| Boiling Point No. 02 - May 1982 |
Wood and charcoal used for cooking account for nearly 50% of the energy consumption in Thailand. Deforestation has become a problem and the continuing population growth puts ever increasing demands on the wood and fuel supply. Any reduction in the fuel used in the cooking process is a factor of considerable importance. This report looks at the performance of a typical Thai charcoal stove used under 18 different operating conditions and identifies the major heat losses.
A typical Thai charcoal stove is shown in Figs. 1 and 2 and the dimensions of the stove used in this study are given in Figure 3. The measurements included the burning rate, air flow, exhaust gas analysis together with temperature measurement on the walls, combustion bed, inlets and exhaust gas and cooking pan as shown in Figure 4. Simulated cooking tests were carried out using water; the heat to the water and heat lost as steam were measured. The transducer outputs were fed to a minicomputer via a multiplexer and processed online. A theoretical model was used to predict the heat balance.
The following stove parameters were varied:
Area of the air aperture (A)
A1 = 2,500 mm
A2 = 5,000 mm
A3 = 10,000 mm
Distance between the charcoal bed and the bottom of the pan (V)
V1 = 100 mm
V2 = 150 mm
V3 = 170 mm
Number of holes in the grate (G)
G1 = 16
G2 = 37
Diameter of holes 15 mm
The best configuration of these tested was A1V1G1, i.e. the stove with the smallest air aperture, the smallest distance between the charcoal bed and the pan bottom and the grate with the fewest holes.
There was a 40% increase in the charcoal used from the best test (AlV1G1) to the worst test (A3V3G2). The heat input into the pan did not vary much from test to test. The difference in heat output from the charcoal between A1V1C1 and A3V3G2 was 3,015 kJ and 2,532 kJ of this extra heat appears in the flue gas of A3V3G2 as seen by comparing the two sankey diagrams of Figures 5 and 6. With this configuration, the two main heat losses were, the heat to generate steam and the heat stored in the stove wall. The loss by steam could be reduced by using the air control to reduce the heat input-to the pan after the water has reached boiling point. It is however, typical Thai cooking practice to boil away a large amount of water while cooking the rice.
The other major loss to the stove wall could be reduced by making the stove smaller; the test stove was made large to accommodate the changes in dimension between the charcoal bed and the pan bottom. The distance of 100 mm between the charcoal bed and the pan used in the best configuration is about the minimum that can be used without losing some of the stove's versatility e.g. for cooking large quantities of food and sometimes wood is used instead of charcoal
The Thai stove, as developed over the years, is probably not far from the best that can be achieved using readily available local materials. It is much superior to the simple metal charcoal stove found in Africa (Openshaw 1979) which is little more than a metal "radiator": There can be little doubt that a stove could be built using heat shields, modern materials and insulators which would be superior to the Thai stove, but the cost would put it out of the reach of most users and the local material content would be largely lost.
The heat input to the pan is not sensitive to dimensional changes. If sensible use is made of the air aperture control by closing it down after the fire has become established, the charcoal used can be reduced by at least 30% without any significant reduction of the heat input into the pan.
The economic use of charcoal is very much in the hands of users. Recommendations can be made, such as:
1 Proper use of air control
2 Do not overload the stove with charcoal
3 Keep the stove away from draughts
4 Keep the bottom of the pan black
However, the real spur to economy will be price and scarcity. While charcoal is available at affordable prices, old practices are not likely to change.
Heat stored in the stove walls is significant if the stove is only used for a short period. Over 50% of the heat produced during the 30 min test period when starting from cold goes into heating the stove walls, a modified-design which reduces the heat capacity of the stove without any loss of its insulating properties might be worth pursuing.