This is theory at this point to me, but i thought i'd ask the experts if anything has ever been tried with this idea. I think it could work for greenhouses especially and substitute for the more expensive (energywise) transfer of heat into sub floor storage mass using fans etc, with relatively simple materials and setup before building the RMH. Note that this is probably most effective in places with short winters, the amount of stored heat dependent on summer temps and amount of mass insulated in the ground and to some extent the transfer rate,(higher/hotter stack promotes faster/greater transfer).
The idea is to use several (probably a lot) heat conductive metal rods or plates(rebar?) driven deep into the ground and connected to the metal duct work of the rmh exhaust.
the last part of the exhaust needs to go vertical through the roof,painted black, and acts as a solar stack drawing air through the rmh as a normal draft would do
vertical insulation needs to be installed several feet into the ground outlining the base of the RMH, creating a large thermal battery under it.
in the summer the superstructure of the rmh is totally covered with insulation and the solar stack continuously/passively pulls hot air through the entire system of the rmh, and all summer long, whenever there is heat there will be heat transfer going on into the RMH bench and into the ground. Some sort of temperature sensors can shut the stack every time the mass stored heat exceeds the ambient air temp. at the intake, open when ambient temps are greater.
In the winter insulation is removed from the rmh whenever heat is necessary.
Depending on the building, there may need to be an extra added section of pipe installed connecting the intake of the rmh to the outside (possibly hotter air)
with sensors the whole operation could be automatic, with near zero energy input (except of course for the sunlight and natural warmth of the air.
This could also be a simple added dimension to the RMH and allow for longer periods between fires (like going away for a week or something)
It shouldn't be too difficult to use a trombe wall with pipes or something similar to replace the active fans. You could probably even run the intake pipes across the bottom of the trombe wall with a slight uphill pitch to preheat the air, go through the mass, and then vertical through the trombe wall to create a draft. Something like an Arduino could power flaps or valves to cut off air flow and be very power efficient.
I don't think adding metal is going to be very effective or economical as the mass is already fairly conductive. There is only a small temperature difference using outside air comparatively to burning wood in a RMH, so squeezing 1 or 2 degrees extra out of a given amount of air will be difficult and probably expensive. Adding more than one pipe would greatly increase efficiency. The thing about a large thermal mass is the surface to volume ratio. If the mass is more circular or square it won't give off heat as fast as one that is really long and wide and thin at roughly the same volume. Adding surface area in the form of more pipes will give more contact for the air to dump heat in to the mass. One could be the regular RMH system for supplemental heat and another one or more for airflow only.
For a greenhouse, you probably wouldn't want a low bench, but rather something closer to square in cross section and closer to a more normal working height to serve as a work/plant space or perhaps with 'steps' for different tiers to hold plants. This would give off heat a bit more slowly but also take longer to lose heat. Given the soil all of the plants use then a full greenhouse would have much more thermal mass than many typical modern homes. The temperature swings shouldn't be drastic, so the mass won't need to be shaped to give off that much heat. In an emergency, the bell of the RMH can provide fast heat, but most of the time the mass should be sufficient if designed right. Getting the vents to open and close at the right times will be key to this.
I have been looking into this idea from the opposite perspective; I want a cool mass to moderate the hottest temperatures of my climate. Instead of preheating pipes with a trombe wall, I would use geothermal cooling. I want to take the night air and cool it further with subterranean pipes to then pull heat from a large mass. The idea of the mass and pipes is identical. Having a large monolith that isn't too thin in any direction and enough pipe surface area should be able to accomplish our goals.
Van Powell on YouTube has a lot of interesting videos, including making a greenhouse with a rocket mass heater. It isn't terrible large and doesn't have the system you are looking to implement, but there are a few ideas there that could be useful. You can insulate the north side of the building to retain heat, or maybe use charcoal to darken the surface of the mass in spring/fall to absorb heat from the sun. Hopefully some of these ideas will help you out.
Hi Allen, for some reason I am not being notified when people add posts to topics , so I'm sorry for the delay in responding.
Yes, I was thinking about VA when I came up with that line of thinking, and have seen those diagrams from the links you included many times in the past. The point I was looking at had specifically to do with the creation of the Thermal storage battery with little real effort as far as deep burying pipes and then using active /forced circulation to make the system work--like some of the more recent greenhouses in canada that are able to do deep storage of heat with a buried insulation layer on top of an earth thermal battery that stores summertime heat for winter.
I haven't really thought much about this recently, but noticed these replies to my original idea, and after a recent talk with a greenhouse operator who was having lots of trouble with heat expenses, I started to think about it again.
RMH ducts charge and discharge primarily from clay within a 6inch distance. The metal rebar rods are useful in that it is not necessary to dig up all that subsoil to make it an active part of the thermal battery and install deeper pipes and greater numbers of pipes. A little like some of the recent solar hot water collectors that use metal rods to transmit the heat to a central core that exchanges heat with a single water pipe running through.
There are two possible shortfalls I see with the system that would likely require trials, one is the height of the solar stack necessary to draw the air through the RMH (and storage battery) and the resulting cooled air out the stack. That air would need to be reheated by the stack to provide the energy to keep the air moving. The second question is how closely the rods need to be spaced in order to conveniently charge and discharge the thermal battery optimally.
My guess is that it has to do with average temps in summer and winter, length of daily cycles, etc
I have a rocket mass heater in my home and have been using it for several years now. While soil or clay may conduct heat somewhat, it is still much slower than metal. The idea of the rebar driven into the ground with insulation in a trench around this "battery" is to facilitate the transfer of the heat at relatively lower temps than a Rmh
In normal operation the RMH absorbs large amounts of very high heat quickly, then radiates it more slowly at a lower temp, but would likely not be fast enough to bring up the deepest heat charge from the thermal battery over the winter season without the conductive metal.
The trombe type solar chimney effect might be an option to passively move the air, and like you suggested might work more effectively at a lower position than the falling (cooler)RMH exhaust, which would then be heated by the trombe apparatus causing it to rise and drawing the air through the whole thing .
I'm not sure what you meant by Arduino, unless you were referring to some improvised electronics.
Yes, I do envision the greenhouse RMH being almost counter height, so the above ground part would look more square on its end than most house RMHs, with the full thermal battery going deeper into the ground.
Again, I was thinking about trying to store the heat of the summer for use in the winter like the newer canadian greenhouses, only without such extensive, expensive underground "plumbing". Those deep heat exchanging tubes being replaced with rebar conducting heat more passively.
My dream is the Earth Tube Bill Mollison talks about, which might do the air conditioning you are thinking about. basically just a tunnel under ground about a meter deep and about 100-300 feet long. The solar stack in the house draws air through the tunnel. The cold earth precipitates moisture out of the air while it cools the air that is then exhausted into the house. In doing research on harmful molds that can plague some of these systems, it appears that most of those problems appear when the natural bacteria and fungus in the earth are isolated away from the air corridor. Bill recommended earth walls of the tunnel, not Poly pipes, and my research confirmed his preference, although if anyone has direct experience or knowledge about this it is a topic I'm trying to learn more about
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