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Thermal battery for small scale domestic application

Thermal battery for small scale domestic application

Thermal battery for small scale domestic application


I was reading about different types of thermal battery implementations for small scale domestic use, and wanted to come up with a way to improve on them without adding too much complexity or danger. In the course of researching, I happened across rocket mass heaters, which gave me an idea. For the majority who I assume have never heard of these things, they are simple wood stoves designed such that a second insulated burn box after the primary burn box also combusts the flue gasses, which are then often routed through a large masonry mass (usually in the form of a bench). What comes out of the chimney after that point is CO2 and water vapor, at around 50 C (ostensibly; these are claims made on the internet, and I don't know that they have ever been studied formally).

A drawback to any radiant heat source is that it tends to be uncomfortably warm when you're next to it, and totally unnoticeable when you are in the next room.

Skipping to the idea, I thought that if the mass and combustion chambers were more insulated, and the mass made larger, it could heated to much higher levels, and that heat could be stored for an extended period of time. If it were underground, and surrounded by insulation, probably months. The temperature of the flue gasses can exceed 1000 C fairly easily, and I calculated that a 3 cubic meter mass of concrete heated to 500 C above ambient can hold almost 10 GJ, or nearly 10% of what an average household consumes in this area in a year. (I'm not sure how feasible it is to contain it, even underground away form the house, at those temps.)

For those who don't have an abundant supply of woody biomass, excess electricity from solar energy can also be used to heat the mass. To make it more practical, I want to allow for hot water to be a possible input, but I don't want to use compressors, so I imagine a mass of water would need to be inside the battery (not for drinking). Also, to prevent the hot stone from heating this water to a boil, a third chamber with glycol could sit in between them. This chamber could double as a hotter output, for the purposes of cooling the system at medium speed, and perhaps melting snow. Lastly, I thought it should have an emergency rapid cooling system.

I drew something on a napkin, but my experience is in writing software, so please don't hold it, or anything I've said thus far, against me:

It's not intended to be comprehensive, e.g. I forgot to put an exhaust to atmosphere on the hypothetical generator, or show public water by default to the water heater. I just want to know if I'm on the right track, and that I'm not overlooking something that would make this dangerous or impractical generally. Assuming it's more or less OK, then I have much to learn, but if I could ask one question now: I don't know of any way to exchange the heat between the hot and medium chambers while being sure that the glycol would not boil, and would appreciate any suggestions as to that.


RE: Thermal battery for small scale domestic application

Interesting. I worked for the state of California judging solar houses for a nationwide contest. We had every kind of wild plan you couldn't have imagined that we had to work thru the physics of. Things like thousands of suspended one gallon bottles of water for thermal storage.

Several of the designs used multi-ton piles of granite as annual-ish thermal stores and performed fairly well. They merely used air as the transfer medium. I suggest you maybe skip the intermediate temp store and the water/glycol hassle and work with air as the medium. I'd also dump the emergency cooling scheme as unnecessary. If you've got that mass safely isolated and insulated there's no reason to include the resulting undesired thermal leak and never used complexity it would add.

On schemes like these you should hash out every consivable feature then consider each one and decern:
1) Which you can do without.
2) Every simplification you can possibly manage because aging of everything is what will come back and bite you hard in the future. Even a simple pipe joint can get you.

The ultimate system should have a lot of elegance in it.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Thermal battery for small scale domestic application


What you say makes a lot of sense. When the novelty wears off, it would be no fun to be servicing the thing. Also, a large amount of PG is expensive whereas concrete is dirt cheap, and holds four times as as much heat by volume, so I could use more of that. It may be possible in the springtime, when less heat is needed to cool a chamber too

I assume that it would be fairly simple to exchange that heat to water "on the surface", for a hydronic heating system and human consumption. Could that transfer enough heat fast enough to serve as your primary source of heat in a 2500+ sq ft house? On the other hand, could it foreseeably get too hot for a forced air system?

I'm excited to draw this when I get a chance.

Thanks a bunch for the input!

RE: Thermal battery for small scale domestic application

I went ahead and redrew my rudimentary diagram per itsmoked's suggestion. Part way through it occurred to me that there was no longer any way to get thermal energy out of hot water, which would seem to be lower hanging fruit then flue gas, as in the summer there is no need for central heat. So on the right side I have a separate battery for hot water, which has one continuous pipe to circulate water, in order to keep things simple.

Aside from fireplace inserts, you can find water jackets generators, and I presume that it would be simple enough to recover heat from less conventional sources. Off hand, the radiator on a central AC air handling unit comes to mind. (If I'm not mistaken, this would actually make it more efficient, if* done correctly). I used a fireplace insert with a water jacket in the diagram, since that is probably the most common example.

I think I see a problem with this, which would also apply to the flue gas heat capture: while concrete can hold a lot of heat, I don't think it makes a very good heat sink for this application. In the case of flue gasses, I don't imagine there is much you can do that does not add a lot of complexity and cost. Burying a large mass of copper hardly seems worth doing the cost calculation. (Maybe there's another solution?). However, in the case of a hot fluid, the best I was able to come with is to hold the liquid in a tank until the heat from the previous circulation has a chance to diffuse through the mass somewhat. I show PG instead of water for this on the diagram, because I think it will be easier to prevent boiling if it is going to be recirculating the fluid. If I'm not mistaken, using a liquid for this purpose would be better, given that the temperature change needed is relatively small. Still, I feel like there is a better way that I should be able to see, but can't.

I looked into parafin as a medium in order to take advantage of the phase change, but there don't seem to be any commodity water-wax heat exchangers. I have, however, stumbled upon a proposed design after some cursory searching.


Update: I found an article that describes how adding 5% aluminum wool to paraffin increases the thermal conductivity, and making it suitable as a phase change material (otherwise the non melted part acts as an insulator, preventing a full melt with reasonable temperatures). I calculated that a 9 M ^ 3 mass of paraffin heated 70 C (from 20 to 90 C) would hold 10 GJ. I also realized I miscalculated what a similarly sized mass of concrete would hold by a factor of 3. It's actually almost 30 GJ. Together, that represents 40% of average annual heat consumption for the area, if you can charge and extract it. To that end, I imagine a paraffin based thermal battery for water would look essentially identical to the concrete battery for flue gas, but rotated 90 degrees so that the liquid paraffin will not leak out. One continuous piece of pipe for water in and water out should greatly reduce the possibility of failures.

I doodled a cube with "knockouts", which I imagine could be filled with a medium and could be fitted with exchangers for gasses and liquids as the end user sees fit. If you want to use some exotic boiler to move heat from one battery to another, you do that on the surface, or it can be as simple as using water as a medium and water as a heat exchanger.

RE: Thermal battery for small scale domestic application

If you have electricity, you can get rid of the pipes, for the most part, and the concrete too- for the most part. You can just use washed river gravel in a very well insulated vault. Pass the air directly through the rocks and embed heaters in the pile of rocks. Low watt density to keep the temperatures of the heating elements low enough even at your peak storage temperature. As long as the structure is well enough insulated, with both conductive and radiant barriers, the heat will eventually distribute itself in the pile of rocks even though the thermal conductivity is comparatively low- but a high temperature circulating fan inside the enclosure, suitably sealed, would spread the heat out faster if needed.

Using flue gas adds a lot of complexity and some safety issues. Using concrete adds cost and embodied emissions that are otherwise unnecessary.

RE: Thermal battery for small scale domestic application

Thanks moltenmetal. I have begrudgingly agree that an all electric/forced air version is best, at least to start.

I assume there will be times when I have some excess electric. Of course, it won't be anything on the order of GJ, but perhaps it would make sense to use a wood gasifier to turn Woody biomass into electricity.

It seems counterintuitive to not use wood to directly make heat, but at first glance it looks like they are about 75% efficient at turning wood into gas, which isn't terrible, and maybe 35% efficient in making wood into electricity. The byproduct is non-toxic and can be used as fertilizer, which is good because unfortunately I'd make tons of the stuff with any serious electrical output.

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