Mass heater with water heater.

I'm thinking about making a rocket mass heater with a water heater and was looking for some wisdom on my idea.

Its going to be outside, here in the UK we can't have them indoors.  So the mass heater is not for space heating but to continue heating the water for a prolonged time.

My idea is to line the outside of the first chimney/burn chamber with sand as a thermal mass with water pipes embedded in it running round the chimney, then pass the hot gasses into a highly insulated barrel(or possibly a gas bottle as mine will be probably be smaller) and out to a chimney.

The idea is to heat the sand up so I can capture more heat and have it continue to heat the water after the fire has gone out therefore requiring a shorter burn.  

Possibly also running the chimney horizontally as described on this site to recapture more heat with water pipes in as a preheater befor going to the primary burn chamber.

I will try to upload a diagram later when I've got home.

Any thoughts welcome

Thanks
Jon

Hi Jon,

Welcome to Permies.

Hi Jon, Welcome to Permies!

Nice last name by the way.

Here is a smattering of links that might be of interest to you.

https://permies.com/t/22260/wood-burning-stoves/RMH-principle-heat-water

https://permies.com/t/22895/wood-burning-stoves/ideas-Central-Rocket-Heater-provide

https://permies.com/t/22471/rocket-stoves/Large-Rocket-Mass-Heater-Boil

https://permies.com/t/22229/rocket-stoves/Instant-Hot-Water-systems-Rocket

https://permies.com/t/21721/rocket-stoves/rocket-bath-water-heater-filter

https://permies.com/t/22031/rocket-stoves/Quick-Easy-Rocket-Stove-Hot

https://permies.com/t/20844/rocket-stoves/Water-Mass-RMH

https://permies.com/t/12378/rocket-stoves/rocket-stove-oven-hot-water

https://permies.com/t/19487/rocket-stoves/outdoor-water-heater-RMH-project

Thanks John and Timothy for your welcome 🤗 and links, I will look through them tonight!

Thanks
Jon

Good luck with your project, I think you need to re think your sand idea though.
Sand is neither a ‘good’ insulator or a ‘good’ mass, the grains trap air and air is a good insulator.
So sand is sort of in between  and is pretty neutral.
Any form of pipe longer than a few feet, filled with water and then heated can become a dangerous design, so think through very carefully.
I look forward to your progress…..

Water heater... I seem to have a link to the design of Tim Barker. His water heater is reliable and can't go boom.
Ahh... here it is. http://permaculturenews.org/2012/11/23/rocket-stove-hot-water/

Fox James wrote:Good luck with your project, I think you need to re think your sand idea though.
Sand is neither a ‘good’ insulator or a ‘good’ mass, the grains trap air and air is a good insulator.
So sand is sort of in between  and is pretty neutral.
Any form of pipe longer than a few feet, filled with water and then heated can become a dangerous design, so think through very carefully.
I look forward to your progress…..

Thanks Fox
I have been reading alot about sand being used commercially for heat storage.   It maybe the news reports have used the word sand rather than sand like substance, maybe some more research is needed.

I have seen a couple of long form YouTube clips of people using sand as a heat battery successfully, but they may in reality be horribly inefficient.

I will read around the subject.

Thanks again
Jon

AMENDMENT.
I've just been reading this which broadly agrees with your assessment.   https://protonsforbreakfast.wordpress.com/2022/07/21/a-sand-battery-not-obviously-a-great-idea/

Peter van den Berg wrote:Water heater... I seem to have a link to the design of Tim Barker. His water heater is reliable and can't go boom.
Ahh... here it is. http://permaculturenews.org/2012/11/23/rocket-stove-hot-water/

Thanks for the link

I will have a good read, my only reservation on a cursory glance is in my experience trying to heat up a large amount of water in one go can be miserable.   I would like to be able to do short burns as time allows, to just heat up the top of the thermalstore if needed.  But I know very little about this subject  and any new knowledge is welcome.

Thanks again
Jon

Hi Jon, Great post. I was about to ask the same question when I found your post. I am looking to do something very similar. The sand topic is really interesting. I live in Finland and have been keenly following the sand battery news.  One often over looked benefit of the sand, it that it can be very easily emptied and reused if the design changes.  Cob can also be reused, but not as easily.  The point made above about cost on a smaller scale is a good one. Maybe the better mass for a heat battery on the home scale is a combination of rocks and pebbles with the gaps filled with sand.  If the sand trapping air causes an insulating effect maybe finer sand is a better option on a smaller scale.

Heat storage medium aside, the most unique thing about using a RMH in this way is that we want to eliminate as much heat loss as possible. There is no benefit to heating the space, we want to insulate the RMH and store as much heat as possible.

I am currently leaning towards a bell design, as a bench is just more surface area to lose heat.  How to pack as much mass around that bell, how to insulate that mass and then how to safely exchange the heat in that mass to water as still very much work in progress

Good luck and keep posting your finding on the forum please as it sounds like we are working towards a similar goal.

Hi Dan, please post your findings as well.

I have quite a few storage heater bricks which are I believe are rated for 250c, that might be enough heat for a short hot burn, used in combination with sand or other material.

Safe water heating is a bigger issue with that model.

I had thought about running coiled pipes through a larger mass of open vented water, but my feeling is it would be inefficient and act as a giant heat sink.

Thanks
Jon

Jon, a heat sink is *exactly* what you want here. A body of thermal mass that absorbs as much heat as possible from your burning wood, and in so doing becomes warmer than the temperature in the building. That makes it a heat source, and, thanks to the way heat always flows from a source to a sink, it gives off that excess heat to the room and its occupants.

Whether you have a masonry bell, a cob bench, a box filled with pebbles, or a tank of water, the differences are matters of logistics, construction decisions, and tradeoffs. They are all heat sinks to the combustion gases of the RMH, and heat sources to the space that they are installed in (until you get to summer, but that's a feature and not a bug).

Yes, I understand that we are always creating a heat sink. But all the designs I have seen so far gradually release that heat into a space. The larger the mass, the longer it takes to heat but also the longer it releases heat into the space.   My point was that I am trying to minimize this, as I am not wanting any of the heat from the mass to go into the space, but to be available when I need it.  A heat battery if you will, not a new term I know.  This is still built around a RMH and a heat sink but the difference is that the sink is designed to be insulated rather than to radiate heat into a space.

So I think the type of heat sink does matter, as you want to minimize external surface area. So I think a bell design naturally suits this better. That said if you want a big mass, then it has to be above the RMH, which I guess can be quite an engineering challenge.  The bench, although more area to insulate has the advantage of being easier to construct on the ground.  There is also the issue of where you capture your heat from the RMH, directly from above the main insulated riser, or from the longer chimney like the one is a bench, or both.  Then it also needs to be considered if any areas being insulated will effect the draw or the efficiency of the burn.

This is all new to me and I don't have the answers yet.  I am still working on a design for what I want to do.  As this is such active community with so many interesting idea already tried and shared, I am assuming that this has probably been done already, or at least similar things. So I am trying to find as many as possible and learn as much as I can before I start building.

Phil Stevens wrote:Jon, a heat sink is *exactly* what you want here. A body of thermal mass that absorbs as much heat as possible from your burning wood, and in so doing becomes warmer than the temperature in the building.

In my situation I would prefer not to shock the thermal mass or allow it to seep out, but give it a chance to heat water for a longer time, as it won't be in a building.   That way I can do short high temp burns and heat a high temp thermal bank to continue to heat the water after the fire is out.

I think alot of the principals you mentioned still apply, just the slow release of heat to water rather than a building.

Thanks
Jon

@Dan Pagan

Have you also thought about controling the temperature of the water?   I think boiling the water and it turning to steam will have a big energy transfer and waste.  Where as heating to say 70-80c would probably be plenty.   But doing this over time with decreasing heat available in the heat bank is tricky.

I'm probably over thinking it and it looks like you are further forward than me in planning.

Jon

You don't state WHY you need to heat the water and having it continue to heat for a long time ?
If we understood the final use it might be easier to give advice

just a few things off the top of my head

Sand doesn't "accept heat" or give it up very quickly = it is not a good conductor of heat.
sand has a large amount of AIR between the particles. You can get a"feel" for this by putting 1 cup of sand into a kitchen measuring jug and then seeing how much water you can also get in there until the water level reaches the top of the sand.
Air is a good insulator especially when trapped in small spaces = foam type insulation and rock wool etc get their insulation from trapped gases or air respectively.
Think of a solid like a rock (which conducts heat well) now mash it up into sand and it becomes an insulator.
Another example: if you have ever tried to walk bare foot on sand during a hot sunny day your feet will get burnt. Flick off the top 1/2 inch and its not even uncomfortable = poor conduction

why not just capture the heat from the fire into the water and keep it simple (KISS) principle. Water stores allot of heat (high specific heat) then you have a compact store of heat you can use anyway you like to over time for any purpose. water filled radiators on walls for radiant and convective heating, water to air radiator with fan to heat air etc etc

@Jon Norton
It's all just ideas at the moment, but I am thinking to have several elements. The rocket stove, the insulated heat battery, a heat exchanger and a water tank.  My idea is to heat the mass of the heat battery to several hundred degrees and then control the release of that heat energy to the heat exchanger to keep the water tank at the desired temperature. The mass is a better place to store the heat than the water as it can be more compact, and therefore easier to insulate... with the added benefit of not turning to steam and exploding.  The water tank will also be an open system as well in case something goes wrong and too much heat is transferred into the water tank.

@Stephen Greenwood
They are excellent points, but I am reading so much conflicting information when it comes to sand as a heat store. I understand your points, but in your example I think heat rising, reflecting and radiating away might have more to do with the depth at which the heat transfers. The one thing I can shake though is why the worlds first commercial heat battery is being made using sand.
https://polarnightenergy.fi/news/2022/7/5/the-first-commercial-sand-based-thermal-energy-storage-in-the-world-is-in-operation-bbc-news-visited-polar-night-energy
You can even see some of the piles of spare sand in the article, and it's just regular sand.  I remember reading somewhere that the type of sand, even really fine kiln sand didn't effect performance much. I can't believe that they haven't done their research for the material to use as their heat mass.  I'm sure costs can be a factor, and maybe a mass of solid rock would be more efficient, but impractical, but I think there is more to this, perhaps to do with the higher temperatures involved.  Since yours and other comments about sand on here, I have been planning to try using a mix of sand and stones to see how that works. At the moment, I am still building the rocket stove section, so I have a long way to go before I can test heat battery materials.  

Thanks again for your comments and sorry for the tardy reply. Life got in the way again.

When you get into tens or hundreds of tons, cost gets important. Also, sand would be easily removed and replaced if necessary for maintenance, moreso than other materials aside from water.

Yep, I agree about the cost savings at scale, and the physical practicalities and benefits of using sand for such a large project.  My point was more that sand can't be a 'bad' material to store heat in.  It must work to at least some degree. Stone or cob might be more efficient, but by how much.  Some of the posts and info I have seen online seem to suggest that sand is a bad idea, and that it won't really work.  Others that it is the best thing since sliced bread and that you'd be mad to use anything else.  I'm stuck in the middle struggling to reconcile these two seemingly opposing points of view :/

As mentioned I'm not sure how accurately I'll be able to measure it, but I plan to test between using sand, stones, and a sand and stone mix. On the small scale I am working on it's relatively easy to remove and replace both of those. Cob, not so much, so I'll leave that one for now

If you already have some form of heat source then you could do a simple experiment for yourself.
Perhaps a loaf tin filled with sand Vs a fire brick heated on top of a wood burner or even in an oven.

There are so many analogies  to be made but using said loaf tin could be one….. if you fill the tin to the brim with sand and then add water, you may be surprised how much water it takes to fill all the air voids.
If you fill the same tin with concrete it will still absorb some water over a period of time but very little.

I think you need to do some form of practical test to find out for yourself (and for us to know).

I'm not sure if the oven if the best test, as my hunch is that highers temperature will have an effect.  This is just a hunch though, and doesn't really hold with the previous example I shared, as in that case I think heat was put into the system using a heat pump.  I don't know if it was air or ground source and at what temperatures though.   As you say, the way to be sure is to do tests. So I will, and I'll report back here once I have.

Good idea, I might of missed the bit saying how you are going to heat the sand above oven temperatures?
Most benches or bells wont get that hot at all, directly above a batchbox riser can reach 1000f but only in a very restricted area.