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Discussion Starter #1
This is something I've been planning for a long time, and I'm going to actually start working on it soon.

Solar heat in Europe and especially in Portugal where I live, is a racket. Setups I've seen can't possibly work as advertised. Most use heat pumps and are really just very expensive electric heaters.
Our climate is similar to the southern US.

I've done testing and research, and worked some numbers.
I got a black metal plate in a box with a glass top to 110C by putting it in the sun on December 22 [the solar solstice].
Solar heat at my location should be achievable, but few people if anyone has actually done it. I guess because it's hard to make and sell such a system for a profit.

"Solar" heat tanks I've seen sell for insane money, like $7000. And they're only 1/2 ton. My calculation is that I'll need 1 ton of stored hot water to heat my house for 24 [cloudy] hours. So those expensive tanks will barely bridge the night.

It was suggested to me I buy 4 solar collectors, a total of 8 M2 [about 90 square feet]. My calculation is that I'll need at least twice that.

Ok, the details go on and on so I'll try to keep this part short. I'm just wondering if anyone here has built a significant solar heat gathering and storage setup?

I plan to fabricate practically everything, since it's all sold at exorbitant markups here. There is a big stainless tank for sale at a nearby scrapyard, and I might go for that instead of my first plan of mild steel. But it's only about 2 tons and I wanted 3 or 4.

System insulation is one of the major expenses that I can't avoid.
 

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A major consideration for water type solar collector systems for home heating is, what do you do with them in the summer? The area required is substantial for use in the winter, and the longer hours of summer sunshine will drive the temperature up to scalding level unless there is a constant flow of water to keep it reasonable.

Another thing to keep in mind is that there is a significant difference between using a solar collector for temperature or for heat. I used solar to heat my pools for over 20 years. A relatively high flow rate collects heat at a lower temperature. A relatively low flow rate collects temperature, not so much theat.

When heating for a shower, relatively high temperature is required and a low flow rate will handle that.

When heating a 100,000 litre pool, you want as much heat as possible out of the system, so you have to run high flow rate to take advantage of the low temperature differential between inlet and outlet, and the panel. More heat transfers with the low temperature differential in this application.

I had 420 sq-ft of home made panels on the garage roof. At peak heat production in July, they produced 65,000 BTUs per hour between about 11am and 2 pm. Heat gain at 9 am was minimal, if anything, and at 5:30 pm, was about 0.5 F°, probably due as much to the ambient air temperature as to solar collecting. The flow rate was the full output of a 3/4 hp recirculating pump, about 1700-1900 US gph, and the temperature differential between the pool and the outflow was 5 F° at peak. The plastic hose and header panels leaked and the leakage was collected by the rain gutter and directed back to the pool.

I have seen one home brew solar collector system for home heating that used several 120 US gallon plastic tanks in the basement for heat storage. At the time, they were not insulated. Ground temperature (41° F at a depth of 6') in the winter around here makes direct earth burial for a heat retention tank undesirable. Perhaps that may be a viable alternative in Portugal.
 

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Discussion Starter #3
My plan for heat control is fairly simple; the panels will be at a 50-55 degree angle for maximum winter heat gain [sun is at 60 degrees at solstice]. In summer the sun is at a high angle and there will be much less surface area as well as a lot of reflection. If I still get too much summer heat, I'll add a partial shade, or have an in-ground heat sink loop, or both.

I've done much research, and some experimentation, on stratified heat stores. Domestic hot water will be heated from the hot water at the top of the storage tank, while my underfloor heat loops will take water from the lowest level in the tank where the water is hot enough. A fairly simple system of thermostats and electric valves can accomplish that.
Here's a diagram I made earlier;



I found various ideas for stratification valves. My first idea was to use a complex set of electric controls, but then I found this system which has been in commercially sold solar heat tanks for a while.



I made a test rig to see it work for myself;


I put it in a barrel of cold water and fed hot water in, then cold water. Sure enough, the top flap opened for hot water, and the bottom one for cold water. The barrel stayed stratified.




It's important to keep the velocity very low to avoid mixing, as well as pressure differentials that would just open all the flaps. A challenge is to build one that will operate in hot water without failure for a long time.

I plan to have a series of thermometers down the side of the tank so I can keep an eye on the thermocline.

How to adjust/control the circulation pump from the collector header is still not decided. Ideally, it should run if the temperature there is higher than that at the tank bottom. But I think in reality I'll just set it to run at 80C collector temperature to begin with and see how that works. It will cycle on and off in weak heat gain times, which is fine.

I'll need a box full of adjustable 'feeler' thermostats, electric valves, and much more...
I don't want to use electronic controls or a microprocessor if I can avoid it. That would make it imposable for anyone else to service it.
 

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That stratification is interesting, although I have some doubts as to using 80° C domestic hot water. My tank is set at 50° C and that is more than enough to get my attention if someone flushes a toilet when I'm in the shower.
 

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Discussion Starter #5
I haven't decided how the DHW will be taken off; the simplest is to put a heat exchange tube set right through the holding tank, where cold domestic water would enter at the bottom and draw hot water through the top.
2 problems; 1, there is serious danger that such a setup would ruin the stratification in the tank. 2, the heat system is going to be some distance from the house so that would create a slow response when there's hot water demand. No one likes that.
What I'll probably do is have a hot water heater in the house, with an internal heat exchanger for water from the solar tank to circulate. I'd need a thermostat to prevent cold water entering the heat exchanger [which would cool my water instead of heating it]. This adds more stuff but solves a few problems too.

With both the DHW and central heating, the returning [cooled] water would have to enter the storage tank through a stratification tube, as it could be just about any temperature. So in order to have the possibility of efficient solar heat gain during heat draw off, I need to fabricate [at least] 2 stratification tube assemblies.

With all these differing posable designs, it's going to be tough to get it right in one try. It's the sort of thing you could get working really well by the time you build the third or fourth installation!

I've learned a lot from online sources, but there's a huge variety of plans and people with different design criteria.
Commercial builders keep their research close to the chest, and they're about the only the people who built more than 1.
And theirs have to be priced to sell and sized to fit through standard doorways.

Jeez, this just goes on and on...
 

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It's definitely going to be an "interesting" project. :banghead3

Keep a supply of headache pills available. :fing32:
 

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Discussion Starter #7
My local scrapyard has a 2 ton stainless tank for sale; I was up there yesterday to look again; I think it's too thin but my efforts to measure the skin didn't succeed.
I think it's 2mm, but it could be 3.

So I'm still hunting for a suitable tank, the alternative being to fabricate it myself.
I'd have to start by making a plate bending roller. That's really going a bit too far, even for me.

The prices asked for old stuff here is just crazy. When I do find something, it costs way too much.
I found a perfect LPG tank at another scrapyard, but the guy is nuts. Maybe it's because I'm a foreigner, they think they can stick it to me. He has a yard full of them and they've been there for years.

Anyway, this will be a long project and I have some other work to finish up, so no hurry.
 

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If it was a pressure vessel, then you would want a thicker skin. Since it is a holding tank, the thickness is not so critical and 2 mm should be lots. The wall of my 24' dia. x 4' pool containing 6 tons of water was less than 1 mm.

Fast conversion:
- 2 mm = 0.079"
- 1/16" = 0.063"

The important thing to remember is that the pressure at the bottom of the wall increases with the height of the water column. This article in an engineering forum may have some useful links.
 

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Discussion Starter #9
The main problem is my need to weld on the tank; I need a closable access hole and there will be many ports.
Welding material that thin is tough [I don't even have a mig] to do reliably so it won't crack and leak into the insulation later.
And there will be pressure; just a little, but even 1 bar [15lbs] would put undue stress on metal that thin.

I like heavy metal.
 

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Discussion Starter #11
The tank is round; not that flanges would be imposable, but they would be a real challenge.
I need to add 2 large stratification tubes inside, and a row of ports for draw-off. And thermometer ports too.
It's just not the right tank. There is some doubtful welding on it already and it's smaller than I want. It's mounts are horizontal and I need it vertical.

I'm not really sure the advantages of a stainless tank outweigh the disadvantages. The SS tanks I find here are made for wine fermentation and from the plate distortion I can see near the welds in the photos, I think they're all made of very thin material. I've been looking online; most are 3-4 hours drive away and over my budget so I'm not even going to look.

4mm or thicker and it's strong enough so I don't have to worry about it. A closed system won't corrode they tell me; I may ad some glycol if it's not too expensive. If I find a thick walled SS tank I'll give it serious consideration, but that seems unlikely now.

I've been to several scrapyards recently and they all seem to be run by persons of doubtful sanity; I don't know how they finance themselves, but they have mountains of stuff they refuse to sell for prices that people are willing to pay. I need to find a private seller or I'll be forced to fabricate from new plate.

This one is 2.5 cubic meter capacity and an hour away from me;

It's overpriced but I might go have a look.
 

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Discussion Starter #12
In between trying to get my other projects under control, I've done some new searching and found a couple of new stratifier solutions have come onto the market.
This one is a Dutch startup, I really like this idea but it's a bit expensive for me;
Thermo-differential stratification valve | conico valves

145 euros each + tax and shipping, and I'll need 6 or more valves. But they screw on from the outside which means that they can be replaced without draining the entire tank. And someone else has done the R+D, testing, and manufacture.
Hmmm.

Then I found this;
StratiFlex?

I don't like it as much but it looks cheaper. I've written to the company but haven't gotten a reply yet.

I have another possible tank, a uses LPG can. I have to decide on the size I want and do more numbers.
 

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So Mark, just thought I'd throw out something to muddle the waters even further before this conversation gets deeper in the technical stuff that will confuse me even more. This is a lot simpler than what you are doing, but it worked on our house for some 28 years before the two panels gave up the ghost due to leaks and us taking them down to have the roof shingled. Always thought we would replace the panels but 6 years later we haven't. Mainly cause of studies showing that domestic solar hot hater doesn't have the payback that it once did when we got the 55% tax credit. Conventional wisdom now says add another couple solar electric panels and just use the electric to heat the water. Since we already get most of out heat and hot water (six months /yr) from a gasification wood boiler combined with our age, payback isn't something we are looking at. Maybe I'll replace the panels, probably not but who knows?
Well enough of that. Here is what controlled all of our hot water (controller pictured below). Had a number of thermo couples throughout the system, collector, top, bottom of tank, in and out water pipes, all wired into controller which turned on circulator pumps depending on temperature differential. The closed heating loop was filled with antifreeze which eliminated any draindown situations at night.
I know this was a basic simple system using mid 70's technology but just thought I'd pass it along to either prime the pump of confuse things even more.
Always reading your posts with interest.
MikeC
 

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Discussion Starter #14
Hi Mike;
I'm trying to do something difficult, which is to heat my house with solar radiation.
Here in Portugal the weather is fairly mild and sunlight strong, even in winter. My calculations are that I should be able to meet between 70 and 90% of my heat and hot water energy needs if I can get everything to work just right, without excessive heat loss.
Solar thermal is like the orphan of the energy world; it should be effective and inexpensive but it seems that salespeople prefer to sell more costly solutions for some strange reason.

Energy in Europe is very expensive. Wood is still relatively cheap and I might build a backup wood furnace one day.
At the moment I spend around $1,500 per year on heating fuel, which is a big chunk for me. I could change to anything else and save money.
Portugal has announced their intention to completely stop using fossil fuel in the next few decades; I don't actually bevel that, but there will probably be further increases in the exorbitant taxes we already pay.

I need a new heating system for sure, and I've decided to try to build a solar thermal combination unit.
There will be 18m2 of absorber panels [200 square feet] and a 2.5 ton storage tank.
Let's see if it works...
 

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Understand completely, at least the why. My thought had been looking at the controller and pumps as perhaps a less expensive way to do it rather than depending on stratification for circulation the water. Yes I know that there will be a certain amount of movement just from temperature variance. When we built our log home 35 yrs ago, I spent the better part of the winter tracking the sun and taking compass readings, and ultimately sited the house for the greatest gain in the winter. With just two 4 X 8 panels the water temperature in the (small by what you want to do) 75 gal tank hovered around 110 - 120 degrees in the winter and in summer was 90 -110. (degrees F) Another reason for the generator. Most of our power outages happen in daytime, hence with no power to the panels, there were several popping of the safety valves as the temp soared in the panels.
When we put in our system solar thermal was indeed the darling of the energy world. Several years later there were so many doing it that the govt decided to end the 55% tax credit and let the market stabilize with the better, stronger companies hopefully expanding and that competition would drive the price down. Unfortunately, that didn't happen. Without the rebates, the market tanked, companies closed their doors and solar thermal became a non entity for a good ten yrs. It is still the orphan child of the solar industry as solar electric became the darling. All I can say is that it was a great having it for the 27 yrs or so that we did, and part of me would still like to put two panels back up on the roof and turn on the hot water spigot again. Will be faithfully following your adventure in solar.
MikeC
 

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Discussion Starter #16
I bought the steel tank.
I used the seller's old JD tractor for loading, I took my pallet forks from home. That's one loose funky old machine...

It's 6mm [1/4"] thick, weighs 1/2 ton empty, and is 2,500 liter [660 US gallons] capacity.

I was thinking of a bigger tank at first, but then I figured out that the increased heat loss would be too great, even with 8" of insulation.

I have some other work to clear before I get to it, but I'm working on the design; like, how do I figure out the best flow rate through the collectors?
 

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A faster flow collects more heat. There's a lower temperature differential throughout the collector.

A slow flow rate drives the temperature up faster within the collector. The higher temperature differential doesn't collect BTUs (or calories) as easily.
 

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Discussion Starter #18
Thanks Bob, you got me thinking.
What you said is correct, for a domestic hot water heater; the most heat will be collected, although it will be 'thinned' in a larger quantity of water.
A hot water heater tank will [if necessary] make up the difference with its electric element, using less power that it otherwise would.

My situation is different. Because my tank is quite large, it will take more than an entire day of clear sky to heat it in winter. A stretch of 4 or 5 clear days to heat it up completely.

So in my case, I think I need to set the flow so as to supply water at [least at] my minimum usage temperature [+ loss through the pipes], where it will be fed into the top of the tank, leaving cooler water on the bottom to be heated when possible.

That way I'll have usable heat after some hours of sunlight.

What you said still holds; as I hit the target temperature, the pump should speed up to hold it there, until all the cool water has been drawn through and starts recirculating. At that point, the temperature across the tank will start to rise until it gets to maximum.

So now I have to work out the minimum and maximum flow required to do that.

I will also have electric elements near the top of the tank, that will only heat a small portion as a last resort, but enough buffer to last me through the day so I can use the cheaper night rate electricity.

Eventually, I hope to add a wood burning backup heater. That will likely heat the whole 2.5 tons of water to a high temperature to last 2 or 3 days in winter.
But that's a whole other thread!
 

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I did my experimenting with solar water heating for our 14,000 gallon pool 40 years ago. The panel I built wasn't all that efficient, but it could raise the pool temperature 5-6 F° per day, with the aid of the sun beating on the water surface for the first half of the day. The flow into the pool was aimed at the bottom to eliminate stratification.

The panel consisted of a pair of 10' 1.5" ABS drain pipes with 30 pieces of 1/2" polyethylene hose between them and would deliver about 12,000 BTU per hour at peak heating times with the full flow of both a 1/2 and a 1/4 hp pump going through. Temperature rise between inflow and outflow was about 5 F°, if I recall correctly. (Getting old and the memory is fading.) While I could get a higher outlet temperature using only one pump, the drop in flow rate reduced the heat collection significantly.

The thing to remember is that the temperature differential between inflow and outflow does not change to any serious extent at a given time, even as the tank temperature rises. The angle of the sun does change the temperature differential. At my location, from 1 F° at 9:00 am and 5:00 pm to 5 F° from 12:30 - 2:30. You're just a tad further south than me and should get better numbers.
 
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