Existing Situation¶
The existing design for wood fired stoves used at climate camps in the UK are elbow rocket stoves constructed using a number of 20 liter vegetable oil drums and other similarly thin but easily obtained metal along with an insulting material such a pearlite or vermiculite.
The stoves are not widely used, in fact pretty much only the London neighborhood make use of them routinely and only for heating water for hot drink and washing up. The vast majority of cooking at climate camp makes use of bottled gas from fossil fuel. Gas is convenient and much more predictable and controllable than using firewood* but better designed stoves might go some way to convincing people to use them instead of gas at climate camps.
(We could give up on firewood and switch to charcoal instead which is more predictable and convenient than wood but inefficient in terms of the energy already used to produce the charcoal.)
The current rocket stove design used at climate camps is simply to build and requires only tin snips, drill and rivet gun to construct. Its construction is a good example of reuse and DIY culture but it is far from perfect in use
The Problems¶
- The thin metal on the interior burns out quickly so the stoves are short lived and effectively disposable.
- The construction limits the size of the pots that can be used due to the fixed diameter of the skirt.
- They require constant attendance in order to maintain heat by pushing the fuel into the combustion zone as it burns through.
- They are uncomfortable to use being so low down – especially when starting as the user is likely to get a face full of smoke.
- They must be used outdoors due to having no chimney and being smoky at start up. Not only does the wind outside reduce efficiency but not having a chimney limits the amount of draw and reduces overall efficiency even further.
- Being outdoors they are exposed to rain or dew which makes the insulation damp and reduces thermal efficiency massively as they dry out.
Possible Improvements¶
Based on the above observations, the following improvements would seem desirable:
- Construction should be based on more robust materials which will not burn out. These might be thicker metal, stainless steel, clay and/or refractory cement, ceramics etc.
- Either utilise one standardised pot size or make use of different size or adjustable skirts to fit the desired pots. The skirts should ideally be insulated, especially if the stove is used outside.
- Use a vertical fuel shoot rather than horizontal if less frequent attendance is required.
- Make more efficient use of heat produced by the fuel used by increasing efficiency. One significant way this could be done would be by providing two pot positions rather than one.
- Greater consideration to ergonomics, ie. make the device a more comfortable height for the person feeding the fire or stirring the pot.
- Incorporate a chimney so they can be used inside and/or pay attention to preventing the insulation getting damp from rain or cooking spills etc.
Summary of new design direction¶
A new design would aim to enable cooking, not just the heating of water. It would have a chimney and be designed to use inside. Ergonomic considerations such as the location of the fuel feed would be factored into the design. The materials and construction method used would result in a more robust device suitable for longer term use. It would be designed for specific sized cooking pots large enough for mass catering and might have more than one cooking position in order to get as much usable heat from the stove as possible before lost up the chimney. Ideally it would produce hot water for washing up etc as a bi-product making use of the heat not used for cooking. It would be great if it could also be used for baking but perhaps that is asking too much from a single design ;-)
Factors that improve efficiency¶
Efficiency requires two main elements; 1) the fuel must be fully combusted to maximize the conversion into heat, 2) as much as possible of that heat should reach the item we want heated rather than escape into the surroundings.
To maximise combustion we must ensure the fire burns hot and supply an adequate and well mixed air supply to the right parts of the fire. This requires the following:
- Dry fire wood (otherwise energy is wasted driving off the water and the resulting lower tempature prevent full combustion of gases given off by the wood).
- Insulating the combustion chamber and where appropriate, the chimney.
- Don’t try extracting heat from the combustion chamber, do so later in the heat path.
- Avoid letting in too much air which will cool the fire.
- Preheating the required air intake as much as practical.
- Design secondary air intake to introduce turbulence as it mixes with unburnt gases in smoke.
To maximise desired heat exchange and reduce waste elsewhere:
- Avoiding wasting heat in heating the mass of the stove structure itself (unless desired).
- Insulate the entire heat path.
- Shield pot from wind or convection. Insulate if possible.
- Direct smoke into direct contact with the pot.
- Use high conductivity pots such as aluminum or copper where possible.
- Maximise surface contact areas, eg. us the side as well as bottom of pot.
- Use larger pots to increase available surface area.
- Use multiple pots to further increase surfaces extracting heat.
- Use final lower level ‘waste’ heat to preheat water for later boiling or washing up.
