Your choices:
1 Biomass (digestible sludge); 2 Electricity; 3 District heating
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| 1 Biomass (digestible sludge) | District cooling | Comfortable indoor climate |
| Biomass (fermentable sludge) | 3 District heating | 2 Electricity |
| Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| Geothermal | Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| Sunshine | Fuel: Liquid | Process heat (150 - 1000 °C) |
| Water | Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | Local heating (ind. house) |
Even in such cases when electricity is considered the main product, the heat should be used and, if possible, sold: If the electricity efficiency is 20% and losses are assumed at 10%; then the heat will represent 70% of the energy contained in the biogas.
The heat can be used locally, at the production site, for example to maintain the correct temperature in the digesters or as comfort heat in the buildings, or the heat can be sold externally via a district heating network.
In the municipal scale – digesters at wastewater treatment plants or at dump sites designated for organic waste – district heat production with or without simultaneous electricity production is the major option.
The crucial factors for district heating and district cooling networks to be viable are the line load and the heat density:
The line load is defined as the total amount of energy delivered through the network during a specific period of time, typically one year, divided by the total length of the network in meters. The unit then becomes energy/time*length, for example MWh/year*m.
The heat density is defined as the total demand for heat energy for a specific period, typically one year, divided by the area that the houses to be supplied cover. The unit then becomes energy/time*area, for example kWh/year.m2.