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
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
