Now I am not here to argue what the word efficiency means, I am simply talking about capturing 100% of whatever heat is generated through combustion such that the exhaust is cooled down below average room temperature before departing.
Here is how simple it can be.
Use a blower fan to produce a forced draft and draw your exhaust through a series of heat exchangers (could be as simple as 55 gallon steel drums filled with brick pieces or large gravel). Each “container” would be filled with smaller sized pieces and the last one or two would be filled with sand. It is even possible to use wet sand to more quickly and effectively capture the last remaining heat as long as you provide the means of moistening the sand before each burn. BTW . . . the air is cooled before it ever reaches the fan.
Of course there are many things to be considered such as:
• Burn has to be as complete as possible (technique)
• Fuel needs to be as dry(or clean) as possible
• Looping heavier smoke back to burn chamber during early moments of burn would help eliminate most of what little deposits would be generated.
• Generally speaking, the cost to run a fan is far less than the cost to generate heat.
• Since you are capturing all of your heat, a wide open clean burn will not waste heat.
• Draft can be totally controlled by fan speed rather than by opening vents.
• Design must allow for air inlet to close should power to fan go out. ( one or more ultra light weight “doors” that swing open during draft and fall closed when suction stops).
Now this may not work on every combustion heater in the world (gas, oil, wood, pellet, coal . . .) but it would work on a lot of them. If we saved the 10% to 50% of heat that is going up all of those chimneys, the impact would be tremendous. Also, the potential is here to eliminate buying/building an expensive chimney while making the combustible heater safer to operate at a fraction of the cost.
It takes masonry heaters to a whole new level . . .the final level. Cooled down exhaust can be vented almost anywhere outside or even underground. What about heating a greenhouse with a wood stove and then venting almost pure CO2 and H2O vapor back into the house???? What do you think?
Read more outside the box ideas at www.pandscorp.org
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Ellen Page on Perma-culture
{ 10 comments… read them below or add one }
i was with you up until the final sentences, about venting pure CO2 into the house. i prefer my CO2 exhaled, thank you ;-)
but anyway, approaching 100% efficiency is a great goal, though getting the last fractions of a percent presents an economic battle likely not worth fighting. i’ve considered various systems similar to what you describe, with multiple HX stations, low-voltage solar-powered fans and what-not. i may well even prototype such a system to test it out.
if i may, i’d like to add a few thoughts to the mix. these don’t distract from the goal, but rather offer a few additional design considerations.
cleaning. the media that gets the first soot has to be very easy to clean, be highly durable in terms of surviving the thermal expansion and contraction, easily removed, and ideally, easily replaceable. the rest of the system also needs to be able to be scoured regularly to avoid build-up and the risk of fire (i’ve seen chimney fires, and one would be really frightening if it came out the action end of what you thought was a guaranteed cool tube).
size. not everyone has the room for a trio of 55-gal drums in their off-grid cabin. thus the media and containers need to be scalable. of course it still needs to be cheap.
re-circulation. while its effects on combustion could be adjusted to some degree, recirculating exhaust gases into the chamber seems to be a tricky proposal. if your HX system is efficient enough at radiating (which uses your off-grid cabin as a heat sink), or storing (which still does but delays it), then recirculating would only be really useful for emission-control.
construction. whatever you use should be highly thermally conductive for a radiating system, and at least middling for a storing system. the burn chambers of traditional stoves, with thick iron walls, are a nice middle ground. since you’d be treating the exhaust gases to the HX approach, presumably the main burn chamber does whatever it does… still though you want your exhaust gases as cool as possible already, to avoid unnecessary stress on your system.
upside-down plumbing. have you considered a vat of water through which an upside-down tube formed like a sink trap is routed? that would be basically the first substance that your hot exhaust gases hit, excepting of course the tube. a fan HX is fine, but still needs a power source. why not heat up a lot of water with the gases first, avoid so much rock or other material, and keep the system much more passive?
anyway, just a few musings. i’m going to diagram this out a bit. fun to think about it. thanks for your article.
An economizer is the term used in industry. Waste heat is used to preheat water going into a steam boiler and other applications.
CO2 is deadly in high enough concentration. You would probably last a couple of hours on a cold day.
Use a a catalytic unit to ensure the exhaust gasses are completely consumed. Rhodium based catalysts can even convert NOx into Nitrogen and Oxygen and Gold based converters will convert any CO into CO2.
Combustion products are quite corrosive, so Stainless Steel is in order if you don’t scrub the gasses first.
It’s basically a good idea, just don’t kill yourself. Get a Carbon Monoxide monitor and keep warm clothes handy in case you have to evacuate the house in the middle of the night.
I appreciate what both of you had to say.
1st let me say that the thought of somehow retaining the CO2 by venting through plants was only nothing more than a lofty dream goal . . . but perhaps feasable if a wood stove were used to heat a large greenhouse.
The next most important thing I have to say (and even more strongly to others I have shared this idea with) is that I wasn’t picturing gleaning the last bits of heat from an efficient system (as many take my idea). but rather an option for the many who burn and do not yet have the most efficient system . . . and can’t afford to just walk out and by “state of the line”. Not everyone has deep pockets or available resources to do what is BEST and are simply lokking for a very low cost way to IMPROVE upon what they do have.
I totally agree that the biggest potential obstacle is the access to clean out whatever soot would accumulate, especially considering, again, that not everyone would be able or put out the effort to burn as clean as they possibly can.
I too like the idea of drawing the exhaust through water but couldn’t see it giving off enough heat quickly enough as it would by going through the substantially increased surface area of loose aggragate. Perhaps this would make a great “first step” for the exhaust to follow.
I also agree that for some the space that the “storage mass” would require would be an obstacle. My thoughts are that unless you have enough mass to draw out nearly all of the heat, your fan will not hold up on the other end.
Burning hot can be as effective as a cat. converter and a lot less expensive. The whole point of drawing the air with the fan is to create the environment for a very hot burn with “no smoke” after the innitial startup.
For those that do not agree that the wasted heat going up SOME chimneys would not warrant the electricty needed for a fan . . . A. Not everyone already has a highly efficient system nor can they afford one. B. There are other ways to run a fan beside electricity.
I try to share my ideas with many in hopes that they might benefit even just a few.
Thanks for YOUR comments though (Leif, Mainah). I appreciated your support for the basic concept that I wrote about. Let me know if you have the chance to dabble a bit to any extent.
4un4me, thanks for your e-mail and response.
i think an interesting and useful test of such a system could be accomplished using a heat gun, a kill-a-watt meter, something like an old car radiator (cleaned out, dry), and a thermometer. hook it all up in that order, with the thermometer at the far end of the system. you know what your energy input is, since the kill-a-watt monitors that for you. if you’re fortunate and have a pyrometer or laser thermometer you could even monitor the temperature of the air blown from the heat gun.
measuring the ‘exhaust’ would be difficult without a fairly elaborate setup, but assuming it’s fairly linear isn’t a bad start. thus, if one radiator is capable of cooling 1500 watts continuously, you could do the math on higher energy inputs (e.g., wood or pellets, which have known caloric content) to determine how much you need to scale your system.
a car radiator has many advantages and disadvantages, many entwined. i’m sure you can think of more, but here’s a start: accidental permeability due to corrosion; inability to be opened for cleaning; appearance; etc. perhaps one could take the vanes off and use those. i’ve never attempted such a thing but it could be fun (if handling porcupines is fun). something nice about making the vanes of the test system removable is that one could adjust the surface area until the exhaust is within the desired range of room temperature.
on the topic of carbon deposits: yes the hotter the fire the better for controlling that. soot deposits not only accumulate, but if i remember correctly, smoke creates an acidic environment (someone check me on that please). unless the exhaust is chemically neutral, the HX design would need to be prepared to deal with that in addition to all of the other stresses we’ve discussed.
re the inverted water ‘trap’ as the first HX: a flattened, wide exhaust tube (versus one having a roughly circular cross-section) should greatly assist in heat dispersion. furnaces often use this flattened approach to great advantage in their HX. if it weren’t so late, i’d do some math on the water, to see how much of it one might want, to store excess heat. something for tomorrow.
I hope I didn’t come across as negatively defensive about the concerns regarding CO2 emissions. That is one of the greatest challenges of writing any words since the person reading them can’t see your facial expression or attitude while you are writing.
There is no doubt that the CO2 issue and for that matter the potential for CO are critical. I also know that it has been proven that plants grow better when CO2 levels are high.
I share my thoughts and ideas in hopes that at least someone can benefit from them directly in their own unique situation, or to stimulate a new wave of thought precipitated by something I wrote.
I only get frustrated when people write back and tell me I am totally “off” just because what I wrote about doesn’t make sense in their own narrow situation or perspective.
I have had several people do this to me about this idea when I shared it with a few Yahoo groups focusing on wood heating. First they tell me that there is no potential for my idea with gas furnaces because gas furnaces are already high efficiency and already put out cool exhaust . . . well mine isn’t and doesn’t!!! And I am going to stretch and guess that there just might be someone else with an older less efficient furnace who, like me, cannot afford to replace it any time soon.
The Masonry Heater “expert” told me I was “wrong” because it would cost more in electricity to “draw” the exhaust than the last little remaining heat is worth.
Well, again, not everyone in the cold winter zones have Masonry Heaters or high efficiency wood stoves and not everyone can afford the thousands of dollars to implement such. Not everyone lives in areas where emissions are tightly regulated or carefully enforced, but most people would want to get more from their fuel supply if they could. (Not to mention that a fan does not HAVE to run on electricity.)
What about all of the people who still burn inefficient smoldering night time fires because they can’t afford to burn hot and fast since all of the heat would go up their chimney?
My article was only meant to suggest that this idea might have merit and value for some people and might help lower wood consumption and volatiles submissions somewhere in the world . . . even if a variation to what I wrote were implemented.
Ken
I see where you are going with this and even if the
engineering doesn’t work out, don’t give up. I am
(was)on the bleeding edge of tech and now I guess my
midlife crisis is to go retro. I will use a wood furnace
and if the misses concurs a wood stove in our next home.
If nothing else you have given me some food for thought.
I was even toying with the idea of using the drawn off heat in a sort of car radiator type arragement for forced air
heating of other spaces.
A properly designed boiler attachment can steam drive a
modified turbine to pull the air.
The water cooling section mentioned sounds like a kind
of smoke ‘bong’. Probably not efficient, but an interesting
aside. Thanks for sharing.
I once lived in a house that had a natural gas furnace installed in the basement. The house was built by a production builder in 1982, so the thermal efficiency of the furnace was probably about 70 percent. The exhaust gas came out the top of the furnace and ran up a single wall exhaust pipe that was about three feet long. It connected to a double wall exhaust pipe that ran between house walls up through the center of the house and out through the roof. When the furnace was operating, so much heat was going up the exhaust stack that you could not touch the single wall exhaust pipe where it connected to the double wall exhaust pipe with your bare hand. I knew a lot of natural gas energy was being wasted. To make the system more efficient, I purchased about ten feet of aluminum flexible pipe. The type of flexible pipe that is used to exhaust hot air from a clothes dryer. I one end to the furnace exhaust and looped the ten feet of flexible pipe around in one complete circle that was slightly rising and connected the other end to the double wall exhaust pipe. I then suspended a large fan (fan size was about 2×2 foot) from floor joists so the fan would blow air over the newly installed circular exhaust pipe. Note, clothes dryer exhaust vent pipe has lots of ribs which make it flexible, so these ribs provided lots of surface area to transfer heat from inside the pipe. Aluminum is a good conductor of heat. The electric plug of the fan was connected to the electric circuit of the central heating system blower. Therefore, when the furnace heat exchanger got hot enough to turn on the central heating system blower it also turned on the fan I had positioned to blow air over newly installed clothes dryer exhaust pipe. Heat was now extracted off the clothes dryer exhaust pipe which heated the basement. Since heat naturally rises, it heated the floor above. So much heat was extracted that I was now able to continually hold my bare hand on the exhaust pipe at the location where it joined the double wall pipe. I never took any before or after temperature readings to know how many degree the exhaust temperature was reduced.
If I understand your idea correctly, the fan is used to draw exhaust gases out through several heat exchangers to extract all available heat. In this design, the fan will be subject to all combustion particles/soot/carbon buildup etc. that comes through the exhaust pipe. Consider a closed, and sealed, combustion air loop system where the fan is located at the intake air end. This way the fan takes cold dense oxygen rich air from outdoors and forces it into the combustion chamber. The exhaust gases are forced through heat exchangers to extract as much heat as possible.
If you still desire the fan located on the exhaust side, then consider using a squirrel cage type fan.
Your idea of storing heat is a good idea and there are many texts on the subject. Most that I have date from the 70’s when the price of oil spiked much like it did recently.
The important thing to consider is Specific Heat of the storage medium.
http://en.wikipedia.org/wiki/Specific_heat_capacity
http://hyperphysics.phy-astr.gsu.edu/HBASE/thermo/spht.html
Water has a Specific Heat of 1 calorie per gram per deg. C.
You can look up the energy content of the fuel you are using and calculate the amount of water it will take to absorb the heat generated for a given rise in temperature. To get more heat capacity than water requires things that become expensive, toxic or exotic. The other cheap alternative is to take advantage of the property of salts and their water of hydration. This allows a given volume of water and salts to absorb more heat for a given temperature rise.
When the storage medium warms, it loses more energy due to radiation and more insulation is required to retain it.
When it comes to modern heating, efficiency has not been a problem for some time now.
Some of the current conventional systems already achieve 97%+ efficiency, the loss of -3% is negligible. It sounds like a lot of additional things and I wonder just how beneficial it would really be to shoot for the remaining 3%.
I believe when considering areas to concentrate on within efficiency, I’ll continue my focus in areas such as the Internal Combustion Engine which is usually only around 30% efficiency.
So the effort involved in trying to absorb an addition 3% in heating is better spent trying to capture energy where most effective. That effort in itself can be considered well spent mind and labor efficiency:)
Bottom line, where to draw the line.
In addition to the above… when it comes to efficiency it’s typically measured by a devices ability to deliver work from a given fuel/potential energy source. Heating itself can be further enhanced with regard to overall system efficiency by simply incorporating more insulation in the product to be heated.
With a car or other mechanical device, avoiding friction is the key in that aspect and I doubt we’ll overcome that which is provided by the atmosphere or wheels, but there’s always a need to break too. In such a case inertia can be stored in angular momentum but I doubt we want giant 600 ton lead wheels turning within our automobiles. Bizarre as it may sound, that’s somewhat similar to what trying to capture the remaining 3% lost heat is similar to. Then; if you can get that remaining 3%… WHAT ABOUT THE ENERGY WITHIN THE SOUND OUTPUT FROM THE COMBUSTION ITSELF? Why not try to capture that remaining .0001% energy too? ;)
Where does one draw the line?