Matthias Rascop

wrong post - didnt find he delete button, yet!

1) For passive convection of water you need 2 things: a) Vertical difference in height of of some meters. b) Some serious cross sections (at least 3 inches width) of the piping, two have the water moved by the minimal differences in pressure. You don't need that much flow, because the heat up of 1 liter per minute from 140F to 200F means ca 2,5kW of power. The colder the water gets at the low end, the better for the convection. Generally its possible, but I'm not sure if it can be done with tinkering means...

2) You can basically take every pump you find, because you dont need much power. If you use a sterling engine and a car alternator, possible pumps would be out of the computer-watercooling or aquarium area. Most of them run on 12V, are quiet, efficient, have sufficient flow, and some are capable of continuous duty. I'm not sure, if they are available in the States, but one really good thing for that would be the "Laing vario D5" Its not really cheap, but it'll run, and run, and run...

3 more things:

1) If you get heat riser temperatures of >2500F, maybe it would be be useful to reduce the heat riser insulation a little. Thus, more heat is conducted away, lowering the material stresses, while the temperatures of 1800F still do it to ensure a complete combustion. Has someone ever tried that?

2) I just did some "coaster-calculations" (some fast assessment) on the power of a Rocket stove. A 10''-System would be able to reach 40 kilowatts. With a 1meter heatriser, 2200F Tmax, 77F room temperature. It surely somewhat idealized, but is that halfway realistic? To get it more precise it would take me much longer...
1. Volume flow equation from Stack effect
2. Ideal gas law (gas is still of nitrogen majority) to get the density of air at 2200F
1+2 => mass flow through the stove
3. "heat capacity of air" * "mass flow" * deltaT = ~40 kilowatts... If thats true, who personnally needs more?

/edit:
3) I've somewhat surprisingly found the rocket stove book in a library. It`s the 2006 edition. Are there any major changes since then?

Well, nearly every steel is molten at 2750F (i know no "steel" that isn't) and subject to fast oxidation even significantly below that. One of the best materials sometimes available on the consumer market (2.4605 or "Alloy 59") is resistant to scaling up to ~2200F, and this is a LOT! Wouldn't expect the wall temperatures higher than that, even if the gas is hotter, because the metal looses energy by conduction. Normal stainless steel (1.4301) starts scaling somewhere above 1100F, non-stainless steel much below. Very slow at first, but roughly doubling corrosion speed every 20F. Thats just a rule of thumb, to get the picture... exponential development is not our friend, again...

So its not very remarkable to destroy even a stainless steel-heatriser in a matter of hours, if you really push it. This adds energy to the combustion, but it cannot go that high. If it scales, theres oxygen left, which means, its not a perfect combustion for maximum temperatures. If you could reach the melting-temperature of steel, it would be quite awesome with such a design and fuel!

a 10 inch system is going to burn at around 4500 degrees in the hot spot of the heat riser on a regular fuel load.

What fuel and what oxidizer? I don't think you use pure oxygen, do you? Because 4500F is roughly, what you could possibly expect from that... hellfire...

Wood and air are strictly limited to ~3600F => adiabatic flame temperature
The value is the upper limit, because you can't reach it in a real setup.

Because Sterling Engines don't care, where the heat comes from, it should generally work. To get the hot end of the machine directly on the heat riser, the stove needs some redesign, but why not?

Your plans to charge a battery bank suggests, that you plan an off-grid solution. This is hard to accomplish with a Sterling-Engine and one- or three-phased AC with tinkering means. You need an exact rpm-control, because most of your machines expect a certain grid-frequency (50Hz or 60Hz). Sterling engines react very slow on regulations, so you need a synchronous generator with a seperatly controlled excitement. Quite a challenge, I think!

The cheapest method in my mind to get usable electricity out of a sterling engine off the grid would be an off-the-shelf car-alternator/charge-controller-combination. This is fed directly to your battery. From there on, you can feed every DC-capable device directly and use an invertor for your AC-needs. You dont need to care for rpms or any home built control.

The efficiency is quite low, though... lets assume 10% of the heat, transferred to the engine is electricity afterwards.

/edit: found some interesting source of information for a Stirling motor, but its a little too small for a car alternator.
http://ve-ingenieure.de/projekt_st05g_cnc_engl.html

Actually, it does really good, but as always when you get new knowledge: More Questions pop up than are answered

I want to build a house in around half a year to a year. Building a house is a complete different philosophy in the states as it is in germany. I realized that in my terms in Miami. Here its more like you try to build it for all time being, and live there to the end of your days, and I'm quite comfortable with that thought. Following this way, I want to design all the installations to last a long time. It just doesn't fit in there, to build a stove with known issues remaining. But I expect those issues to be easily solvable by picking the right materials. Therefore I'd like to ask you about some more details:

*Which flue material did you use. In the videos it looked like standard stainless steel (1.4301 or x5CrNi1810). Did this material corrode? If yes, the problems are bigger than thought, but I dont really expect that. Its common flue material for oil and gas fired stoves.
*Did it corrode uniformly all over the flue, or is it worse in some parts than on others? Beginning? End?

I'll build the heat riser either from a highly heat resistant steel tube or chamotte. Whatever is available cheaper. I buy the stuff needed, and will not depend that much on what I find by chance.

at cob:
Although cob as a building material has a long tradition, even today in rural germany ( timber framing => german "Fachwerkhäuser"), I'm not sure, if it will be part of my house, especially if not plastered over. I just don't know it from first hand living in such a house. I'm especially suspicious to the cob-bench, because I would expect it, to be easily abrased. Basically its just dried mud. Thus I'd expect having the dust of it everywhere, at the clothing, in the cushions, the floor, etc. pp... Maybe thats just prejudical, but I wouldn't take the risk, until convinced otherwise. Additionally i want to mate the stove with a water heat exchanger, to support the central heating and to use the heat everywhere in the house. Therefore my thermal battery will be a hot water tank.

Thanks for the answers! I'll look at the links today...

I just didn't see any "maintenance hatches" (don't know if the word fits here, I'm no english native) on the concept drawings. Even the cleanest combustion needs some cleaning every now and then.

I try to stick with the engineers view of the concept, because thats where I'm good at.

at oxidation:
The good material for the heat riser would definitly be firebrick. Especially if you try to improve the burning temps, like described further on. You can buy round chamotte tubing here, so... this is perfect for keeping the turbulences. Not everybody has access to highly heat resistant materials.

at condensed water:
Some part of the RMH-concept is the improved efficiency to regular wood stoves. To push this advantage further on, you need to optimize the process. You want the highest possible burning temps, and the lowest possible exhaust temps. Both goals increase the water condensing problem. This means on the reverse, if you don't have condensing water in your flue, there is room for efficiency improvements...

To get the highest possible burning temps, you need the exact amount of air for the burning wood. Because the fire is sometimes burning more intense than on other times, some kind of air regulation is generally desirable. Theoretically the heat riser should do some of that task on its own, without human interference, and maybe it can be designed to do the job really good. But I guess that would be more complex, than slightly oversize the heat riser and integrating a lambda sensor and a control system, because thats easy. But maybe some of you wouldn't want such technology in a RMH.

Flame temperature of 2000F are definitly reachable. Theoretical Limit is the adiabatic flame temperature of wood and air at ~3600F so there is room for improvement (but be aware of the NOxes)...

If you have managed a perfect combustion (complete combustion and very few excess oxygen in the exhaust gases, no NOxes), even when you have well dried wood at 15 mass% water, if you drop your exhaust below ~60C/140F, water will condense AND stick to the flue piping. At the beginning of the firering, when your thermal mass is cold, I guess exhaust temps will be significantly below that temperature. Thats just theoretical thinking. I've never seen a RMH in reality. Please correct me, if I'm wrong! If thats true you are building up amounts of water within the flue piping, during burning. Thats wanted for efficiency reasons, but needs to be drained, to keep the piping clean. One easy way would be, to lead the last third of the piping in the thermal mass on a downward angle into the exhaust, where it can drip out.

The thoughts need to buzz around my head some more, and then I try to build a RMH on my own... at first in the garden...

Hi everyone,

I stumbled over this concept for the first time last week, and it keeps coming to my mind... Its an extraordinary clever idea! I'm an engineer, and quite used to technical combustion solutions, but this is new to me. I didn't believe it at first and did some calculations of the heat riser draw... and well... I'm convinced... I'm quite sure, that this concept does not comply to the building rules, where I live (germany), but I'm maybe willing to try anyway...

But I have some questions left:

1. What about the condensed water in the mass heater piping? There has to be quite a lot of water, depending on the watercontents of the wood and the end-temperature of the exhaust. Is this removed somewhere?

2. How do you get the ashes out? Isn't there any ashes pulled into the mass heater piping? How do you get that out?

3. Does the barrel or the heat riser oxidize on the interior, due to the high heat? Whenever I see oil barrels, they are heavily rusty, and thus made of quite reactive steel.

Greetings!
Matthias