[Stoves] Improving Thermal Efficiency (TARP-VE)

[Dean Still]

 Hi All:


This winter the new International Working Agreement (IWA) was ratified in
The Hague. Four health based tiers divide stove performance. A top rated
Tier 4 stove (PM and CO) without a chimney is estimated to protect human
health in a small kitchen with 15 air exchanges per hour. Carbon Monoxide
isn’t as bad as Particulate matter when inhaled so it’s a lot easier to get
a 4 (an “A”) for CO but only super clean stoves can make Tier 4 for high
power PM. (Low power PM is easier.)



Most improved stoves that we have tested recently get 2’s and 3’s for PM (a
little bit of PM causes serious respiratory problems) but even relatively
‘normal improved' stoves can get into the 4 category for CO. It’s also
possible for stoves to get a “4” for thermal efficiency. The stove has to
score above 45%. We used to think that this kind of score was almost
impossible to achieve. But now the understanding of heat transfer
efficiency has been ratcheted up a notch.



 Here’s how we try get a “4” on the IWA report card for thermal efficiency:



We use an acronym (TARP-VE) to remind us of the most important variables
when designing a stove that uses the least fuel to boil and simmer water.



1.)    *Temperature*: increase as much as possible the temperature of the
gases/flames touching the pot.

2.)    *Area*: increase as much as possible the area of the pot touched by
the gases/flames.

3.)    *Radiation*: Don’t forget how powerful radiation is! While Rocket
type stoves depend on optimizing convection, if the pot can be exposed to a
hot radiating surface, water will boil a lot faster.

4.)    *Proximity*: Force the hot gases into narrow channels as close to
the pot or griddle without decreasing the velocity of the flow.

       5.) *Velocity*: Slowing the hot gases decreases the heat transfer
efficiency. Keep the hot gases flowing as fast as possible without
decreasing the temperature of the gases. Insulate the heat flow path.

       6.) *Evaporation*: We add this reminder that, while a very large pot
has the advantage that more of the pot is exposed to the heat, a larger
surface of water is evaporating which can make it difficult to achieve full
boil. That’s the reason that traditional pots have large bottoms and
smaller tops.



 OK, dear friends, that’s our newest check list for making stoves more
thermally efficient.


Best,


Dean
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