Your choices:
1 District heating; 2 Process heat; 3 Biomass (solid)
| 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) | 1 District heating | Electricity |
| 3 Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| Geothermal | Fuel: Gaseous | 2 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 usually 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. In the case of one or two major industrial customers, the localisation of the plant needs to be chosen so that the total efficiency is maximised, and then it sometimes comes out that placing the plant farther away from the city but closer to the industrial site is favourable. The district heating plant must also be controllable so that it 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.
Even if a specific industrial customer happens to be minor in the system, the customer will still only be one among all the customers, but if the contract guarantees this customer a certain quality (i.e. temperature) and quantity of heat to be supplied, the district heating producer must always answer up to these demands. Thus, the district heating system must be flexible with respect to flow rates and to temperature levels and the central boiler(-s) must also answer up to this.
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.