Heat Storage For Low Temperature Solar Thermal

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New Member
Jan 30, 2023
Rocky Mtns
I'm planning to build a home, and in my particular location (a rural--read "no natural gas"--part of Climate Zone 6B) it seems like 90% solar hydronic with either electric or propane backup will be the cheapest over 30 years. And yes, I believe it will be cheaper than a heat pump, at least if I'm going to have radiant floors--which I do want. If folks are interested I'm happy to break down my cost estimates but I want, in this post, to focus on my main question, which is about heat storage:

I'm under the impression that most folks who use a heat bank use a water tank. However with a solar thermal system in Climate Zone 6B, being able to heat one's heat bank only to 85 or 90 degrees F is quite advantageous in terms of the panel efficiency--25%-50% on a 30 degree F day when compared to heating water to 125 degrees F depending on the cloud cover, last I looked. Moreover, the higher gains happen on the colder days, meaning that panels go from "only useful in the winter on sunny days" to "useful almost any day the panels aren't covered in snow."

But using 92 F water from the panels means having a heat bank that's large enough to absorb a night's worth of heat without going over 90 degrees or so. That's pretty huge. For example, lets say I need 250 kBTU stored on my worst day. The formula for heat stored in water is

8.33 * X Gallons of Water * Change in Temperature in F

If we assume that the solar panels are heating the water to at least 90 degrees (probably using what is originally lets say 92 degree water) and that the water can be used to heat the home down to 80 degrees then I'd need 250,000/(8.33*10)= 3,001 gallons of water. Or I could bump the temperature up a little more or lower my expectations for how much heat I can save for the worst night--either will lower the size a bit--but you get the picture. Probably I'd need 1,000-2,000 gallons of storage capacity to heat the home using solar thermal panels that are operating efficiently by putting out fairly tepid water.

So that's an option, but an expensive one. A propane tank that large would be difficult to transport into or out of a home. It might be easier to build an enclosure around the tank, but it would need to be built to something close to the standards that the home was built to in order to insulate the tank and keep out the weather. So it winds up being a lot more expensive than just getting a tank and wrapping it with some fiberglass.

An alternative I'm wondering about is creating a sort of "insulated sandbox" in the crawl space. In other words I'd build my crawl space like a basement in that it would have an insulated cement floor. I'd insulate outside of it just as one would a conditioned basement. And then I'd fill it with sand and pex tubing, and top it off with a little more insulation--foam glass of some kind probably--and then put a cement slab on that with radiant tubing.

I've done my best to price this out and it seems like the price is probably about $8,000 to $9,000 for a 1000 cubic foot sandbox, which should be large enough sandbox to provide enough heat storage that on the worst night I'd need a delta T of about 11 degrees F (meaning it would need to be heated to 75+11=86 and the output from the solar panels would need to be slightly higher than that). For context, I believe that's the equivalent of 2,700 gallons of water.

Compare this to the price of a 2,500 gallon water storage tank. The tank itself might cost $1,870. Plus shipping which lets be generous and assume it comes in pieces so lets say $200. So a bit over $2,000. Then there's the housing. This tank is 8 feet around. Figure I'll want 12" of insulation because it's meant to store heat, not bleed it off and insulation is cheap. Then figure I'll want 1.5 feet of space on each side so someone could get behind it if they needed to. Then figure that if I put it in my house any space I stick it in will be a square. So that's a 8+2+3=13 foot square which is 169 square feet. For this house I might be paying $92 per square foot of empty space so that's a bit over $15,500 to stick it in the home. Plus however much I pay for insulation and if I want to get fancy something to cover the insulation or protect it from mice.

It probably costs about as much to put it outside the home, and of course then the heat lost into the environment is truly lost, no portion of it gets "lost" into the home where is essentially space heating.

So to me it looks like if I want this kind of low-temperature thermal storage the subfloor sandbox is the way to go!

However...other than this guy at builditsolar, it doesn't seem like many people have gone this route. Is it just because solar doesn't add to the sales value of a home and homeowners can't retrofit a home this way?

Does anyone have thoughts on building a "thermal sandbox"? I welcome all thoughts. If it's a dumb idea, I'd prefer to know now :)
...and Bob Rohr just put me onto Bob Ramlow for sand bed storage. He's got a book so I'll check that out. But I'm still interested in hearing people's thoughts!
You can use a galvanized culvert installed vertically with a poured concrete bottom and a tin top as a storage tank. I have had two in-ground 3400 canadian gallon cisterns for potable water done this way. One is about 30 years old and the other is 20 years old with no issues. A 12' x 8' culvert buried in your basement/crawlspace might be an option.

sometimes a culvert gets rolled wrong and ends up slightly out of spec, size-wise so the yard may sell them relatively cheap. My more recent one is very thick steel (made to support earth and heavy traffic) and was quite a bit cheaper than the thinner first one because it was a reject.
I am in favor, at least theoretically, of having the heat storage tank inside the insulation envelope of the house, but not insulated from the house.

Once the hot water returns to the storage tank from the solar panel, the BTUs are inside your envelope. Some of them (BTUs) are going to come out through the wall of the tank - but they will be inside your house and the infloor radiant won't have to carry all the load.

If your site can accommodate a walk out basement, even partial, I would be sorely tempted to put the water tank in there. You could frame the walk out wall for an overhead garage door, get your tanks in, and then build a regular wall inside the framed door opening instead of installing the overhead door.

I am used to seeing one piece plastic tanks up here, 1,000 gallons is the common size. An alternative would be two propane tanks at 1k gallons each. Those are very very pricey for me local, but they don't weigh much empty. Certainly moving an empty 1k propane tank around your not a garage in your not a walkout basement with a small I beam on the ceiling and a half ton chain hoist should be doable. I am not sure how much steel work you would need to purchase to stack a 1k gallon propane tank on top of the one on the floor. Full of water they are going to have some weight to consider.

Inside the envelope, one thing I strongly suggest is some kind of thermal break between the storage tank and the floor. Even used railroad ties, just something to prevent the hard earned BTUs from leaking out through the floor.

Today you are the athlete on the sand and I am the spectator. I have no idea what your local prices are for steel and RR ties and etcetera.

Good luck and best wishes.
Dig your basement an extra 4 ft deep, line it with 12" of foam, spray polystyrene foam or both. Line that with a water impermeable membrane. Pour a concrete floor 4 ft above the bottom of that.

Essentially you're building a large insulated water tank under your basement floor. I'll try to find the article but there was one built here a few years back, 24,000 gallon I believe was its storage capacity. I'll see if I can find the article.

IMO it's beneficial to have more than a few days energy storage for cloudy days, or cold days.
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That would be a chore to remove the formwork. Hire some little people!
IMO it's beneficial to have more than a few days energy storage for cloudy days, or cold days.

I rarely disagree with ABMax, and honestly cannot think of a time I did disagree with him.

While it is lovely to set up your solar powered system to meet most of your needs most of the time, everybody 'knows' you need a back up.

If you can, economically, store 3-4 days of BTUs that is great. But someday you are going to need 5 days of BTUs on hand. Whether you are going to burn cordwood or propane on that 5th day is really up to your local conditions and prices.

I had never before considered putting a swimming pool under my garage floor for heat storage. It is an idea that probably does work in some situations. When I get to my site I will at least pencil it out as a possibility.
I followed your ( @eweb ) green building advisor link in post one and see there is probably not a lot of trees growing near you.

If you do have trees nearby, would it make sense to run an OWB once or twice daily to heat perhaps a 1k gallon tank to 150 dF or so? I am guessing not.

Second, for you in your local conditions (rabbit hole) does solar hydronic really beat the pants of photovoltaic? I get we are up against a moving target.

At the end of the day I remain in favor, if it makes any kind of sense at all, to put the heat storage inside the envelope with the people and their stuff.
It would be ridiculous not to put the heat storage inside.
What kind of collectors are you using? What is your building and or engineering background? Are you planning PV panels? Who is building this? What maintenance cost are you accounting for?

I have spent a good bit of time learning about building efficient homes. Have you read the “pretty good house? If not you really should.
Pretty Good House https://a.co/d/amgaNW0

I really really don’t see how with the cost of electric solar panels a hot water system would be cost effective unless it’s is a home brew system. Maintenance of glycol systems is significant.

I have not watched this yet but I will tonight. Not to side track your thermal storage question but I do think it’s relevant.

Furthermore here another one.

All of this depends on having excellent construction methods.
This a good write up for solar thermal DIY collectors and storage in a cold climate.

While I am too skeptical of solar thermal space heating being cost effective vs solar PV+heat pump, esp if you spent the extra money on the envelope... I note that you are in the Rockies.

Out east the winter is CLOUDY. I think being west of the Mississippi is key for this sort of approach. I assume you have looked at winter solar thermal resources for your location already.
If I were building a home from scratch I'd be using the passivhaus principles...thick walls, tight, no heat transfer, etc. Heating and cooling systems can be significantly downsized as a result, and you could probably heat the house with radiant floor heated by a small, efficient source. Shut that off and the house will maintain temperature for a long while.

Do you have any flexibility in design, or is that already locked in?