81C).
G-

[artv]

Hi thanks for the replies,..Ross that explanation seems to make sense so I'll do it that way.
G-  I'm  going to build my box on the roof with R-40 floor and walls the top is the old double pane (thermal pane) glass out of a sliding door. The only thing is the glass is a little smaller than the rad. So I was planning on keeping the rad about 8-10" off the bottom of the box and lining the inside with mirrors.
So another  ? would it be better to do the mirrors, or just paint everything flat black, or do both??
I had no guages in place during test, but when permanetly installed the guage readings should tell me when to circulate and how often. I have another ? about transfering the heat from storage tank to house tank, but ask here or start a new thread??......thanks for the help always appreciated.......artv

[zap]

The only thing is the glass is a little smaller than the rad. So I was planning on keeping the rad about 8-10" off the bottom of the box and lining the inside with mirrors.

Maybe I'm not quite understanding something... but if your piece of glass is smaller than the radiator then all of the radiator's surface wouldn't be exposed to sunlight.  Right?
I would think the best place for the mirrors would be outside the box acting as a funnel?

Any pictures?  They seem to help a ton.

I would ask about the tank heat exchanger on this thread for now.

[artv]

Hi Zap,..sorry I don't have a camera ,but will work on getting one.
My thoughts were to build the box with ~10" of space around the rad, stand the rad at 45 degrees, put the glass top on at ~30 degrees, and use mirrored walls and floor to reflect the heat all around inside the box.
Since the glass is not as wide (it is the same length) less of an angle than the rad should let enough sunlight in I hope.
My ? about transfering heat from solar tank to house tank ; I only have a 40 gal tank for solar will wrapping a copper coil around the outside of the tank transfer enough heat to my house tank?? Or should I cut open the solar tank and place the copper coil directly inside?? I'm sure the 2nd option would be better, but if I can get enough heat with the 1st method it would be simpiler to do.Thanks for your help..........artv

[TomW]

Almost certainly the exchanger tubes inside the storage would be better efficiency.

I think.

Maybe.

But then, I ain't no HVAC Engine Ear so don't bank on my theories.


But that is how I would [will] do it when I copy GaryGary's design.

Static pressure storage tank with heat exchanger coils immersed in the storage liquid.

No basement, no attic and a 500 sq ft home and it can get to -40 C or F here for days on end so outside is not an option so creative placement will be needed and I haven't sorted it out yet or I would be speaking from experience.

Had a pal who sorted out how to insert half inch soft copper through the pipe ports on a big tank and then get it sealed to the tank ports using ferrule fittings and pipe nipples. I think he got 25 feet in there and I never saw him doing it but he claimed it was "not bad". He was using it to make drinkin liquor (Moonshine) and he pumped water through the tank to chill the coil to condense the alky.  I would have sawn off the top and placed the coils in as it did not need to be a pressure tight tank.

Just an idea I have no idea how it was done.

Tom

[ghurd]

Looking at Gary's site would give you some better ideas.

I do not believe the mirror in the box is a good idea.  Painting it flat black would be better.

Double pane glass is not usually a great idea.  Again, Gary's site will have more information about the implications.

Are the 2 panes siliconed together?  Or other goop holding them together?
Could possibly cut then apart with a razor knife, lay them side by side, cut off the extra glass?

Make the box larger than you think it needs to be. 
Mine look fine on paper, but the boxes tend to shrink as assembly gets nearer to completed.
G-

[artv]

Hi TomW,...that idea of putting the collector inside the storage tank via the ports sounds interesting,  but I don't know how you could do it?? I checked to see if 1/2" pipe would even go in the outlets on my tank, they didn't even go in,..maybe on a bigger tank....
G-....I've spent alot of time on Gary's site (Excellent site)!!....I could cut the panes apart easy enough, the black goop disolves(or breaks down) by spraying with laquer thinners , just use a guitar string , like a saw......But a single pane will let more heat in, than a double pane,..whereas the double pane will take twice as long to lose heat as the single pane....Am I right in this thought ??

Just looking for ideas before I build......flat black it is............thanks for the replies.........artv   

[ghurd]

"a single pane will let more heat in, than a double panel"
Yes.

"the double pane will take twice as long to lose heat as the single panel"
I don't know about 'twice as long', but longer is accurate.

If you want to step it up a bit, some university student (India?) did a pretty nice study about "things better than just flat black paint".
There is/was a pdf on the internet, but I do not have the link anymore.
He found fiberglass furnace filters painted black was excellent, and 3 layers was most cost effective, slightly beating 2 layers.
It is cheap to do.  Great bang for the buck.

All of my experiments use furnace filters painted flat black.
I can post a couple basic photos if you like.  Nothing ground breaking or very technical, but it shows how I do it.
G-

[rossw]

a single pane will let more heat in, than a double pane,..whereas the double pane will take twice as long to lose heat as the single pane....Am I right in this thought ??

As Ghurd says, yes, a single pane will let more in, and a double pane will lose heat less quickly than a single - but far and away your best return is *more capture area*.

Don't put the collector (radiator) at 45 degrees just to "fit it in" - the cosine rule tells us you're down 30% for a start.
Secondly, there is (roughly) 1kW per square metre of exposed surface, square-on to the sun. So if you can make your aperture larger, you will capture more energy. Mirrors and things inside the box might help use whatever gets in, but its far better to let in as much as you can.

Blacking surfaces is a science all in itself. "Selective surfaces" that absorb as much as possible while reflecting and re-radiating as little as possible, range from "cheap and not very effective" through to "very expensive and very effective" and lots in between. "Bang-for-buck" is probably the goal for DIY.

Certainly, minimising heat loss through convection, conduction and re-radiation are good returns on investment. Making the box completely airtight is asking for trouble because of the pressure differences, unless you put a balloon or bladder in the box, vented to outside.

[GaryGary]

Hi,
Just to start with what I think would be your best bet if you can:
- Separate the two 21 pipe assemblies so that you can lay them out next to each other rather than in front and behind.  This basically doubles your collection area, and is a hands down winner over anything else you could do (I think).  All things being equal, it will double the hot water output of the system.
- If you only have one of those sliding door panels, use a utility knife to cut the edge seal and make two single panels that can be used to glaze your now 6 ft by 6 ft collector.
- The R40 is overkill.  You have R1 or R2 glazing, so you really don't need the sides and back to be much bette than (say) R7 (one inch of polyiso insulation board) to get the heat loss through the back and sides to very quite small compared to what you are going to lose out the front no matter what you do.  That is, when you have 10 BTU going out the front for every 1 BTU going out the back, you don't gain much of anything by cutting that 1 BTU out the back down to half a BTU with very thick insulation -- you are still losing nearly all the heat out the glazing.  This will make your box thinner and more managable (and cheaper).
- Be sure you use high temperature insulation inside the collector -- the white, pink, or blue insulation board (polystyrene) will melt in collectors.  Use Polyisocyanurate (polyiso) -- sounds exotic, but most lumber yards have -- they may not know it by than name, so go look at the panels -- they will say polyiso on the panels.

You are kind of designing this like a batch solar water heater:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#Batch
But, that does not make sense, because it only has about a gallon and a half of water in it -- better to think of it as a collector -- like these:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#Example1KSystems

You can do the copper coil wrap around the tank -- here is Doug's system that does that:
http://www.builditsolar.com/Projects/WaterHeating/DougThermosyphon.htm
I think that it works pretty well for him -- I'm sure its not as effective as a coil inside, but it may be good enough.

Another option is to do the system like this one:
http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm
Basically, there is a big, unpressurized tank that stores the solar heated water.  When you are not pumping fluid through the collector, the water in the collector drains back to this storage tank. 
To get the heated water into your regular hot water tank, the system uses a single pass heat exchanger immersed in the big non-pressurized solar storage tank. The incoming cold water passes through this single pass heat exchanger on its way to the regular hot wate tank.
I use a 300 ft coil of 1 inch PEX for the single pass heat exchanger -- others have used copper.

Anyway, if none of that is feasible and you just want to continue with the radiator the way it is:
- I'd say inlet to the back coil and outlet from the front coil -- but I'm not really sure.
- Painting black inside or using reflector is about the same by tests done years ago in batch heaters.
- R40 is still way more insulation than you need.
- I think you want to run the pump much more often -- If you have about 18 sqft of glazed area, the 1.5 gallons of water in the radiator is going to heat up (very roughly) at 3 F per minute -- so, in 10 minutes it will be up 30F.  You don't want to leave in in much longer than that or the radiator will get so hot that it will just lose a lot of heat out the glazing.  It would be better to run a small pump all the time.  A good pumping rate would be 0.04 gpm/sqft or about 0.7 gpm for 18 sqft.

Have you thought about how you are going to protect the collector from freezing? Or, maybe that is not an issue?

Gary

[ghurd]

What Ross said.

Myself, I usually start with a box, then fit something into it.

I like the furnace filter idea.
The guy who did the study concluded almost all the light made it down into the filter some distance, some energy was absorbed and some reflected.  Of the energy that was reflected, most of it hit filter media (absorbing and reflecting) before getting out, then most of that again hit filter media (absorbing and reflecting)...
The light sort of gets in the filter and bounces around until it is absorbed.

Bigger is usually less $/W.
You have 2 pieces of glass.  May as well use them full size?

I have some down time at the moment, so...
Free heavy-duty vaccine shipping cooler (ask your vet) 15x17x20"tall, outside dimentions.
Four pack of 20x28"(?) furnace filters ($2.75 at WallyWorld).
One can of Wally's Own 97-cent flat black spray paint, which is JUST barely almost enough (and stack the filters on each other, tipped off the ground so paint can get into and through them... I lean them on the phone pole in my yard).
And one 52-cent piece of flat black poster board from Wally behind the filters.
It will house about 9.5' of 3/8"OD 1/4"ID copper tubing ($7 / 10').

Someone who knows more about hot solar boxes than me is pretty sure the Styrofoam cooler will melt, and I believe he is correct, so almost everything is recyclable into another project.
At best, this only needs to last a couple days in semi-reasonable sun.  And I am only out a free cooler and a 52-cent piece of poster board.

The extra chunk on the bottom makes 3 layers.  Leftover piece of filter.







I see someone posted while I was typing, mentioning the white stuff will melt.

In addition to freezing, I would have a bit of concern about overheating.
A large shoe box with a black dish towel in the botton, glass containers with candle wax in the box, and a broken piece of window glass on top, will liquefy the wax in an hour or 2.

I recommend doing a lot of tests, reading, and more reading before venturing into home modifications.
G-

[artv]

Thanks for the replies,....Ross ,the reason for the 45 degree is with the slope of the roof ,at high noon the collector will be directly facing the sun.
G- ,I'm going to just go flat black for now ,but can always add or try different things once up and running, the glass lid will be removable.
Gary ,I've spent alot of time on your site, great info, read those links and many more. I thought about cutting the rad but, that would mean 42 solder joints and my soldering skills aren't the best, so I think I will leave it as is. I want to run anti-freeze in it because 6 months of the year it is below freezing. So like TomW said I think I will cut the top off the 40 gal tank and put the collector coil directly in the anti-freeze.The only concern I have is , will there be any fumes ,will I have to seal the lid back on the tank after I install the coil?? The tank is going to be in my porch( I don't have a basement) plus that is where my circulating pump and shut-off valves for the in-floor heating are.I'm thinking of hooking the pump on the drain valve of the tank, pump up to the bottom in-let of the rad ,have the top outlet of the rad piped into the top of the solar tank. Then circulate the anti-freeze at certian temp readings. Just concerned about open fumes from the anti-freeze.
I'll do some more reading .........artv 

[Bruce S]

artv;
Though it really depends on the type of anti-freeze you use, for the most part the fumes can be a health problem.
Any time we worked on cars that were having overheating problems, broken radiator hoses etc, we used an oscillating fan to get the fumes away from us.
Some of the RV stuff is less toxic (read also more expensive too). Since closing the top of the collect will help in keeping the captured heat in, and this unit will be around people ( porch) a good tight fit will help tons.
If once up and running the collectors get to where it starts producing fumes having a good tight fit will stop about 98% of the fumes as it will collect on the lid and turn back into liquid.
IF at anytime you walk in and smell a sweet-acidic odor, grab a small fan to disperse it, OR even better if possible build a small vent using old small 12V 1/4 watt or smaller computer fan to vent to out doors.
The tight lid will be the best in the long run, it'll serve two functions, 1) Keeping captured heat in and 2) recycling any possible fumes back into unit as liquid.
Hope this helps
Bruce S

[artv]

Thanks for the reply Bruce.
I decided not to use anti-freeze. I've just got my cold out from the house feeding the collector. It has seen ~16 hours of sun so far. I been trying different schemes for circulating. It's looking like a 45 sec. circulation every 1/2 hour. But as the stored water gets up to house temp (110 F) , I think this will change to longer runs more often, maybe even constant circulation.The stored water reached 115 F by 4:00 yesterday afternoon. This morning it had dropped to 93F. My roof top box is'nt finished yet and none of my pipes are insulated yet, so once this is done I'm sure the temp drop over night won't be as bad.
 The one thing I noticed was that when not using the house water and, circulating the solar water the hot solar water was back feeding into my cold out, warming up my cold water. I'm not sure if this is an issue or not??  But as soon as my wife started getting supper ready (washing vegys and stuff) the warm cold line in almost instantly went back to cold so I'm not sure if the back feeding really matters or not???..........maybe somebody else has seen this before...any thoughts welcome......thanks ....artv   

[artv]

Hi all,..I drew this picture so as to give a better idea
both the cold in and the hot in are under house pressure, but when a hot tap is opened in the house, will the release of pressure at the tap allow the stored hot water in the solar tanks to enter the house (that is if it is up to proper temp via the mixer valve)??
Not sure if it will work this way.........thanks for any input......artv