# Design Optimization of a District Heating Network Expansion, a Case Study for the Town of Kiruna
## 2 Case Study
### The Expansion of the Existing District Heating Network  
The town of Kiruna is currently undergoing a deep urban transformation due progressive geological deformation caused by iron mining activity. Some of the town districts are planned for relocation to safer zones, and a new town center, which should host 7500 person equivalents, is already under construction within a ground area of approximately 420,000 m\({}^{2}\). The new building blocks are shown as green polygons in Figure 3. While the new areas are being built, the old ones will still be used, so that there will temporarily be a higher heat demand to be satisfied. A new bio-oil boiler, indicated as \"New Boiler\" in Figure 3, will be installed, the capacity of which was among the variables of the design optimization problem.  
According to the plans prepared by Kiruna municipality and TVAB, the piping that serves the new town center will have two connections to the existing network, one at the North East corner of the large south-eastern loop (just close to the location of \"New Boiler\") and the other at the feeding branches of the user area that is immediately West of the new town centre.  
Figure 4 shows the hypothetic piping superstructure that is considered in this paper to supply the buildings of the new town center. These buildings, which are shown as green polygons, are grouped  
Figure 3: A sketch of the district heating (DH) network in Kiruna: pipes belonging to structure loops are shown in blue, consumer feeding branches in red, new heat production plants with factory and boiler symbol, and new building blocks in green.  
into five user areas, and the delivery junctions of these user areas are shown as red double squares. The potential pipes of the superstructure, which are shown as black lines, are placed to connect a series of nodes, shown as black dots, and the delivery junctions of the user areas. The nodes are intermediate piping junctions that are placed to provide a number of alternative paths for the hot water to reach the delivery junctions of the user areas. One of the purposes of the design optimization is precisely to select which alternatives will be implemented in the final piping layout.  
The black pipe heading North from the southeastern loop of the existing network has been already built and has a diameter of 400 mm, whereas the existence of, and the diameter of the other pipes in the superstructure, are among the variables of the design optimization problem, and will be determined as the solution of that problem. For sake of simplicity, the series of pipe segments connecting the end of the built 400 mm pipe to the feeding branch of the user area immediately West of the new town center are considered to belong to one single pipe, and all have the same diameter.  
It is worth noting that the considered superstructure was conceived with the following spatial constraints: the connections with the existing network (as planned by TVAB and Kiruna municipality), the 400 mm pipe already built, and the delivery junctions to the new user areas (that are located approximately in the barycentre of each area). The layout of the pipes and nodes in the considered superstructure follow the criterion of minimizing distances among these fixed locations, as longer than necessary pipe segments would not be part of the optimal design solution anyway due to higher investment costs.