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Underground heat storage.
Last Post 22 Jun 2011 04:46 AM by zehboss. 109 Replies. |
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dragmit
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Posts:64
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| 04 May 2010 01:45 PM |
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You are of course correct Dana. I have left out elements to this on purpose. I am looking into different solutions that would increase the boiling temp of my solution. Anti freeze for example. If I must increase the pressure in order to stabilize the system, then so be it. But I would rather keep it as cheep as possible. How do you explain being able to store high temp water in ~45-50F subsoil in an uninsulated tank without huge conductive losses
The subsoil will eventually stabilize and losses will reduce.
How do you expect to be able to collect 300 degree ...
magnifying glass onto a black receptor cooled with the system solution. Yes, some solar tracking here.
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 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
I don't. The pvc will be used as the external insulator in supply lines. Metal pipe, possibly internal coated copper will be enclosed within the plastic, and closed foam insulation will surround the supply lines inside the plastic. One issue is the fiberglass limitations underground. Working on that ... Maybe high temp spray foam? ...
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
Pretty close. But remember, I do not need water tight like a swimming pool. As long as exterior water pressure is diverted, slight leaks will not be an issue.
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
I disagree ... Convection is my worst enemy here. I need it, but I need to keep it controlled, especially as the internal energy is removed. And as long as the solar is inputting, wether its into the house, or into the bed, it is being collected.
In my experience, liars make numbers and numbers make liars Dana. The greatest scientists on Earth, not unlike yourself, insisted the Earth was flat at one time. " Uh dude" they said, "just look at it! You cant sail that way". I say you can.
Before you can get out of the box you must poke your head out and free yourself from the limitations of others, and simply deal with the limitations of science. This box is the greatest millstone around the necks of most scientists today, and yesterday. Give it a shot Dana, peek out! Its liberating! But if you say "I can't" you will not. You will be trapped by the omnipitance of others! |
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eric anderson
Basic Member
Posts:288
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| 04 May 2010 04:10 PM |
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OK hear is how you do it.
Take the 10,000 gallon tank and bury it in the yard. Instead of using water, fill it with a more appropriate fluid.
I would use #2 fuel oil myself.
Spend the money you were going to spend on all those tubes and solar collectors on the oil. Now you have about 1.38 billion BTU's of storage. That should go through the winter no problem.
Lets say a house uses 50 million BTU/ year.
Storage tank is 10,000 gallons best case scenario.
You could get the tank water up to150 deg with solar assuming you can use water down to 90deg F
You have 60degrees *10000 gallons* 8btu/gallon/deg or 4.8 million btu's Possible storage at the beginning of the heating season. 1/10th of the hypothetical heat load needed for the winter This is the same as 36 gallons of oil. Too much crap to capture that little heat. No way this works out well. Add in acutal heat loss to the surrounding ground and the idea is not going to fly at all.
This one does not even sound good on paper.
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Dana1
Veteran Member
Posts:4576
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| 04 May 2010 04:57 PM |
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The subsoil will "stabilize", reducing losses how? The temp at any distance from the tank will be determined by the net flux of the heat moving from the tank toward the cooler subsoil. A depth of 40' of soil is more than sufficient to seasonally average air-temps over a year, due to thermal mass per unit-R issues but 300F water against 45-50F subsoil is a very different beast than ~ 70F summertime air average in your neck of the woods. There are crummy/better/best methods of modeling this, but if you're just going by the seat of your pants you're likely to miss by an order of magnitude. Show me the model.
SPF is the highest temp foam insulator commonly available. Methinks you're looking at rock wool for the layers nearest the pipe.
Antifreeze will raise the boiling point, but wouldn't extent the thermal storage capacity nearly as much as the higher delta-T since it reduced the net specific heat. to get to 300F without boiling over it would have to be something like 90-95% propylene glycol by weight. You can't use ethylene glycol without running into a whole bunch of regulatory issues due to it's toxicity. (It's heat transfer capacity gets pretty sucky in high concentrations as well.) I'm not convinced that it'd be a net gain without the arithmetic, it might be, but if it is, it won't be by much. Keep working it, but if there are lots of workable fluids out there with reasonable specific heats & densities that won't boil at 300F they're probably in use somewhere. If you're lucky the cost of the working fluids at the temps you actually need to provide the seasonal BTUs you need (still an unstated, but easy number to calculate) will be less than the value of your house.
Show me the math on how convection inside the tank increases the losses, (and in the unlikely event that it does, how it makes up for the net loss in storage capacity by adding the rocks).
Slight leaks into the 40 foot dryfield boundary will not be an issue because... (?) And you're guarnteeing by design that they'll drain, how?
Concentrating tracking collectors don't increase the collection efficiency issues per unit area, but will reduce the number of evacuated tubes necessary. The hardware cost per BTU/annum collected will still be way higher than lower temp higher-efficiency collection.
I agree, show me some numbers- the numbers count. The modeling of thermal stuff is a lot more evolved and precise than the cosmology of armchair thinkers 3000 years ago. I'm not saying the earth is flat, only that what floats in the bowl doesn't necessarily run uphill when you flush just 'cuz you'd like it to. I'm saying gravity works, heat flow issue are moderately well understood, and I'm hoping you have the sense to do at least the napkin-math on it before you dig a big hole in the backyard to dump all of your money into.
And what do you really know about "most scientists today"? (Hamstrung by the math, and decades of test results, are they? :-) ) Do you know any scientists, and understand their work & methods in depth? Most of the scientists who ever existed are alive and working today- there is no lack of intelligent creativity out there, and the tools for doing the real work are better than they've ever been. Science isn't a limitation that one needs to be free of- it's building on the understanding of what is verifiably true or not.
One good measurement trumps all models, but I'll put a cheap napkin-math model of the fundamentals up against your free-of-science-limitations omnipotence, if you're giving odds. ;-) I design & test stuff that actually works, predictably, because the math works. For me science isn't a limitation, the yoke around my neck, rather, it's what gives me the confidence & freedom to design stuff that really works in the very real world. Most of the thermal transfer stuff at this level isn't even hard math, is well understood & modeled, and verified literally millions of times every day. But if your omnipotence knows better...
Math is your friend- this is a tractable, knowable, designable problem, but not without the math. When you figure out the cost of what it actually takes to do a high-temp storage system that really works, doing the math on what it takes to super-insulate your house to do a working low temp system may look considerably more attractive from net-present-value point of view. It'll be simpler, smaller, more efficient, and cheaper (but still pretty expensive). At only an order of magnitude smaller, with a reasonable low-temp radiation for your heating system and a few big envelope upgrades it might look something like the Drake Landing Solar Community's district heating low temp aquifer thermal-storage in Alberta. (google it) |
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dragmit
New Member
Posts:64
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| 04 May 2010 06:28 PM |
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Hey Eric. So, if I up the size of my tank by 10 times, I will never have to pay to heat my house in NY again? I can do that. |
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dragmit
New Member
Posts:64
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| 04 May 2010 08:03 PM |
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Excellent Dana! Delta T heat transfer. As soil permeates heat, the rate of heat transfer reduces at Delta T. Minus other insulation. I can not use asbestos, but there are other elements. Again, I'm working on that. Someone suggested old tires packed with dirt. Maybe closed cell foam would work better. Sounds weak, but maybe. Cheap for sure. I'm still looking.
I know this really hurts to approach with such an open mind, but I believe it can be done. Most systems are designed to simply 'work' and little time is spent trying different approaches. Its easier to claim the mouse trap is already at its best, and sell the ones we have. Its just easier, I know. I've serviced mouse traps for years and I always saw better ways to do things, but there was really no reason. The mission was to kill the mouse, and they killed the mouse quite well. Off to the ball game. But for years we have taken energy for granted. 4 inch walls were the norm, and in the 60s anyone talking about 6 inch walls was from lala land. But its different thinking that comes up with new ideas. Does this mean different thinking always comes up with a better answer? Of course not. But if you never try, you never make changes. Make a boat from steel? Are you a nut? Sometimes our education is based on fixed parameters, and fixing those parameters closes off any possible alternative solutions. I am sure many of the things you speak of are true. No, probably most of them. But if I throw two degrees of tilt to the plow blade, all your fixed numbers go out the window. Thats all I'm trying to do is change the plow angle. It might not work. But then again, it might tune up if I do not give up. You see, it is this spirit that caused Thomas Edison, Nicola Tesla, Henry Ford and thousands of others to accomplish the things they did. And the experts laughed at them. After all, they were not the experts. And I will bet there are thousands of others who were not successful at their crazy ideas so you and I will never know their names. And that does not even mean they were wrong. Maybe they just had a bad heart. Or maybe they had to deal with raising a family so they simply could not follow through. Or maybe, most sadly, someone brow beat them down to a point that they simply gave up. But their failure does not mark the spirit, because everything you see and touch today is a direct result of this spirit. So you can assume I do not understand something, and you may be correct. But you might be missing something. No one knows everything except God ... and just in case you are completely up to speed with the Old Man, good for you! I am sure he has a good reason for encouraging my 'folly.' But I am pretty sure you are not that close to God. And when that fuel oil you speak of goes up to 200 dollars a barrel, and it will, the price of a 100,000 cubic foot buried heat storage device will become feasible.
Now, the truth to you. My first response to seeing your post is "damn" My final response is "Hot damn!" You are the best at making me engage this old brain to solve problems. You make me think, and that is my favorite hobby. Thank you Dana, I really do appreciate it! Hang with me there Dude and we can change the world! |
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Brock
Advanced Member
Posts:599
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| 04 May 2010 09:47 PM |
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Sitting here and trying to keep an open mind the analogy to this would be something along these lines.
My VW TDI wagon (diesel) has a life time mileage of 54.98 mpg over the last 93,678 miles and I typically go over 800 miles between fill-ups (this part true). Now, and here is when things go terribly wrong, my plan is to take my super efficient small diesel engine and put it in a semi truck so I can make hundreds of thousands of dollars driving across country hauling freight getting 50+ mpg and driving 800 miles between stops. This just isn't going to work, and not because I have a closed mind. It's not just the engine, but the size of the car, the power to weight ratio, driving technique, powertrain, drag and so much more.
You simply can't throw a valid idea at something, force it in a system and make it work, you have to take in to account everything going in to the equation and that is what seems to quite simply be ignored in this case.
Again I have 16,000 gallons of off peak geothermal heated indoor pool and it works great as a load shaver and it could even carry me maybe 2 weeks in the dead of winter, but somehow you plan use 10,000 gallons to carry you through the entire winter? I would have to heat up my pool to over 400F, not taking in to account the massive losses to the ground and my pool has 2 inches of foam around it as well as the 2 inches out at the footings and poured basement walls as well. |
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| Green Bay, WI. - 4 ton horizontal, 16k gallon indoor pool, 1.8kw solar PV setup, 3400 sq ft |
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Como
Basic Member
Posts:128
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| 04 May 2010 10:08 PM |
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I am putting in 3000 gallons, in the depth of winter it should last overnight.
High temperatures - pressurised storage - cheapest option is used propane tanks. |
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dragmit
New Member
Posts:64
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| 04 May 2010 10:42 PM |
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I hear you Brock. So how about ten of these containers, all heated to say 350 degrees. Or 30, or 40. The units in the center will lose heat to the ones surrounding them, thereby 'saving' this energy for use. Remember, I am using this 10,000 gallon tank as part of the recovery device of this stored heat. Much of the heat will be stored in the ground around the tank. (So I don't really think I will need 40, 30, oe even 10) But even still, if I had to use ten of these tanks and 1000 square feet of solar collectors, wouldn't that be better than no heat at all? Do you really expect fossil fuels to last forever? When oil reaches $200 dollars per barrel, just how concerned are you going to be about the cost of ten used fiberglass tanks? Yes, the losses you all speak of are real, and they must be dealt with. I am not imagining some sort of Warner Brothers cartoon here, I am considering reasonable solutions just in case the whole 'windmills are wonderful' theroms collapse. And I see that as a much more likely scenerio than stored local heat apps. I only have to heat my house, they have to heat the entire community. Good luck with that. Now, how do we fuel our damned cars? Thats the real problem. |
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Como
Basic Member
Posts:128
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| 04 May 2010 10:53 PM |
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http://www.scanhome.ie/research/archive/Seasonal_store_of_solar_heat.pdf
This link given before says it all.
First mitigate your load, then everything becomes easier. |
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Dana1
Veteran Member
Posts:4576
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| 05 May 2010 11:28 AM |
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Posted By Como on 04 May 2010 10:53 PM
http://www.scanhome.ie/research/archive/Seasonal_store_of_solar_heat.pdf
This link given before says it all.
First mitigate your load, then everything becomes easier.
That's right! For the the amount of money you'd spend on insulating super-temp 300F adequately you can insulate the house to the point where adequate heat can be had at more moderate temp, which adds up to higher collection efficiency and lower standby loss. Working temps that high aren't only inefficient, they're dangerous in the event of a leak or steam explosion, or exposure of the plumbing to touch. Going with a "Keep It Simple Stupid" approach will make it affordable & reliable. Running it at the temperature limits of the materials used, running at temps that require exotic or expensive materials, and extra safety control/construction just runs up the capital expense- make sure you really NEED those measures before going down that road. It's not clear where the design storage temp of 300F came from, or whether that's an even adequate amount of BTUs for the seasonal load the system has to deliver, but since dragmit won't tell us what his seasonal fuel use is, we can't even estimate it. We don't know the subsoil type or the distance to the water table, only it's approximate temp, so it's hard to put a make a back of napkin estimate on the heat loss over months of time either, but first-principles say that the heat loss of 150F water against 50F subsoil is about half that of 250F water/antifreeze for a given surface area of tank. Even utilizing moderate temp water & much more modest delta-Ts in the district-heating scheme in the Drake Landing project they're talking three years before the subsoil & aquifer are fully stabilized. Dragmit's could take a decade to stabilize if 300F is truly the design temp for storage, and losses from subterranean water flow could sap it's storage capacity via standby loss beyond all practical use. If you don't know how big the load is, you'll never know how much storage you need, which bears directly on the peak storage temp necessary. Underground seasonal storage is not some new idea, and there have been many projects built & studied over the past ~35 years. There's plenty of room for creative work on this, but ignoring the knowable-testable physics isn't a good place to start. In all things solar, temperature & efficiency run counter to one another. In all thermal storage, delta-T & and efficiency also run counter to one another. There is no advantage to collecting or storing heat at temps any higher than the heating system's radiation requires at it's peak load. Using 300F fluids to maintain 70F room temps can work, but it's the antithesis of efficiency. And with an uninsulated tank in the ground it's the thermal mass & insulation value of the dirt, not the water that's your primary heat storage medium- the skin of the tank being just another heat exchanger in the system. |
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jonr
Veteran Member
Posts:3334
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| 05 May 2010 12:09 PM |
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Forget 300F - just go bigger. A 10K gallon tank would be useful for a few days of solar or off-peak electric heat storage. If you want to store a seasons worth of heat using water, you need much larger tanks (perhaps 500K gallons) - expensive to build and insulate (with dirt or regular insulation).
Dirt isn't as good (volume or input/output wise) at storing heat, but the price is much better. So just putting heat underground in the dirt is your best bet.
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Dana1
Veteran Member
Posts:4576
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| 05 May 2010 12:40 PM |
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Posted By jonr on 05 May 2010 12:09 PM
Forget 300F - just go bigger. A 10K gallon tank would be useful for a few days of solar or off-peak electric heat storage. If you want to store a seasons worth of heat using water, you need much larger tanks (perhaps 500K gallons) - expensive to build and insulate (with dirt or regular insulation).
Dirt isn't as good (volume or input/output wise) at storing heat, but the price is much better. So just putting heat underground in the dirt is your best bet.
Yep, if he's not insulating the tank, putting the heat into the dirt is EXACTLY what he's doing, and the size of the tank is only relevant in terms of it's surface area, not volume, as a measure of it's effectiveness as a heat exchanger between the dirt & working fluid. |
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dragmit
New Member
Posts:64
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| 06 May 2010 11:25 AM |
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What is the general consensus on being able to hold these temps in dirt if I am able to isolate this dirt from ground water? I am thinking about putting up some sort of heat barrier on the outer field edges such as blown closed cell foam against a wire support. Wasted effort? I will be doing this on the top surface. I am thinking most heat loss would be vertical. Any idea how critical delta T issues will be once the field ‘stabilizes’? I am thinking of circulators to draw field heat into the main exchange area (fiberglass tank), from the exterior field, if the internal tank temps get lower than 20 degree difference. I’m thinking that returning heat to this internal field will reduce conductive losses. 20 degrees OK, too little, or too much?
Remember, I am thinking of the field as being dirt, but I was planning on insulating this dirt from the exterior of the installation to reduce delta T into the external field. Possibly even a secondary barrier another five feet from this first barrier. But if I do the first barrier this may be ineffective.
Jonr, if 300 degrees is less expensive than going much bigger, which may not make financial sense, why not 300 degrees? This elevated temp is kind of critical to the theory. |
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Dana1
Veteran Member
Posts:4576
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| 06 May 2010 11:42 AM |
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Posted By dragmit on 06 May 2010 11:25 AM
What is the general consensus on being able to hold these temps in dirt if I am able to isolate this dirt from ground water? I am thinking about putting up some sort of heat barrier on the outer field edges such as blown closed cell foam against a wire support. Wasted effort? I will be doing this on the top surface. I am thinking most heat loss would be vertical. Any idea how critical delta T issues will be once the field ‘stabilizes’? I am thinking of circulators to draw field heat into the main exchange area (fiberglass tank), from the exterior field, if the internal tank temps get lower than 20 degree difference. I’m thinking that returning heat to this internal field will reduce conductive losses. 20 degrees OK, too little, or too much?
Remember, I am thinking of the field as being dirt, but I was planning on insulating this dirt from the exterior of the installation to reduce delta T into the external field. Possibly even a secondary barrier another five feet from this first barrier. But if I do the first barrier this may be ineffective.
Jonr, if 300 degrees is less expensive than going much bigger, which may not make financial sense, why not 300 degrees? This elevated temp is kind of critical to the theory.
If you 're blowing even 1" (R6) of closed cell foam on all sides of a 40' x 40' x 20' deep right rectangular prism (a big box surrounding your tank) you're looking at well north of $6K in foam alone- foam that might be more cost-effectively applied to your house. (Alternatively, applying the same dollar amount of foam to your tank would be lower-loss.) You still haven't made the case for 300F water/other fluid why DO you need it, and why would you take the huge efficiency hit to get there? high temp, high tech=> high maintenance, high cost, low reliability low temp, low tech=> low maintenance, high reliability Spend the first 15-30 grand getting serious about fixing all the heat leaks in the house, which will cut your annual heat-energy requirements by 30-50% or more. That will lower the operating temp, increasing the efficiency of whatever system you concoct to for seasonal heat storage. Please, PLEASE do some of the simple math on this! Even storing 150F water for months at a time will be neither cheap or easy, but 300F is nuts. |
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WoodNotOil
New Member
Posts:11
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| 11 May 2010 09:52 PM |
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I have 1000 gallon heat storage tank that I made for use with my gasification wood boiler. It allows me to burn for about 10 hours a day (2 loads of firebox) and get all of my heat and domestic HW for the day in the winter. In the shoulder seasons I can space out single load burns and in the summer I can heat the storage and get nearly a week of domestic hot water. My house is old and drafty and I will be doing some insulation upgrades this summer. I hope to cut my heat load in half...
I too agree that you need to button up the house extremely well if you intend on trying to store enough heat for the whole winter... I have to tell you that there are guys out there with super insulated houses and heat storage that heat the house and domestic year round on 4-6 cords of wood. Add your solar collectors into the equation and the wood goes down. There are proven installs out there that already have the kind of liberation from fossil fuel you are talking about. Sure there is some wood involved, but not much... just a thought. |
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| WoodNotOil.com |
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popawheelie
New Member
Posts:7
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| 01 Jun 2010 08:45 PM |
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I may be wrong, but I think you are asking for trouble using liquids.
I worked on a home that used a large hole in the ground for heat storage.
The collectors and storage used hot air.
Hot air collectors are relitavely easy to build since air is less prone to leaking.
Hot air piped into a large pit full of rock is petty easy to do and works dependably.
Once you have a large pit of rocks heated it is pretty easy and dependable to tap into.
You want something that will work reliably over a long time. |
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dragmit
New Member
Posts:64
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| 02 Jun 2010 10:15 AM |
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I may be wrong, but I think you are asking for trouble using liquids. ...
Hot air piped into a large pit full of rock is petty easy to do and works dependably.
Once you have a large pit of rocks heated it is pretty easy and dependable to tap into.
You want something that will work reliably over a long time But trying to extract heat from air is problematic. We can move that energy around easier and more efficiently with a liquid than a gas. We will be drawing fluid from this container rather than pumping and pressureizing it so leaks are more likely to contaminate our field rather than leaking it. And stabilizing the field will reduce this possibility greatly. We do not want to pressurize these containers in any case. |
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shortly
New Member
Posts:11
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| 11 Jun 2010 12:36 PM |
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Is there any closed-cell polyurethane foam that can tolerate 300° f? The product (very well-known manufacturer, tanker truck quantities) we use begins to degrade at slightly above 200°, at 250° it all but disappears after a day or two. |
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Dana1
Veteran Member
Posts:4576
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| 11 Jun 2010 01:20 PM |
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Posted By shortly on 11 Jun 2010 12:36 PM
Is there any closed-cell polyurethane foam that can tolerate 300° f? The product (very well-known manufacturer, tanker truck quantities) we use begins to degrade at slightly above 200°, at 250° it all but disappears after a day or two.
Short answer is "no". Some of the fireproof foamed cement stuff might cut it, but there's still only negative benefits to 300F storage. More volume & much lower temps takes less insulation, and radically improves the solar panel efficiency. For the cost of the insulation required to store enough heat at 300F to heat a leaky 1980s house in dragmit's location for the winter you could instead insulate the house to the point where it didn't need a heating system. And that's before you get into the size & expense of the solar array capable of delivering that heat into the storage. (It's probably even cheaper & more efficient to use 15-20% efficiency PV panels and resistance elements in the storage that it is to run 300F hydronic loops at high cost & high loss.) The standby losses at high temp are bad enough, but the cost of 300F fluids & insulation only make it ridiculously non-cost-effective. A PassiveHouse type building envelope retrofit would cost less, and be far more reliable, and could be heated with a very modest low-temp water storage. |
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Rosalinda
Basic Member
Posts:332
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| 12 Jun 2011 01:24 PM |
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Dragmit, did you ever actually build an underground heat storage system of any type? If you did, can you post what you did and what your results were?
-Rosalinda |
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| Sum total of my experience - Designed, GCed and built my own home, hybrid - stick built & modular on FPSF. 2798 ft2 2 story, propane fired condensing HWH DIY designed and installed radiant heat in GF. $71.20/ft2 completely furnished and finished, 5Star plus eStar rated and NAHB Gold certified |
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