Your choices:
1 Comfortable indoor climate; 2 Biomass (digestible sludge); 3 District heating
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| 2 Biomass (digestible sludge) | District cooling | 1 Comfortable indoor climate |
| Biomass (fermentable sludge) | 3 District heating | 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) |
To be feasible, a district heating system must have a minimum size. This means that the digester must not only be large enough to supply at least a very significant portion of the total energy needed in the district heating network, it must also be situated in close conjunction to the city or the municipality. So this is first and foremost an alternative for such cases when the digester is located at the wastewater treatment plant or when biogas is the contracted fuel. To increase the gas production, collection of organic waste from households and restaurants may well be feasible.
District heating is distributed by the aid of hot (>100 °C) or warm (<100 °C) water.>
The internal distribution of heat in a building with a central heating system is often done so that warm water is produced in a local boiler and the warm water is distributed to the individual rooms and given the chance to heat up the room through radiators, floor heating or other systems.
In buildings without central heating systems the heat is instead distributed using warm air as the distribution medium. However, there will still be a need for hot-water production for tap-water and for hygiene.
The practical difference when the central AC-unit in – for example – a school or an office building is replaced by district heating is that heating function in the main, central, air-conditioning unit is replaced by a heat exchanger. The heat exchanger will need two separate circuits: one for heating of the ventilation air or for the radiator water and one for the production of tap water.
In case the district heating plant is not big enough for tri-generation, so that cooling must be produced locally using air-conditioning unit, but the district heating plant is big enough for CHP-production, then the electricity delivery should be contracted so that also the electricity used for the AC-unit is provided from the biogas-fired CHP-plant.