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.
• 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 radiation increases heat transfer exponentially. *See chart below.
• 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.