Author Topic: solar domestic water from roof heat  (Read 20905 times)

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mikey ny

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solar domestic water from roof heat
« on: July 19, 2005, 01:38:42 AM »
hey there everyone,

   Just had a silly thought about obtaining solar hot water from the underside of my roof. I am not excited about the look of the solar panels attached to roofs. I have worked in the building industry for about 20 years now and in the past 15 years or so there has been a big emphasis on attic ventilation in relation to the longevity of roofing materials. Most if not all roofing companies will not warrantee a roofing product unless the roof is properly ventilated. Now for my hot water thought. I was thinking of installing pex tubing or black onx pipe directly on the underside of my roof sheathing in the attic, kind of like you would in a radiant floor heat system. Inslulate beneath the pipe and collect the solar heat absorbed by the roofing shingles. If I ran domestic water through the pipe to preheat it for before my hot water heater it would cut my bill down considerably. The big question is will it trap too much heat under the roof and fry my roof shingles, or will it actually act a coolant or heat exchanger and cool my roof? If I had a 20 year roof i woul not acually know until it failed early or lasted longer. Anybody ever try this or heard of it. It sounds too simple to be good. I am just thinking about saving a buck without all the stuff on the exterior of my house. I don'nt live in an upscale area so appearance is not really an issue, just a crazy thought.

                                                          Mike
« Last Edit: July 19, 2005, 01:38:42 AM by (unknown) »

farmerfrank

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Re: solar domestic water from roof heat
« Reply #1 on: July 18, 2005, 08:01:50 PM »
I think just running some pipes in your attic would suffice as most attics get up to 150 degrees. Insulating directly under the shingles is just asking for trouble. An uninsulated tank placed in the rafters would collect more than enough heat and would act as storage too. Actually a couple hundred watts of energy will supply a house while a large roof has 100,000 watts of energy falling on it on a long summers day.
« Last Edit: July 18, 2005, 08:01:50 PM by farmerfrank »

pyrocasto

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Re: solar domestic water from roof heat
« Reply #2 on: July 18, 2005, 08:20:08 PM »
I wonder if a big tank type of thing would be enough to preheat your water? It wouldnt freeze in the winter also. Huh...
« Last Edit: July 18, 2005, 08:20:08 PM by pyrocasto »

Kwazai

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Re: solar domestic water from roof heat
« Reply #3 on: July 19, 2005, 07:43:29 AM »
I too have been thinking along these lines, only slightly different. There is a place on the net that sells wide rolls of radiant barrier (foil faced kraft paper) and it would be an easy matter to create a plenum to the attic vents at the ends, where a condenser coil/radiator could be put with PV powered attic fan to heat water. You would get the bonus of the radiant barrier in addition to picking up the hot water from the attic exhaust. I also have been looking at it as a source of hot air in the winter- the underside of my roof in NC was blanket of warm(almost hot)air on a 28degF day last winter for about the 1st inch or so. The window box heater I saw in a previous post would be easy to do on the underside of the roof in my attic (up to about 600sqft worth), relatively cheap (with the radiant barrier and insulated ductwork and PV powered fans) and I believe currently a tax break (energy efficiency-radiant barrier). When it cools off next fall, I'll be up there installing a test run between a coupla trusses. just my .02$

Mike
« Last Edit: July 19, 2005, 07:43:29 AM by Kwazai »

rotornuts

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Re: solar domestic water from roof heat
« Reply #4 on: July 19, 2005, 08:59:09 AM »
Perhaps as illustrated below.





If you opened up the ridge and put in a ridge vent then perhaps you could go from 1/3 the run up to the ridge with the baffle. If you allow for 1 1/2" air flow between the pex and the baffle then your in no different a situation than if you built a house with an occupied attic with the insulation between the rafters, other than of course the rafters are deeper to accommodate the insulation and required vent space.


Btw, while placing a tank in the attic is an interesting idea it's not really feasable because at over 11 lbs per gallon your not going to fit a tank of any appreciable size in the attic without some significant structural work. I'm sure you can imagine how fun it would be to put bearing posts down to a footing to allow for an additional 2000 pounds in the attic in about a 5 x 5 space.


Mike

« Last Edit: July 19, 2005, 08:59:09 AM by rotornuts »

farmerfrank

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Re: solar domestic water from roof heat
« Reply #5 on: July 19, 2005, 02:39:50 PM »
I would think that a pressure tank sitting in an area surrounded by 140-150 degree air for any length of time would absorb alot of heat. And yes in any cold climate it would have to be drained in the winter but so would pipes and the like.
« Last Edit: July 19, 2005, 02:39:50 PM by farmerfrank »

pyrocasto

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Re: solar domestic water from roof heat
« Reply #6 on: July 19, 2005, 03:05:08 PM »
It doesnt get much colder than about 10 degrees F here so I'm doubting the temperature in the attic would even get cold enough to freeze, but I'll have to test with a bowl of water I guess.

« Last Edit: July 19, 2005, 03:05:08 PM by pyrocasto »

pyrocasto

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Re: solar domestic water from roof heat
« Reply #7 on: July 19, 2005, 03:08:51 PM »
A 30 gallong tank would only weith 300lbs, so I think it could easily be done. I dont need 180 gallons in my attic, I would think enough for shower would be fine for my house.
« Last Edit: July 19, 2005, 03:08:51 PM by pyrocasto »

elvin1949

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Re: solar domestic water from roof heat
« Reply #8 on: July 20, 2005, 01:38:02 AM »
 Check the house real close.

Mine has a load supporting wall 56ft long,right down the middle,with a sill under it on concrete pier's spaced 6ft apart.

 Old lumber [the stud's are REAL 2 by 4's] cut in 1890.Southern longleaf pine heart wood.Super strong.

later

elvin
« Last Edit: July 20, 2005, 01:38:02 AM by elvin1949 »

Kwazai

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Re: solar domestic water from roof heat
« Reply #9 on: July 20, 2005, 12:13:13 PM »
only problem I could see with the pex tubing(radiant floor stuff) was when you need to reshingle the house-nails ...


The storage would probably be easiest with a long piece of 8" pipe along a bearing wall (a coupla gallons per foot of length)-horizontally- could be put in in manageable sections. You would definitely want some type of pan under it in case it leaks-similar to an attic mounted hot water heater drain pan.


on a 28degf day I had a good inch of warm (90F) air flowing on the underside of the roof- somewhere around 1ft/s velocity- is there a small windgenny/pump that could make use of it for a gallon an hour or so of water flow(PV seems cheapest so far). also would one of the stirling(fluidyne) pumps work off the temp difference between the roof and barrier and the barrier and ceiling (assuming the heat is exhausted out the vent)??

« Last Edit: July 20, 2005, 12:13:13 PM by Kwazai »

mikey ny

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Re: solar domestic water from roof heat
« Reply #10 on: July 22, 2005, 01:48:45 PM »
I'm in the Finger Lakes til Sunday or Monday Fishing and having Fun with the Family

be back sun or mon

Mike
« Last Edit: July 22, 2005, 01:48:45 PM by mikey ny »

GaryGary

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Re: solar domestic water from roof heat
« Reply #11 on: July 28, 2005, 09:52:26 PM »
Hi,

I think this is a pretty interesting idea.


I don't see why insulating below the roof sheathing should be a problem, in that many chathedral ceilings (mine for instance) are done that way.


This doc on my web site will give you some actual temperatures measured on my roof surface, and on the back side of the sheathing.  

http://www.builditsolar.com/Projects/LowTechCol/LowTechCollectorR1.pdf

(scroll down to the "sample test panel results" part)

The temperatures on the back side of the sheathing with the insulation you describe will be higher because of the insulation.    If you live in a cold climate, while the roof may get hot enough on a sunny winter day to produce some useful water preheat, it will radiate heat to the sky very quickly on a cold night, and some form of freeze protection will be a must.  You could make the system a drain down type.  In these systems, when the roof is not hot enough to heat water, the pump stops, and all the water in the PEX would drain down to the drain down tank.  See this linke for more info on how this would work:  http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm

You could control the circulation pump with a simple $10 thermal snap switch that turns the pump on when the roof temperature gets above its setpoint, or you could use a more expensive differential controller (like a Goldline) that sences the difference in temperatur between your roof sheathing temperature and your storage tank temperature.  


Dawn Solar does a commercial system that is a little like what you are proposing.  They claim that the system can be used for water preheat, and for space heating.

http://www.dawnsolar.com/home.html


You would, of course, want to do this only on the south facing part of your roof.  If you find that the PEX tubing does not make good enough thermal contact with the roof sheating to pick up heat well, you could consider using some of the aluminum plates like these:

http://www.pexsupply.com/index.cfm/action/catalog.browse/id_category/1d7dd316-8145-4e10-b7d9-f2b5bb9
061f8/category_header/1

I think you will need descent thermal contact between the sheathing and the PEX to make this work well.


I think your scheme might work pretty well.  If you go ahead and do it, I'd be interested in getting some pictures and a description to put here:

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


Gary

« Last Edit: July 28, 2005, 09:52:26 PM by GaryGary »

nothing to lose

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Re: solar domestic water from roof heat
« Reply #12 on: August 03, 2005, 12:44:57 AM »
Hmm, for such smart people too :)

You and pyrocasto are both about wrong, just a little over 8lbs to a gallon for water :)

 So 30 gal is abit over 240lb but less than 270lbs. 20gal around 160lbs.

Also connecting several small tanks together you could spread out the weight anyway you wanted around the attic, 100-200 gallons does not have to be all one tank. You could have 5 20gal tanks along the front and rear walls for 10 tanks and 200 gallons if you wanted to do that. Each 20gal tank only weighing about 160lbs each, less than if I was up there walking around. Course I dought I'd want that much water over head in case of a leak, could ruin walls and ceilings pretty fast. Just another way to think on it, use more tanks and spread out the weight.


Also I think smaller tanks would gain heat (and lose it) faster than larger tanks. Depending on shape maybe, long thin or short fat, surface area has an effect.

« Last Edit: August 03, 2005, 12:44:57 AM by nothing to lose »

alain

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Re: solar domestic water from roof heat
« Reply #13 on: August 31, 2005, 05:51:07 PM »
Hi All,

while i understand why you may want to preheat your water,another way to reduce the bill is to connect a timer switch to your existing electric tank.Depending on the number of people, the power on 2 hours a day is enough for 2-3 people .

a timer like that is $37 CAD at home-depot. You don't need to heat water while you are sleeping or at work.Last week while we were away on vacation i noticed when we

returned ,the house was using 4 kwh a day with fridge ,freezer and hot water on.


Alain

« Last Edit: August 31, 2005, 05:51:07 PM by alain »

steak2k1

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Re: solar domestic water from roof heat
« Reply #14 on: September 13, 2005, 09:34:05 PM »
Alain..bonjour.


The timner idea is a good one and should be easy to do.  I am curious if there is a possiblity of there being in existence one for a Nat Gas fired water heater.  I shall have to see.  It seems to be true, I drop the house Nat Gas temp on thermostat at night to 14 deg C in Calgary to reduce consumption...so why not on the water heater too..??


Yup gonna go to Home Despot and see if they have one.!


merci,


stk

« Last Edit: September 13, 2005, 09:34:05 PM by steak2k1 »

Nando

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Re: solar domestic water from roof heat
« Reply #15 on: October 10, 2005, 07:48:12 PM »
It is shame that the roofs use shingles instead of copper flashing with a copper tubing to harvest the heat energy available during the day.


The tank can be on the ground like in the garage, or a closet.


Regards


Nando

« Last Edit: October 10, 2005, 07:48:12 PM by Nando »

Kwazai

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Re: solar domestic water from roof heat
« Reply #16 on: October 21, 2005, 07:25:48 AM »
I had run across this site a few years ago- finally found it again


http://www.solarattic.com/engineer/scientific_model.html

« Last Edit: October 21, 2005, 07:25:48 AM by Kwazai »

alain

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Re: solar domestic water from roof heat
« Reply #17 on: November 07, 2005, 05:28:35 PM »
HI, why not turning it off the whole nightfrom maybe 9.00 pm until say 5 .a.m until it is time to get ready for the day. personnaly, it is on 1 hour in the morning and one hour at supper time and it is enough for the day. what is expensive with hot water is not to heat the water , it is to keep it hot when you don't use it.

goodluck

Alain
« Last Edit: November 07, 2005, 05:28:35 PM by alain »

Kwazai

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Re: solar domestic water from roof heat
« Reply #18 on: November 15, 2005, 11:49:24 AM »
« Last Edit: November 15, 2005, 11:49:24 AM by Kwazai »

GaryGary

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Re: solar domestic water from roof heat
« Reply #19 on: November 16, 2005, 08:37:50 AM »
Hi,

Quite an interesting paper.


Its nice that you can't see any evidence from the street that there is a solar collector embedded in the roof, and also nice that the materials are pretty simple and could just be part of a normal roof construction routine.  


But, it also seems like you pay a pretty hefty penelty for the no glazing over the top of the collector.  I suspect that there is also some loss in efficiency because  its hard to get a good thermal conduction between the PEX and the alum.  It seems unlikely that it would work for northern climates (see below).


I did a little experiement with very simple ($1 per sqft) glazing over siding as a very low tech solar heating scheme: http://www.builditsolar.com/Projects/LowTechCol/LowTechCollectorR1.pdf

I wonder if this could be used as a way to improve the performance of this scheme while still keeping a pretty low visual profile?

You could even envision someone manufacturing a panel that had all the elements already built into a roofing panel -- maybe even a SIP with solar collector/roofing applied?


All in all, it does seems like an idea with a lot of promise.

Any other ideas on how to improve it?


---

Cold climate performance:

Tamb = 0C,

Tinlet = 50C

Isun = 800 watts/m^2


eficiency curve parm = (Ti-Ta)/Isun = (50C-0C)/800 watts/m^2 = 0.0625

The efficiency for this value is right around 0


Gary

« Last Edit: November 16, 2005, 08:37:50 AM by GaryGary »

Kwazai

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Re: solar domestic water from roof heat
« Reply #20 on: November 16, 2005, 03:20:57 PM »
I took it as data on a possible retrofit. at 3.9+/- kwh/day(7000+ btu/day) from essentially one set of rafters(trusses)(41sqft). I have been doing some internet digging into Nick Pine's solar thermal closet storage techniques and trying to put some costs together for an even lower tech version of this. radiant barrier, polyisocyanurate foam board box full of 2-litre bottles. I should be able to safely put about 55gallons worth over a four truss area ( I design trusses for a living and checked the set on my house). Looks like the water would need to go at the top of the 'collector'- in a closed box (thermo-siphon type closet) and then forced air to the house from the 'solar closet'. in thinking about it, there's a few other oddball items to add--- attic water heater drain pan, hacked automatic foundation vent (open at 150F?)(bottles get deformable above 140 to 150F), duct booster fan and flex duct, temperature controlled relay (24v control wiring in house plus box temp set point(short cycle the standard hvac unit with additional solar heat).

ther is bound to be something I missed.

The pex tubing is fairly expensive($1/ft?) and eliminating it would also solve the reshingling nails problem. a foil radiant barrier would pay for itself in a few years from AC savings and with the 'closet' box at the top closed would heat the water (probably only a few hours of space heat after dark).  I have enough trusses in my attic for the functional equivalent of five 55gal. drums of water (35000 btu/day). admittedly you would get at least twice the heat gain from a glazed one(if not more). The booster fan(250cfm) and associated hardware per 'closet box' is somehwere around the 150$ mark, add radiant barrier, drain pan and foam (and 2-litre bottles-last I heard you could get people to pay you to take them). Much like one of the window box units or the r1 low tech collector without it being visible (about half the efficiency, but a lot cheaper). anyway my .02$.

L8r

Mike
« Last Edit: November 16, 2005, 03:20:57 PM by Kwazai »

GaryGary

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Re: solar domestic water from roof heat
« Reply #21 on: November 16, 2005, 07:51:42 PM »
Hi Mike,


"I took it as data on a possible retrofit. at 3.9+- kwh/day(7000+ btu/day) from essentially one set of rafters(trusses)(41sqft). "


I think if you live in a warmish climate with fairly high ambient temps, and you can use relatively low collector output temps,  that might be true, but the lower your ambient and higher your collection temperature, the less you will get.  If go to the efficiency equation they provide, it is:


efic = 19.1 - 525(Tinlet - Tambient)/Isun


As (Ti - Ta) goes up, the efficiency drops.  So, what is the maximum Ti-Ta you can get out of the collector before the efficiency drops all the way down to zero?  

If you assume 800 watt/m^2 sun (about of 80% of  full sun), then your efficiency would drop to zero (ie no heat output) when Ti - Ta = 29C or 52F -- that is:


efic = 0 = 19.1 - 525 (Ti - Ta)/800


(Ti - Ta) = 19.1
0.656 = 29C   (or 52F)


 So, if its 30F outside (an average mid-winter HIGH where I live :-)), then the max output temp would be 80F, and you would get precious little of that (since this is the zero efficiency point).  Not too many things you can do with 80F water or air?

This does not work for me, but it might work for you if you live in a warm climate.

Or, maybe I am missing something?


"I have been doing some internet digging into Nick Pine's solar thermal closet storage techniques and trying to put some costs together for an even lower tech version of this. radiant barrier, polyisocyanurate foam board box full of 2-litre bottles. "


I guess the 2 litre bottles are OK for life?  I tried a little experiment with adding some thermal mass to my air thermosyphon collector using gallon bottles of water from WalMart, and they were all falling apart in a few months.


"I should be able to safely put about 55gallons worth over a four truss area ( I design trusses for a living and checked the set on my house). Looks like the water would need to go at the top of the 'collector'- in a closed box (thermo-siphon type closet) and then forced air to the house from the 'solar closet'. in thinking about it, there's a few other oddball items to add--- attic water heater drain pan, hacked automatic foundation vent (open at 150F?)(bottles get deformable above 140 to 150F), duct booster fan and flex duct, temperature controlled relay (24v control wiring in house plus box temp set point(short cycle the standard hvac unit with additional solar heat).

ther is bound to be something I missed.


"The pex tubing is fairly expensive($1/ft?) "

I think the half inch is more like 40 cents a foot.


"and eliminating it would also solve the reshingling nails problem. "


Had not thought about that.


"a foil radiant barrier would pay for itself in a few years from AC savings


Yes, that's a nice double benefit.


"and with the 'closet' box at the top closed would heat the water (probably only a few hours of space heat after dark).  I have enough trusses in my attic for the functional equivalent of five 55gal. drums of water (35000 btu/day). admittedly you would get at least twice the heat gain from a glazed one(if not more).  The booster fan(250cfm) and associated hardware per 'closet box' is somehwere around the 150$ mark, add radiant barrier, drain pan and foam (and 2-litre bottles-last I heard you could get people to pay you to take them). Much like one of the window box units or the r1 low tech collector without it being visible (about half the efficiency, but a lot cheaper). anyway my .02$."


Again, I think the only problem with the scheme is if you have low ambient temps and/or want high storage temps - I think either kills this scheme by lowering the collection efficiency to nothing.  But, again, maybe I am missing something.  


Maybe a small test panel would be in order - you could just do one bay with the radiant barrier and foam (you could even skip the foam board on the test panel), and see what kind of flow you get.  You can do a half way descent job of calculating output with a couple cheap thermometers (one at the input end and one at the output end), and one of these $25 air velocity meters.

http://www.dwyer-inst.com/htdocs/airvelocity/Series480Price.cfm

(a really helpful instrument for solar air people)


Output = (Tout - Tin)(Duct Area) (Duct Air Velocity)(Air Density)(Air Specific Heat)


Eg

Output = (110F - 60F)(1 ft^2)(90 ft/min)(0.07 lb/ft^3)(0.24 BTU/lb-F) (60 min/hr) =  4500 BTU/hr.


You can even estimate efficiency of the collector by doing the test on a good clear day, and looking up the solar input in a book, or use the RadiationOnCollector program at my site.  Then:  efic = (Output from above)(Collector area)(Isun)(Cos (IncidAngle))

If you do this, please post the results you get.


Another thought would be to bag the storage, and just use the collector as a way to collect warm air to go directly to the house.  This way, the relatively low collection temps would not mater too much - it would still be warmer than the house air.  The house thermal mass will store some heat if you let the temperature drift up to 75F or so.  Nick Pine likes ceiling mass that has a low emissivity surface facing the room.


Gary


Here is another of the no glazing roof collectors:

http://www.dawnsolar.com/

The Dawn Solar collector was tested by either the FSEC or the SRCC, and the results published.  As I recall they were similar to the one in the paper - OK for collecting luke warm water in warm climates.

« Last Edit: November 16, 2005, 07:51:42 PM by GaryGary »

Kwazai

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Re: solar domestic water from roof heat
« Reply #22 on: November 17, 2005, 02:01:17 PM »
hi Gary,

appreciate the comments. I do live in moderate climate (North Carolina). didn't really expect to get more than about 20% of my heat from the collector.(the radiant barrier is actually a better investment, but I have a source for free or almost free foam). I hadn't really tried to put any roi numbers to it yet. Most days around here are 50f or above in the winter, there's just a month or two of freezing temps and only an occasional day every few years below zero. supplemental solar (if you will). not real sure about the uhmw in the 2-litre drink bottles. I've had some outside ,full of water, for about 2 years now and the only thing thats torn them up is my dog. I want to (like you said) run a test batch over my garage area before plowing into the whole roof, but its going to be a while before I get to it.(a friend just cut a well tank in half for me for a little stirling pump collector-not sure its big enough, and a couple bike projects with higher priority).

I have been doing some thinking about the thermosiphon and Nick Pine's comments on flap dampers. The idea would be basically just to trap the warm air (comfortably warm on a 28F day) at the bottles. I have been looking at this stuff for several years now, been collecting car condenser coils , etc. etc. This one appears to be fairly cheap in terms of do-able solar. best guess about 3 months a year i'd save 20$ or so on heat and 3-months a year i'd save 20$ or so  on ac (120$ a year??). based on what you know does that sound reasonable? that would put the heat side payout (roi) at about 2-1/2 yrs.

When I can get something up and functional, I'll post results. thanks for the link.

Mike
« Last Edit: November 17, 2005, 02:01:17 PM by Kwazai »

Kwazai

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Re: solar domestic water from roof heat
« Reply #23 on: November 18, 2005, 06:17:16 AM »
my bad. 12-1/2 yr payout. (5 units)
« Last Edit: November 18, 2005, 06:17:16 AM by Kwazai »

GaryGary

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Re: solar domestic water from roof heat
« Reply #24 on: November 18, 2005, 08:53:00 AM »


Hi Mike,


I'd say the savings are hard to predict.  If you were doing conventional collectors with storage, I'd say well in excess of 50% of your heating bill would be a reasonable target (thats my target for my system, and its supported by some simulation work).  Your system will have a somewhat experimental collector design (which is good!) -- it may do nearly as well on saving at a much lower initial cost, or it may not do so well, but should still be good investment.  I'm not familiar enough with the Air Conditioning end to make  even a guess.


One thing you might consider is not using the heat storage at all (or at least not using it until you know you will get enough heat to justify having it).  You could just have an lightly insulated space or plenum above the outlet of the collector area, and when the air in this space gets up to 80F or so, turn on a fan that pulls air from the heated area through a duct to your house.  This could be controlled by a $15 replacement attic fan control (Home Depot sells them).  This might turn the whole project into a build it in a day affair, and it would let you see how it works out befor investing the time and effort into a storage scheme?


I use this themosyphon heater on my barn/shop:

http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm

The heated air tends to gather in the rafter space just above the outlet before dispersing out to the rest of the room -- on a sunny day, the temp in that area gets over a 100F.  You could just create a similar area at the top of your collector, and pick up the hot air there and duct it to the living area.


A rough rule of thumb is that the collector area can be up to about 20% of the living space area before you need heat storage.  With this collector design, this might be more like 40%?


---

Maybe in the interest of science, you could get one sheet of 2 ft by 12 ft corrugated polycarbonate from Home Depot ($30), and temporarely fix it over one of the collector bays (maybe with silicone) -- then see how much better that bay does than the others :-)  I may have to try that myself on my south roof.


---

Being an experimenter type, you might like this book:

"Low-Cost Solar Heating -- 100 Daring Schemes Tried and Untried"

http://www.builditsolar.com/Experimental/ShurcliffPart1/TOC.htm

Even thou this book dates back to 1980, its full of very well thought out schemes, including some interesting attic ones.

Its my all time favorite solar energy book.

Dr. Shurcliff (the author) has allowed me to make it a free download.


Gary

« Last Edit: November 18, 2005, 08:53:00 AM by GaryGary »

Kwazai

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Re: solar domestic water from roof heat
« Reply #25 on: November 18, 2005, 01:34:54 PM »
"One thing you might consider is not using the heat storage at all (or at least not using it until you know you will get enough heat to justify having it).  You could just have an lightly insulated space or plenum above the outlet of the collector area, and when the air in this space gets up to 80F or so, turn on a fan that pulls air from the heated area through a duct to your house.  This could be controlled by a $15 replacement attic fan control (Home Depot sells them).  This might turn the whole project into a build it in a day affair, and it would let you see how it works out befor investing the time and effort into a storage scheme?"


sounds like a plan.

The Shurcliff book looks like it has a lot of ideas in it I would never have thought of. will definitely have to find a copy of it (I've hung onto most of my school books too- just seems easier to read from a book instead of the net).

 I have harvested some parts out of a couple of hvac units, I'll have to look and see what I've got (temp switches, fans, etc...) and see about making it lobuck as possible.

The polycarbonate panel would (hopefully) do what in terms of collector performance?


L8r

Mike

« Last Edit: November 18, 2005, 01:34:54 PM by Kwazai »

GaryGary

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Re: solar domestic water from roof heat
« Reply #26 on: November 18, 2005, 07:52:18 PM »
Hi Mike,


"The polycarbonate panel would (hopefully) do what in terms of collector performance?"


Glazing over the outside of the roof shingles with polycarbonate would make it behave more like a conventional collector.  The glazing reduces the heat loss from the roofing surface, which means the roofing surface has to heat up more to get rid of the heat its absorbing from the sun.  If the roofing surface runs hotter, than more heat gets transfered through the roofing to the inside where you collect it.


If (for example) you replaced the asphalt shingle roof with a metal roof, and glazed over it with polycarbonate, and then let air cirulate in the channel you have made behind it (with the radiant barrier), you basically have a full fledged conventional hot air thermosyphon solar collector.  Glazing over an ashalt shingle roof gets you closer to this.


As a ball park guess, I think it might double the performance (because it more than doubles the R value for heat loss outward), and it would also allow you to collect useful heat when the outside temperature is lower, or when its windy.


The only thing I would be concerned about with this arrangement is how hot the surface of the shingles might get.  On that little test sample I made, the shingle surface got up to 175F -- I'm not sure when you start effecting the life of the roof?


--

Amazon.com used books has the Shurcliff book -- last time I looked it was $11 or $12.


Gary

« Last Edit: November 18, 2005, 07:52:18 PM by GaryGary »

Kwazai

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Re: solar domestic water from roof heat
« Reply #27 on: November 21, 2005, 07:37:27 AM »
I live in a covenanted neighborhood and as long as the 'stuff' is not visible from the road I can do pretty much what I want in terms of 'projects'. unfortunately my sun side roof is facing the road. its a 5/12 pitch rood(22.5deg) and I had thought about the backside as vertically mounted collector space using condenser coils for hot water heat. There is a lot more to an outside system though in terms of cost for the components (drain down, or antifreeze, etc. plus heat exchangers). That would be on a future projects list. The stuff I have read on the net in relation to shingle life and roof interior temps. unvented attic max at 150f, vented attic max at 130f. anything above 140f will shorten the life of the shingles (approx. 1% for every 10degF above the 140 number (all ballpark figures). The data is relatively scarce, I guess due to the variation in houses, roofing materials, etc.

Their are some code requirements in relation to unvented attics and crawlspaces and attaching particular types of foam to combustible surfaces and I'm still researching them. Most of the requirements boil down to having a foil barrier between the foam and the wood(or concrete). not sure why this is. (U.L. flame spread ratings maybe?). I had originally thought about just putting the foam board to the bottom of the roof and seeing what happened, but figured the radiant barrier would be worth the extra money on the AC side of the money equation. I'll be pulling together some rough calcs in a few weeks. This weekend is dedicated to additional storage space in the attic (flooring so to speak). after that I'll have room to manuever in the attic.

 I also got to thinking about the costs(this weekend) of the air to air collector ductwork and fans and should be able to reduce the costs with a thermostatic damper at the top of the collector feeding a duct to a solenoid damper(heat mode only) at the return air plenum in my house. It would be ideal if I could get my gas pack to not heat the air when the return air temp was sufficiently high to provide heat(supplemental solar heated air).not sure what kind of electrical /tehermostat control would be necessary.

 I had checked some of the convective airflows for a 1sqft duct raised 50F.

http://www.engineeringtoolbox.com/natural-draught-ventilation-d_122.html

http://chuck-wright.com/calculators/stack_effect.html


from what I had read on an hvac site they actually get only about 60% of the calculated flow velocity, but up to 45 degrees from vertical it is a good predictor-not sure how my roof slope will affect it.


there are some payout numbers here as well as some of the control system stuff-

http://www.westol.com/jbrown/jimsspot.htm


I'm one of those people that if I plan every piece out to the last detail -it works and works better than expected, but if I just slap it together it fails- so naturally I'm a little trepidacious about just filling the attic up with a bunch of 'solar' stuff.

L8r

Mike

« Last Edit: November 21, 2005, 07:37:27 AM by Kwazai »

GaryGary

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Re: solar domestic water from roof heat
« Reply #28 on: November 22, 2005, 08:48:29 AM »
Hi Mike,


" ... There is a lot more to an outside system though in terms of cost for the components (drain down, or antifreeze, etc. plus heat exchangers). That would be on a future projects list. "


This is my "outside" project -- its been a lot of work, but it does allow you to get the collector orientation and tilt perfect.  It is a pretty simple system -- plain water, no heat exchangers.

http://www.builditsolar.com/Experimental/InWorkshop/SolarShed/solarshed.htm


The other thought is that the approach of just using a sheet of corrugated polycarbonate over the existing roof shingles (perhaps spaced off the shinges by 3/4 inch) is that it would show very little from the street.  It might get by the covenant police?


"The stuff I have read on the net in relation to shingle life and roof interior temps. unvented attic max at 150f, vented attic max at 130f. anything above 140f will shorten the life of the shingles (approx. 1% for every 10degF above the 140 number (all ballpark figures). The data is relatively scarce, I guess due to the variation in houses, roofing materials, etc."


Good to know.


"Their are some code requirements in relation to unvented attics and crawlspaces and attaching particular types of foam to combustible surfaces and I'm still researching them. Most of the requirements boil down to having a foil barrier between the foam and the wood(or concrete). not sure why this is. (U.L. flame spread ratings maybe?). I had originally thought about just putting the foam board to the bottom of the roof and seeing what happened, but figured the radiant barrier would be worth the extra money on the AC side of the money equation. "


You might want to also consider the Polyisocyanurate that has the alum foil on one or both sides -- seems like it should serve as both a radiant shield and insulation?

It also has a much higher temperature capability than the Polystyrene.  I think that Celotex might be one brand name(?)


"I'll be pulling together some rough calcs in a few weeks. This weekend is dedicated to additional storage space in the attic (flooring so to speak). after that I'll have room to manuever in the attic.

 I also got to thinking about the costs(this weekend) of the air to air collector ductwork and fans and should be able to reduce the costs with a thermostatic damper at the top of the collector feeding a duct to a solenoid damper(heat mode only) at the return air plenum in my house. It would be ideal if I could get my gas pack to not heat the air when the return air temp was sufficiently high to provide heat(supplemental solar heated air).not sure what kind of electrical /tehermostat control would be necessary.

 I had checked some of the convective airflows for a 1sqft duct raised 50F.

http://www.engineeringtoolbox.com/natural-draught-ventilation-d_122.html

http://chuck-wright.com/calculators/stack_effect.html


from what I had read on an hvac site they actually get only about 60% of the calculated flow velocity, but up to 45 degrees from vertical it is a good predictor-not sure how my roof slope will affect it.


there are some payout numbers here as well as some of the control system stuff-

http://www.westol.com/jbrown/jimsspot.htm"


Thanks -- I'll take a look.


"I'm one of those people that if I plan every piece out to the last detail -it works and works better than expected, but if I just slap it together it fails- so naturally I'm a little trepidacious about just filling the attic up with a bunch of 'solar' stuff."


Probably the best approach -- I like to do some experiments along the way -- seems like you always learn something useful -- even if its not what you thought you were going to learn :-)


Good Luck with the project!


Gary

« Last Edit: November 22, 2005, 08:48:29 AM by GaryGary »

Kwazai

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Re: solar domestic water from roof heat
« Reply #29 on: November 23, 2005, 07:45:44 AM »
"My biggest quandary at the moment is deciding whether it is worth paying the extra money for absorber plates that have a selective surface. "


cool project. I have a shed in my backyard (10x12) that I insulated last year. during the winter it gets full sun and is also out of view of the covenant police (actually there is a lot of stuff I already get away with). being that most days I squeek when I walk, I am wondering how the aluminum condenser coils from car air conditioning compare to the collectors you describe. they are essentially finned aluminum heat exchangers (radiator like except a lot cleaner than a car radiator).

 I have ordered Shurcliff's book and will be digging thru it when I get it.

L8r

Mike

« Last Edit: November 23, 2005, 07:45:44 AM by Kwazai »

GaryGary

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Re: solar domestic water from roof heat
« Reply #30 on: November 26, 2005, 01:29:42 PM »
Hi Mike,


I think the auto condenser coils would work OK as collectors.  I think the hard part would be getting enough of them to get enough area to make a difference.  The Maine Solar Association provides plans for a collector that you can build thats pretty cost effective (around $3 per sqft):


http://www.builditsolar.com/Projects/WaterHeating/MSClosedLoop.pdf


I an told these go together pretty fast.  I did a little experimenting with a similar home-made one here:

http://www.builditsolar.com/Experimental/CPVCCollector/cpvccollector.htm

I used CPVC, which, in hindsight was not such a good choice, but the same technique would work with copper tubes.


Another possibility is those $4.50 per sqft absorber plates from SunRay -- they seem like a pretty good deal to me.  I have been running a simulation on various types of collectors (eg with and without selective coating on absorber, single and double glazing, and evac tubes vs flat plates) -- my initial conclustion is that you don't get as much from the high tech features as the makers would have you believe, especially if you live in a not very cold climate.  


In using solar water collectors for space heating, I think collector area is the name of the game.  Its a bit of work to put these systems together (pumps, plumbing, air purgers, expansion tanks, ...), and to me it just does not seem worth the effort to do this if you can't put up at least a hundred sqft of collector area -- more is even better.  


I'm sure you will really like the Shurcliff book.


Keep in touch,


Gary

« Last Edit: November 26, 2005, 01:29:42 PM by GaryGary »

Kwazai

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Re: solar domestic water from roof heat
« Reply #31 on: November 29, 2005, 07:04:09 AM »
"In using solar water collectors for space heating, I think collector area is the name of the game.  Its a bit of work to put these systems together (pumps, plumbing, air purgers, expansion tanks, ...), and to me it just does not seem worth the effort to do this if you can't put up at least a hundred sqft of collector area -- more is even better.  "


ditto.my available roof collector area comes to about 560sqft. storage space is mostly done, now its time to put together a couple of trusses worth of collector(radiant barrier) over my garage. will keep ya posted...


I have been digging into temperature controls, etc. and I beleive that the IAT sensor out of a car fuel injection system has the correct temperature range.

it would only need a transistor and relay to provide an available heat fan switch.

these are GM, but I'm sure the others are similar. It would wire into the 24v control wiring of the hvac unit.


http://www.diy-efi.org/gmecm/component_info/sensors.html


any way still thinking. my 'garage' heater will only have a bathroom vent fan hooked to it- while I figure out some of the other pieces...


L8r

Mike

« Last Edit: November 29, 2005, 07:04:09 AM by Kwazai »

Don Cackleberrycreations

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Re: solar domestic water from roof heat
« Reply #32 on: December 06, 2005, 07:41:43 PM »
Ok heres the problem.

first off if your attic is that hot it means you dont have enough vents .

So in the summer your house is twice as hard to cool. Now for some bad news

If your attic is that hot your new 20 year roof will last around ten years .

Any savings you gain off of harvesting the heat for hot water will cost you 3 times as much trying to keep the house cool .

since its winter the best thing to do is go out and look at your roof first thing in the morning. If there is frost on the overhangs but none over the living quarters you need both insulation and ventelation . Do not insulate the rafters to the botome of the roof sheathing  this will cut the shingle life in half. Also be sure there are vents in the soffit  so heat can thermosiphon off . Then install some roof turbines ( with a little imagination you can make a micro alternaor and the turbines can power a few LEDs) Keep in mind you can not over ventilate your roof.

 Start with insulation to stop heat loss during winter and make cooling easier in summer . to further lower cooling costs install a good attic fan. .

This wont get you anymore hot water but will save you a bundle onheating and cooling as well as prolong the life of your roof.
« Last Edit: December 06, 2005, 07:41:43 PM by Don Cackleberrycreations »