Bright Stove - Smokeless, Flueless Condensing Wood Stove

This page is about an experimental wood stove designed and built by Hughes DesignLtd in the UK. It works, but there is a long way to go before it could be anything like a practical product

Smokeless, Flueless Condensing Wood Stove 1WHY 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. (Find out more about using wood as a fuel source) 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?

No, not quite. The Hughes Condenser Stove captures about 97% of the heat energy in the wood. Compare that to the, say, 35% for a power station, 27% for a car, or, at very best, 70% for an ordinary wood stove. It means the same heat for about ½ or 1/3 less fuel.

WHY SMOKE MATTERS

Smoke is just little tiny specks of fuel which haven't properly burned. It isn't just nasty to look at, smoke is very, very poisonous. The Hughes Condenser doesn't emit any smoke, which means it doesn't need a chimney. This matters a lot, not just because you can fit it anywhere and save the cost of a chimney, but because smokes are mutagenic and possibly contribute to a quarter of a million deaths each year in Europe alone.

Bright Stove- Smokeless, Flueless Condensing Wood Stove 2WHERE CAN I GET ONE?

The Hughes Condenser Stove is built and working, but it is still a long, long way from being a production product. Copies are being manufactured for trials, but don't expect them to be in the shops by next week.

HOW IT WORKS

At the back of this stove is a slim tube down which water is sprayed at high velocity, this creates a negative pressure draught which sucks the smoke and waste gases from the fire into the tube. The fast-moving water droplets collide with smoke particles and carry them down to a settling tank, where they form a scum, which is skimmed off the top and dribbled into the drains.

This technique, called a 'wet scrubber', is not new, it is widely used to remove smoke and grit from industrial chimneys. What has been tricky has been miniaturising it. The problem is that in order to knock out smoke, the water has to be jetted at a very high velocity, and that generates high pressure, a pressure far too high for a domestic stove to work safely or effectively.

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.

EFFICIENCY

There is a practical limit to how efficient an ordinary stove can be. Although curiosities of the European test procedure can show claimed efficiencies of 80-90%, around 62% is more usual, with the absolute maximum being about 75%. It is possible to burn wood with almost perfect efficiency on an ingeniously-designed conventional stove, but what isn't possible is to capture all the heat it produces. There are two reasons for this;

First, a certain amount of heat must be lost into the chimney so that the gases in there are less dense and so less Bright Stove- Smokeless, Flueless Condensing Wood Stove  3pulled-down by gravity than the surrounding air. (Find out more information about your chimney/flue) 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.

 

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