How close are we to the limits of efficiency? What's next? Will newer catalysts provide significant gains? Will automatic adjustments filter in to provide better burning? Will better materials be the answer? Lot's to ponder!
New Technology
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EbS-P
Minister of Fire
My thoughts are all the low hanging fruit was pick some time ago. The 2020 regulations and the 2021 tax credit will pretty well clean out all the middle fruits and Cost and complexity will put the high fruits out of reach. Wood-burning gas byproducts are not clean. Maybe a new stove will come with its own venting system that’s super insulated and creosote won’t stick to it?
Evan
Evan
brenndatomu
Minister of Fire
In my mind about the only real gains to be had now would be done with automation of the controls...like where things have went with wood fired boilers and furnaces
I agree the low hanging fruit are long since picked. The manufacturers are on the tail end of the efficiency gain for woodstoves. Active controls like O2 sensor loop may buy some efficiency albeit with increased cost and complexity. There is a possible gain by going with condensing heat exchangers on the exhaust but that means a very expensive and maintenance heavy device. As long as wood is a non uniform fuel we are close to the peak. Pellets were the solution to make wood uniform and wood boilers with storage and low temp radiation is probably the top of the heap for efficiency. The Europeans have condensing heat exchangers on some of their larger systems so as long as there is bypass for startup and shutdown that would gain a few points of efficiency.
Automation and may a scrubber for some of the strictest controls? @John Ackerly may have some insights on what is coming down the pike.
IMO it all starts with electronic controls. There are a lot of good stove designs that operate on passive control, or on some rudimentary mechanical control like a thermostat. These designs could be much better and significantly reduce emissions if active supervision was incorporated into the burn control.
After that efficiency and everything else falls into place. Creosote is no longer an issue if unburnt combustion products no longer escape the stove, meaning better heat exchangers and better heat scavenging from flue gases. Maybe it even comes to a point where wood stoves could be vented out a wall like a pellet stove or a high efficiency natural gas furnace. But I really don't see a wood stove or furnace ever surpassing 90% efficiency.
The kicker will be cost, it will be hard to convince existing stove owners to pay more for less emissions, especially with higher maintenance costs. What really determines the future of wood heat will be the cost of alternatives, if electric, heat pump, propane whatever becomes cheaper than wood, wood heating will disappear. If all other heating methods increase in cost, which I believe they will increase in cost significantly in coming years, then there will be a resurgence in wood heating, and will be driven by the younger crowd that want to control and view every aspect of the stove on their smartphone, and will pay more for these features.
After that efficiency and everything else falls into place. Creosote is no longer an issue if unburnt combustion products no longer escape the stove, meaning better heat exchangers and better heat scavenging from flue gases. Maybe it even comes to a point where wood stoves could be vented out a wall like a pellet stove or a high efficiency natural gas furnace. But I really don't see a wood stove or furnace ever surpassing 90% efficiency.
The kicker will be cost, it will be hard to convince existing stove owners to pay more for less emissions, especially with higher maintenance costs. What really determines the future of wood heat will be the cost of alternatives, if electric, heat pump, propane whatever becomes cheaper than wood, wood heating will disappear. If all other heating methods increase in cost, which I believe they will increase in cost significantly in coming years, then there will be a resurgence in wood heating, and will be driven by the younger crowd that want to control and view every aspect of the stove on their smartphone, and will pay more for these features.
Several years ago someone modified their PE stove with servo controls. It looked fairly effective in operation. The kicker for me would be reliability. Stove electronics need to be hardened and protected against surge currents, etc. Electronics failure is a too common problem with pellet stoves. It's not like it can't or isn't done. Furnaces have a better reliability record achieving the same functions.
MongoMongoson
Feeling the Heat
I remember seeing that thread or threads. I thought to myself that if I built that, it would be gravity or spring-loaded to shut the air down in the event of a power failure. That wouldn't work with a servo (at least not the servos I have worked with), but there are other options to move a lever.
Electronics are one obvious route to take, but another option would be to offer the service to tune the system after install.
We all know that the chimney is what drives the stove, and the stove is designed to work with a draft that, chances are, doesn't match your draft. It happened to me. If my stove is running through a full load in 4 hours and sending 800 degree gases up the flue, it's not meeting its EPA-established efficiency numbers. It's probably not putting out a lot of particulates, but it is wasting a lot of heat.
So we take it upon ourselves to tune our stack using dampers, but the stove manufacturers could, conceptually, develop a "tunable" stove and authorize stove technicians/engineers to perform the tuning upon installation at which time it would be "locked". It would be a service that would incur a cost during install, but I guess I'd rather pay for that than to have gizmos and gadgets running my stove.
That's probably way over simplifying things, maybe BKVP will show up and tell me I'm off my rocker! It is just something that occurred to me, since these newer stoves are so finicky about draft.
If people burned only dry firewood properly and maintained their stoves well, it would go a long way toward reducing pollution.
My guess is the next step us capturing the heat from a fast, hot burn and slowly giving it off via thermal mass. Maybe they don't give us controls anymore. It seems to me that the weak point in the operation is the operator, whether its wet wood, choking to a smouldering hunk of charcoal, etc. If there was 1 or 2 thousand lbs of mass to absorb the heat, and we could load up for a full day, we may be quite happy with a light it and forget it stove.
A fast hot burn would move a lot of heat through the flue to the great outdoors.
Proper air control allows to load up and walk away for a day already.
Proper air control allows to load up and walk away for a day already.
Not necessarily, the right heat exchanger, and most importantly somewhere to put all that heat like into a thermal mass, could reduce emissions while maintaining low stack temps.
It does however necessitate more cold restarts, which could create more emissions in itself.
Heating up thermal mass is slow (by definition, or it's not sufficient mass...).
Gas flow thru a hot fire is fast (because making it less hot is done via decreasing the flow).
You can't heat up thermal mass quickly and have it give off heat slowly.
Gas flow thru a hot fire is fast (because making it less hot is done via decreasing the flow).
You can't heat up thermal mass quickly and have it give off heat slowly.
I like this idea as well, and have thought of a few different ways stove builders could make kits to tune the stove to the chimney setup.
The problem comes with the EPA regs, stoves are supposed to be deliberately build so the end user can't tamper with it. Think of carburetors, it used to be that instructions and parts were provided by the manufacturer so the end user could tune the carb, now it's to the point you need a special tool just to adjust the mixture screw.
Sure you can, that is the entire principle behind using a boiler with thermal storage, heat it up and allow it to slowly release. Masonry heaters also work on this principle.
Building a stove with say 1 1/2" plate would prove this point, it would take a couple hours to get it up to temp, but it would release heat all day.
It would just require the correct heat exchanger design, current stoves with a flat plate top are literally the simplest, and most inefficient means of transferring heat from the fire to the surroundings. They just continue to be used because they work well enough, and are efficient enough while also being reliable and cheap to build.
fvhowler
Feeling the Heat
I have been amazed at the technology gains in the last 15 years or so. As said, however, perhaps to most important part of clean burning is still properly seasoned wood. I'm not too excited about the automation of wood burning. All I know is the smarter the stove gets, the more I'm disconnected from the process.
Or more practically with a cast iron jacket on a steel stove. That allows the stove to heat up quickly and the cast iron jacket to soak up heat over time.
It has been done for a long time with masonry heaters in Europe. It does work but they are huge structures that the house is pretty much built around.
Ancient history that I mention on occasion is the UMaine work in the 1980s. It was the Carter Administration when burning wood and renewable were big and funding was available. Richard Hill a professor had a project to make the worlds most efficient woodstove. He and his grad students started from scratch and ended up with a wood boiler using water for thermal storage. It was an extended batch burner that burned very hot very quickly then grabbed the heat from the burn with a seperate extended lo mass heat exchanger. The only thing that went up the stack was water vapor that came in with the wood, O2, CO2, Nitrogen and trace gases that came in with the air. There were no creosote issues as it was close to complete combustion. The problem with a high mass burner is that its takes longer to get up to temperature. The rocket mass folks use a similar approach of very hot intense combustion but their use of thermal mass for heat recovery has its limitations compared to water.
If you search Richard Hill on this site you will find a few threads on Professor Hill, or Dick Hill and I found this on YouTube . He was talking global warming long before most have heard about it. He sometimes looks like a grumpy old man but spend some time listening to him and he was incredibly good at explaining complex energy concepts with a dry sense of humor.
If you search Richard Hill on this site you will find a few threads on Professor Hill, or Dick Hill and I found this on YouTube . He was talking global warming long before most have heard about it. He sometimes looks like a grumpy old man but spend some time listening to him and he was incredibly good at explaining complex energy concepts with a dry sense of humor.
SpaceBus
Minister of Fire
I think the Wittus Twinfire is probably the most efficient freestanding stove design without getting into automation or heating up some form of thermal mass.
I agree that water works, and a masonry heater too - though to me those are not stoves. Those are furnaces (water) or "a home with a fire in it".
The point is that "capture with thermal mass" has been misconstrued as being effective a lot. Soapstone (the most advertised "accessible" thermal mass material) has only twice the heat capacity (per weight) of cast iron. (and about 1/10th that of water). I.e. it "stores" only twice the energy per weight as cast iron does. Not very effective.
Yes, you can heat up stone with a hot fire. But, a quick hot fire by definition (unless one has a convoluted flue path as in a masonry heater), pushes a lot of the heat generated from the fuel out into the outdoors. Hot fires are only hot by the grace of burning a lot of fuel in a short time, for which a high gas flow is necessary. As that gas is hotter, and flows faster, the heat flux going out the door is larger as compared to a longer less hot burn in the same system.
The thread is "new technology". And the remark was made that [heavy mass] allows to load and walk away for a day. My remark was that that is not necessarily more efficient, AND that that already exists with systems with proper air control.
I load and walk away for a day.
And it's more even than an exponential decay in heat out put from a large mass.
The point is that "capture with thermal mass" has been misconstrued as being effective a lot. Soapstone (the most advertised "accessible" thermal mass material) has only twice the heat capacity (per weight) of cast iron. (and about 1/10th that of water). I.e. it "stores" only twice the energy per weight as cast iron does. Not very effective.
Yes, you can heat up stone with a hot fire. But, a quick hot fire by definition (unless one has a convoluted flue path as in a masonry heater), pushes a lot of the heat generated from the fuel out into the outdoors. Hot fires are only hot by the grace of burning a lot of fuel in a short time, for which a high gas flow is necessary. As that gas is hotter, and flows faster, the heat flux going out the door is larger as compared to a longer less hot burn in the same system.
The thread is "new technology". And the remark was made that [heavy mass] allows to load and walk away for a day. My remark was that that is not necessarily more efficient, AND that that already exists with systems with proper air control.
I load and walk away for a day.
And it's more even than an exponential decay in heat out put from a large mass.
Sorry but something about what you said just isn't sitting right with me. What definition? It's hot, it generates more heat. Where the heat goes is up to the technology not the fire.
Not much higher. The total amount of oxygen needed for complete combustion is the same whether you burn fast or slow. The key to maxing the output from your stove is to give it just enough oxygen without carrying excess heat up the chimney, and it is doable at a high or low burn rate. Thanks to technology.
I feel like this is conflating heat flux with total heat loss. Maybe heat is leaving a bit more rapidly, but the duration is shorter. The total heat lost is not higher with a hotter fire, in fact I think in many cases it is the opposite. Depending, of course, on the type of system you have to capture the heat.
It takes a certain temperature for CO to ignite. Generally all of the other volatiles in the wood burn at lower temps so if you burn the CO everything else is long gone. If the firebox temp is less than the minimum, the CO goes right up the stack. Even if that minimum temperature is met there has to be enough turbulence for the CO molecules to directly interact with oxygen (close is not good enough they need to touch). Cold air coming in cools down the temps so preheating it keeps the temps up. That usually means a hot fast fire to get full combustion. All a cat does is reduce the ignition temperature of the volatile gases and the intermediate combustion gas CO. Once the reaction happens there is a blend of hot gasses that have to interact with heat exchangers to transfer some hat sink like a room, a block of mass or tank of water. Ideally this heat exchange is done through a series of heat exchangers with low mass. If its a clean burn there is no creosote but there is water vapor mostly from the water in the wood and some trivial amount formed in the combustion process. If that vapor is condensed there is a bit more energy to be grabbed (1000 btus per pound of water that runs through the stove). The device to do that is called a condensing heat exchanger. If there is low temp demand for warm water the stack temps can be brought down to very low temps to the point where this a fan needed to get good dispersion out of the stack. During start up and shut down there can be corrosive gases that make it past full combustion and they in combination with water vapor can make conditions acidic so the heat exchanger is usually made out of stainless and has fairly open clearances and a way of cleaning it.
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