Your choices:
1 Geothermal; 2 District heating; 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) | 2 District heating | Electricity |
| Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| 1 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) |
The over-all characteristics of district heating systems are independent of the actual energy source.
Central heating in single-family houses can be water-borne – which is preferred in case of district heating – or it may be air-borne. In case of water-borne heating, the production of tap water is normally integrated in the same heat exchanger as for the production of radiator water but – of course – with a separate heat exchanger coil. In case of air-borne heating, there will be a need or one water-to-air and one water-to-water heat exchanger circuit.
Though an apartment house in itself constitutes a number of households is the peak need for heat not necessarily equal to the sum of the peak need for heat in the individual households. The reason for this is that chances are that the peak loads in the individual households occur at different times. Though the short-time peak loads will thus tend to even out will the slower daily variations resulting from social life and working hours sum up. Even if the individual households do not have their morning shower at exactly the same time will they all have their showers at about the same hour of the day.
Office buildings will quite frequently have a pronounced need for cooling during office hours because of the excess heat delivered by office electronics and by the fact that the number of persons per m2 in an office building is usually more than the number of people per m2 in homes. The indoor climate in an office building should be adapted to the fact that most people in the building will be sitting most of the day.
Shopping centres, sports centres, schools, hospitals, official buildings and such, aimed to host a large number of people of varying ages and constitutions and not primarily a cadre of sitting middle-aged people, will again pose new demands on the indoor climate control.
In reasonably large district heating systems, the travelling time for the water in the distribution loop amounts to several hours. Thus, in case of a sudden temperature drop and a subsequent increase of the heat demand in the system, the customer at the farthest end may experience that the heat supplied may become insufficient for a number of hours depending on the thermal inertia of the individual building. To avoid this, it is recommended that buildings connected to district heating systems should be equipped with water-borne central heating systems. This is also the system solution providing the highest possible total efficiency. In the case of large variations and extreme demands on the district heating systems there may even be a need for accumulator tanks to increase the thermal inertia in the individual buildings.
With air-borne heating systems in the individual buildings, the thermal inertia is close to zero and there may be a need for a back-up unit with a short response time in the individual house.
Hence the total system performance becomes a combination of the characteristics of the buildings connected and of the district-heating system layout, size and control.