Your choices:
1 Biomass (solid); 2 Comfortable indoor climate; 3 District heating
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | 2 Comfortable indoor climate |
| Biomass (fermentable sludge) | 3 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) |
District heating is distributed by the aid of hot water or, in some cases, steam.
The main advantage of district heating is that the production unit becomes large
and hence may accommodate not only advanced environmental control but also advanced
process control, depending on the scale. There are no strict limits for the scales
but from a qualitative point of view one may distinguish three general categories
for water-borne systems, the minimum demand being that there are at the very least
two separate buildings involved so that there is a meaning to the word "distribution":
Small systems.
In the current context, a system will be considered small if the thermal load for a
full year is too small to accommodate steam production. This type of systems might
for example provide the heat needed for tap water and comfort heating in a university
campus, for a number of public buildings in a cluster or for the buildings in a
hospital area.
Intermediate systems.
In the intermediate scale, the system may incorporate steam production. There can be
several reasons for this: There may be a process industry or a laboratory needing
steam among the customers, or the system may simply be big enough to be able to
carry the extra cost associated with a steam boiler.
Large systems.
A large system is one large enough to make power (i.e. electricity) production a main option. In this case, the main focus is often switched from heat sales to electricity sales but it is a mistake to look away from the heat market.
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.
Combined heat and power production allows for a flexibility in product mix and a large and enough advanced boiler will provide for fuel flexibility.