Storage (buffer) in the soil by thermal rollers?

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Aumicron
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Storage (buffer) in the soil by thermal rollers?




by Aumicron » 08/04/10, 11:18

On an other subject, dedeleco has given a link already presented by minguinhirigue but that had gone a little unnoticed:

http://www.earthshelters.com/Ch_1.html

This page presents an experience in the US Montana summer storage of heat in the ground for reuse in winter in the house.
Personally, I never understood how this system can work without fan. Any ideas?
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loop
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by loop » 08/04/10, 12:37

Bonjour,

The heat displacement time, called the heat front from one side to the other of a wall is called the phase shift.
This wall can be a wall, heated on one side by the sun, and "cooled" on the other by contact with ambient air on the other.

If this principle is applied to a very thick wall such as the ground, the time that the heat will put to go up can be several months. It depends on the thermal conductivity of the material (earth, chalk, rock etc).
The heat is diffused in all directions if there is not at a given place of insulation.
What I remembered is that the speed of the heat front in the ground is close to 30 cm per month (it seems to me little but hey, take that as hypothesis)

In summer, if I create a hot zone in the ground, the heat will gradually diffuse at the speed I indicated above, but slowly losing its temperature.
If the diffusion is continuous, the last wall encountered (here the ground) will be "reheated".

I do not know the rule which makes it possible to determine the final temperature of the wall (diffusion essentially by radiation and convection) but one can suppose that it is higher than the natural temperature of the soil (of the order of 10 ° C)

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by Christophe » 08/04/10, 13:04

I never believed in storage by "pebbles and hot air" for various reasons, the first being the stored energy which is quite low, the second the very bad heat exchanges and the third the health risks ...

On the other hand, using rocks or the ground to store energy is a good idea (provided to control the underground water infiltration which would cool the buffer) and by using "heatpipes" with phase change ca can become very interesting ... technologically speaking because economically ... the installation could be very expensive!

For an individual not to be profitable ... not to mention the fact that it must be done almost inevitably at foundations.

In addition, it takes 4 times more "solid rock tonnage" to obtain the equivalent heat capacity of liquid water: http://fr.wikipedia.org/wiki/Capacit%C3 ... e_massique

Substance (solid phase)
Specific heat capacity J · kg-1 · K-1
Asphalt 920
840 brick
880 concrete
Granite 790
Gypsum 1090
880 Marble
Sand 835


Liquid water being at 4180 J · kg-1 · K-1

Even in "volume" it's not really interesting to use rocks in comparison to water ...

In short: I think that a large buffer of water with a phase change material (PCM) to "dope" and create a T ° buffer bearing is much more interesting econologically speaking!

For the MCP refer to the following subject: https://www.econologie.com/forums/stocker-de ... t7421.html
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by swift2540 » 08/04/10, 13:07

Hello,
just a track, my english is too bad to understand the link. : Cry:
The warm air rises (this is the principle of the balloon).
So stored under the house, the heat tends to rise to the ground.
That's why with a conventional heating (pc is different) it is always warmer on the ceiling.
That's also why roof insulation is so important.
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by aamgr » 08/04/10, 13:41

Hello,
Here is a link that talks about storing heat in pebbles:
www.forums.futura-sciences.com / habitat- ... galet.html
Thank you
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by dedeleco » 08/04/10, 16:09

In agreement with Christophe, it is necessary to calculate the good orders of magnitude of heat capacity and thermal constants according to the materials.
It is necessary to avoid final judgments, given the wide variety of possibilities.
But a fact is clear, underground to more than a few meters deep (amount of land without limit at zero price) the heat is stored over several years naturally (Canadian well / provencal effective giving the average temperature over one year, 13 ° C as in the typical caves in France).
So if in the summer we heat up more, this earth strongly (40 ° to 80 ° C, above or in provided by a solar collector even simple) over several years, we will increase this average and we will have 20 ° C in winter, we transporting to Africa virtually !!
On the other hand, the heat is simple for the key points, for each material, there is the heat capacity, then the thermal conductivity whose ratio gives the thermal diffusivity with a penetration time or diffusion of the heat through a wall who believes like the square of the thickness.
http://fr.wikipedia.org/wiki/Diffusivit%C3%A9_thermique
and corresponding links.
More complete and not necessary if one wants a largely sufficient order of magnitude:
http://fr.wikipedia.org/wiki/Conduction_thermique
This law of the square is fundamental and easy to remember
So if we multiply the thickness by 10 we multiply the penetration time by 100.
This allows to have 6 months (or 180days) by multiplying by 10 the thickness that gives 1,8 day of penetration, for example 30cm of earth or rock, (function of its composition and its humidity) with 1,8 day passes to 6months with 3 meters of earth or rock !!
Obviously if the basement is full of fast flowing water (underground river), the heat will not be well guarded, but it is not usual except flood zone. Then by injection of waterproofing agent (cement, resin, etc.) we can stop this rapid circulation of water in a limited area underground.
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by dedeleco » 08/04/10, 19:09

But to fix the ideas, it is necessary to store what energy

We store the energy of the sun in the summer that falls on the roof where solar thermal sensors that drive the heat underground (heat exchanger conventional coolant, (water alcohol etc ..), to the earth at depth, while in winter it heats the interior of the house!
One can have a good heat conductor between the roof (or an outdoor slab well heated by the sun) and the earth deep down. A good conductor metal, aluminum, copper, iron at the same time, even massive concrete, convective heat exchanger, can also drive this heat and heat the earth deep to restore it in winter, but it must be able to cut in winter good contact thermal summer.
The principle is simple: heat the earth in hot summer (or swimming pool, or heat accumulator) well insulated deep, to recover this heat in winter in the opposite direction.
The means to realize it are immense by using the good known and varied heat conductors and insulators between the hot surface in summer (roof, slab, sensor, etc.) and deep underground storage (3m and more) or any other means provided that it is well insulated to keep the heat between summer and winter !!! (isolated swimming pool, tank with chemical or physical reaction, etc.).
The large number of possibilities at various prices must not block achievements, which appeal to the imagination of all.
The challenge is to choose the cheapest form, even if it is not at maximum yield, increasing the size.
The land under the house or the garden at depth costs only the boreholes a few meters to bring the heat. The rudimentary solar collector (black plate with water pipes glycol exchanger, even without glass) is one of the cheapest and effective to be heated strongly by the sun in summer (but insufficient in winter where the heat stored in summer replaces it).
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by dedeleco » 08/04/10, 20:12

looping writes:
What I remembered is that the speed of the heat front in the ground is close to 30 cm per month (it seems to me little but hey, take that as hypothesis)

The heat has no velocity in an immobile material because it diffuses (random movements of electrons and acoustic waves, which go in almost equal quantities in opposite directions) and therefore the time taken to cross a wall believes like the square of the thickness, so if an earthen wall of 30cm takes two days to transmit heat (typical a few days depending on the humidity and the composition), a 10meters 3 thicker thickness will transmit this heat (or let it loose in winter) in a time 10x10 = 100 times more is 100x2 = 200 days is 6 months and 20 days !!!
If the speed existed constant it would take 10x2 = 20 days which is experimentally false and archi false !! (we could not insulate anything thermally, given the speed of acoustic waves of several km / s)
The apparent velocity of heat thus decreases as the inverse of the thickness and the phase shift or penetration time increases as the square of this thickness.
This basic fact is fundamental to understand the thermal effects.

What I remember is that the speed of the heat front in the ground is close to 30 cm per month

If on 30cm of soil it takes a month to heat on the other side (special dry floor actually), a thickness of 3 meters (10times plus) will 10x10 = 100 times 100mois or more 8ans 4mois !!!
and not 10 months !!! The difference is fundamental.
It's crucial to understand this fact well that you can check with your hand, by heating the torch, on one side a thin plate of 1mm or 10mm, with your hand on the other side that feels the time taken to burn and remove the hand !! If a second at 1mmm, with 10 times more we go to 100s 1min 40s with 10mm (taking what you want, wood plate, iron).
Finally swift2540:
The warm air rises (this is the principle of the balloon).
So stored under the house, the heat tends to rise to the ground.
That's why with a conventional heating (pc is different) it is always warmer on the ceiling.

mixes convection in the rising air gas, because lighter if it is hot with what has nothing to do, the diffusion of heat in a hard rigid material such as the floor or a wall, where even if a little lighter in hot, it can't move like a hot air balloon freely !!
In a rigid material the heat diffuse taking a time as the square of the thickness is 100 times more for 10 times thicker !!
The convection in the air or water, with increasing speed movement with the temperature difference is much faster than the diffusivity and thus destroys the thermal insulation.
For the air a thickness of more than 1cm causes the convection and removes its good insulation, which requires taking insulators with pores less than 1cm filled with air or double or triple glazing spaced less than 1cm !!
I say this because I am amazed to see thermal insulation touts tell errors that show ignorance of these basic physical fundamentals.
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by loop » 08/04/10, 21:51

Good evening,

Thank you Dedelco for these clarifications

If on 30cm of soil it takes a month to heat on the other side (special dry floor actually), a thickness of 3 meters (10times plus) will 10x10 = 100 times 100mois or more 8ans 4mois !!!


The value I have chosen must be a simple average established by dividing the earth's thickness by the time it took for the heat to rise.
It is true that with a law squared of the thickness, one can quickly be mistaken of several months, to see years !!!

This storage solution, I think more and more seriously.
As I can not demolish the floor of my house (tiled concrete slab), I had thought to do horizontal drilling from the cellar, which is only a small part under the floor area on the ground floor.
The soil in my area is in chalk and we could quite easily place a solar circuit discharge exchanger for the summer.
No more overheating problems due to over-sizing, and natural heating floor for winters.

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by Aumicron » 09/04/10, 12:07

On the excellent site fiabitat.com which is a wealth of information, I found additional information on this page:

http://www.fiabitat.com/construction-ma ... me.php#5b4

According to them, the principle of storage in the summer by natural convection underground is as follows: in a pipe whose end is connected to the house (hot spring) and which goes underground (cold source) to lead to lower air free, the air is sucked to the ground and warms it. The phenomenon is reversed in winter, the air of the house colder than the earth.

The example is presented with 2 storage areas for more performance, but it must work with one.
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