2.4. The energy demand

Although simple biogas plants embody multi-functional agricultural technology, the benefit of energy is almost always primary in the farmer's decision to buy. The demand for fuel in the household - and with this the need for biogas - varies extremely from region to region. Whilst gas from household plants is used for cooking, lighting and cooling, larger plants could be interesting as agricultural inputs. Where there are larger quantities of gas (over 15 m³ per day), motors, incubators, heaters or generators can be operated by the gas. The size of the biogas plant will essentially be linked to the amount of substrate occurring. The gas consumption is then adapted to the gas quantity available. The recurring consumption patterns which are feasible and, primarily, which are realistic is a matter which will have to be investigated.

There are no overall and generally valid values for gas consumption since the consumption patterns and quantities depend on other energy supplies and on the possibilities and preferences for the utilisation of the gas. Lighting is particularly attractive for farming families in regions where there is no electrification. 0.5 m³ gas/per day should be assumed per biogas lamp.

The household demand for energy is greatly influenced by eating and cooking habits. Gas demand for cooking is lower in regions where e.g. preserved vegetables are eaten with white bread or millet soaked in milk than in areas where rice or beans are part of daily nourishment. The supply of energy from biogas plants reaches the limits of its capacity when cooking is carried out only once or twice a week and then as a supply for several days. This method is common e.g. in Central Africa for beans. The biogas plant would then have to be designed for this peak consumption pattern. A disproportionally large gasholder combined with a larger digester would correspond to this peak demand. For reasons of construction, this would lead to high costs particularly for the fixed-dome plant.

Although the women become quickly accustomed to using the biogas stove and can adapt to the use of this new source of energy without any trouble, substitution of conventional sources of energy by biogas also has its limits. In this respect, the following questions have to be answered: can the biogas plant, due to cooking habits, only meet a relatively low share of the energy demand? Does the source of energy which is otherwise used (firewood, charcoal) serve as a source of heating for the inhabitants during winter months? Does the smoke from the open fire conserve foodstuffs or control insects? Does the kitchen also serve as a smoking chamber?

The possible and probable substitution of firewood by biogas should thus be investigated particularly in programmes with specifically ecological objectives where the saving of firewood is a central question.

To define the demand for gas, the previous energy consumption can be taken as a rough value. Here, the profiles of utilisation are to be compiled and these converted into biogas equivalents using standard figures and more exact measurements. The standard figures which are based on experience can be taken from the following list (cf. also the chapter on "Data collection":

Gas for cooking in an Indian or West African household:

1.3 to 2.5 m³/day (depending on family size and eating habits which also depend on the prosperity of the family).

Gas for cooking in an East African household: 2.4 - 3.0 m³/day Gas requirement for one lamp for three hours: 0.5 m³/day Gas requirement for one refrigerator (100 - 160 1): 2.8 - 3.5 m³/day Gas requirement to produce electric energy in a generator: 0.8 - 1.2 m³/kWh Gas requirement for power engines with 20% diesel: 0.5 m³/h* hp.