Packing Heat

Yeah those absorption chillers are one tech that I haven't quite got my mind around. My understanding is they are fairly inefficient (20% or something?) but since your solar collectors are probably producing way too much heat during the summer, a 20% conversion rate is better than just wasting them. I suppose if the 20% number is in the ballpark and your collectors have a 75% solar capture efficiency, you can get maybe 15% efficiency out of the collectors when used for cooling purposes. Which isn't too far behind the 20% neighborhood for PV. There you go, still another way to make solar collectors productive during the summer.

Did a couple go rounds exploring PCMs. Definitely interested but there always seemed to be some drawback, discharge cycle limitations etc. Some youtube guy did periodic updates on his sunamp, was definitely excited about the tech and wanted to make it work, but iirc had to call the company back for leaks a few months later, and then again a few months after that. Looks like the function of the PCM in the Latento is to increase capacity and also buffer against vaporizing? Everything I've come across so far has circled back to water or slab being a better option than PCM for primary TES for buildings, but it makes sense to me that it could find a good niche as a supporting actor.

Sooooo… I noticed I have so far failed to sell anyone on my concept of using the ground under the house as thermal storage 🤣. But for anyone who has made it this far in the thread, I am trying to come up with why it wouldn't work. Now, I understand why it would almost always be a bad idea. In general it's a terrible idea, because it would be — by far — inferior to a water tank when it comes to volumetric heat capacity, heat exchange rates, and insulation. My hope is that my (rather specific) set of conditions may be enough to tip the scales at least as far as "maybe a chance in ****."

I mean specifically in the context of these particulars: Moderate climate with dry summers, will be installing the hydronic-solar combisystem including radiant cooling that I have described at length, and that system at every turn is designed around the aim of reducing required water temperature differentials. As I understand it, best predictor for undisturbed ground temps at 20' deep or so is average annual temp, which is 62 here. And the average temp below a building will be warmer than that; somewhere between the average surface temperature of the slab and 62. The footprint of the house including crawl space is about 2000 square feet, and so I suspect I have a good 40,000 cubic feet of dirt before I get down to 62f temps, on a temperature gradient that's probably around 70 at the dead center of the slab and (I know I'm oversimplifying here) then ranges towards 62 as you go downward and outward. My guess is that during the summer I have an almost limitless supply of the 66f floor temperatures to draw down from, just sitting there right at my feet. The limiting factor is how quickly heat from the interior/surface of the slab can migrate into those depths, which is why I think it will be necessary to renew it nightly with 10 hours of night cooling from the solar array. I think in the long term all that ground would be almost self-renewing on its own, but in the short term the surface of the ground needs nightly recharging.

The winter situation is less favorable because the base temperature scenario of all that ground is lower than we need it to be. But not by all that much. By the time cooling season ends, we will have already begun to lift the average water temperatures we're cooling the slab and crawl space to each night. As fall progresses, we will be running as much heat into each as thermal comfort in the interior allows. I'm making a starting guess that by the time peak winter arrives we can have a lot of cubic feet of dirt down there warmed up to perhaps somewhere in the 70-80 range? There's going to be a lot of downward and outward heat loss still to be sure, but remember the TES is built around the concept of heating a huge volume of slab and dirt near the surface only up to something like 85 degrees and being able to survive the night on that. Using volumetric heat capacity of dirt being equal to the slab, and both being 1/2 the capacity of water as a first approximation. Then, the first foot of depth (ie, something like 6" deep of slab and then 6" deep of ground), that top layer has a heat capacity equal to:

2000sqft x 1' depth x 7.48 gallons per cubic foot x 0.5 = 7480 gallons of water

which can hold:

8.33 x 7480 = 62,308 BTUs per degree Fahrenheit

which should be enough to carry us through a peak winter night even if the array only heated that top 1' of surface up to 85f. Yes, a significant amount of heat will bleed away any way you look at it, but we are looking at heat loss into a giant cube of earth that has been conditioned all fall so that it might range downwards with a temp gradient of something like 80 to 70 over the first 10 feet. At this point though I'm only making up numbers, need CFD modeling to answer these questions.

But maybe they're at least questions worth asking? When the temp we need to increase the ground to is so low, I think the heat loss may prove an acceptable tradeoff. The massive volume of the earth underneath the house — and the fact that it's free and already there — may outweigh the fact that it's a far worse choice than a water tank on a per cubic foot basis. If — and only if — the temp we need to heat it to is fairly low. No way this works at all back east, or other climates where the starting point of the earth temperatures are far less favorable.

You have my permission to try this. If that is what you're asking :)

Well maybe not exactly. Although if you were to forbid me, I don't think even Apollo could make this thing pan out

ah that makes sense. Yeah even Martin Holladay said something like storage doesn't pan out unless you're only looking to store for 4 days. When I saw that I thought 4 days would be great for my purposes! Although I am hopeful this concept lends itself to extending that a little further… just long enough to get a running start in the fall. Even if 80% of the heat we pack in during the late fall goes to waste, that would still be a win if all the heat were generated using the array's spare shoulder season capacity.

Calcium chloride is good for this because you can change the melting point just by changing the proportion of water.

and I see without water the melting point is 84. I'm using ASHRAE's barefoot floor temp limits of 66-84 as a constraint, so it sounds like it could be formulated to hit the sweet spot for both summer and winter. Mouthwatering!

Roger that on the shifting. Need to take into account both rates and relevant supply temperatures.

On the dirt, volumetric heat capacity is the better metric here, yeah? I've been going by volume. Will depend on specific soil composition but I think by volume it's closer, at least for something like an "average" soil composition. For prototyping I'm using water has double the VHC of dirt, and dirt has double the thermal conductivity of water. And slab concrete works out to about the same VHC and thermal conductivity as dirt. Obviously these are simplifications but at this point I'm only looking to land in the neighborhood.

No digging would be involved, except — possibly — for some perimeter insulation around the exterior of the slab, going down maybe a couple of feet. My understanding is that retrofitting insulation under the slab of an existing building is feasible from an engineering standpoint, but disruptive and expensive and the numbers basically never pencil out. No digging for the piping either; the radiant flooring would act as a heat plate on the surface of the ground, but the pipes wouldn't go downwards into the ground like geothermal. This is significant because the arrangement will concentrate heat at the surface and the isotherms will presumably show a declining temperature gradient the lower down you go. Whereas an analogous geothermal arrangement would disperse the heat more evenly throughout an imagined "cube" of dirt under the house, with 5 sides of the cube exposed to something like 62f base subsurface temperatures. Pretending those sides are like walls around a tank for the thought experiment, even though it's all just dirt, because the ground underneath conditioned space is a different animal than the ground adjacent to it. Anyway, with the "heat plate on the surface of the ground" configuration, there is much less surface area where high temperature soil under the building is exposed to the low temperature boundary at the sides of the cube. The lower you go, the lower the temp and the less heat you're bleeding. dirt stratification i guess

Anyway, I get an R-Value for dirt of about 1.5 per foot. With a 50'x40' footprint, in some sense it is already well insulated, especially toward the middle. Different parts of that cube of dirt would variously function as storage medium, transfer medium, and insulation medium. Compared to alternative materials, dirt doesn't do a good job at any of those functions, or I would say even a fair job. But I suspect the perimeter heat losses near the top may prove acceptable given the sheer volume of thermal storage we're talking about, AND given the fact that the dirt thermal store in this arrangement becomes useful even at VERY low temperatures. Like, even if we're only lifting the surface temp of the ground to 70 degrees and relying on a W2W Heat Pump from there, we're still talking about significant improvements compared to an A2A heat pump drawing from outdoor air ranging from 35-55. The more the ground warms up, the more heat our cube will bleed, but keep in mind the concept here is Big Dirt, Small Temp Lifts. It would be a different animal than typical TES arrangements like water tanks designed to get up close to 200. The hottest part of the DIRT-TES, the surface, may never even top 90 degrees. Because if the top foot of dirt is at 90, the next foot is at 88, then 86, etc all the way down to 62… That's a lot of BTUs packed in there. With temp differences between the store and the surroundings an order of magnitude lower than conventional arrangements, it should be able to get away with an order of magnitude worse overall… U-Factor? Can't remember if that's the right term here.

And with a few feet deep of perimeter insulation to bolster this arrangement available as a doable retrofit if that's what's needed… man the more I talk it out the more it just seems like there's got to be something there. We'll see but as best as I can put it all together so far it seems like the question is not whether there's something there, but rather how well it would stack up in terms of performance and cost compared to alternatives.

Right well I can't see how it could work in a cold climate. Slab heating under interior room would be limited to what thermal comfort will allow. The basement/crawl space could be allowed to get a little warmer, and/or insulation could be applied on top of the slab in that area to reduce heat flow.

I will use the heat pump you linked to and build on the cost analysis that DC did below to build and come up with a TCO for going this route. That will become my target cost to beat.

 @BTUser has been emphatic that practicality cannot be a consideration!

😁😂🤣 This entire thread in a nutshell right there.

My objective is to bring our natural gas consumption to zero and our electricity consumption down by an order of magnitude, to the point that we can get a second electric car and still have our 6.48 kwh PV system wipe out all electricty spending including home car charging. Currently despite the PV system we've average $3250 per year energy spending over the last 2 years, excluding nonbypassable charges. A second electric car charging only off-peak would add about $3000 to that. So I'm looking at about $6K per year on the table to wipe out. Will build out the numbercrunch for the PV+A2AHP+envelope improvements route once I get proper load calcs and post here. That should be relatively straightforward and I'm guessing piece of cake for you guys to glance at and see if it's reasonable.

Modeling for the approach I've described will be much tougher, more debatable, larger error margin. The conventional approach will be a useful firmer reference point to compare against.

Here’s all the numbers I could think of to gather so far. I was off on my estimates for the second car. Looks like we’d be at $4000/year to PG&E if we were to swap out my wife’s car for a second electric, but otherwise make no changes. New car may not happen until a bit further down the road, she’ll want to keep what she’s got until it dies.