Off Grid hydronic with clay battery

Benzdoc

Looking for discussion or ideas for the following project

2 stores 1200 sqft house built on slab in northern Alberta Canada. The lower level will be a garage and the upper level will be the living area.

I am thinking that I can build thermal storage underneath the garage pad and heat it up using evacuated solar collectors, charging it up during the day and using the heat during the night

Has anyone out there done this? What are some considerations?

ethicalpaul

I think the inefficiency and heat loss outside your system would more than devour any potential gain.

Slightly less hopeless would be to heat water in a storage tank in the home’s envelope but still the ROI is bleak.

People smarter than I have recommend photovoltaic solar instead

Jamie Hall

Perfectly sound concept. The two biggest problems with any design of this sort — and I've been involved in several — are transferring the heat from where it is collected to where it is stored and then back, and then ensuring that you actually have enough storage and that it is insulated well enough.

Both of these problems are manageable. I'm not a really big fan of dedicated collectors, however. With reasonably informed architecture, one can collect enough heat simply from well-placed glazing to provide more than enough heat (and yes, I have done it). Then the storage problem can be reduced somewhat if at least some — if not all — of the storage is accomplished within the usable envelope rather than in some other location — to which the heat has to be moved and then, later, moved back).

It is important to realise, though, that the average architect — never mind builder — is not going to be much help.

Now if one does really want to use collectors, rather than building the whole thing as a passive design, that can be done too. It is considerably more difficult, however, to actually transfer the heat from the collector to the storage. @ethicalpaul mentioned that using a water tank storage is often preferred to solid state (in your case clay), but not the reason: with water you can have a completely mixed tank, whereas with solid state you are very much dependent on conductive heat transfer within the storage. Not that that can't work well — it is the way that solid state storage works when the storage is within the usable envelope — but getting the energy into and then back out requires a good deal of thought and engineering as to the piping design to minimize the mean distance to storage, as well as potential problems with such trivia as leaks. There is also the problem that response times can be very long.

Photovoltaics are an exceedingly poor idea for space heating. If you are off-grid, you will want some — and the accompanying battery banks — to power your electrical needs. However, the inefficiency of photovoltaics vs. some form of actual energy capture makes them a no-go for space heating.

Larry Weingarten

Hi and welcome. I'll add that if you make the shell of the building far more efficient than is considered sane, you'll keep the heating load minimal, simple and inexpensive. I built off grid and use solar thermal to keep a 1000 gallon tank warm for space heating. 80F water will keep the house at 70F, but my energy usage per square foot of living space is about 1/10th of the normal house, so a simple solar thermal system works. I also have backup wood heat and burn about 1/4 cord per year. A really efficient shell is key.

Yours, Larry

ethicalpaul

I agree Jamie, I wouldn’t try to directly run heating with PV but rather bank the energy into batteries or the grid

DCContrarian

About 10 years ago I wrote an essay entitled, "Is Solar Thermal Dead?" where I made the case that the price of PV solar had come down so much that they were cheaper per delivered Watt than evacuated tube solar collectors, and that collecting electricity had so many practical advantages over collecting hot water that it just didn't make sense to use thermal collectors. In terms of dollars per delivered BTU, it was cheaper to run the output of a PV cell through a resistance heater than to collect hot water in an evacuated tube.

The only area where thermal excelled was output per square foot. If you only have a limited amount of space to mount solar panels then that may be a consideration.

At that time evacuated tube collectors were widely available, it was easy to do a Google search and get pricing and output information. I just went to update my statistics, and evacuated tube collectors are barely being sold any more. I'll add that my local Craigslist often has solar collectors being given away for free. I think the market has spoken.

With a hot water collector, you have to worry about it freezing in the winter and overheating in the summer. You get most of your output when you need it least and the least output when you need it most. The efficiency is lowest when you need the most output. With PV you get your output in the form of electricity, which can be put to work in all kinds of ways. And the efficiency is essentially constant.

DCContrarian

People have been trying these sort of heat-storing schemes for at least 50 years, the landscape is littered with failed attempts. Almost all of them would never had been built if they had been properly engineered, because the engineering would have shown that they were never going to work.

There exists no technology for feasibly storing heat on a residential scale. If it did exist, it would be widespread.

I recommend this article, which revisits an attempt build a house heated passively by solar energy, ten years later:

https://www.greenbuildingadvisor.com/article/the-sunrise-house-in-fairbanks

Benzdoc

Further to my initial post….
My idea for the thermal battery would be to dig down 3-4 feet below the grade of the slab, put insulation down below and to the sides of the excavation, backfill with clay and integrate pex tubing to transfer the heat into the battery. My initial idea would have been to use PV direct to an electric water tank and circulating to the battery, but after watching this video of a greenhouse build in my area I thought it would be more efficient to use the evacuated tubes. Thoughts?

Appreciate the discussion

https://youtu.be/0x-1hLmAmBQ?si=WfSuFj8B3pEAvERY

Jamie Hall

The houses and other buildings which I and my father-in-law designed and built, getting on for 60 years ago now, are still 100% passive solar and are still happily being lived in and used, @DCContrarian . All in New England. We worked with William Shurcliff — a rather odd duck, but a really bright guy. Look him up.

It's not that it can't be done. It can. It does require careful and custom engineering, and considerable care (though not much extra expense) in construction.

Perhaps one of the problems was that the resulting structures looked very normal… no fancy frills for people to point to and say "wow! Look how advanced we are".

Our biggest problem was figuring out simple passive ways to handle overheating.

Jamie Hall

And a bit more… the biggest problem with the clay battery concept is the extremely slow response times. For a residence there are better ways to store the heat.

Most of which have been known and common knowledge in the passive solar community for at least 60 years now….

Hot_water_fan

@DCContrarian Martin Holladay?

Hot_water_fan

check out phase change materials! Find one that freezes around 70F and it can capture whatever type of thermal heat you have easily. Put it in the envelope

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1815209#Comment_1815209

I was all excited about phase change materials about two and a half years ago. But it didn't pencil out. Details here:

https://www.greenbuildingadvisor.com/question/calcium-chloride-hexahydrate-for-phase-change-heat-storage

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1815208#Comment_1815208

Am I Martin? Sadly, no.

DCContrarian

This is Martin's article:

https://www.greenbuildingadvisor.com/article/solar-thermal-is-really-really-dead

Larry Weingarten

Hi, I challenged Martin back then as I'd been working with low cost solar thermal, which is simply poly coils under a single glazing, and it helped bring system cost down to around half of the typical cost. The approach is simple as well, with little freezing or overheating concern.

Yours, Larry

Hot_water_fan

https://www.greenbuildingadvisor.com/article/solar-thermal-is-dead

I was thinking of the original !

hot_rod

Solar thermal drainback systems work well in any climate. I have none on my shop and one on my home. No glycol, no freeze issues.

As far as storage, water is your best, least expensive option.

Storing under a slab in dirt would require insulation between the storage and slab to prevent summer and fall over-heating.

I've spoken with all 3 US solar thermal manufacturers this summer. Business seems to be good for them. I suspect mostly commercial and states like Hawaii where ST is required on new constructions.

The most recent Coffee with Caleffi webinar talks about thermal storage and a bit about phase change tanks.

https://www.youtube.com/watch?v=BDspXgtzLZU&list=PLuuV0ELkYb5VE0I4evUZ30b5U78CRlRdg&index=2&t=3926s

water vs steel air sand etc.png

Hot_water_fan

@hot_rod ha how is that chart accurate? Of course the ice phase change stores more energy than water. Not that it makes sense for most residential installs either way.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1815275#Comment_1815275

It's for ice kept in solid phase. If you change phase from ice to liquid ice has about 180 times the heat density of water.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1815217#Comment_1815217

On second thought, this might be worth looking into for off-grid.

If you read comment #11 by Paul Wiedefeld, his objection is that the amount of energy saved doesn't justify the cost, the juice isn't worth the squeeze. He guesstimates the long term cost in the $1/kWh range for the saved energy. If you have grid electric, that's not worth it.

But it might be worth looking into off-grid. Off-grid, everything is more expensive.

That posting anticipates storing heat for both heating and cooling so it uses a phase-change material with a melting point of 72F. If all you're doing is storing heat for heating you'd probably want something with a higher melting point, maybe around 120F, which would make it easier to get heat out. The nice thing about calcium chloride hexahydrate is you can dial in the melting point by adjusting the mixture.

leonz

With your weather and the fact that you have huge reserves of clean burning Western Sub Bituminous coal to use as home heating fuel which is less expensive than gravel you should think about using a coal stoker boiler.

An AHS S130 coal stoker, steel pipe and using steel or cast iron surplus salvage radiators freom an architectural salvage dealer for a top fed gravity hot water system to heat your attic, living space, garage, basement and eventually a driveway would cost you much less money and provide a faster return on your investment and you would have plenty of reserve heating power for heating a greenhouse.

The S130 operates using 110 volt power and if you use a top fed gravity heating system with the boiler in the basement or first floor you do not need a circulator to move the hot water and you will have slow even heat.

Larry Weingarten

Hi, This might be relevant, somewhat. 😏 https://www.heatinghelp.com/systems-help-center/another-solar-myth-bites-the-dust/

Yours, Larry

Jamie Hall

That rumbling noise you hear in the background is my father-in-law, Bill Shurcliff, and others in the really early passive solar game (1960 to about 1975) turning over in their graves. The solutions were there. The houses and dining halls and hangars were built and are still in use. They work just fine, five and six decades on.

They are simple. They are passive. The marginal cost was typically less than 10%.

But they aren't sexy. They don't scream green. They don't scream at all, in fact. Just work.

Oh well. It was fun while it lasted… and I'd build another one today, but I don't need a new house…

Larry Weingarten

Hi @Jamie Hall , I don't remember your ever saying that Bill was your father in law. Nice roots! I corresponded with him some, and Steve Baer of Zomeworks considered Bill a friend. I was on Zomeworks board of directors until they closed up shop, so both Bill's and Steve's ideas rubbed off on me. Bill's books, from Brick House Publishing were the sanest reading available on solar thermal and the ideas around how to implement it. Thanks for bringing him up. 🙏 No doubt his books are findable for those looking for elegant ways of getting solar done.

Yours, Larry

DCContrarian

So I've been trying to find a copy of my essay, so far unsuccessfully. I did find the spreadsheet I used to create tables and charts for it (which is dated December 2012, so earlier than I had remembered.)

Here's a couple of tables. First is Heating Degree Days by month for my location, Washington, DC.

			Heating Degree Days (2)
	january		906
	February		741
	march		562
	april		269
	may		72
	june		5
	july		0
	august		1
	september		19
	october		202
	november		467
	december		755

Second is daily available solar, BTU/day. This is based upon a collector of 128 square feet, facing south and angled at 45 degrees, with a water temperature of 120F. This takes the amount of sunlight hitting the ground (insolation) and corrects for the angle of the sun and the outdoor temperature.

	Daily available Solar, BTU
january	36,236.1
February	54,195.8
march	76,295.6
april	97,962.5
may	120,736.3
june	129,261.0
july	135,274.9
august	129,237.1
september	106,569.5
october	75,816.6
november	48,536.3
december	33,287.8

This graph puts them together, the Y-axis is percent of annual total.

And it shows the crux of the problem. When you need the heat is when it's least available. (Which makes sense, that's why it's cold in winter.) If you size your system to meet demand in the coldest months, it's way oversized the rest of the year. Which means you have to figure out a way to get rid of that excess heat. It also means you're paying for a large collector and then throwing away most of its output most of the year.

Larry Weingarten

Hi @DCContrarian , This is the crux of one of the "ah ha" moments I had. By using inexpensive, inefficient, and oversized collectors, I get around much of the problem. It's somewhat twisted thinking 🤨

Yours, Larry

hot_rod

Even in the sunniest of states with a long heating season, the mountains of New Mexico, 283 average sunny days, for example. The designers/installers they would shoot for 30% of the heating load with ST.

100% never makes sense from the sheer size of the array and the $$

ST should be sold and promoted as a supplement to a fuel powered hydronic system.

So by default with a heating design you get a good % of the DHW.

With a drainback there is no over-heating issues in the summer. Or freeze protection concerns. Only downside is they can be noisy as they drainback. A pool or hot tub can be a good summer load. Use it or lose it is the ST motto.

The best way to do these projections is with a solar simulation program. F-Chart is a basis program, RET screen from Canada is, or used to be free. The nicest program is T-Sol out of Germany. It has a worldwide weather data base, a bunch of product specs, and npiping drawings..

A ST collector is fairly easy to build. A frame.copper tube insulation, and glass. This is the one I built for my shop 28' long, 2.5' wide

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Jamie Hall

I think I'm going to cry. With reasonably intelligent architecture and no dedicated collector (or at least not an obvious one!) the overheat problem turns out to be confined to the late fall months, and is manageable. Completely manageable..

Read this.

The History of Passive House: A Global Movement with North American Roots | Article | EESI

The Saunders in "Saunders-Shurcliff" is my father-in-law; Bill was a close friend.

They died disappointed men.

They gave up, and I can understand why…

DCContrarian

The idea put forth in that article that a simple awning can be put at some "just right" angle where it lets sun in when it's needed and blocks it when it's not is a cruel hoax.

Another table, angle of sun at midday by month:

	Altitude of sun(3)
january	34
February	42
march	50
april	58
may	66
june	74
july	66
august	58
september	50
october	42
november	34
december	26

It's particularly important to look at this in concert with the earlier table showing heating degree-days by month. So in September, the sun follows the same path as in March. But in my climate September averages 19 HDD, while March has 562. Similarly, October has the same path as February, and it's 202 vs 741. April at 269 HDD is the same as August, with 1. Put simply, March and April are heating season here, August and September are cooling season.

Is it possible to design a building where the sun contributes to the heating in the winter? Absolutely. I live in one. You can read here about how my house is heated solely by the sun on a design-day temperature on the second-shortest day of the year:

https://www.greenbuildingadvisor.com/question/hows-this-for-solar-gain

But that's a far cry from a building that self modulates to maintain a comfortable temperature.

hot_rod

A company in Switzerland sizes ST for 100% dhw and heat. Tanks get quite large :)

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Jamie Hall

Um… well, I beg to differ. @DCContrarian . Here are two houses, both of which I was involved with building, both of which I have lived in, and both of which did a fine job of "self-modulating

There are two others I'd love to show — the Winterkeeper's House on Star Island, and the Shrewsbury House, but I don't have good pictures of them.

But I suppose I'm deluded, and they don't work, and I'm just imagining things…

And I might add, since no one seems to believe me (or my father-in-law, or Bill) I can see why they lost interest in discussing how it was and still can be done. Sorry.

Larry Weingarten

Hi, I have no problem believing you. My house has an active component, but does modulate also. The only real problem is this sort of building requires a very different mindset and attention to natural forces that we have not been accustomed to thinking about since energy became cheap and pretty simple to get.

Always, the problem with being an innovator is that most people think you're sorta crazy. "If it were such a good idea, why hasn't it been done before?" is something innovators hear a lot. I just build things and let those who are able to, notice what's different.

By the way, there are pictures of Shrewsbury House in William Shurcliff's book, Super Solar Houses.

Yours, Larry

Larry Weingarten

Hi, Apologies for taking this thread a bit sideways. Personally, I much prefer water as a heat storage medium. It's a LOT more responsive and versatile than anything solid. It holds more heat and can be moved around to deliver that heat as needed. All you need is a system that doesn't leak. 🤠

Yours, Larry

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1815400#Comment_1815400

So Jamie, I want to make sure we're not talking past each other. The second one looks to have a quite large chimney coming out of the roof, with what looks like several flues. Is passive solar the only heat source or does it complement something else? Because if it's the second I have no doubt that it contributes to the heating of the house.

It's when you try to build a house where passive solar is the only heat source that the numbers just work against you.

Jamie Hall

Building codes and mortgage bankers are queer cattle, @DCContrarian , and where that second house (the salt box) was built the code required (and still requires) that there be a more or less conventional heat source in the structure which meets their computed heat loss requirements — or no CO or mortgage, both of which are useful.

Neither the building inspector nor the bank agreed that passive solar heat would ever work…

So yes, there is a chimney in that house, with a flue. And there is a nice big Jotul stove in the living/dining area. In the ten years after we built it, we used it once per year, just to make sure it worked each year (the current owners don't bother). Otherwise the stove was and remains completely superfluous.

Now as it happens that nice big chimney, which is mostly solid masonry, is also part of the passive solar design (it's part of the heat storage "system") but with a mass of upwards of 40 tons, it better be passive… It didn't have to be there, but since we had to have a chimney and stove and flue, why not work it into the architecture?

And I agree, if you try to work against the physics, it will defeat you. If you work with it — as we did — it helps you every time.

DCContrarian

Interesting. Can you say what state or even what climate zone that house was in?

I'm surprised the inspector allowed a stove as the heat source, normally they want something thermostatically controlled.

Jamie Hall

northwestern Connecticut. Design low temperature is -10 F, but it routinely gets colder (for Connecticut it's at a high elevation).

The building inspector took a fair bit of arm twisting. All amicable (he was, and remains, a good friend), but he had rules he was obliged to figure out how to follow…

Jamie Hall

I might add, @DCContrarian , that if I were to build that place again there are a few minor changes I would make. Nothing major, but there were a couple of spaces which stayed a little cooler than was really desirable, particularly one bathroom in the northeast corner which stayed about 5 degrees cooler than the rest of the house (I'd underestimated the amount of winter shading on that corner — overoptimistic assumptions about how much the leaves coming off a hedgerow to the east would help… )