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Efficiency question again - care to help me summarize?  RSS feed

 
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Efficiency is such a difficult word... everybody thinks they know what it means. Very few people have a sense of what's involved in an apples-to-apples comparison of efficiency across different technologies.


I'm working on an article here:
http://www.ernieanderica.info/rocketstoves/rmh-efficiency

to answer the same old comments posed after this article here:
http://www.treehugger.com/clean-technology/learn-absurdly-efficient-and-ridiculously-cheap-method-heating-home.html

The author loves rocket mass heaters, but the first and several following comments are from people who seem sold on very conventional, and misleading, ideas about the differences between wood heat and other options.
Which, if you are following the fracking and peak oil scenarios at all, or even if you just look at the CO2 emissions numbers, is dangerous mis-information.


I would love your comments, and suggestions of images or citations to make the article better.

Or, if you're really a science geek, I'd love to see a better workup of the carbon footprints (including externalized costs if possible) of average American lifestyle choices. Happy to give credit.

Anybody know if xkcd.com already did one? I love his stuff.

Here's the text of the article:



RMH-Efficiency

We often get asked to produce numbers to verify the efficiency of rocket mass heaters.
People hear someone like Paul Wheaton say they use 1/10 the wood of a conventional woodstove, and they say, But modern woodstoves are rated at 75% efficient, up to 90% efficient, how can you possibly be claiming these are 750% efficient? That's not possible.

Well, let's discuss what "efficient" means in this context, and why rocket mass heaters seem to deliver more efficient performance in use compared to typical woodstoves.

This article is sponsored by my deep and abiding interest in the success of two 2015 Kickstarter projects:
- Paul Wheaton's new, improved 4-DVD set: http://kck.st/1JaD8a1
- our long-awaited Rocket Mass Heater Builder's Guide: https://www.kickstarter.com/projects/rmhbuildersguide

A) Rocket mass heaters have been tested privately with the same equipment used for woodstoves. Results have been posted on permies.com and proboards.com, among other places. Most models are at least as efficient as the best modern woodstoves. Some are cleaner and more efficient than pellet stoves or natural gas furnaces (see graph below - the pink line is CO, the bad stuff, and the red line is efficiency between 87% and 95%).
Some prototypes suck, of course, and we walk away from those. Also note that instead of burning refined fuels that have added costs to extract, manufacture, and transport, these are burning only local, renewable cordwood.
Peter's Batch Box, heating a 2000-sf shop space with CO emissions lower than a candle.

The EPA does not test stoves for efficiency. Their required tests focus on emissions, and the woodstoves' hot burn rates, to determine safe clearances. Most users confuse the EPA or UL rating sticker with actual independent lab tests- my understanding is that the EPA gives a generic efficiency rating by type, but doesn't test this independently for each model.

Manufacturers may publish their own test results in product data. They are allowed to take off a factor for the legal minimum exhaust temperature, 350 F, so a light-weight woodstove that was rated 100% efficient would actually be about 85% efficient if you considered that it still emits the legal minimum of 350F constantly. So a 75% efficient stove would really be 63% efficient.... This is still pretty good considering most automobile engines are lucky to get 40% efficiency converting heat into mechanical movement.
But heat-t0-heat efficiency is much easier than mechanical or energy transformation. The results should be better for straight combustion heat delivery than for multiple-transfer process, like mining and transporting and burning coal to make steam to make electricity in successive voltage levels to transmit across hundreds of miles of wire to run through a resistor... to make heat.

Given that heat is the final end state of almost all physical energy processes, the real questions are why are we not getting more heat for our efforts. And that mostly comes down to the design of the home, and to trying to compare numbers like square feet, BTUs, or thermostat temperatures with actual delivered comfort. The more space we put between ourselves and the source of life-sustaining warmth, the more of it is wasted before it ever reaches us.

A bigger problem is that real woodstove owners don't like letting the fire burn out at a faster, cleaner pace, and will often scandalize a stove to make it smolder all night, resulting in much worse fuel efficiency in practice than was suggested by the lab tests.

B) The rocket mass heater burns for only a few hours per day, so any excess air is only an issue for 4 hours in 24, at about 200 F. Compare sending smoke (unburned fuel) up a woodstove chimney 24/7, above the legal minimum of 350 F the whole time. Compare 4 hours of chimney draft with NO heat production if the woodstove goes out at 3 AM. Some rockets have tighter air controls than others; most experienced operators constrict the air to the specific percentage that makes their stove run best. But in most stove models, too small an air intake runs the risk that it could be clogged by fuel or ash, and suddenly you go from optimal performance to smoldering creosote factory.
In the balance between allowing a little excess air to ensure complete combustion of your fire wood, and trying to hold on to all that warm air for the full 3 hours it could otherwise spend in the house... this is not a win-win situation. The less time you have to make this tradeoff, the better. Run the fire briefly, shut it down when not in use.

C) Compare the heat capacity of air vs. brick. Losing some warm air is a negligible problem when thermal mass maintains warmth for days. We still have 2 feet of snow outside, and we are only running our heater every other day.
To have good indoor air quality, avoid structural rot, black mold, depleted oxygen, and other toxic situations, most modern homes must be ventilated to ensure a minimum air exchange of 1/3 their volume per hour. Living in a zip-loc bag just is not a good idea. As Paul summarized it after repeatedly asking this question of half-a-dozen different rocket stove experts, "Why would you breathe stale air and farts, and feed the clean air to your stove?" Only if you have gone off down the rabbit-hole of chasing the wind. Air is a poor conductor, that's why most insulation consists of foams or fibers that trap lots of air. It's hard to force heat into air, and even when it's at a high temperature, it doesn't actually store or deliver much heat. Trying to store heat in a bag of air is a wasteful exercise compared to using dense thermal mass like brick or stone or hot water: something that has the heat capacity and conductivity to actually build up a thermal storage worth protecting.
Imagine you are about 6 years old and just came in from the cold. What's going to warm you up faster, asking your mother to blow on your hands, or some time in her lap with a hot mug of cocoa?

D) The real question is, what kind of efficiency can a heater deliver in practice? Is it comfortable? Does it get the heat to the people who need it, rather than up into empty space?

From the results we've observed in the field, thermal mass heating works awesome. It is compatible with passive-solar design (which I consider the ultimate in efficiency, as it's literally zero fuel usage). It provides all-night warmth without the dangers of unattended fire, or the inconvenience of all-night fire tending. It provides super-efficient and comfortable contact warmth, like a full-body heating pad or heated seats. Ever notice the difference in comfort between a car with heated seats and one that just blows hot air in your face while you drive? On long car trips, if I can get my toes warm and keep my head cool, I stay awake much easier, to the benefit of everyone on the road. Sitting on the rocket bench is kinda like a mini-sauna for whoever wants it, available at all times, no need to change clothes or argue over the thermostat.
All these add up to delivering comfort directly to people, where they need it most - warm butts, a safe and warm night's sleep, warm toes in the morning. It stops mattering what the thermostat says, if the cats and puppy are all stretched out, you know it's warm enough inside.

In some climates, the mass heater completely eliminates heating and cooling bills for several months of the year. Over the remaining part of the year when heat is needed, it's common to see owners use about 1/4 the wood (8 cords down to 2 cords). Getting to 1/10 the fuel is not unusual in coastal and mild climates, where the heater can 'coast' more months: users go from 4 to 5 cords down to 1/3 to 1/2 a cord. Where the home was already storing the woodstove's heat with thermal mass, or an efficient passive-solar design, they'd be using less energy already, so the rocket mass heater results may be more like 1/2 the former fuel usage.

I've never seen anyone use anywhere near the same fuel in their rocket mass heater, compared with a previous wood-burning stove or insert. Or spend a fraction of the money on cordwood they used to spend on gas. If you already have a masonry heater, I would probably not switch. If you can do passive solar, or triple your home's insulation, go for it. But if it's just comparing the heating unit itself, woodstove or natural gas vs. rocket mass heater, the benefits are huge.

NUMBERS GAMES:
I've asked Randall of xkcd.com if he will do some carbon emissions comparison of lifestyle changes a person could make, like riding a bike instead of driving, or switching from natural gas heat to rocket mass heater heat. From the preliminary numbers we've worked out, just grabbing statistics off the Interwebs, it might look something like this:

Carbon footprint of heating with natural gas: 55 tons per year
Carbon footprint of heating with conventional wood heat: 10 to 12 tons per year
Carbon footprint of heating with a rocket mass heater: 1 to 5 tons per year*
*in predominately small, efficient homes, so let's use the 5-ton figure for fairness.

Carbon footprint of driving a typical modern car (23 to 28 mpg) 12,000 miles per year: 3 to 4 tons per year.

This means that switching from natural gas to a rocket mass heater could save as much carbon emissions as if 12 people completely stopped driving their car or truck. If the user was able to place the rocket mass heater in a conventional home as part of a conversion to passive solar (for example, just the right distance back from a south-facing picture window), they could get the same savings as if 23 people stopped driving their cars.

There are also substantial concerns about resource extraction, contamination of groundwater during extraction and transport, and losses along the utility grids that mean up to 2/3 of our fossil energy is simply wasted on the way to our door.

In terms of lifestyle changes, using a quieter and more comfortable form of heat could just be the biggest single thing you could do to save the world from climate change, resource wars, and preserve clean air and water for future generations.

What would happen if a million households switched to rocket mass heaters? If they stopped using natural gas, electric, or oil-based heat, and were able to use super-efficient, renewable, low-input local biofuels instead? What if burning wood really is the cleanest option on the market, and millions of people can do it without releasing smoke or wasting fuel and effort?

In 2015 we are encouraging our readers to support two Kickstarter projects:
- Paul Wheaton's new, improved 4-DVD set: http://kck.st/1JaD8a1
This supports Paul's efforts to promote this technology, including an annual Innovator's Gathering where we continue to work on ways to make these heaters better: easier, more affordable, even cleaner and more reliable, and find compatible ways to use them for a wider range of functions like cooking, hot water, and a wider range of heat outputs for specific needs.

- our own upcoming launch for the long-awaited Rocket Mass Heater Builder's Guide:
https://www.kickstarter.com/projects/rmhbuildersguide
This book will put the current state of the art into the hands of a lot more people, make it simpler for first-time builders to avoid known errors, and for professionals to customize a heater for each new site.
A stretch goal is to use the proceeds from this book to get the ball rolling toward EPA approval, eventually getting the best models out there with that reassuring sticker that your local authorities know and love. (We'd like to test the most popular DIY versions, too, so that these heaters can remain affordable to a much larger number of people.)






 
Erica Wisner
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Here's a good article on a similar topic: housing energy use.
Read it with the idea of how your heat delivery might change with a large warm rocket in the living room (automatic zoning in both time and space) vs. a furnace in one corner of the basement.

http://shrinkthatfootprint.com/shrink-your-housing-footprint
 
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Just a quick typo in "But heat-t0-heat efficiency", small o instead of "zero".
 
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Erica, you could separate the mentions of burning efficiency, and heat recovery efficiency!

Someones attacks you about their favourite stove, burning cleaner than yours. You reply simply, what is the heat recovery efficiency of your stove? Do you know that? Can you give numbers?

People love to have numbers shoved down their throats, keeping thoses for future bragging rights. When you turn their numbers to zilch, they don't like it. I'm exactly into that situation over at free forum 4x4, a french forum i hang out at. The guy i'm argumenting with is a pro pellet furnace fitter. But may be i'll end up making him understand.

I've asked Peter and Matt for numbers.

http://donkey32.proboards.com/thread/1555/peter-matt-combustion-analysis-screenshots

Hth.

Max.

 
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Erica,
Your mentioning of the batch box thingy looks like an island. Yes, I know you think the thing is awsome, I think so too, but the phrase is hanging in mid-air.

Good article though, sitting on a warm bench you'll care less about air temperature, being part of the way to use less fuel. Less convection of warm air and more infra-red radiation is helping too.
 
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For me, the lack of soot or odour plus the visibly undeniable existance of a vigourous fire says very efficient combustion, and the low temperature of the gases leaving the building says excellent heat recovery. In my open fire and every woodburning stove that I've seen in the flesh, I can see smoke, and soot in the chimneys, and I can tell from the heat shimmer over the chimney (outside), that there's a hell of a lot of wasted heat. For me the image of someone with their face ridiculously close to the exit point, with a fire roaring at the other end, speaks volumes!

imho the figures are just bamboozlement. Intended to make normal stoves look way more efficient than they really are. The 15% 'allowance' clinches it. I mean seriously; 100% should be only for genuine 100% not 100% - 15! And indeed totally ingnoring the wasted heat going outside. . .



That 350F minimum thing is confusing me. I suspect it's because it was badly thought out (I'm getting wise to legislation being bs, both here and in US) but surely it should be maximum, not minimum.
 
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The 350 F minimum chimney exit temperature is to prevent the unburned gases from depositing in the chimney as creosote. Even that, of course, is not completely effective, not to mention that there should be no unburned gases wasted up the chimney in the first place
 
Erica Wisner
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Thank you, everybody, for some excellent suggestions!
I'm off to do a little "soul labor" on an organic farm and I'll be back to work on this after. Hope to have an improved version up before the weekend is over.

Any links to similar explanations you've done elsewhere, I'd be delighted to post them with credits. Let me know if you want your citation to be a different name than your Permies.com user name.

Yours,
Erica

 
Glenn Herbert
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I have added a number of clarifying responses to some of the comments on the treehugger article... it may be good for some others to do the same so it doesn't sound like a personal crusade by a few people.
 
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Good article. Just to note the obvious that, in terms of convincing those who are already leaning towards accepting an alternative lifestyle, this will be a much easier sell. For those not thinking about it, efficiency will not matter. It will come down to personal cost and convenience and how that fits into the 8+ hour work-day-religion. Focus on selling the technology to as many who will implement it in the field as possible.....beta testing in as many climates and conditions and dwelling layouts as can be imagined and keeping in touch with as many installations as you can to assist in working out bugs. Ultimately, their successes will sell others on the idea. In particular, as it relates to whole dwelling "home economy", things like re-routing plumbing (cold climates) since there is no forced-air heat, re-thinking room divisions, etc. to maximize use of heat, would be really good to include.

"What's going to warm you up faster, asking your mother to blow on your hands, or some time in her lap with a hot mug of cocoa?"

Well now....I can tell you haven't met my mother! I'll be in the bathroom warming them under a stream of tepid water.....
 
Erica Wisner
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Alan Lamborn wrote:Just a quick typo in "But heat-t0-heat efficiency", small o instead of "zero".


Thanks, found it, fixed it.
Probably installed a few more in the process...

-E
 
Erica Wisner
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Peter van den Berg wrote:Erica,
Your mentioning of the batch box thingy looks like an island. Yes, I know you think the thing is awsome, I think so too, but the phrase is hanging in mid-air.

Good article though, sitting on a warm bench you'll care less about air temperature, being part of the way to use less fuel. Less convection of warm air and more infra-red radiation is helping too.



I think that's the alternative text for one of your graphs that I posted - it looks better on the article, one of the few images I've put there yet.
I suppose I could include "Picture: Graph of..." to make it clearer if someone is reading it through a text to voice program or something.
-Erica
 
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Excellent article Erica. Reading it from the perspective of someone relatively new to rocket mass heaters, perhaps finding their way to the article via a search, link, or etc., begs a few additional details.

Item C, where you write about your rocket mass heater, firing it only every other day or so, with snow on the ground and etc. tends to be somewhat ambiguous. The more astute readers will want to know more details. Maybe add a link to your Cabin8-RMH page, or add a footnote briefly describing RMH size/mass, square feet being heated, dwelling insulation values, climate/zone, weather conditions in terms of daily highs and nightly low temperatures.

Suggested edits:

Item B, "The rocket mass heater burns..., ...at about 200° F. <add here--> "exhaust temperature">

For a more "polished" look, copy-n-paste this [ ° ] degree character to the temperature numbers throughout the article which I've done in the above item B, "200° F".
 
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Erica Wisner wrote:
NUMBERS GAMES:
I've asked Randall of xkcd.com if he will do some carbon emissions comparison of lifestyle changes a person could make, like riding a bike instead of driving, or switching from natural gas heat to rocket mass heater heat. From the preliminary numbers we've worked out, just grabbing statistics off the Interwebs, it might look something like this:

Carbon footprint of heating with natural gas: 55 tons per year
Carbon footprint of heating with conventional wood heat: 10 to 12 tons per year
Carbon footprint of heating with a rocket mass heater: 1 to 5 tons per year*
*in predominately small, efficient homes, so let's use the 5-ton figure for fairness.

Carbon footprint of driving a typical modern car (23 to 28 mpg) 12,000 miles per year: 3 to 4 tons per year.



You might want to double check these numbers. According to the EPA, using a therm of natural gas releases 0.005302 metric tons CO2. Looking at my heating bill for January, in Indiana (low of -8F), for an 1800 square foot house I used 122.28 therms for a grand total of 0.64 metric tons of CO2. February of 2014 (low of -17F), I used about 160 therms. Considering the furnace is only need 4-5 months out of the year, I'm not sure how you would get to 55 tons per year unless you were in a drafty mansion in Maine.
 
Glenn Herbert
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Does the carbon footprint number for natural gas include the energy of extracting and transporting it? These are definite costs, even if they are not obvious to the end user nor easy to calculate in "carbon" terms.
 
Erica Wisner
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John Wolfram wrote:

Erica Wisner wrote:
NUMBERS GAMES:
...


You might want to double check these numbers. According to the EPA, using a therm of natural gas releases 0.005302 metric tons CO2. Looking at my heating bill for January, in Indiana (low of -8F), for an 1800 square foot house I used 122.28 therms for a grand total of 0.64 metric tons of CO2. February of 2014 (low of -17F), I used about 160 therms. Considering the furnace is only need 4-5 months out of the year, I'm not sure how you would get to 55 tons per year unless you were in a drafty mansion in Maine.



Thanks for catching that on both threads. I got the number the same place Cassie did, with bad calcs off a website that, retrospectively, were too bad to be true.
Editing out that entire section, until we can get better numbers....

I would welcome a recommendation for reliable sources that are doing real-world research to find the carbon footprint of things. The easy armchair calculations seem to rely on a ton of assumptions. It's not easy to get accurate data on the total actual impact.

-Erica
 
Erica Wisner
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... and yes, I want it to be easy.
 
Alan Lamborn
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All of you commenting on treehugger are patient saints with your handling of the comments over there! Much more than what I could do.
 
pollinator
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John Wolfram wrote:

Looking at my heating bill for January, in Indiana (low of -8F), for an 1800 square foot house I used 122.28 therms for a grand total of 0.64 metric tons of CO2. February of 2014 (low of -17F), I used about 160 therms. Considering the furnace is only need 4-5 months out of the year, I'm not sure how you would get to 55 tons per year unless you were in a drafty mansion in Maine.



To be fair to drafty old homes, and I've know a lot of them,

I've seen places run through 5 or 6 propane fills in a season no problem. Heating home and water with it. As an experiment to see if I could dry things out at all here several years ago after some near major renovation, I set the thermostat to 66*f Three days later the tank was empty from a reading of +a half or more. No more hot water for Landon. Defiantly never investing in gas heat for my home ever ever again. I like the draft. It keeps things fresh and allows the wood to breath. It's and old home.

Mass , on the other hand, really is good in drafty situations. I know this for a fact. From experience. I've never taken it to the extreme of using a rocket or heating a literal ton, but that seems like the logical conclusion for heating given my circumstances.
 
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