The plumber was never going to design the pressure reducing valve that would work to spec at the most reasonable manufacturing cost that met all of the other requirements for potable water plumbing, etc., despite the ability to install it, and a gut feel as to how the thing worked.

Physicists aren't engineers, but but having worked with several of both, I'd hazard more physicists than plumbers could get the engineering rightr, eh?

The energy transfer & storage stuff we're talking here doesn't take high math, but it takes SOME math, and the fundamentals are dead-obvious, easy to dig up on the web when you're not sure:  How much heat is available to be captured for a given location, what heat-capture efficiency you might be able to achieve with standard solar technology, how much heat energy the structure needs, how much energy needs to be drawn from storage to meet the seasonal shortfall, how much water volume at what delta-T can actually store the seasonal shortfall, the how much insulation the storage vessel needs given the heat input/outputs for a given structure & solar array etc. are all calculable numbers, the methodologies of which can be found online.

Then, do the cost/benefit of insulating the structure more vs. more solar, more storage at lower tank-insulation vs. higher-temp higher-R storag vs. solar efficiency at lower/higher temp etc.  before plunking down hard cash to hack something in place then tweaking it into working-mostly. For new construction it's pretty clear that it's more cost effective in most of the lower 48 to super-insulate the house and design it for modest passive gains than it is to build ever larger & more complex solar + storage systems to get the thermal loads down to net-zero.  As retrofit to existing homes the break point is in a different place, but you'll never know where that is until you quantify the energy demands and what it takes to lower them vs. adding larger solar & storage. 

Some simple arithmetic:  A typical home in the mid-atlantic states uses 50-80 million BTUs for space heating in a year.  To even store 50MBTU in a 20,000 gallon tank at temps above room temp would take a storage temperature of ~500F, even at zero storage loss, and the pressure requirements of keeping it liquid at 500F are gia-normous.  Cut that heat load in half, and you're still S.O.L., but you're getting closer to the point where the size of the solar array to support the load month-to-month fits on the building lot, but storage temps for the seaonal shortfall are over the boiling point with only 20,000 gallons. Cut the load by 2/3 and the solar easily array fits on the house, with storage temps below boiling, and you can start doing trade-offs between collector area, tank size &  insulation & operating temp (which affects solar efficiency) etc. etc.  Cut the heat load by 3/4 or more and you may have something.

Think I'm rong? Do the math an PROVE it. (No rocket science required.)

I didn't mean to downplay the intelligence of engineers, after all, 'I are one', however I have met 'plumbers' who were just as sharp and less distracted by digits. Not to compare my Dad to a plumber (He would have a fit, he was a steamfitter) but a fellow I met told me about the day when the engineers, were on a job site with their slide rulers computing how many feet of heat pipe they would need to run around the perimeter of a collage campus they were working on. When they finished their calculations my Dad produced a piece of paper from his top pocket with the exact same number they had just calculated. They asked him how he did it and he smiled through his Camel cigarette smoke and said ‘experience’ and left the shack. The fellow tells me the engineers were dumbfounded. True story.
Now, I hear and understand what you are saying. You have done some of the work for me here, and as I suspected, I may need to increase my field size for storage. But I guess I am missing something. Let me elaborate, and maybe you could explain where my thoughts are going astray. Lets say after all is said and done, the only way I can do this is to install a field size of 80,000 or even 100,000 gallons, and I must build and install 1000 square feet of solar heat collectors to hold my house at a comfortable level throughout a winter season. Lets say it runs me $30,000 dollars. If the life expectancy of the installation is 100 years, would that not make sense? Especially since natural gas prices are very unlikely to remain the same over the next 20 years, and may not even be available after that according to energy scientists like Duncan and Youngquist. I am basing my storage field construction on inexpensive materials coupled with longevity characteristics. Take plastic pipe for example. The trade off in speed of heat transfer through plastic instead of copper would not be an issue if the fluid spent six hours in a circulation path within the storage field. Plus plastic is much less expensive. Or another option would be to isolate the field from outside corrosion influences so we could use copper. But that would probably require a much more expensive installation. Does that make more sense now? What are your thoughts?

My thoughts are, as always, do the math, even if it only a first-order approximation scribbled in pencil on the inside of your cigarette pack or lipstick on the bathroom mirror, whatever works best for you. This isn't hard to do even if you only passed 8th grade algebra with a C.

And when you assign the dollar numbers to it you'll likely find that 1000 square feet of active collector costs a heluvalot more than $30K, installed price.

And bigger storage volume means more tank, more tank insulation, which is even more money.

Superinsulating your house and incorporating modest amounts of passive solar to where you don't NEED a heating system costs less than $30K in additional cost, once the cost of the now-absent heating system is factored in (new construction, 2000 square foot house). At that point 1000-2000 gallons of well-insulated storage and a couple hundred feet of active solar collector can easily cover the cold-snap or cloudy-week thermal shortfalls for climate like Buffalo or Irving if you manage it well.

PEX heat exchangers are not rocket science to design, but like any coil heat exchanger you have to pay attention to total surface area as well as the amount of head presented to the pump. If you need a high rate of heat exchange it takes a higher flow as well, which usually means multiple balanced-flow coiled plumbed in parallel. But again, if you lower the heat load on the structure by insulating & sealing it, the rate of heat exchange you need from the coils falls dramatically. (Domestic hot water flows during showers/tub-fill are easily 3-4x the peak heat exchange required for space heating in my not-even-close-to-superinsulated house in Worcester, MA.) Getting the heat into/out of the tank won't take a huge heat exchanger if your house has any reasonable insulation and doesn't leak like a sieve. You can calculate on a napkin a very close approximation of what your peak BTU/hr rate needs out of the heat exchanger are if you correlate your heating fuel consumption to heating degree-day weather data, using your furnace or boiler's efficiency rating to measure the whole-house heat loss, and applying it to your coldest-hours-of-the heating season low temp.

Again, simple math, no rocket science involved, but you'll at least not over or under design the heat exchanger, and it'll work better. Stabs in the dark tend to either

A: not work well, or

B: cost too much.

The math will set you free...

Calculate how many BTUs you need.

Calculate the peak rate you'll need to pull it from storage.

Calculate how much you can collect on-average per square foot of collector in January/February at temps that your PEX heat exchangers can handle.

Then start playing with tradeoffs between collector efficiency & storage temps, as well as storage losses at what temps at what R-value.

Then assume the system efficiencies are going to be 25% lower than what the lipstick on mirror calc shows, and your storage losses 25% higher, and use that to determine how much collector and how much storage insulation you REALLY need, and you might get it to work.

But when you start assigning dollars to the various bits you may want to readjust the approach.

Net Zero energy houses aren't some illusive Holy Grail, it's been done several ways, at dramatically different costs, both as retrofit and as new construction. None that I know of have used 10,000 gallons of low-temp storage, but Riverdale house in Edmonton Alberta had a substantial tank built into the house that operates at below room-temp much of the time, using a small heat pump to put the heat into the space. By operating large volume storage at very low temp the efficiency of the solar collectors was high enough that the collector area requirement became low enough that it could actually fit on the building.

It often pays to learn from the approaches taken by others, both those that succeeded in their design goals and those that came up short:

http://www.src.sk.ca/images/Vancouver%20Talk.pdf

http://www.alidp.org/resources/Riverdale+NetZero.pdf

http://www.riverdalenetzero.ca/Riverdale_NetZero_house_--_project_profile.pdf

http://greenedmonton.ca/MillCreekNetZeroHome

http://www.nowhouseproject.com/index.php

Agreed, 10,000 gallons isn't very much heat storage at low delta temperatures.

In general:

hard to beat water for storage - the price is right and performance is reasonable.

small amounts of storage (like 2K-10K gallons of water) are useful when you have free or low cost heat available at times and want to capture it for later use (typically within hours to a couple of days). Solar, wood stove, daily temperature swings, low cost electricity at night are examples.

If you want to use the ground for a full season of storage, then you should look at a typical geothermal system - it puts heat into the ground in the summer and extracts heat in the winter.

Dana and Jon,

Thanks guys. Yes I have a bit of research to do. But let me pump your expertise a bit. If I have a 220 degree f. hard surface, about 2,500 cubic ft. and I am trying to delta T 80 degree water up to 110 degree, is this actually not going to be a relatively small delta T? I am not working with minus ten degree to plus seventy two. I see the biggest challenge as keeping this temp intact outside underground. The computations for energy requirement is the simple part, and when the time comes I will pull out local temp history records and correlate them with my heat bills, but my focus is to cheep design an energy storage element. The focus here is in the collector designs, handmade, not purchased with a 100% markup. Storage made from natural materials not manufactured. I understand if I buy this stuff out of the box the price is going up about 100 times, so I am looking for alternatives. PVC pipe, computer boards? Obviously these things must be purchased and built. Collectors will be built, not purchased separately. I am pretty sure I can acquire a large number of Fresnel lenses for $5,000. The rest is pipe, glass, grit, wire and nails. The computer controls are basic logic design. I can build this, not buy it. Valves, switches, hydraulic system controls, all individual parts used to create handmade components. The Fiberglass tank cost me fifty bucks. (I got two and could have sold one for $500.00 but that does not count.) Delivered. $10,000 to $20,000 range is what I am looking to do here. ($15,000 target) My waste will be in design errors, but it is the mission that counts here. My application is probably not worst case scenario, and yes, I could insulate the hell out of this structure, but that is not my mission. I am looking to design an inexpensive, highly aggressive system without spending a fortune. Know why old houses leaked like a sieve? Because energy was so cheap no one cared when they were built! And the vast majority of those houses still exist. I am doing lab work here guys, I know there are already ways to solve this problem; I'm just trying to think outside the box here. The efficiency of developed systems already stem from these old structure designs. But remember, garbage in, garbage out. And again, we must not forget that future energy will most likely be much more expensive than it is today, and eventually nonexistent according to some. Can you guys tell me where you see this going over $20,000? I might already have an answer to those points ...
Net zero for the 'poor guy' is the mission. I can solve any problem with enough cash. $2.5 million a year and I can find a gaggle of naked wenches to hand crank generators for ya when it gets cold! If it wasn't for my wife ...

hey:

I am the guy who started a 'dirt battery' trhread near here. For me the big revelation has been that I can heat my house all winter with well water maybe. I'll bet if that well water always stays above 40 degrees even when its minus 20 outside.

The house already has baseboards on thermostats and grid power so I can install a system and if it fails the baseboards kick in. All I really need to add for the second year is some solar thermal panels and some hot water storage to bring it up a couple degrees.

I'm a dolt greendreams. I saw your title, but never checked it out. I will back track now.
Energy is one of those things we have always taken for granted, and as a result, research has always had a tilt that says 'you can't do anything else'. I as you, disagree. Nicola Tesla said basically that there was so much energy on Earth that some day it would be free. I like Nicola, even if he did chase pidgeons. (I had the same problem with women when I was younger, nuc, nuc, nuc) If however you just spent a million bucks on a system that would replace itself forever you might have a bit of future concern. Westingshouse's problem with Tesla designs. But to use a popular phrase of that day, poppycock! If I sold a machine that would forever eliminate the need for home heat, how much would you pay for it? $10k, $20k? $100k? I would finance $50 k immediately any system that would remove forever the need to purchase energy for home heat. And every day that passes it becomes easier to say that because the price of energy only goes up over time as it is. The Sun came out in Western NY today and I could already feel heat on the back of my hand. More heat than would feel comfortable if it were that temp in my house. Mid Feb.

Dragmit: Interesting topic! And what a large project you are about to undertake. You guys lose me on all the technical stuff.
I doubt this will help you but I have a very, very, simple heat system: I bought a 1,000 gal concrete septic tank, sprayed it with 2" of polyurethane foam, coated it with black mastic, and buried it about 40' from my house. The copper supply and return lines are also sprayed and coated. Since I am on a South facing hill I placed six 4x8 flat panels (bought used for $100.00 each) below the bottom of the storage tank. The solar hot water transfers to the storage by way of capillary action (no pump needed). I have a jet pump inside the house that is used to pump the heated water through pex tubing on 16" centers buried in the concrete floor. Each room has a manual valve to control the heat for that room.

Heated floors seem so much more efficient than heated air and it's nice to walk barefoot on warm floors in the winter time. Once the heat builds up it is fairly easy to keep the temp consistent. In Oklahoma we generally have 2-3 days of sunshine in the winter and that is sufficient to keep my floors warm. This past winter we had an unusually cold winter with 8-10 days of overcast and I had to run the backup heat pump on a few occasions. I don't feel it would warrant the cost to store more than 1,000 gallons of hot water. You may be surprised to find how much it costs to antifreeze 10,000 gallons of water. I am not an engineer and I don't have monitoring equipment so I can't tell you different temperatures on particular days--I just know it works for me.

BTW I should mention that I insulated the floors under the pex before pouring the floors AND my home is built with metal clad SIPS. Because of the tightness (is that a word?) of the house I also have 400' of 10" corrugated tubing buried 6' down for fresh air. That is piped into my heat pump blower cabinet that runs on low 24-7 year round. That automatically supplies about 10 per cent of my return air as fresh air at an average temperature of 67 degrees (I do monitor that).

Sometimes simpler is better.

Posted By Reddirt on 31 Mar 2010 11:32 PM
Dragmit: Interesting topic! And what a large project you are about to undertake. You guys lose me on all the technical stuff.
I doubt this will help you but I have a very, very, simple heat system: I bought a 1,000 gal concrete septic tank, sprayed it with 2" of polyurethane foam, coated it with black mastic, and buried it about 40' from my house. The copper supply and return lines are also sprayed and coated. Since I am on a South facing hill I placed six 4x8 flat panels (bought used for $100.00 each) below the bottom of the storage tank. The solar hot water transfers to the storage by way of capillary action (no pump needed). I have a jet pump inside the house that is used to pump the heated water through pex tubing on 16" centers buried in the concrete floor. Each room has a manual valve to control the heat for that room.

Heated floors seem so much more efficient than heated air and it's nice to walk barefoot on warm floors in the winter time. Once the heat builds up it is fairly easy to keep the temp consistent. In Oklahoma we generally have 2-3 days of sunshine in the winter and that is sufficient to keep my floors warm. This past winter we had an unusually cold winter with 8-10 days of overcast and I had to run the backup heat pump on a few occasions. I don't feel it would warrant the cost to store more than 1,000 gallons of hot water. You may be surprised to find how much it costs to antifreeze 10,000 gallons of water. I am not an engineer and I don't have monitoring equipment so I can't tell you different temperatures on particular days--I just know it works for me.

BTW I should mention that I insulated the floors under the pex before pouring the floors AND my home is built with metal clad SIPS. Because of the tightness (is that a word?) of the house I also have 400' of 10" corrugated tubing buried 6' down for fresh air. That is piped into my heat pump blower cabinet that runs on low 24-7 year round. That automatically supplies about 10 per cent of my return air as fresh air at an average temperature of 67 degrees (I do monitor that).

Sometimes simpler is better.

In Oklahoma your subsoil temperatures are  60-66F (15-19C), which minimizes your tank's heat losses considerably compared to dragmit's 48-50F subsoil temps.  (Your 67F exit air temp on the earth-coupled ventilation air is another indicator of high subsoil temp.  Even if the undergound ducting is all under your heated-insulated slab,  the effect of being under conditioned space with R10 would only raise it a couple of degrees at most.)  You also have considerably higher wintertime insolation and milder temps, and your air-tight SIP house is probably at least 2x as efficient as dragmit's antique, all of which adds up to a much more favorable wintertime solar input/thermal-load ratio. 

The slab itself has comparable thermal storage capacity as your 1000 gallons of water, so your total thermal storage may be roughly double what you might think if you were only considering the .  (We'd need to know how many square feet x how thick to hit it with any precision.

What you have is a hybrid active/passive solar heating system:  Passive heat collection, with active heat distribution.  This isn't the same as dragmit's seasonal-storage concept- f'rinstance YOURS actually works, whereas dragmit's doesn't (provable in simple math on the back of a napkin- even Tesla would agree. )    Your seasonal-storage is natures-own- the annual average temp is over 60F, which is why your subsoil temps are so moderate, and very near comfortable conditioned-space temps.

Dragmit would get much better mileage out of his solar & storage dollar bringing his clear-wall R-values up to R30+, then doing a ~200 square foot solar thermal array like yours.  His seasonal heat load is currently something on the order of 4x what yours is (he won't tell us is actual wintertime fuel use) and he wants to use less collector, bigger storage, in hopes of taking summertime heat in to use it during the winter.  The simple math sez the storage losses over the months on tanks even 2x as high can't get you there economically, since the storage temps would have to be too high, driving the insulation requirements ridiculously high (better to put that money into insulating the house better, not the tank.)

BTW: That's a thermosiphon (not capillary action) driving the flow between the collectors & tank.  For thermosiphons to work efficiently (=keeping the collector temps down with higher flow) it usually requires some fairly big-bore pipe- what size did you use, and how far is it between the array & tank?  Do you have pictures of the installation in progress, or the finished setup?

Dana 1: Thanks for that correction (thermosiphon) Guess I don't know the dif but it still works. It's only about 4' from the top of the collector array to the tank inlet. 1 1/4" is the pipe size.

Don't know if I know how to post pics on this forum but I'll try. This installation has been complete for 2 years.

OK, found it--here are the pics

Let's try that again!!

Mayhaps putting them on a pic-hosting site, and putting a link to it here would be easiest?

Great topic - here is a word to the wise from personal experience with one of these fuel storage tanks. In Siskiyou County, CA, the fire marshalls had the brilliant idea of scooping these up and offering them free to homeowners who lived far from municipal water supplies - which, in that county, is nearly everyone. We had one installed on a property I lived at, above ground, and filled with 10,000+ gallons water. The fire department installed a hydrant. It was a resource they could count on for fire suppression in the whole neighborhood.

For several years, maybe 5 or so, we were proud of our tank and the assurance it offered us and the neighbors. We were in a high-risk wildfire zone. But then...
along comes... someone from some agency...and guess what? The tank is deemed a hazard, though it had been cleaned prior to installation, much in the same manner you have described yours having been 'cleaned' by the guys who moved it. The upshot - WE have to pay about $5000 to get the thing emptied, dismantled, and hauled off by a Hazardous Waste disposal outfit. Again --- WE PAID --- not the fire marshall or the county who gave it to us.

There was no choice in the matter.

It was irrelevant that the tank was the only source of fire suppression in a wickedly flammable summer landscape for a radius of 2 miles or more. It didn't matter that wildfires occur every summer in the National Forests that cover much of the county.

I sure hope you get that sucker cleared for any possible future scenarios where some agency may deem it a hazard to the environment. You should consider federal, state, county, and municipal regulations. (Plus who-knows-what future legislation yet to be decided.

Oh, by the way, one of the persons who lived on the property with me had a chemistry degree. He wanted to do exactly what you have described - build a submerged heatsink - with one of these tanks from the county. We already had 5 separate banks of solar water panels supplying most of our summer hot water, including a dozen or so showers, commercial dish washer, etc.. We could easily have excavated with our backhoe and we had people who welded, plumbed, electricians, etc. that knew how to design and build all the heat exchange engineering, etc. They had already supplied the property with several homemade wood-fired boilers and homemade heat exchangers that also incorporated solar heating capacity. That whole side of affairs worked great. But the ex-diesel storage tank - a real disappointment, and an expensive learning experience.

But it didn't end there... our insurance company demanded that we have reliable fire suppression. So... yup... more money, big bucks again, many 10's of thousands of dollars to get brand-new water storage tanks scattered in strategic locations with all the necessary hookups on the property. Now I know this fire suppresion issue doesn't apply in your situation. But doesn't it just highlight how the unexpected can come along in these projects, with a certain charm, don't you think?

I've heard some awful things about California in this regard. This is just another example of over educated idiots with a need to justify their overpriced and undemanding government jobs. You guys could solve all your state budget problems by simply sending these dorks out of town … just not NY, we got enough of our own. In my opinion you need a large collective group of people to organize a lynching of one of these groups then keep the rotting corpses staked out on a pole right at the front gate. (This is figurative for those not bright enough to realize) This has worked for centuries keeping criminal types in check. We all understand the need to protect the environment, but the stupid are not capable of making reasonable decisions, howeverthey have been known to suck wildly off the state teat. The teeth are a good sign that it is time for weaning.

I looked at mass storage, using concrete tanks.

But the cost benefit simply was not there.

We have good solar, but very large delta's . The concrete tanks were not the main issue, the cost of evacuated tubes and the number of them would be much more than the rest of the building put together.

Water is the best source for storage, stratification is a major issue. You would need a water treatment regieme, have a look at what Garn do for their Boilers.

A large used fiberglass tank cost me $50.00, and the company that removed it cleaned it so it was accepted for nonpotable water in NY State. I intend to bury it 20 feet below ground, pack tight soil around it, fill it with porus stone and circulate solar heated salt water through it all summer, then see how long that heat lasts in the heating season. Your mistake in CA was 'out of sight out of mind', the above ground tank. My largest potential cost is mostly in the construction equipment, which is what my brother does for a living. Afterthat its plastic pipe, black paint, valves, computer programming, glass and my time. Not everyones option, but not a problem for me. If it doesn't work then I am out a few thousand bucks. But if it does work, I'll have to look at the feasability of building this thing for others. If that does not pan out I will just have to learn to live with a $20 gas bill in January. My wife will not cook with electric. And I will not cook at all. Raw meat is just fine. <;-P

Posted By dragmit on 02 May 2010 10:41 AM
A large used fiberglass tank cost me $50.00, and the company that removed it cleaned it so it was accepted for nonpotable water in NY State. I intend to bury it 20 feet below ground, pack tight soil around it, fill it with porus stone and circulate solar heated salt water through it all summer, then see how long that heat lasts in the heating season. Your mistake in CA was 'out of sight out of mind', the above ground tank. My largest potential cost is mostly in the construction equipment, which is what my brother does for a living. Afterthat its plastic pipe, black paint, valves, computer programming, glass and my time. Not everyones option, but not a problem for me. If it doesn't work then I am out a few thousand bucks. But if it does work, I'll have to look at the feasability of building this thing for others. If that does not pan out I will just have to learn to live with a $20 gas bill in January. My wife will not cook with electric. And I will not cook at all. Raw meat is just fine. <;-P

Again, stone inside the tank will reduce it's thermal storage capacity, and if you don't insulate the tank at all, your storage losses will be quite high. These are numbers that you can calculate on the back of a napkin (crayon on the wall, lipstick on a mirror, whatever method/material works for you.)

Insulating the plumbing to/from the tank is essential too.

If  it's even partially below the water table the tank losses will be ENORMOUS.

Seasonal thermal storage sometimes works without insulation on a scale 2-3 orders of magnitude using deep natural aquifers that have very low or no flow.

The only seasonal storage system using underground storage in an insulated tank anywhere near your size is a demonstration project in Galway Ireland, with much milder climate than yours and a very tight very well insulated PassiveHouse type building, not a retrofit on a leaky 20th century standard-issue house. See this.

Don't hold your breath waiting for that $20 January heating bill...

Hi again Dana,


Again, just how do you explain heating water to 300 degrees without huge convection problems? I must use stone because stone can hold higher temps than water while reducing convection losses. My exchange medium will be water, but salt water. The machanical heat loss of fluid is much greater in a fluid because of heat transfer through convection. Cool water will move down forcing warm water up to the loss of heat point the original water was subjected. I will use that somewhat to gather the heat from the top of the unit, but the heat transfer will be retarded by the rock.

Now, the rock. What I am planning to use is rail road ballast. It is very porus which will hold the waterand temp in place reducing conductive losses. I apologize for not making that clear at every question, but if you look at previous posts I believe I did mention rail road ballast, although I did not elaborate on the reasoning. Sorry about that.


Again, you are using conventional low temp designs. This is a nonconventional saline design. The amount of energy used is to be inserted over an 8 month period, unless the maximum temps are reached, where I will shut down the collectors. With salene density limitations that could reach several hundred degrees, although I do not intend to go anywhere near that. And while the tank is ten feet in diameter, the field I am setting it in is closer to forty feet. This area will be 'sealed off' from ground water flow using clay and packed soil along with a ' buried plastic roof ' to deflect ground water away from the site.



I agree. The supply lines will also be deep in the ground, and shielded from external water sepage. I plan to enclose these lines in a 12 inch plastic pipe filled with expanding thermalfoam like that used in boat construction. (Closed cell.)


Correct again. It will if my water table is that close to the surface, which it is not. But is this unsurmountable? Swimming pools do a fine job of keeping water in and out. But thats wat too expensive forty foot hole.

If this works, it will be pretty impressive, but there are few things that would cause me to hold my breath. But I do know one thing. The world is not flat, no matter how loudly the Church says it is!

So, basically I agree with everything you have said. I'm just not doing what you think! Think 'unconventional'! The basis of all R&D.

Uh, dude, how do you explain heating the water to 300 degrees without a massive STEAM EXPLOSION?  High pressure plumbing?  What pressure is the tank rated for?  Saturated saline at one atmosphere of pressure boils at ~230F- you'd need considerable pressure to bring that up to 300F.

How do you explain being able to store high temp water in ~45-50F subsoil in an uninsulated tank without huge conductive losses?

How do you expect to be able to COLLECT 300F solar in 50-85F air at any reasonable efficiency?  (You'd be looking at average collection-efficiency under 20% even with evacuated tubes.)

How to you intend to run high-temp fluid through plastic plumbing only rated for 180F (PEX) or less (PVC/ABS/HDPE), insulated by SPF rated for only ~220F max?

If you're using 40 feet of soil as the insulator, subterranean water need to be kept completely out of the insulating layer, not just away from the 200F tank.  Does this mean you're digging a 60 foot hole, waterproofing it, and backfilling 40 feet before inserting the tank?  Swimming pools crack, and are repairable- how are you going to guarantee that this huge cube of soil remains impervious to groundwater and never takes on seepage?

Allowing the tank to stratify is usually good thing- the heat content of the storage remains the same, but you can tap off the hottest water for your space heat, and feed the coldest water to the solar, boosting it's net efficiency.  Adding the rock won't keep it from stratifying, it'll only reduce the net heat storage capacity due to less favorable specific heat/density issues compared to water.

Got math (yet)?

There's nothing "conventional" about seasonal storage of low-temp water like the house in Galway, but from even first-order estimates only high-volume/low-temp is the only approach that is likely to be affordable,  once you consider the material costs, efficiency losses, and standby losses of low-volume/high temp approaches. 

"Unconventional" isn't the basis of all R & D, doing the math on the available resources & technologies  (conventional or otherwise) is.  (Trust me, I've been working in both the hard-sciences or engineering since the 1970s- stuff has to work on paper before you build/test/modify, and even when it works on paper, the secondary factors can still get you on critical performance.)  Near as I can tell you have yet to take a stab at the first-order estimations, like even how much heat you even need to store (which is easy to estimate using the fuel use & efficiency ratings of your existing heating plant), and how hot your given volume of water/stone needs to be at the outset to satisfy the season load.  This exercise is necessary to determine whether it's even feasable at that volume.  This isn't rocket science, takes no hard math, but puts a realistic stake in the ground of what's necessary or possible.   Violating the laws of physics might be "unconventional", but it's not actually possible, and not a worthwhile approach to R & D. 

Clever ways of working with/around the physical realities are possible, but you have to know where the limits are first, then design within them accordingly. No, you WON'T be able to raise the temp of the water to 300F and store it in a low pressure tank (with or without high temp solar collectors), and it's doubtful that even 300F water at your volumes would store sufficient seasonal heat for your house, even with no standby loss (which is also not possible.)  Do some math to find the theoretical boundaries, and work it back from there. Ignoring first-principles is a recipe for wasting time, money, and materials for a net-negative result.