Testing in the laboratory and in the field
It is important to remember that in the Aprovecho lab, the 3 Stone Fire has used less wood and made less pollution than cooking fires and high mass stoves operated by cooks in the field. The fires in the lab tests were carefully made using dry and uniform sticks of Douglas fir fed into the fire in a controlled way to optimize the performance. Well-constructed 3 Stone Fires protected from wind and tended with care scored between 20% and 30% thermal efficiency.
Open fires made with moister wood and operated with less protection from the wind can score as low as 5%. The operator and the conditions of use largely determine the effectiveness of operation. Stoves have to be tested with careful repetition in order to achieve statistical confidence in the results. Because there are so many differences between laboratory and field results, it is difficult to use the results of laboratory testing to predict exactly how stoves will perform in the real world.
However, side-by-side comparisons can be used to generally estimate performance. An automobile that gets 40 miles per gallon on a dynamometer is more likely to use less gas driving down the highway than a car that only gets 20 miles per gallon in the same test. A cooking stove that used less fuel or made less pollution in a standardized test will, one hopes, translate into reductions in the field, but field surveys are needed to establish the actual performance. Field tests are essential to also learn lots of important things, such as if the cook likes the stove, whether the stove product will be successful in the market, and how much PM2.5 and CO is inhaled by members of the household.
In our opinion, no lab test can replace going into the field and learning from reality. And, being taught about stoves from cooks is one of the most fun parts of this job.
Clean Burning Biomass Cookstoves: A Quick Summary
- The stove body and interior (including the combustion chamber) is low mass and insulative. The heat from the fire goes into the cooking process and is not diverted into the stove.
- The heat transfer efficiency is close to optimal resulting in over 40% thermal efficiency. One successful technique is to combine moderate firepower (2.5kW) with very small channel gaps (6mm) around the pot. Burning less wood results in fewer emissions.
- Emissions are reduced by increasing combustion efficiency. An appropriate amount of wood gas is made. The rate of reactions is controlled by adjusting the primary air or by metering the fuel.
- A zone of mixing of air, gases, smoke, and flame is created using jets of secondary air. The jets of secondary air can be powered by natural draft in a Top Lit Up Draft stove or by forced draft in both Rocket and TLUD stoves.
- Increasing the velocity of the jets of air can improve the effectiveness of the zone of mixing.
- The cooling effect of the secondary air jets is not allowed to decrease thermal efficiency below 40%.
- The amount of flame, air, and wood gas entering the zone of mixing is adjusted until close to optimal combustion efficiency is obtained.
- Emissions in the exhaust stream can be further reduced with a catalyst.
- Removing the emissions from the living space in a chimney is mandatory in the United States. The ARC stoves have chimneys to comply with new WHO guidelines.
- The prototype stove moves through an iterative development process by testing one change at a time under the emissions hood. The Water Boiling Test and the Controlled Cooking Test are both used to evolve a stove that is clean burning, fuel efficient, and cooks as well or better than the local model.
- The cooking function of the stove is designed by local users. The market viability of the product is determined by field testing involving stakeholders such as distributors, manufacturers, funders, consumers, etc. Market testing precedes and informs manufacturing.
- Reducing adverse health effects requires the new stove to be a successful intervention. The intervention involves many infield factors that influence the effectiveness of the whole package. Identifying these factors begins the process of creating the successful intervention.
- The stove is only one part of the successful intervention! A recent study in Malawi (commentary, article) of a clean burning stove found no decrease in PM exposure related illnesses, but less than 50% of the stoves were in use.
From “Clean Burning Biomass Cookstoves” found for free at aprovecho.org
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Aprovecho Research Center
PO Box 1175
Cottage Grove, OR 97424, USA