# Heat sink solar heating system



## Jim S. (Apr 22, 2004)

Here's a hypothetical I have been wondering about that I might try out in a few years. I have been thinking awhile about this, and I cannot see how the design idea could fail provided the heat sinks were big enough.

I have a farm shop with in-floor water heat. Why couldn't I get 2 to 6 prefab septic tanks, dig a long pit in the ground, line the pit with rigid foam insulation, and set the tanks in the ground.

Then fill them with rocks and run PEX tubing through them, then fill them with RV antifreeze and water and seal the lids.

Solar panels would heat the water and rocks and there would be a thermostatically controlled valve that would direct the hot water to the shop when it is needed to heat the floor and building. But when the stat hit 70 in the shop, the valve would switch to running the water into the heat sinks. Once the temp got close to boiling, the pump would shut off so heat sink pressure build would not be a problem.

At night, the heating water needed to maintain building temp would come from the heat sinks rather than the panels.

It might even be possible, if the heat sinks are sized right, to run the PEX coils through the septic tanks and then just simply fill the entire tank with pea gravel as the heat sink, with no fluid. Or even fill it with solid concrete, like my shop floor is.

A gas water heater (which I use now to heat the shop) would serve as backup.

Been running this system back and forth through my mind and I can't see why it won't work unless the heat sinks are inadequately sized to store enough to heat the shop. For long cloudy periods, the gas heat would take over.

Thoughts?


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## sevenmmm (Mar 1, 2011)

Very difficult to calculate an exact answer as there are a lot of variables.

Water - without the rocks - will absorb more heat faster and will give it back faster, should it be needed. If you put the insulated tanks underground, you do not need antifreeze in the tank, but still need anti-freeze in the heat exchanger. 

To fully understand the type of system necessary, you have to calculate the BTUs needed per 24 hour day to heat the building on a typical winter day. Of course, every solar collector should have a BTU per hour output rating. Depends on your latitude, but here in Wisconsin we can only count on 4 good heat-producing hours per day during winter. Also, because of the angle of the Earth during Winter Solstice, the sunlight during this time passes through more atmosphere and has less value, so a calculation here is necessary as well. 

Right. How many BTUs of storage you will need to figure!

Other than actually doing the calculations, I could tell you if you are sizing the project to heat all of the building during the coldest day, you will be disappointed in just how many collectors and how much storage is necessary to get through the night. 

So, I would downsize the system to do the building on moderate days, and totally bypass the storage with the gas heater during cold days (another-words, don't heat the storage with the gas heater, just heat the building).

The other idea would be a highmass system. Just heat the building infloor starting in August, and if the footings are insulated, the ground will heat up and hold it for a time into the fall.

Not the best input I am sure, but it is my input!


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## SteveO (Apr 14, 2009)

A couple of questions?
What would you be useing as a collector??
What is the purpose of the RV antifreeze ( you are uderground already)
I had a house in CO completly solar heated 95% of the time
8 4x8 collectors on the roof heating a5ft diameter 4 ft tall tank filled with water there were 3 heatsink radiators dumping heat and 2 heatsinks taking it out for heat and 2 more for domestic hot water.
It would be cheaper to get a wood fired boiler and heat both the house and your shop or dig a deep trench and put your piping in it. Down 6 ft the temp stays 65 year round I seem to remember.
The collectors were 1k each and the control package was 500 so I had close to 12k in a heating system. And when we sold the house the new owner took it out what a waste. It was a sweet system
Steve


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## Jim S. (Apr 22, 2004)

I live in the South, in extreme southern TN. It rarely reaches zero here (but it sometimes does), rarely snows and the sun is good in winter. For example, last night's low was 53.

This is just a hypothetical idea. The collectors would be homemade. The idea behind the system is to make it on the cheap. I had intended at some time to supplement my gas heat with solar panels; adding the heat sinks would make it work for a longer period. I am not trying to be "off grid" with it, just have a major supplement.

The antifreeze would be to keep the tank fluid from freezing, as the tanks would just be one foot down to the lids. I already have it in my current gas system.

True on the ground temp. I could have it bored and put in coils. My BIL installs such systems. That would run me $10 grand, for a farm shop. Not happening.

I thought if I matched the cubic footage of the shop slab with the cubic footage of the heat sink tanks, I'd be storing double the heat and that would go a long way to supplementing my gas water heater.

I am very satisfied with the current system, and I do not want to have to load wood all the time in an outside heater. I also know several people with those syetems, and based on their experience they are inefficient compared to just a woodstove indoors. 

Just kicking around an idea.


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## pancho (Oct 23, 2006)

Cost would probably be pretty high. Dependability would be another question.


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## Cyngbaeld (May 20, 2004)

Skip the septic tanks. Dig your holes, line with something heavy, like the billboard tarps (cheap and very heavy) then insulate with straw (not hay) bales. Fill with large rocks so you have some good air flow thru them. Have your cold air intake at the bottom and your outflow warm air at the top and use a fan to push it thru. Charge it all summer while it is hot weather and close off the vents to the shop during this time. If you make it big enough, it might take you right thru winter. I read about one in Massachusetts that kept a house warm most of the winter, even with their less than perfect (for the purpose) summer heat.

The article I read was in a print magazine in the 80s and was probably in MEN or one called "Shelter" or something of the sort that was about energy saving (not "Home Power"). I've been wanting to build one ever since.

I made a solar heater once using old shower stall doors from salvation army. It was just for cold, sunny days to supplement the wood stove, no thermal mass involved. Worked pretty well.


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## ||Downhome|| (Jan 12, 2009)

I would skip the expense of the tanks. though if you had a leak proof tank you could skip the pex or any other tubing with in the tank? circulate the fluid through a rock mass.

Its true water absorbs and releases the heat the best but I think Rock and concrete buffer better.

Cyngbaelds Straw insulation is a good one, though straw will rot with moisture unless you take some step to prevent it. Above ground would be one thought. 

if you built the tank above ground you could use ferro-cement construction like they use for cisterns.Again filling with rock. Insulate with the straw bale. Perhaps use some heavy garbage bags and duck tape to prevent moisture damage. then use the tarp or epdm and cover with a layer of soil. 

May be easier to fix any maintenance issues that way.

I would stick with a hydro based system over forced air for the collection.


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## sevenmmm (Mar 1, 2011)

The problem with heating rocks is condensation. I know of systems that emitted very bad smells. 

If you want to go homemade I would suggest doing a simple solar air heater and use a small PV panel to power a fan. Remember, the air flows through easier under negative pressure.



[ame]http://www.youtube.com/watch?v=QLNViUsRCVU&feature=related[/ame]


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## ||Downhome|| (Jan 12, 2009)

I dont see how bad smells would be issue with a closed system.

a "water tight" tank filled with rock for mass, with the working fluid circulated through for collection and extraction. the rocks would be within the working fluid, so there is no condensation. 

a manifold system with solenoids and thermocouples and/or stats could regulate it with out high tech circuits. perhaps a few relays included.


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## sevenmmm (Mar 1, 2011)

||Downhome|| said:


> I dont see how bad smells would be issue with a closed system.
> 
> a "water tight" tank filled with rock for mass, with the working fluid circulated through for collection and extraction. the rocks would be within the working fluid, so there is no condensation.
> 
> a manifold system with solenoids and thermocouples and/or stats could regulate it with out high tech circuits. perhaps a few relays included.


Sure. But if you insist on a closed system then no need for rocks. Water with an anti-freeze mix is the best method for transferring and storing heat. If it is closed and the entire storage tank has the anti-freeze mix, then the fluid can be pumped through the collector as well as through the infloor system.

It should be noted anti-freeze is expensive.

The smelly system is an open system that flows the building air over the storage mechanism, in this case rocks and whatever.


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## sevenmmm (Mar 1, 2011)

Oh, and overheating would be an issue, so one would have to build a drainback or cover the collectors when this becomes an issue.

Type in solar differential controller into a search engine, this is a mechanism that would regulate circulation through the collectors and the floor.


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## Jim-mi (May 15, 2002)

Moisture and MOLD in a "rock" storage system can get very nasty..................
Not something you want to be breathing.
Getting rid of mold in an existing system would be one heck of a task . . . . . .

Earth temp around these parts is more like 53-54 F


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## ||Downhome|| (Jan 12, 2009)

sevenmmm said:


> Sure. *But if you insist on a closed system then no need for rocks.* Water with an anti-freeze mix is the best method for transferring and storing heat. If it is closed and the entire storage tank has the anti-freeze mix, then the fluid can be pumped through the collector as well as through the infloor system.
> 
> *It should be noted anti-freeze is expensive.* [*B]"One Reason for the rocks"*[/B]
> 
> The smelly system is an open system that flows the building air over the storage mechanism, in this case rocks and whatever.


see the quote for reason one. _*second is I think the rocks offer a buffer to the system..*_

I do not disagree that water or a water based heat storage is very efficient,it wants to give up the heat as fast as it can absorb it. not really a good thing if you want storage.


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## sevenmmm (Mar 1, 2011)

||Downhome|| said:


> see the quote for reason one. _*second is I think the rocks offer a buffer to the system..*_
> 
> I do not disagree that water or a water based heat storage is very efficient,it wants to give up the heat as fast as it can absorb it. not really a good thing if you want storage.


Insulate the tanks, the water will hold it.

The issue with rocks is it doesn't hold as much heat, and if you really need a lot of heat fast, it won't give it to you.

Look, I am not typing this won't work, but should one have the wherewithal, the water storage will pay for itself.

Now, if someone has no money, they wouldn't have a shop to heat neither. 

It is the same old story, a guy rushes out and builds a monster house, or building, then expects to heat it with the sun for free. Not typing this is the case here, just typing it happens all the time. Sorry, but having spent countless hours hawking solar at shows, I am sick of it.

Do the stupid rock thing then come back and tell us how well the experiment went.


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## ||Downhome|| (Jan 12, 2009)

everything a trade off in the world? 

I'm not saying your wrong because you are right but there's more then one way to skin a cat.

Of coarse someone with a wheelbarrow of cash isn't going to squabble bought a few bucks on the heat bill.


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## Jim S. (Apr 22, 2004)

So sorry to have spawned an argument. It really is a scientific hypothetical, not a money vs. no money issue. I did not rush out and build my shop, I took over 15 years planning it out before I built, and I carefully chose the existing heating system for economy and also to suit my needs.

I am more than happy with my in-floor PEX heating system now, which runs on natural gas. It works beautifully and is comfortable set on 50 degrees. Currently it is run by a 60,000 btu water heater, so I could get more efficiency when the heater goes out by replacing it with an on-demand boiler. I could not afford the double price of that boiler over a water heater when I built the shop.

I could simply add solar panels controlled by a solenoid-actuated valve to heat the water in that system, using the water heater tank as some storage and as a gas backup. This is proven old skool solar tech, nothing new here. The solar panel valve opens via a thermostat when the fluid temp falls to a certain level. When the temp reaches a certain peak, it closes. A small circulator pump moves the volume. The heating system loop runs off its own thermostat on demand according to shop temp.

That would work while the sun shines. What I ponder is adding a system where PEX pipes run through a rock or concrete mass to store additional heat that then can be released into my shop slab in nighttime or cloudy hours.

Lining with rigid foamboard is also standard solar practice for insulating heating mass in contact with earth. The same is done under in-floor heat slabs. What I could do is make a grid of PEX in a dug trench -- say 4 x 4 x 20 -- lined with foamboard. Then pour the whole thing solid with concrete to create the heat sink. Using RV-type antifreeze is also standard operating procedure in in-floor heating systems to preevent freezing if there is a power failure, etc.

Again, a thermostatically controlled valve would open the heat sink to the floor system when needed. This could be based on time, building temp or both.

I would guess the heat sink principle is solid, because obviously that is what makes the in-floor heat work. The concrete floor acts as a heat sink. Key would be to size the additional external heat sink to store enough energy to be worthwhile, and also to devise a system that would be as inexpensive and trouble-free as possible so the payback time would be shorter.

I could probably set up a small experimental demo sometime and see how it could work. Thanks for all the input.


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## sevenmmm (Mar 1, 2011)

I wouldn't use a heat sink at all in your case. Just allow the solar to operate when the sun is shining, the natural gas when it it not.

A simple differential controller can look to the collectors first, if there is not heat in the collectors, then call for heat from the furnace. 

Think about this, your infloor is already a heat sink. So with an additional heat sink you would have to increase the amount of collectors to have the ability to heat 2 heat sinks. For what? 30 days of production? Maybe? Much better to design the system straight to the floor, using the system for 120 days (or whatever) and spending a lot less time, money, and effort. 

Also, allow the collectors to heat the floor an additional 5 degrees (or whatever)(now you are utilizing your additional heat sink idea) from your furnace setting, and, be careful not to overheat the collectors during summer months.

Just my input, based on past experience with what people normally will chose after exploring all the options.

You can always add an additional heat sink and more collectors later.


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## sevenmmm (Mar 1, 2011)

And don't be sorry. I have a contingency of critics nipping at my heels.


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## cmcon=7 (Mar 7, 2010)

my plan was to dig down 4' like a shallow basement, insulate and back fill with pipes in the fill, the heat would radiate from the floor. no fancy stuff needed.
I have done this with a 2" slab and liked it very much, I ran the radiator water from my rabbit diesel(waste oil) generator and it would be nice 12 hours, with 4'
I bet it would be nice for a week.


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## sevenmmm (Mar 1, 2011)

cmcon=7 said:


> I ran the radiator water from my rabbit diesel(waste oil) generator and it would be nice 12 hours, with 4'
> I bet it would be nice for a week.


Oh! I like it!

Best idea yet.


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## cmcon=7 (Mar 7, 2010)

that's not all, I ran the hose through a 55 barrel to extract the the best heat for domestic 
uses then through the floor. I used 3/4" pvc in a manifold about 8" apart @ 0 psi, that gave me 14 lines so back pressure was very low. I used it like that for 3 years until the motor wore out, it still ran but had a bad habit of running away and burning the crank case oil and making tremendous amounts of smoke. I then used a 78 civic on gas for a year but I was spoiled on the free fuel and I upgraded my inverter to 1 sw1225 which is 12v so this method would only give me 500 watts instead of 1kw with the 24v system.

to convert the alternator I put a tail lamp in series with the field coil and set the idle to 40 [email protected]


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