This page is about an experimental wood stove designed and built by Hughes Design Ltd in the UK. It is a test bed we built to try and see how close we could get to a carbon-neatral, zero emissions, 100% source of domestic heat. It works, but there is a long, long way to go before it could be anything like a practical product
|WHY WOOD IS GOOD
Wood grows, so it is renewable for ever, and burning it can never emit more carbon gases than the tree itself took in when it was alive, so wood is the greenhouse-neutral fuel. Wood accounts for about 14% of Europe's domestic heating, (but only about 1% in the UK) and is expected to become the largest single source of renewable fuel.
REALLY 100% EFFICIENT?
CAN I GET ONE?
The ingenuity here has been to use not one but two scrubber tubes, working in opposite directions relative to the gas flow. So the pressures they generate partly cancel each other out, leaving just enough (about 10 Pascals) for a domestic stove to work perfectly, at the same time as having double the smoke-scrubbing power.
First, a certain amount of heat must be lost into the chimney so that the gases in there are less dense and so less pulled-down by gravity than the surrounding air. It is this 'Chimney Effect' which makes sure the (poisonous) smoke and waste gases are drawn out to the the atmosphere and that fresh oxygen is drawn into the stove at a rate sufficient to make it burn at the 'unnaturally' high temperature needed to break-down smoke and potential pollutants.
The second reason is that solid fuels contain a lot of water. Even very dry wood can be 20% water - about a cupful in every log - and the flue gas must be kept hot enough all the way along in order to prevent this moisture condensing out. If it partially condenses it produces a sticky, very acid, and very corrosive tar which rapidly eats into or blocks flue-ways.
In the Hughes Condensing Stove the water jet generates the draught, so no heat is needed for that, and the whole flue gas is completely condensed, producing merely slightly dirty water rather than tar. Most of the heat normally lost into the chimney is here captured by the water jet, a simple heat exchanger transfers it to the room.
HOW A FAMOUS SCIENTIST GOT IT WRONG
Wood stoves look simple. They're not.
Like Bertrand Russell said, they seem "something so simple as not to seem worth stating" which on investigation leads to "something so paradoxical that no one will believe it." Stoves have so far resisted all attempts to usefully describe them mathematically - unlike their gas or oil-fired competitors, or major industrial coal plant, the performance of which can be predicted on paper. We've known the actual efficiency of apparently identical stoves to vary from 17% in one case to 83% in another, with the unskilled eye being very hard-pressed to spot any difference between how they're constructed or even how they appear to burn.
Benjamin Franklin thought he could increase the efficiency of wood fires by simply extracting more heat from the waste flue gas, he wrote at length about the simple wisdom of his brilliant idea and produced detailed plans. Fine, until someone actually built one and found that, far from giving more heat from less wood, it barely produced any heat at all, but did make huge clouds of toxic smoke. Franklin gave up on stoves.
DIFFICULT WAS THIS TO DESIGN?
ELSE ON WITH THIS?
They have mostly settled on using an electrostatic precipitator, where an electric charge is applied to the smoke, making the particles stick to a collecting plate. This does work at getting rid of smoke, but it adds to, rather than reduces, the required energy input (it reduces, efficiency). One research group is trying to use a cyclonic separator, where the flue gas is spun at high speed to throw the smoke particles out.
Who are we?