Off Grid hydronic with clay battery

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

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

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.

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….

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

Am I Martin? Sadly, no.

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

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

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.

With your weather and the fact that you have huge reserves of 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.

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…

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.

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.

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.

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

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:

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.

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.

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

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.

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…