So I created a post Why Rocket Stoves are so efficient. That was not the original title but since someone (moderator?) gave it that name, I felt compelled to explore the entire subject more completely. I had no idea it would become the behemoth (massive word wall) that it became. I kinda made it a 6 chapter post. First chapter being the original post and the 6th chapter being a summary. And everywhere in-between it is full of "sciencey" and "mathy" stuff (if you like that sort of thing).
I just finished the 6th chapter and realized, most people will never read the conclusions of the study (because it is such a word wall). So In the name of efficiency I am offering this post so people do not have to read a bunch of stuff to get the salient points.
This post contains the sections "Executive Summary" and "The Numbers" from chapter 6 of that comment stream. Feel free to go over to the original post to see all the support to this post.
The main topic of this discussion is "efficiency" and the perception of it for heating systems. I believe, with my limited research skills, enough anecdotal data and scientific research exists to "prove" or at least empirically define what efficiency actually means in this case and what people have mistakenly think it means.
"Design efficiency" is defined as the efficiency of the wood burning device to turn the chemical energy in the wood fuel into heat energy. "Transport Efficiency" is defined as the ability of the wood burning device to transport the heat generated in the device, to the room which it is heating. "System Efficiency" is the total of or combination (product) of the two other efficiencies (SE = DE X TE). In my estimation, wood burning manufacturers specify the "design efficiency" and people perceive that at the "system efficiency". This causes large confusion and "heated" debates (pun intended) based on misinformation (or incomplete information).
While that may be the case, the estimates of efficiency may also be flawed by the testing instrumentation or procedures causing major inaccuracies (uncertainties). Also, "design" efficiency is usually compared to what theoretical limit of the design, not the theoretical limit of the energy stored in the wood fuel. Likewise, most people embroiled in this type of argument fail to analyse the wood fuel and waste of the wood burning device and ignore this portion of the efficiency calculation. This wood fuel does not magically appear next to the wood burning device and the ash/waste does not magically disappear from the device.
My limited knowledge of thermodynamics and the barriers the wood device industry places on transparency have hindered some of these effort but I feel I have made a reasonable effort for the time constraint (do not want to do more) and limited resources (I have never even touched a rocket stove). I am reasonably assured, without further investigation, that a "rocket" - "mass" stove (compare to wood or pellet) IS the most efficient heating alternative of the three. That is, from the standpoint of conversion of chemical energy into heat energy filling a room. Likewise, it is my opinion that it also is the most efficient from a fuel gathering and waste minimization perspective in the parameters of labor, transport fuel, garbage created and money spent. While I did not do a complete analysis, I believe the initial creation and end of life environmental considerations also point to the "rocket" - "mass" stove being the most advantageous.
I compared the three systems and gave a rank for many factors for the "wood stove", the "pellet stove" and the "rocket-mass stove". I submit Figure 1 as a table of that ranking. Likewise, I tried to come up with estimate of actual numbers corresponding to the issues raised in the Executive Summary. I submit Figure 2 as a table of those number. I would like to point out, these are best efforts numbers based on the investigations of the previous chapters. In the end, these are just (educated) guesses. I assume in the first line of "design" efficiency the numbers are compared to 8,600 BTU/lb for the wood fuel.
Notice, I do give a "system efficiency" estimate for the three. I would like someone else to provide some rigger into verifying some of these estimates. Since these are guesses, I do not want people to state the number to others as if they were gospel. If there is at least a little consensus with the people that work with the stuff, that would be good.
Thank you for your attention.
Permaculture People - Good; Evil People - Bad; Evil Permaculture People - Trolls.
Thank you for the information and time spent,
My two cents, i see the word "efficiency" comparing sci & fi of woods burn capabilities and guesstimations concerning any wood burning designs other than the rocket design and think garbage...........
My understanding to get wood burning it has to be heated to a high enough temp that the wood starts giving off vapors, they eventually ignite and that is what starts the cellulose to burn, the higher temp's obtained due to the rocket design causes an entrapment of heat allowing for complete combustion of all vapors and solids that are mostly lost to all wood burning designs except for the rocket design, my conclusions are based on the terms "crown fires", moisture content, creosote, and oxygen levels
Has anyone mentioned air flow, BTU = CFM x 1.08 x delta Temp. Does a rocket stove have a good draw? and therefore an increase in concentrated oxygen levels in a heated space? Now ad the term wood tars to the conversation.
Thoughts to ponder, Only a rocket stove owner would know for sure.
Air flow is best limited to a bit more than needed for theoretically perfect combustion, as mixing is never perfect. More than this amount of air will start to decrease the combustion temperature by dilution with cooling air, therefore tending to lower combustion effectiveness. Wood tars and other volatiles are all burned in a well-built RMH. The RMH gets its combustion efficiency from a combination of insulation to keep the gases hot while burning, and elbows and other features to ensure good turbulent mixing so all the fuel gases meet oxygen.