Secret to how heat powered wood stove fans (Eco fans) work

Is it really possible that the heat from a wood stove can create electricity?
Enough electricity to power a small fan?

What is an heat powered fan?

Yes its true. Heat Powered Fans, aka Eco fans, aka stove top fans, are powered only by the heat generated by the wood stove. They do not use any source of electricity.

A heat powered fan sits on top of the wood stove and as the stove comes up to temperature the fan blades start spinning. Some manufacturers claim that a heat powered fan pushing warm air away from a wood stove will help heat a room up to 18% faster.

heat powered fan attached to flue
Heat powered fan attached to a wood stove flue

The other benefit of a heat powered fan is that by moving air around (and creating a convection effect), the wood stove does not have to run as hot and therefore you can use less wood.

How do heat powered wood stove fans work?

Heat powered wood stove fan work when a temperature differential is created within the fan. A high temperature side is heated by the wood stove, and a cold side is cooler by insulation and a heat sink exposed to room air.

There are two common types of heat powered wood stove fan designs, a Thermoelectric Generator which is an electrical design, and a Stirling Engine which is a mechanical design. Both heat powered fan designs work by exposure to temperature differences, where one part is exposed to warmth (as per a wood burning stove), and the other part is kept cooler.

The Thermoelectric Generator heat powered fan

The thermoelectric Eco fan uses inexpensive thermoelectric generator (TEG) modules to provide a small amount of electrical power. The TEG is an electrical device made of two dissimilar metals separated by positively and negatively charged semiconductor material.

When heat is applied to one side of the TEG and there is a sufficient temperature difference to the other side and electrical power can be generated through the force of thermal insulation. The generation of electrical power in this method is known as the Seebeck coefficient.

Quick video explaining how a Thermoelectric Generator heat powered fan works

A TEG when connected between two heat sinks, one side placed directly on top of the hot wood stove, and the other side thermally insulated to create a temperature difference will create enough electricity to power a small DC motor. Multiple TEG’s connected in series will create a higher voltage, and therefore more power to spin a motor fast enough to connect it to fan blades and push enough air around to assist with heating a room. An this is the simple no input power design of a modern wood stove powered Eco fan.

The Stirling Engine heat powered fan

The Stirling engine, (or Stirling Generator), has been developed with many different working designs based on the fundamentals of expanding and compressing fluid or gas in a closed cycle.

Two pistons connected together contain the same gas. The gas is expanded when heated forcing the first piston in a direction that forces the opposing piston to compress. As the fluid or gas in the hot side starts to cool it’s force is lessened, but there is now a force on the second piston that contains the compressed fluid or gas. The second piston the moves in the direction of the first piston, repeating the cycle.

This cycle continues so long as there is a heat source that continues to expanding the fluid or gas in the first piston.

A rotating crank connected to the pistons turns as the pistons move. Fan blades are attached to the crank to make Stirling engine eco fan.

FAQs

We answer some common questions about heat powered fans.

DIY stove fan – How to make a heat powered fan

Well, there’s just so many ways. This video shows a clever design and warns of the problems with over heating. If your keen to make your own Eco fan, this video is a good starting point.