Biogas Plants in Animal Husbandry (GTZ, 1989)
 4. Balancing the energy demand with the biogas production
 (introduction...) 4.1 Determining the Energy Demand 4.2 Determining the biogas production 4.3 Sizing the plant 4.4 Balancing the gas production and gas demand by iteration 4.5 Sample calculations

### 4.1 Determining the Energy Demand

The energy demand of any given farm is equal to the sum of all present and future consumption situations, i.e. cooking, lighting, cooling, power generation, etc. With deference to the general orientation of this manual, emphasis is placed on determining the energy demand of a typical family farm.

Experience shows that parallel calculations according to different methods can be useful in avoiding errors in calculating the gas/ energy demand.

Table 4.1: Outline for determining biogas demand (Source: OEKOTOP)

 Energy consumers data Biogas demand (l/d) 1. Gas for cooking (Chapter 5.5.3) Number of persons ............. Number of meals ............. Present energy consumption ............. Present source of energy ............. Gas demand per person and meal (Table 5.17) ............. Gas demand per meal ............. Anticipated gas demand ............... Specific consumption rate of burner ............. Number of burners - ............. Duration of burner operation ............. Anticipated gas demand ............... Total anticipated cooking-gas demand ............... 2. Lighting (Chapter 5.5.3) Specific gas consumption per lamp (Table 5.20) ............. Number of lamps ............. Duration of lamp operation ............. Gas demand ............... 3. Cooling (Chapter 5.5.3) Specific gas consumption X 24 h (Table 5.22) ............. ............... 4. Engines (Chapter 5.5.4) Specific gas consumption per kWh ............. Engine output ............. Operating time ............. Gas demand ............... 5. Miscellaneous consumers Gas demand ............. ............... Anticipated increase in consumption (%) ............... Total biogas demand ............... 1st-priority consumers ............... 2nd-priority consumers ............... 3rd-priority consumers ...............

The following alternative modes of calculation are useful:

Determining biogas demand on the basis of present consumption

. . ., e.g. for ascertaining the cooking-energy demand. This involves either measuring or inquiring as to the present rate of energy consumption in the form of wood/charcoal, kerosene and/or bottled gas.

Calculating biogas demand via comparable-use data

Such data may consist of:

- empirical values from neighboring systems, e.g. biogas consumption per person and meal,

- reference data taken from pertinent literature (cf. chapter 5.5), although this approach involves considerable uncertainty, since cooking-energy consumption depends on local culture-dependent cooking and eating habits and can therefore differ substantially from case to case.

Estimating biogas demand by way of appliance consumption data and assumed periods of use
This approach can only work to the extent that the appliances to be used are known in advance, e.g. a biogas lamp with a specific gas consumption of 1201/h and a planned operating period of 3 in/d, resulting in a gas demand of 360 l/d.

Then, the interested party's energy demand should be tabulated in the form of a requirements list (cf. table 4.1). In that connection, it is very important to attach relative priority values to the various consumers, e.g.:

1st priority: applies only when the biogas plant will cover the demand.
2nd priority: coverage is desirable, since it would promote plant usage.
3rd priority: excess biogas can be put to these uses.