Your choices:
1 Biomass (digestible sludge); 2 Comfortable indoor climate; 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 | 2 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) |
In case the total gas production at the individual farm exceeds the need of the farm, then an option may be to sell the surplus heat to neighbouring farms. In case such an excess of heat energy is common to several farms, the best option may be to set up a common district heating plant.
Such a joint venture may open up for a larger production unit, maybe large enough to be viable for electricity production using CHP-technology, maybe large enough for cooling production but definitely large enough for a sophisticated combustion control system and a more advanced environmental control. The drawback is that this type of system solution becomes more sensitive to disturbances than individual house heating since there is basically only one production unit and only one distribution system – and if any of these fails...
The second drawback as compared to individual steam raising is that in the case of district heating, the only realistic option is to limit the distribution to hot water. This means that if any of the farms connected to the system has a need for steam, then this steam will call for a separate steam generator to be installed at that farm.
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.