Originally Posted by
CanBeDone
This statement is false. Have a look on google for "cockle rocket stove" and you will find examples of rocket stoves that have been built for home heating. Not only that - there is even one guy who has measured and quantified the heat output of his rocket stove, coming to the conclusion that it is the most efficient way to extract heat out of fuel to heat a home! I have even seen a Youtube video (from a Russian) who heated his tent (pitched in 10 or 20cm of snow, somewhere in the Urals) with a variant of a rocket stove!
This statement leads to the core of your error, not because it is false, but because you omit from your thinking a key component of any heating system: temperature conversion from the high temperature inside the fire place (1000oC) to the much lower temperature desired inside the home(20-25oC).
For PowerMK, efficient conversion of fuel heat from high to low temperature is not much of a concern, portability of his stove, and thus light weight, is. Thus his design is in steel. For house heating, portability is unimportant, but efficient conversion is, as long as a uniform temperature output - not a uniform heat output- is obtained. Thus, the design of a rocket stove for house heating will be very different from the one presented here by PowerMK.
The first design target for domestic (urban) heating is low environmental emissions. This can be realized if the fire burns at around 1000oC, as near this temperature carbon burns to CO2, not the toxic CO. Particulates (Smoke!) are only produced if the temperature is much lower, and Nitrogen Oxides (NOx) will be produced only if temperatures are much higher.
The second design target is heat output. If the furnace temperature inside has to be 1000oC for environmental reasons, and the furnace outside no more than, say, 50oC, for safety reasons, then the size of the furnace chamber is determined, for a given heat output, by the surface area of the furnace and the thermal conductivity of the furnace walls, and the total conversion efficiency by the ratio of (heat in the flue gas) divided by (heat emitted through furnace surfaces).
The third design target is heat transfer rate, i.e. the change in outer furnace surface temperature over time. As this is controlled commonly by building thick furnace walls, alternative names for cockle furnace are masonry furnace, Kachelofen, Finnish or Russian Stove.
The fourth design target is air flow inside the furnace, and here the Rocket Stove principle shines, because it produces a highly turbulent gas flow, moving fresh oxygen more quickly to the fuel than do other designs, heating the inside very quickly. But because it takes a long time (up to 24 hours, on some designs) for the heat to move through the thick walls and heat the house, it does not matter how long the fire lasts. It only matters how much fuel was burned, and how much heat this fuel transferred to the furnace walls, as it is the heat stored in the furnace walls that heats the house.
Heat content for fuels is:
Wood: 4-4.5 kWh/kg
Coal: 7.8-9.8 kWh/kg
Fuel oil: 12 kWh/kg
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