Your choices:
1 District heating; 2 Biomass (solid); 3 Comfortable indoor climate
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | 3 Comfortable indoor climate |
| Biomass (fermentable sludge) | 1 District heating | Electricity |
| 2 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) |
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.
District heating systems are by necessity placed close to cities or other population centres and need to be of a certain minimum scale to carry the fixed cost for the distribution pipes. At the same time they must be controllable so that they can meet varying demands among the customers. On hot days during summer holiday the heat demand will be extremely low – on cold winter days it will be high. The production of biogas at – for example – a wastewater treatment plan will be relatively constant and if that is to be the base fuel for a district heating plant there must be a possibility to store the gas during low-demand periods. The same thing applies also for the biochemical process of alcohol fermentation – it runs best if the throughput is reasonably constant. Thus, neither of these is very suitable as base-load fuel for a district heating system.
The most practical fuel – from a storage and handling point of view – is instead solid biomass of some quality that is easily available in the region and that can be stored without excessive risks. However – also other renewable energy resources like solar heating, biogas or geothermal energy may well be integrated in the system, but the base-load fuel would usually be solid biomass.
The controllability gained by the use of advanced combustion control and fuel storage – so that the total production can easily be geared up or down – will guarantee that each customer will always have what they have contracted. Reliability and availability must always be the hallmarks of any district heating system.
For the customer one main advantage with district heating as compared to individual heating is that the responsibility is handed over to a central, large-scale, production plant with 24-hour manning and professional personnel. Thus, the risk for breakages and the risk that the central AC-unit must suddenly be replaced at a high cost is minimised. The price paid is, of course, a fixed fee.