Your choices so far:
1 Biomass (solid); 2 District cooling
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | 2 District cooling | 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.
In some cases and is some locations cold water may be at hand naturally: cold ground water, cold water from the bottom of lakes, harbour basins or rivers and alike. In such cases, and provided no ecosystem damage is created by raising the temperature of that water a few degrees, such a natural source may completely replace the production circuit. This is known as "free-cooling" systems.
In many cases, though, the cold water has to be produced and in such cases because of the capital and the operational costs involved in the system, the system must not be too small. For "large-scale" systems (15-20 MWth and bigger) solid biomass in combination with either hot-water boilers or steam boilers is the main alternative. The production of the cold water would preferably be done using an absorption heat pump.
The advantage with the absorption heat pump is that it demands much less electricity for the same cooling capacity than the compressor heat pump. The bad news is that the saving in electricity is achieved by a very high demand for heat. That, in turn, means that the final amount of lost energy to be dumped becomes bigger.
Hence, the core process for the use of an absorption heat pump is the production of heat at a suitable temperature, and it so happens that with a proper choice of brine in the heat pump, district heating temperatures are suitable for this type of systems.
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 the absorption heat pump 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, the absorption heat pump in combination with a CHP-installation 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.