| Boiling Point No. 10 - August 1986 |
By: Ernst Sangen and Piet Visser, Woodburning Stove Group
This recent study was done at the request of the Energy Department c the World Bank, which wanted review of existing literature an comparative analyses of variour stove and user features. During the tests the influence of stove e well as user parameters were assessed and finally 13 stoves were compared concerning performance an. combustion quality.
First problem encountered was the establishment of the testing procedure. Normally charcoal stoves are fired and used with on charge of charcoal only. This charcoal is burned and is usually sufficient to complete a cookie session; sometimes a smalle additional charge is added.
If the stoves are tested accordion to this pattern, it will bedifficult to determine a powe output of the stove. This power output will vary automatically wit the burning stage of the charcoal bed.
Therefore, it was decided-to adopt a testing procedure which will keep the weight of the fuelbed between maximum and a minimum value. This means that testing is started wit a fixed amount of oven dry charcoal (always 300 g) and if the amount ofcharcoal is below a defined level fresh charcoal is added up to 30 g. Doing so, the power level of stove is constant, enabling it measurement.
Apart from power output, also the efficiency and the combustion quality (by measuring the CO/C02 ratio) was analysed. The performance of a stove was judged on the average numbers determined during the total test. Testing was done over the period of time needed to bring the pan contents to the boil and cook it for another 30 minutes. Then the weight of the pan and remaining charcoal were determined in order to calculate the efficiency.
Pan diameter and charge size
First variations in the lower level of the weight of the charcoal bed were determined. For three stoves (Feu Malgache, Kengo Jiko and Thai Bucket) this level was varied from 150, 200 to 250g. This however did not show much variation in performance.
The second variable was the pan diameter and thus the content of the pans. Pressed aluminium pans were used with a diameter of 21.5, 24 and 28.5 cm; with contents of 2.5, 3.5 and 6.0 litres of water respectively. Also these variations did not show much influence on the performance of the three stoves.
These preliminary tests were used to determine the optimal charge size and pan diameter. However the variations in the performances were small, there was a slight preference for the big pan and large charge sizes. Therefore all next tests were done with a pan of 28.5 cm diameter and charge sizes of 150 g.
The second phase of the study was concerned with the influence of fuel characteristics on stove performance. Charcoal size (pieces smaller than 2 cm diameter, larger than 5 cm diameter and in between) as well as the moisture content (oven dry, 10%, 20% and 30%) were varied. For Kengo Jiko virtually no effect could be measured for variation, except for the influence of the moisture content on the ratio of CO/C02. For moisture contents of 20% and higher the CO/C02 ratio increased to unacceptably high values (0.40 instead of 0.08 normally).
User and stove parameters
The third phase of the study was measuring the influence of user and stove variations. The grate porosity was varied by plugging the existing holes (100%, 50%, 25% and 10% of the original porosity) and the air inlet area was varied by closing the opening gradually (100%, 50%, 25%, 10% and 5% of the original opening). These tests were done with the Thai Bucket stove. The results are shown in the figures below. It can be clearly seen that no influence is measured on the efficiency and CO/C02 ratio. Power output, however, can be reduced to half the maximum power output. This gives the possibility to vary the power output of a stove, which may be useful for a simmering period. However, the low power output is still considered to be high, especially in combination with the measured efficiency values.
Comparison of the stoves
The last phase of the study was measuring the performance of thirteen stoves, using the large pan (28.5 cm diameter) and large charcoal charges (150 g.). In the table below an overview is given of the most important stove parameters and the results of the tests. The tests were done as described above and then a low power phase was added for stoves which had a provision for reducing the air inlet area. This low power or simmering phase lasted one hour.
From the table above, the consumption of charcoal for a specific task can be calculated. These calculations however have to be checked with real measurements of charcoal consumption. Measurements of this kind are planned in the future. In general, the efficiencies of the stove do not vary that much, most of them being between 40 to 45%. Big difference in the efficiency however can be found during the low power phase; ceramic stoves show a very high efficiency number, while metal stoves show relatively low efficiencies.
In addition to efficiency other criteria have to be considered for the use of a stove. One of these is the time required for a cooking session. If, for example, 45 minutes is the ultimate period to bring 6 litres of water to the boil, then only 8 of the 13 stoves are acceptable.
The last criterion to be discussed here concerning whether a stove is acceptable or not is-the combustion quality. From the table it can be seen that the stoves which have a ceramic liner have the lowest CO/C02 ratios, metal stoves mostly have higher ratios. Stoves like Unicef Jiko and Sakkanal are unacceptably high, especially if they are used indoors.
The test procedure used in this study differs from the standard test by maintaining a constant charcoal bed level throughout and so the results are not directly comparable with those of other tests on these stoves. This may account for the relatively high efficiency percentages shown in the table. It will be interesting to hear if the laboratory test results are confirmed by field trials.