Rocket Mass Heater for college project (will determine whether I fail or pass!!)

I am trying to design a RMH for a graded project in college, I have had 18 weeks to complete this and am now half way through my schedule. I have not been able to come up with a suitable design for the actual RMH and am wondering what will be best, I am trying to produce enough energy (heat) to allow a stirling engine to run efficiently, this will be connected to a bank of deep cycle batteries which in turn will be running a small house's electrical system, (lights and sockets). The heater will preferably have an oven and a cooker top as well. There might be some thermodynamic couplings attached to provide that little bit more energy if this is possible. The fuel available is wood which can be chopped to smaller pieces. Please can anyone help me out, I have been using this website quite a bit and find the information very helpful and love the ideas that go around, but have not been able to find anything that is similar to what I am looking for with my RMH.

What kind of heat differential is required to run the sterling "efficiently"? I'd wager that what you want to do is realatively easy to do with an L-rocket stove. You want to have an insulated riser to completely combust the woodsmoke, and then slam the resulting jet into the hot side of the sterling's heat exchanger, then probably just vent the exhaust to the side. Use thick metal on the baffle that the jet hits, it will get red hot. Your cold side would be fine with a water barrel, or flowing water heat exchanger. It might prove difficult to charge a battery bank, though; as a sterling isn't exactly a particularly efficient prime mover and a rocket stove doesn't actually produce a huge amount of power (as a BTU rating); but is particularly well suited to efficiency and effective heat storage. A sterling is not going to work well at all off of the stored heat in the mass.

It sounds like you're trying to do 5 or 6 different things, and they may not work in harmony. Do you need to build a working RMH or just submit plans for one? I hope it's the former, because otherwise it will be tough to grade. In either case, I think you'd be wise to ask and answer the question, "Why am I using this?" for every material, every shape, and every dimension in your project. Those are the questions I'm struggling with - not for credit, just warmth.

I on,y need to submit plans for the heater, but my uncle and I are wanting to build one at a later stage, (not for college). The Stirling engine does not have to work I just need reasons why it would be a good idea and then why it won't work, that will be showing that I have done research. I have looked at Stirling engines but as I don't know the average power output of a RMH. If there is only enough heat to warm the room and bench then that would be fine but it would be brilliant if I could get the battery bank charged with the heat given. Would there be any other bit of equipment I could use to change heat to electricity and that will be efficient??

Caryn-Ann Langley wrote:I on,y need to submit plans for the heater, but my uncle and I are wanting to build one at a later stage, (not for college). The Stirling engine does not have to work I just need reasons why it would be a good idea and then why it won't work, that will be showing that I have done research. I have looked at Stirling engines but as I don't know the average power output of a RMH. If there is only enough heat to warm the room and bench then that would be fine but it would be brilliant if I could get the battery bank charged with the heat given. Would there be any other bit of equipment I could use to change heat to electricity and that will be efficient??

Don't expect to be able to do both functions at once effectively with a stirling.

If you want to convert heat into energy via a sterling engine, then you want to build a rocket stove, not a rocket mass heater. The intention of a RMH is to extract the heat into the cob/stone/mass/etc and you want to extract it into the sterling engine. These two are opposing needs and I don't think you will get a good design if you try to accomplish both.

Caryn-Ann L. : I agree with Tom O. Whole heartedly, Not only are you trying to re-invent the wheel, ( why do college Profs love to set their students up like this ? )
in this case you are trying to re-invent the wrong wheel. Go with the Rocket Stove. In case you need it go to rocketstoves.com to download a PDF copy of Ianto
Evans' great book, ' Rocket Mass Heaters ' Tho $15.oo U.S., you will want it again when you and your Uncle decide to build one ! For your Rocket stove you will
need pages 76, and 77 , If you have committed yourself to building a mass heater type Rocket, Go to ernieanderica.info, pick out a model and tell your Uncle
how much you will love him if he can float you a loan ! Kick your Prof in The shin ! GOOD LUCK, Be safe,Keep Warm, PYRO Magically - Big Al

hmm, have you ever though of maybe combining the two so that you can somehow get enough energy out to produce heat and electricity? A stirling engine may not be the best way to change heat to electricity but maybe there is another more efficient way? I was thinking of having the stove on one side and the heater part next to it, any left over heat from the stove would flow through to the heater and you would just have to have a small amount of fuel to increase the heat to the RMH, I may be going very wrong here, but this is what my head is saying? haha

RMH need to be very finely balanced to get the right amount of draft through the burn chamber. By attempting to extract enough heat at the top of the burn barrel to run a sterling engine, or any other system to convert the heat to electricity, would degrade the flow of air enough that I would think you would have a hard time drafting the air through the rest of the system. I remember there being discussions about people trying to incorporate water heating systems into the barrel, and there always seemed to be issues with too much heat being extracted. I'm not saying that it can't be done, just that based on my understanding of previous attempts to do similar things, your idea might not work.

The only way you would know for sure though, is to build it. In 18 weeks you should be able to build many test stoves. Use a large pot of water in place of the sterling engine. If you know the starting temperature and the end temperature of the water, you should be able to calculate the energy extracted from the stove and calculate theoretically how well it could power a sterling engine. The trick will be to see if you can get the stove to burn clean and still have enough draft to pull air through the whole system to heat up a mass after extracting all that energy at the point of the barrel.

Keep us posted on your results!