Heating network plays a significant role in achieving self-sufficiency in heating energy production. Anniina Koikkalainen studied in her master’s thesis completed in May 2015 the most important properties of heating network in a decentralized heat production system.
The most common heating network type in Finland is a two-pipe district heating network, in which heat is distributed from the producer to the network in water, and the cooled water returns back to the producer. At the moment all of the buildings owned by Aalto CRE are connected to district heating network, and thus water-based heat distribution system was chosen as the basis for the study.
Decentralized heat production covers geothermal heat, solar heat, small-scale heating plants and CHP-plants, and surplus heat. Geothermal heat was chosen as the topic, as the geothermal potential of Otaniemi area is found to be good. Based on the results it can be stated that heating network can improve the usability of geothermal systems in local heat production.
The properties of a heating network were studied by conducting applied calculations to Otaniemi area. In the calculations a 11,3 MW geothermal system consisting of several geothermal heat pumps was compared in four scenarios:
1. separate geothermal systems in each building, covering 50% of the peak demand of each building
2. the system of scenario 1 combined with a heating network
3. only 10 buildings having their own system, covering the peak demand of each of those buildings
4. the system of scenario 3 combined with a heating network
In scenario 1 the potential yearly production is 43 300 MWh, which covers 94% of total heat consumption. If heating network is utilized (scenario 2), the same system can produce 44 300 MWh (96%). In scenarios 3 and 4 the benefits of the heating network are much bigger. Without the network the 10 building-specific systems can produce heat only for those buildings (25 300 MWh, 55% of total consumption), and the heating needs of other buildings need to be covered by other sources. If using a heating network is possible, the system can produce 44 300 MWh –19 000 MWh more than a building-specific system and 96 % of total consumption. With the network the need for the secondary heat source in only 4 % of the total consumption.
Heating network improves the usability of a geothermal system significantly. Without the network the need for secondary heat source is 45,1% of the total consumption ja with the network only 4,0%.
When looking at the calculations it is clear that the role of the heating network is significant, especially if geothermal heat is built only in certain suitable areas. Heating network enables the use of decentralized heat not only in separate buildings but also locally in several buildings according to their heat demands. This evens out the peak heat production and demand of different buildings in the network. The heating network also enables using larger centralized heat storages, which often are more efficient than smaller building-specific storages.
When utilizing decentralized heat sources, the most important characteristic of the heating network is the temperature level. The lower the temperature level of produced heat, the better are both efficiency and usability of solar heat, geothermal heat and surplus heat sources. It would therefore be good to lower the temperature level of the network. However, this is restricted by e.g. the power distribution ability of the network in lower temperatures, pressure loss resulting from higher flow rates, and minimum temperature of the water due to Legionella bacteria.
The results of the study are used when planning the energy production system in Otaniemi area. When local decentralized heat production is planned at a local level in stead of a building-specific level, can local synergies in heat and cool production be better utilized. This implies that the role of the heating network needs to be taken into consideration in the heating energy self-sufficiency plans.
Utilizing the current district heating network in Otaniemi also for decentralized heat production is most likely possible. As the role of the heating network in achieving self-sufficiency in heat production is significant, developing the current heating network together with the owner of the network is important.