My Ecofan
If you have ever sat in front a wood burner on a cold winters night there is every chance that topics like energy efficiency and conservation may have come to mind. You may also have noticed that it always ends up warmer nearer the top of the room than it is at the bottom. Which is a bit of a shame as I am sure given the choice we would much rather have it the other way round. Short of installing underfloor heating there doesn't seem like there is too much we can do about this other than fitting a slow rate ceiling fan. The warmed air is lighter than the cooler air which causes it to rise and move towards the ceiling whilst cooler air accumulates lower down generally closer to seating areas. Most people will describe this as convection and its why you often see central heating radiators placed underneath windows.
Eventually the air becomes stratified or layered and slow moving. So, what can we do to disrupt this effect and encourage more of the warmed air into the most used part of the room? One way that many wood stove owners have discovered is the use of stove top Ecofans. Whilst this term was originally trademarked by Caframo of Candada one of the leading manufacturers of stove top fans, the word has become synonymous with these devices from many different manufacturers. Because of the rise in popularity of wood stoves, more manufacturers have entered the market, and the result of this is that since starting to collect the component parts for this project, and actually completing it, the price of a commercially produced Ecofan has reduced significantly to as little as £36 When I started collecting parts the cheapest Ecofan was around £75 - but costs aside, I was inspired by other peoples efforts at making their own. I am also the sort of person who likes a technical challenge and have always been interested in physics.
So what actually is an Ecofan? Put simply there are 4 essential parts: the metal heat sink, an electric motor, a fan and one or two Peltier Modules The modules are held between the flat surfaces of the heat sinks. They are wired in series and the output feeds a small electric motor which in turn drives the fan. The stove top is the heat source which heats up the bottom heat sink. The upper heat sink has a much larger thermal area and quickly begins to reduce the temperature of the upper side of the Peltiers. Due to the thermoelectric effect this creates a current flow and powers the motor which turns the fan. This disrupts the stratification and moves warmed air into the middle of the room.
The heat sinks are reclaimed CPU heat sinks from a Dell server. The Peltiers and electric motor where sourced via eBayUK Because the fan is the one component that is difficult to fabricate accurately and symmetrically I decided to purchase that from a Caframo spares supplier in the UK. Once I had all the parts together assembly was fairly easy. I used thermal paste on either side of the Peltiers before fixing the heat sinks together using single strand 2.5mm copper wire from domestic twin & earth cable. I found this created adequate tension to hold the modules and heat sinks in place. The initial tests where done on my electric cooker griddle with the lead outs soldered together in series and an LCD multimeter measuring output. Interestingly I discovered that blowing cool air through the top heat sink increased output i.e. the greater the temperature differential, the greater the output of the cells. The wiring was put through heat resist braided glass sleeving and held neatly to the heat sink with 15 amp nickel coated copper fuse wire. The motor was mounted on a small piece of alloy plate, again by the use of 1.5mm single strand copper wire, twisted under tension.
My EcoFan in action on YouTube