Your choices:
1 Biomass (digestible sludge); 2 District heating; 3 Comfortable indoor climate
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| 1 Biomass (digestible sludge) | District cooling | 3 Comfortable indoor climate |
| Biomass (fermentable sludge) | 2 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) |
A comfortable indoor climate is maybe the most common energy service and is one of the most important ones.
Since the indoor climate is a low-temperature service, temperatures ranging from room temperature up to 50-70 °C in tap water and at the most 80 °C in radiator water, the demands on the energy carrier are low and low-exergy carriers such as district heating can be used. There is one exception to this, and this is the energy supplied for illumination which is best supplied as high-exergy, i.e. electricity. Typically, this results in a demand for energy where low-exergy sources are dominant.
Though not part of the indoor climate electricity must also be supplied for the use of household electronics.
The heat supply to a cluster of buildings may be planned according to any of two completely different strategies:
- De-centralized (i.e. individual) energy production.
- Centralized energy production in combination with energy distribution.
In the case of farm-scale biogas production, the first option is usually the case, i.e. the individual farm will take full responsibility for its own heat production using its own gas. The individual farm may well use an internal combustion engine for the production, then producing not only heat but also electricity. If a cluster of farms form a co-operative or in case geographical or other factors are favourable, a small district heating system may be established connecting the farms and supplying heat to all the farms from one central boiler.
This opens 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. 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 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.
As a "small" district heating network, one would consider anything with a peak thermal power less than about 5-10 MW, but there is no strict limit. Small systems would usually not only have lower thermal peak load but would also have a lower line load than larger systems.
The heat for a district heating system can be produced either using "pure" heat production in fuel-fired boilers, stand-alone or in combination with solar heating panels, or using CHP for the simultaneous production of heat and electricity. In the case of biogas, the use of internal combustion engines (IC-engines) is the most common.