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Assisted Draught Charcoal Stove

Extracts maximum available heat from fuel.
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I would like to propose a charcoal stove that could be extremely efficient. The idea owes something to my childhood long ago where primary school classes took place in a drill hall heated by two coke stoves with steel chimneys venting through the roof. Much heat must have been radiated from those chimneys.

The main body of the stove would be a vertical steel cylinder, say, four feet high. Attached to the bottom would be an ash container with integral air inlet which would be equipped with a simple flap that closes under gravity. Near the base of the stove and above the ash container there is the combustion zone with a grate so that ash could drop through to the ash container. At this point there would be an access point with a fire resistant glass panel to allow lighting and visual observation.

The charcoal fuel would be stored in a second parallel vertical cylinder that is joined to the stove via an angled tube at the combustion zone. The top of this hopper where fuel is fed in would have an airtight seal so that combustion would not proceed upwards through the stored fuel. As fuel is consumed it would be replenished from this hopper via gravity but would not move much further up into the stove itself since charcoal lumps do not make for a fluid medium.

The top of the stove would join a labyrinth of steel pipes which zigzag up and down and could have fins of steel sheet welded on to assist radiation of heat. The labyrinth would be black leaded to further assist radiation. Finally, the end of the labyrinth would be connected to the outside world via flexible metal ducting.

Such a structure would not be able to act as a chimney and air would be drawn through the system via a small electric fan at the exit, the idea being that by the time the combustion gases reach the exit they would be at ambient temperature. With all heat having been radiated the gases would be cold and relatively non corrosive. If corrosion is felt to be an issue then a Venturi type configuration could isolate the fan. Since all the heat will have been radiated and the combustion gases leave at ambient temperature the efficiency could be near 100%. The length of the labyrinth would be found by experiment. Ultimately, the fan could be powered from a battery that is kept charged via Peltier or thermocouple devices mounted on the stove.

Combustion of carbon monoxide might be assisted by having a bypass pipe from the ash pan area to a point above the combustion zone allowing a small amount of air to bleed past. Carbon monoxide is an important safety issue and external detectors would be mandatory however it will be realised that the system would be under negative pressure and if the fan stopped then the inlet flap would close and combustion quickly cease.

If successful I would anticipate that this idea would be of use where large scale space heating is required. In the UK, where this article is being written, there are many stately homes in the care of the National Trust which could benefit and such places often have associated land where charcoal could be produced. Large ecclesiastical buildings and also greenhouses spring to mind.

One problem that I foresee is that charcoal quality can be variable and tarry residues do build up on barbecues. To assist in cleaning it may be necessary to make the stove and the first leg or so of the labyrinth demountable to allow easy access.

corvuscornix, Mar 26 2012

Franklin stove http://en.wikipedia...wiki/Franklin_stove
[bungston, Mar 26 2012]

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       Is this a Franklin stove with an automatic feeder?
bungston, Mar 26 2012
  

       As I see it the Franklin Stove and other allied devices delayed for a short time the exit of the combustion gases so as to transfer as much heat as possible to parts of the stove from whence it could be usefully radiated. They all were driven by ascending hot gases in a chimney which is wasteful.   

       In this device we take the delay to the ultimate and hang on to the gases until they are actually cold.   

       It would take a very high quality fuel to enable the idea to function hence the choice of charcoal.
corvuscornix, Mar 26 2012
  

       // the fan could be powered from a battery that is kept charged via Peltier or thermocouple devices mounted on the stove. //   

       Now that's ingenious. Bun for ingenuity.
8th of 7, Mar 26 2012
  

       Remember that ambient temperature means heated, building temperature, and that you'll get rapidly diminishing returns as you try to approach equilibrium. In other words, at best, you'll use a very large heat exchanging structure, but still have exhaust gases leaving at somewhat above indoor temperature, and cold external air coming into the building to supply the make-up air. Worse still, the entire building acts like a cold air duct, with a net flow from the chilly exterior, through all the rooms, and out via the fire. This is the classic hot face / cold back castle effect, somewhat reduced by the relatively low air requirements of an efficient stove but still present.   

       You would get a higher efficiency, even with a less convoluted heat exchanger, by using ducted external combustion air, and even more with a heat exchanger between the exhaust and the inlet. In fact, if you're going to make it as complicated as you describe, you might as well bring in slightly more (preheated) air than the fire uses, which not only makes up for the slight inefficiency of the heat exchanger, but creates a net flow of warm air away from the source and out through the more distant parts of the building.
spidermother, Mar 26 2012
  

       Also, I'm going to play devil's advocate re. your electrical system. I find it hard to see how peltier -> battery -> fan is not worse in almost every sense than using (say) mains electricity. If you need to operate off the grid then fine, power the fan from the stove's heat (electrically or mechanically), but please drop the battery, unless you can justify its inherent wastefulness.
spidermother, Mar 26 2012
  

       One assumes the Peltier is between stove and indoor-ambient so there's zero wastage, and that the battery stores just enough for 5 minutes (or however long it takes the stove to heat up enough that the Peltier can then run the fan directly).   

       [ ] The Peltier is neat though I can't help think there's a way to incorporate one of those Dyson fanless fans in there somewhere.   

       However, the Labyrinth is a bit of a sticking point: bitch to clean no matter what solid fuel you use, and when the inside is coated it means less efficient heat-transfer.   

       Maybe a pipe like a curly-straw or curled like a radio-antenna coil: aesthetic.
FlyingToaster, Mar 27 2012
  

       Swedish (and similar) masonry heaters are an excellent solution to many of these issues. You burn your day's (or even more) quota of wood, very quickly, in one go, then seal the stove completely when the fire goes out. The hot masonry releases heat until the next fire is lit. Because the fire burns very hot, there are no sooty or tarry deposits. And because it only breathes for a short period, there is little loss of warm air the rest of the time. Mark Twain was very impressed when he saw one.
spidermother, Mar 27 2012
  

       There's a reason why power stations have neither peltier devices nor batteries - despite fluctuating demand and waste heat. You are better off throwing excess electricity away than storing it in batteries, and you are better off just using waste heat as heat (or even throwing it away) than generating electricity with peltier devices. They both simply waste more money, energy, and resources than they ever save. They are great when you need mobility, or don't care about cost and waste, but otherwise they are worse than useless.   

       That's why, whenever an idea has an element of 'just add batteries / peltier junctions / stirling engine / whatever' I tend to expect some justification - because any concept of saving or conservation is likely to be misguided.
spidermother, Mar 27 2012
  

       This is probably a stupid question, but is there a method for using waste heat to power a Sterling engine which imparts its energy to a flywheel to be stored?   

       Yes, but there are the same cost / benefit problems. As a general rule, it's much better to generate power when you need it, and in the form you need it.
spidermother, Mar 27 2012
  

       On rereading the idea, I don't see any method of reducing the exhaust to outside-ambient temperature.   

       [sm] On rereading your objection to a Peltier-driven fan, I fail to see the actual substance of your objection: that which isn't turned into electricity becomes heat, both on the Peltier interface and in the battery which, unless it is stored outside simply heats the room; again, I assume the battery is just there to be used until the stove is hot enough to drive the Peltier-fan by itself.
FlyingToaster, Mar 27 2012
  

       The substance of my objection is that the cost of ownership of a battery over its lifespan is several times greater than the total value of the electricity the battery will store over the same period. If there's a mains connection, it makes more sense just to plug it in. The same applies to the cost of the thermocouples versus the electricity they produce.   

       Charging batteries with the thermocouples just compounds the problem. You have to sum the costs, and multiply the efficiencies.   

       Look, it's not a major objection, I just wanted some justification for the thermocouples and batteries, because I don't immediately see that they are necessarily better than (say) mains power.
spidermother, Mar 27 2012
  

       Efficiency isn't really a problem: the Peltier is inefficent... that means it lets heat through it... and since the purpose of the stove is to provide heat, that doesn't seem to be a problem. The battery is inefficient, but notwithstanding energy lost to degradation of the chemicals, the loss, again, goes into heating.   

       As per cost/bother... maybe, but it'd be a pain if you couldn't heat the house just because the mains power is off.
FlyingToaster, Mar 27 2012
  

       The fan would be very small when you consider that 1kg of charcoal requires only 11 cubic meters of air for combustion and this puts in perspective the amount of air transferred to the outside.   

       I mentioned possible ways for supplying the fan power as a way of staying independent of the mains electricity supply--so many gas and oil heating sytems go down when the mains electricity fails. For this application I would envisage that a 12 volt fan and battery charged from the mains would suffice.   

       The cleaning of the labyrinth does cause concern. It may be that the combustion zone is small and would burn hot and this, together with the bypass air, would reduce the production of tarry deposits. In a large scale system maybe you could burn it out with pure oxygen from time to time!   

       The idea is envisaged for larger scale applications and maybe not applicable to the domestic environment. However if you do want to go down that route, art deco or art nouveau labyrinths would be tasteful and full size cast iron statues (in the best possible taste of course,) would be a real ice breaker at parties.
corvuscornix, Mar 27 2012
  

       [+] artistic heat-transfer surfaces.
FlyingToaster, Mar 27 2012
  

       //staying independent of the mains electricity supply// Thank you. Objection withdrawn. Sorry for doing a Little Prince on you.   

       Based on a ratio slightly leaner than stoichiometric, and an exhaust temperature 25 K above the external air temperature, you lose about 0.01 of the charcoal's energy; which is not too bad, as you say. But that's a best-case scenario, and it's still good to avoid even a few cubic metres per minute or whatever of air flowing in the 'wrong' direction - a gentle flow away from the heat source is a very effective way of heating, rather than chilling, distant parts of the house, virtually for free.   

       The only designs I've come up with that seemed to warrant a fan use counter-current heat recovery and external make-up air. Once you are actively powering the air flow, why not make it go in the most beneficial direction? An added advantage is that with a bit of careful design, only clean air ever has to pass through the fan.
spidermother, Mar 27 2012
  

       // only clean air ever has to pass through the fan. //   

       What if you live in Los Angeles ?
8th of 7, Mar 27 2012
  

       //and also greenhouses spring to mind.//   

       Don't forget the CO as your greenhouse may pile up with sacrificial horticultural workers!
corvuscornix, Mar 28 2012
  

       Maybe the fan could be powered by a steam engine. Anyone who likes it warm probably needs to drink, and a little water could go to the engine from time to time.   

       A stovetop steam engine with a power takeoff would have the added benefit of providing rotary power for other applications; for example a grinder.   

       You might miss your grinder in the summer, and so some means to avoid heating the dwelling might be nice (stove built into wall with rolldown curtain?)
bungston, Mar 28 2012
  

       I think that at the crux of this idea is the functional separation of combustion gasses and heat-exchanging gasses. In a traditional fire, the two are one and the same.   

       Since the flow-rate of the input gas must equal the venting flow-rate of the output gas, it's a relatively simple step to say that you need a higher volume of heat-exchanging gas in order to more effectively do the heat-exchaning.   

       The question is whether the extra energy required to continually move this higher volume of gas is higher, or lower than the extra energy it would take to directly supply the additional heat. How would we be able to make an informed guess as to which of these values would be bigger?
zen_tom, Mar 28 2012
  

       /In a traditional fire, the two are one and the same. /   

       I thought fires mostly heated by radiance, with convection actually wasting warm room air to combustion (and entrained room air that never touches the fire) and up the chimney. Stoves using indoor air for combustion have the same problem but radiate more efficienty and probably waste less indoor air.   

       I think I have seen long slanting inside chimneys and presumed they were for this purpose - capture more heat from gases leaving the stove.
bungston, Mar 28 2012
  

       The chimney should angle downwards at some point: since the exhaust is slightly denser than ambient it's not going to flow up.
FlyingToaster, Mar 28 2012
  


 

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