graph helps calculate proper skirt gap for best heat transfer efficiency

Thermal Efficiency: How High Can We Go?

From SAMUEL BALDWIN’S “BIOMASS STOVES: ENGINEERING DESIGN, DEVELOPMENT, AND DISSEMINATION,” VITA, 1987

Various stove/pot/skirt combinations are achieving ~ 60% thermal efficiency. 

How high can we go? 

  • Doubling temperature doubles heat transfer efficiency when other factors remain constant.
  • According to Newton’s Law, doubling the surface area doubles the heat transfer.
  • Doubling Radiation increases heat transfer efficiency to the 4th power.
  • Forcing hot gases to thin the boundary layer of still air next to the surface to be heated (Proximity) effectively increases heat transfer efficiency (as above).
  • Doubling the Velocity of gases ~doubles heat transfer efficiency.
  • Increasing the view factor helps, too! (That’s the proportion of radiation that contacts the bottom of the pot.)
  • Prasad and others have suggested a correlation between firepower and area.

There may be other important factors?

  • In a modern Rocket stove at high power, the gases can be around 800C and the velocity can be around 1.2 meters per second.
  • Small, dry pieces of wood tend to make hotter fires and gases.
  • Pots have to have sufficient external surface area to achieve 50% thermal efficiency.

In ARC tests of modern Rocket stoves, a pot with an area of around 800cm2 scored 34% thermal efficiency. Increasing the area to around 1000cm2 increased thermal efficiency to about 40%. With the same stove, a pot with 1200cm2 is expected to achieve above 45%. ARC uses 26cm to 30cm in diameter pots with at least 5 liters of water to get closer to 50% thermal efficiency.

Keep in mind that increasing the surface area of the water in a pot also increases the amount of steam emitted, which makes it harder to bring water to full boil in a larger pot (without a lid).

Thermal efficiency, when burning biomass, seems to top out (so far) at around 60%. Perhaps the gases in the channels at the bottom and sides of the pot loose temperature and velocity, resulting in a theoretical upper limit to normal natural draft heat transfer efficiency?

Since doubling velocity ~ doubles heat transfer efficiency it seems likely that if forced draft increased velocity, without reducing gas temperatures, good things might happen?

We’ll give it a try.

Stick Size Matters!

Small sticks make higher temperature gases, better for heat transfer efficiency, but more smoke

Monitoring many fires seems to show that along with density, moisture, etc., the diameter of sticks has a large effect on both heat transfer and combustion efficiency.  

In a Rocket stove without a closing door, there is obviously a lot of cold excess air entering the fire. How do we raise Temperatures without limiting primary air?

Our observations seem to indicate that burning smaller diameter sticks results in more flame/higher temperatures. However, burning smaller diameter sticks also tends to make more smoke. For this reason, it may be that burning small sticks increases thermal efficiency but decreases combustion efficiency.

Conversely big sticks seem to burn slower making less flame, resulting in lower temperatures while making less smoke. Since flame from wood makes smoke, when the wood becomes charcoal, much less PM2.5 is emitted.

The Jet-Flame can burn 2” by 2” sticks and testing shows that PM2.5 gets lower with bigger diameter sticks. The jets of air make the made charcoal very hot and even big sticks stay lit. In a normal Rocket stove without a Jet-Flame, especially with wet wood, only smaller sticks will keep burning. 

The goal is to create as-hot-as-possible gases flowing next to the heat exchanger (pot) while controlling emissions. The size of the sticks does seem to have a significant influence on thermal and combustion efficiency.