Your choices:
1 Comfortable indoor climate; 2 Biomass (digestible sludge); 3 District cooling
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| 2 Biomass (digestible sludge) | 3 District cooling | 1 Comfortable indoor climate |
| Biomass (fermentable sludge) | District heating | Electricity |
| 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 cooling is an alternative only in such cases when also district heating is an alternative. This means that the digester must not only be large enough to supply at least a very significant portion of the total energy needed in the district heating network, it must also be situated in close conjunction to the city. So this is first and foremost an alternative for such cases when the digester is located at the wastewater treatment plant or when biogas is the contracted fuel for a tri-generation plant. To increase the gas production, collection of organic waste from households and restaurants may well be feasible.
District cooling is distributed by the aid of cold water. District heating is distributed by the aid of hot (>100 °C) or warm (<100 °C) water.
What is needed for a district cooling system to work is that the buildings are provided with a forced ventilation system. The inlet air to the building shall then be cooled by the cold district-cooling water via a heat exchanger prior to being distributed to the individual rooms. This requirement puts a limit to the usefulness of district cooling systems when it comes to single-family houses while it poses no restriction to the use in apartment houses, office buildings and alike.
The practical difference when the central AC-unit in – for example – a school or an office building is replaced by district cooling and heating is that the main, central, air-conditioning unit is replaced by a heat exchanger. For a complete climate control, the heat exchanger may need three circuits: one supplied with district cooling, one for heating of the ventilation air and one for the production of tap water, the latter two both connected to a district heating system.
For the customer one main advantage with district heating and cooling as compared to individual climate control 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.