Closed Loop Water Storage Tank Lifespan

desert_sasquatch

@hot_rod wow that's one huge water tank!

And that's interesting--and frustrating--about the conductivity of sand... I had been assuming that sand would have a similar thermal conductivity to rock and I see that's not the case, or rather that it varies quite a bit depending on what the sand is made from and how fine the mixture is. At least that's what I gleaned from this study. I'll have to talk to local folks and see if anyone knows the conductivity of the local stuff...

So thanks for answering my question. I guess this is why sandbed heat storage is not often done. As far as I can tell (I spend the last two days figuring out how to calculate the heat transfer through a sandbed...not that it's a big head scratcher for folks who know how to do it but I'm new to all this) it seems like this doesn't make it impossible but it does make it less practical. My math--which may be a bit too hard on the sandbed system because I only know how to calculate how much heat will pass from the radiant tube to the middle of the space between radiant tubes--suggests that I'd need to make a few concessions in order to get to the point where a whole night's heat could be stored in the space of four hours. For instance I could make it work by going from 8 inch spacing to 6 inch spacing plus assuming a 10 F temp difference vs a 5 F temp difference plus adding another foot of sand. Or I could negate the temp difference if the sand had twice the conductivity I'm currently assuming...

Aaanyhow, thanks again, I've been wondering about this for a month.

jumper

Multiple tanks are preferable. Otherwise stored heat becomes less useful as water cools.
High temperature water is less dangerous than steam. Cools fast when it flashes.
Still locate tanks where nobody gets sprayed.

Used electric storage water heaters are an an obvious idea. 250° is theoretically low pressure so technically they do not require rating. Hopefully no official will ever be involved.

Storing heat is the ground is interesting. Decades ago off peak industrial electricity was inexpensive. I think the company's name was Indeeco? Bury electric heating elements deep beneath the floor. Warm earth provides radiant floor heating. If you get too warm you can open window or buy a carpet.

I know folks in Los Angeles who are happy with thermal solar storing heat beneath their floor. Somebody calculated correctly. I think the practice there is to install insulation below heating coil. However if you can design correctly it may be better not to insulate. That way heat accumulates over the years.

desert_sasquatch

@jumper

In this scenario with sandbed heat storage I would have a tank for domestic hot water. The sandbed would be used for heating the home, and since that means that the water come out of it is probably useful down to 75 F or so, I'd be much less concerned with the whole "usable heat" thing. Am I missing something?

Interesting that 250 F is considered low pressure I did not know that! That would make storing all of my domestic hot water in a single storage container easier.

I admit that I'm skeptical of the idea that significant heat would accumulate below the home over the course of years. I'm open to being wrong but...isn't the whole idea behind ground source heat pumps that if you push heat into our out of the ground, that any difference from the surrounding ground temperature will dissipate? Maybe not immediately but over the course of a day it should become negligible. And that's because the conductivity of earth is high enough that any major difference in temperature gets smoothed out over time, right? Otherwise earth source heat pumps would have the same issues that we see with water source heat pumps with small water sources where water winds up being heated or frozen or whatever and then the heat pump becomes less useful. (I have no expertise in this, I've read about this).

It just seems like at some point the heat lost to the ground via conduction will reach equilibrium with the heat put into the ground from the heat source. And that point will come much sooner if there's no insulation below the heat bank. It'd be like pouring water into a bucket with holes on the sides; the more full the bucket get the more it leaks. Right? How would the earth around the home hold on to the heat from today next week?

desert_sasquatch

How often do large water tanks used for storing solar thermal have to be topped up? Can someone give me an estimate on how much water I would need to add to, say, a 2000 gallon tank each month?

(I'm curious about how much water the tank will lose to the environment. Which isn't a big deal if it's outside but if it's indoors then it's a source of moisture that could lead to mold--think what would happen to a bathroom with a tub that never emptied. The drywall might develop mold issues just due to the constant high humidity. My guess, however, is that if the tank has a top on it the amount of water that evaporates out will be minimal. But knowing how much water needs to be added each month would tell me in more exact terms.)

hot_rod

If the tank is loosing water from evaporation, it is also loosing heat energy. The entire tank needs to be sealed tightly. Also for moisture issues and possible bacteria growth in the tank.
The unpressurized tanks that you buy haver well insulated tight sealing lids.

desert_sasquatch

Oh, thanks @hot_rod. So the vent in the unpressurized tanks that I read about in idronics 17 (page 19)...those vents are like the vents connected to the pressurized part of the system, they're almost always in the "closed" position?

hot_rod

If the tank isn’t designed to hold pressure there will be an air space to absorb the expansion as the water warms. A small vent prevents the vessel from being pressurized and possibly damaged.

desert_sasquatch

Just to be clear @hot_rod are you saying that if I got one of these unpressurized thermal storage tanks, I'd be able to fill it up once and never top it up again until I replaced the liner? (I do assume there is some antimicrobial something I'd have to add periodically, but in this case I'm just asking about adding water.)

Larry Weingarten

Hi, My experience with unpressurized solar tanks is that they do need to be periodically checked and topped off as venting will allow humid air to escape. As the tank heats and cools, it will slowly breathe and just like people, will lose water. You may only need to check water lever every six months, depending on how hot the tank gets.

Yours, Larry

desert_sasquatch

@Larry Weingarten thanks, how much topping up are we talking each six months? A liter? A gallon? (I know you said it depends on how hot the tank gets, I'm just looking for a ballpark figure)

jumper

Larry Weingarten said:

Hi, My experience with unpressurized solar tanks is that they do need to be periodically checked and topped off as venting will allow humid air to escape. As the tank heats and cools, it will slowly breathe and just like people, will lose water. You may only need to check water lever every six months, depending on how hot the tank gets.

Yours, Larry

Correct. And sealed tank may vent through pressure safety. Hopefully air cannot get in when it cools. Instead there is expansion space. So it will never vent again and one will never have to add fluid.
Sealing out air absolutely can be difficult.
OP says he is unconcerned with cooling. Therefore for lowest floor storing heat in the ground sounds easiest.
Transferring heat from some solid material to upper floors is another matter. I doubt that blowing 75° air can be reliably comfortable. Remember complaints about heat pumps blowing "cold" 85° air? Perhaps it was in another thread that I mentioned electric high temperature heaters?

hot_rod

desert_sasquatch said:

@Larry Weingarten thanks, how much topping up are we talking each six months? A liter? A gallon? (I know you said it depends on how hot the tank gets, I'm just looking for a ballpark figure)

I think surface volume, surface area, temperature, evaporation, humidity all play a part in how much water you loose.
Check it yearly, really no harm in the level dropping an inch or so, until you refill as long as the coils are submerged.

I’ve seen homemade plywood tanks with EPDM liners and the lid lag bolted on top, so very little water loss. Actually no need to vent either as long as there is adequate expansion space. It would be tough to get an air tight seal on liner tanks, there are always some puckers in the liner as it folds over the top.

WillyP

Larry Weingarten said:

Hi, An approach I've used successfully is to use a large plastic tank, essentially for heat storage, and then use copper coils high and low to put heat into the water or remove it. This way you can have a long-lived tank, but not under pressure.

Yours, Larry

This is similar to what I am designing. But I plan to use aluminum tanks not plastic. How many coils (or length of coils does it take in a 1000 gallon tank?

WillyP

First off let me state; I haven't built squat (yet). bt I have done a ton of research. I will say this. After hundreds of hours of reading and learning Hot Rod is by far the best source of information there is.
As for mold. just add something to the water to keep it from growing algae and other nasty stuff.
As for a plastic tank. Most manufactures say their tanks hold shape up to 140 degrees F. After that they start to reshape themselves (that is very bad).

hot_rod

I have done a number of both unpressurized tanks and stell pressurized tanks (former LP tanks) up to 500 gallons.

I did try a HDPE pallet tank and about 140F the plastic would blister out through the steel mesh surrounding the tank. I would not trust any plastic tank above 130F or so.
The best heat transfer is with an external plate type heat exchanger. With a plate you have two pumped flows and excellent heat transfer. You can get within 2° of supply and distribution temperature.

With coils in a non pressurized type tank, the tank water is stagnent so heat transfer is much lower.

It is nice to have the HX outside the tanks for service also.

Some examples and formulas to play with from Idronics 29. Swap solar thermal for the wood gasification boiler shown.