Your choices:
1 Biomass (solid); 2 Comfortable indoor climate; 3 District cooling
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | 3 District cooling | 2 Comfortable indoor climate |
| Biomass (fermentable sludge) | 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 cooling is just like district heating – only that the distribution system handles cold water instead of hot water.
If the cold water has to be produced, because of the capital and the operational costs involved in the system, the system must not be too small. It does not matter for the system as such what fuel is used in the district heating hot-water or steam boiler, but since the system must not be too small to become economically viable, biogas or ethanol are effectively out of question. Hence this type of systems is best installed based on solid biomass as the fuel.
The advantage with tri-generation using absorption heat pumps becomes obvious when it is considered in combination with a CHP electricity production system or in a location where solar heat is abundant. In both these cases may the cost for the heat necessary to run the absorption heat pump be kept low and the electricity saving then becomes the most important feature.
When installed at latitudes where there is a demand for heat during winter and a demand for cooling during summer, tri-generation really comes out the best solution imaginable:
- During winter, the CHP plant runs, producing comfort heat, tap water and electricity.
- During summer, the CHP plant runs but the excess heat (the heat not needed to produce hot tap water with the end-users) is used to produce cooling. At the same time, the plant produces electricity.
Hence, the use of district cooling systems in combination with CHP (known as tri-generation systems) will maintain a high production of electricity during summer while a common CHP-system will exhibit a significant drop in electricity production during summer.
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.