[Stoves] Improving Thermal Efficiency (TARP-VE)

[Sarbagya Tuladhar]

Hi Frank,

Had a similar question. Recently did a CFD simulation for a cookstove. With secondary air inlets got the velocity magnitude in the heat flow channel to moving faster as compared to without secondary air inlets. I reckon with faster gases kinetic energy is higher and hence when it changes to thermal energy its higher as well. Wat ya reckon ?

Sarbagya 

Sent from my iPhone

On 02/06/2012, at 5:29 AM, "Frank Shields" <frank at compostlab.com> wrote:

> Hi Dean,
>  
> Do know why hot gasses moving faster transfers heat better to the pot than slow movement? Just seems the longer the heat (whatever that is) would go into the pot with the most contact time. Perhaps ‘they’ need be pushed up against the pot so they don’t bounce off.  
>  
> Thanks
>  
> Frank
>  
>  
> From: stoves-bounces at lists.bioenergylists.org [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Dean Still
> Sent: Wednesday, May 30, 2012 4:55 PM
> To: Discussion of biomass cooking stoves
> Subject: [Stoves] Improving Thermal Efficiency (TARP-VE)
>  
>  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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