Joao's Clay Vesto
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VESTO Stove
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VESTO Stove
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BACKGROUND
Message to Stoves from Crispin Pemerberton Pigott May 1 2006
CPP>I am reminding people of the project under way in Maputo to make a ceramic stove that can compete in the market against a metal stove. This is a formidable problem I admit because
a) the metal stoves already dominate the market and
b) clay stoves are heavy, breakable and unknown.
The selling point has to be fuel economy and perhaps appearance (pride of ownership).
Tom Reed>The "tincanium" stoves are easy to construct. They have a very low
>heat capacity compared to clay. They are lightweight and not fragile.
>They can be mass produced. I have yet to see a clay stove that
>duplicates these virtues.
CPP>Exactly.
However high thermal mass offers other benefits, in particular the provision of retained heat to the primary air late in the fire's burn increasing the usually low combustion efficiency. The reduction in CO later on is very valuable both to reduce emissions and to increase the heat output basically for free, though some of the heat might have been available earlier on which could affect performance.
Overall clay stoves have long term advantages that may see them gain wide acceptance. First, they can be made cheap and in large quantities in modestly sized businesses so that a standard product can reach the market with a predictable performance. This is valuable when scaling up production in a city and when there is local or regional advertising touting them.
Artisanal stoves suffer from variability / inconsistency.
Clay stoves can have both a fairly high insulative value, giving early flame temperature rise, as well as the ability to store some heat for later when the amount of fuel is greatly diminished. The effect can best be seen when looking at a limited time 'cook' instead of pretending that the stove works in a continuum. For example saying (as a criticism) that clay absorbs heat from the fire in the early stages does not mean that it continues to do so after a time. It is a temporary thing and the heat can be returned later, not to cook without a fire, but to heat the incoming air and assist the burn over a 10 or 20 minute period as the stove body cools. The effect on emissions is pronounced. Late in the burn when a charcoal stove is in the dying-fire stage, heat transfer efficiencies frequently exceed 100% as the heat is recovered from the stove body while simmering. This effect is only minimal with light metal bodies. Thus clay stoves have some inherent advantages over light metal ones.
As for the possibility of a hybrid clay+metal stove, the most efficient wood or charcoal stove I have tested to date was a clay bodied stove with a Vesto fire grate in it. Its overall efficiency (PHU) was 52% and I can say it was not optimised - just the parts we had available tossed together. Joäo Da Conceiçäo made the clay parts and the Vesto parts were standard. I did the testing with charcoal.
The reason for the efficiency is that the metal parts that do the air preheating and fire manipulation are placed in the middle where these attributes are most required. The clay on the other hand is used to provide stability and huge thermal insulation. The stove did not get noticeably warm on the outside after more than an hour. The resulting stove looked a great deal like a heavy clay Vesto. It was stable and cheaper that an all-metal Vesto. A variation on this has been on sale by ICEMA in Maputo for almost a year using locally made non-stainless steel inner parts.
Regards to all
Crispin
Se messages and thread May 2006 http://listserv.repp.org/pipermail/stoves/