Your choices so far:
1 Biomass (solid); 2 District heating
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | Comfortable indoor climate |
| Biomass (fermentable sludge) | 2 District heating | Electricity |
| 1 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) |
Anytime a district heating system is large enough it should be designed for combined heat-and-power production (CHP-production). The common technique for this is to install a steam boiler with super heater surfaces, expand the steam through a turbine and then cool the condenser using the district heating water. The gain is that the energy that would be lost in case of condensing production now becomes a valuable product and can be sold to district heating customers. The price paid is that the vacuum at the turbine outlet is reduced, the steam through flow is reduced and the electricity efficiency drops to about 40% or slightly less.
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.