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Post by paulwheaton on Nov 6, 2017 13:11:18 GMT -8
Here is a video of donkey, peter, erica, mud and i responding to something put forward that a rocket mass heater cannot be as efficient as claimed.
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Post by pinhead on Nov 7, 2017 10:47:53 GMT -8
Only thing I would have added to the discussion is the "excess air" calculation and its effect on heat extraction efficiency. 350°F exhaust moving at 1/2 cfm is more efficient than 150°F air moving at 500 cfm (non-calculated for demonstration purposes only). The argument the "engineer" is making is purely mathematical made with assumptions as is mentioned in the video. The first assumption is the "typical" 75% efficiency of a box stove. Donkey mentioned that we don't know what type of efficiency - is it 75% combustion efficiency or 75% heat efficiency? I would argue that 75% combustion efficiency is lousy when compared to a well-built RMH. The engineer's POV is that in order to use 1/10th the amount of wood of a 75% efficient stove would require an efficiency number greater than 100% which is obviously impossible. However, anybody who's ever been around the typical wood stove know that most people (I'd say the VAST majority) don't run the wood stoves optimally - or even close. Closing the damper, closing the primary air, and "smoldering" the fuel over night to get a slow steady "burn" is closer to 5% chemical efficiency than the "optimal" 75% even with a catalyst - and even if it's fairly heat-efficient with low excess air. I will say, though, that a "box stove" can be build to burn efficiently. I believe it was Matthewwalker who competed against a few of them with his batch box a few years ago. This was under "ideal" conditions, however - conditions that were tailored in favor of the box stoves. The entire argument reminds me of the automotive engineers in the '90s telling us it's mathematically impossible to increase the efficiency of the internal combustion engine. After all, modern engines already burned 99% of the fuel at the time. What they didn't take into consideration is the fact that more than 90% of the fuel is/was dissipated through the radiator and exhaust as heat... Much like the typical box stove.
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Post by briank on Nov 7, 2017 12:00:03 GMT -8
I met Mattewwalker at the wood stove decathalon in DC that year. I was there to pick up a Beta version of Woodstock Stoves new Ideal Steel hybrid model, which has both secondary air and a catalytic combustor. That stove model won the Wood Stove Decathalon that year despite competing against some impressive Stoves including masonry stoves. It is 82% efficient, but again I couldn’t fully explain their efficiency standards. That stove was such a joy to Beta test, I bought one of the first production models the following year. Despite that, I still wanted a rocket heater. From their blog: Monday, March 3, 2014 BTU Output: How is it Determined? Several people have recently asked about how BTU output is determined. BTU/hr rating is basically a speed and efficiency rating. The (1) greater the number of POUNDS OF WOOD CONSUMED PER HOUR, and (2) the higher the OVERALL EFFICIENCY, then the HIGHER THE BTU/hr OUTPUT RATING. For example, if we put a 20# load of Douglas Fir at 22.5% moisture (dry basis), then the dry weight is approx 16.3# (20/1.225 = 16.326). At 8,522 BTU/per pound for Douglas Fir, our TOTAL INPUT is 138,909 BTUs for this load (8,522 BTU/# x 16.3 lbs = 138,909). Ideal Steel Hybrid: If the stove consumes the wood in 2 hours at an overall efficiency of 82%, then its overall output is 52,090 BTU/hr [(138,909 input/2hrs)*.82 efficiency = 56,952 BTU/hr output]. Secondary Air Only Stove (EPA Default Efficiency): The same formula at 63% efficiency is 43,756 BTU/hr [138,909/2*62% = 43,756). EPA tests use default efficiency values for wood stoves unless the stove has been tested for efficiency as part of the EPA test procedure. It is possible to get a very high BTU rating with a small firebox, or a very low BTU rating with a large firebox. BTU output doesn’t have much to do with firebox size (though burn time does). As noted above, BTU output has everything to do with (1) Rate of consumption, and (2) Efficiency. Notwithstanding what most manufacturers say, if you use cordwood, BTU output generally goes down, because the stove consumes fewer pounds of wood per hour than with EPA cribs, in part because there is less surface area exposed than with EPA cribs. A manufacturer can make up an efficiency number (or use LHV, or just one burn rate, or just part of one burn rate) but in our experience, the BTU output numbers that EPA reports are usually not far from the truth. When you see BTU/hr numbers that are double or triple the EPA numbers, they are often pure fiction. Most people just put wood in their stoves and are happy if they are warm! But higher efficiency is a good thing as you can see above. As efficiency goes up, you either get more BTU output, or burn less wood, or both. More here: www.popularmechanics.com/home/how-to/a9734/woodstock-soapstone-captures-the-wood-stove-decathlon-grand-prize-16180264/“In an unexpected gesture of goodwill, Morrissey says his company will share the cash prize with two of competition's unsponsored, noncorporate finalists: Jason Stewart, who brought his affordable stove retrofit (IntensiFire) all the way from New Zealand, and Matt [Walker], who drove in from Washington State with his masonry/rocket-stove hybrid (Walker Stove) in the back of his Subaru. Travis donated its $5000 prize to the competition's organizer, the Alliance for Green Heat.”
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Post by pinhead on Nov 7, 2017 12:41:58 GMT -8
...If the stove consumes the wood in 2 hours at an overall efficiency of 82%, then its overall output is 52,090 BTU/hr... This still doesn't answer the major question as to how they came to that 82% number or even what it means. Do they mean the stove will convert 82% of the fuel into heat (with 18% leaving the chimney as emissions)? Or do they mean 82% of the heat produced stays in the living space? Or is it a combination/average of the two? As Donkey mentioned in the video, it's entirely possible to get an old pot-belly stove to burn highly efficiently (high chemical efficiency). Unfortunately most of that heat will go up chimney (low thermal efficiency). As was also mentioned, it doesn't cover the time that heat is available; a box stove will be stone cold shortly after the fire goes out while a masonry stove will continue to heat the living space for much longer. Are both types of stoves' thermal efficiency measured over the same span of time, i.e. during an active burn? In my honest opinion, the only way to get a "true" efficiency number would be to calculate the input BTU's of the fuel vs the exhaust BTU's, paying special attention to the number of BTU's lost as CO and/or "smoke." If a stove has 100,000 BTUs of fuel put in, and 10,000 BTUs leave the chimney (in both terms of heat and emissions), the stove is 90% efficient. Again, measuring/calculating exhaust heat (BTU's) as opposed to temperature is paramount to maintain accuracy; the exhaust mass must be used in the calculation. Using that metric, I suspect the "82% efficient" box stove is closer to 25%, IMHO.
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Post by martinm on Nov 8, 2017 14:20:12 GMT -8
Help needed! we are few "rocket builders" and we will have a meeting with the local government staff in a major city in eastern Europe, presenting them the concept of rocket stoves. The reason for such mission is that there is need for teaching people in poor neighborhoods to use efficient stoves. I need reliable data on emissions of rocket stoves and emissions + efficiency comparison with typical "traditional'' wood burning stoves. Our aim is to start staging workshops and show and build few simplified versions of efficient wood burning stoves (doesnt have to be the typical gravity fed - and barrel on top of heat riser + bench design. The main concern of the local authorities is the polution from the stoves of poor people burning anything in whatever stove they have a hold on. Climate here (Bulgaria) is moderate and some pooor people are entitled for government assistance for heating in form of certain amount of wood for heating. Will appreciate any input. W here or via email : martinmikush@gmail.com. thank you
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Post by Orange on Nov 16, 2017 2:26:37 GMT -8
let's focus on the combustion efficiency. Weather someone keeps the heat inside or lets it out of the chimney is their thing.
Can we have some simple numbers like this:
classic wood heater 65% burn efficiency classic wood heater with secondary air 80% burn efficiency insulated wood heater with secondary air 90% burn efficiency?
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