# Design Optimization of a District Heating Network Expansion, a Case Study for the Town of Kiruna
## 4 Results and Discussion
This anomaly could be clarified if the investment and operating costs of all the design solutions forming the new population in last generation of the optimization algorithm were re-evaluated for all the considered capacities of the new bio-oil boiler. The results of this re-evaluation are plotted in Figure 7. It can be seen that there exists a Pareto front of optimal compromises between the investment and operating costs for each of the considered bio-oil boiler capacities (dashed lines are plotted in Figure 7 along each of these fronts to help the reader identify them). According to the concept of Pareto dominance, it is logical that the overall Pareto front shown in Figure 6 is the envelope of the Pareto fronts for a fixed value of the new bio-oil boiler capacity. It is apparent in Figure 7 that the fronts obtained for 11 and 12 MW do not contribute to forming the envelope of the Pareto fronts, because the design solutions belonging to these fronts result in higher operating costs for similar investment costs if compared to some of the optimal design solutions  
Figure 7: Pareto fronts of the best compromises between investment and operating costs for each of the considered capacities of the new bio-oil boiler (dashed lines do not represent the actual fronts, they are just added to help the reader locate the solutions having a given boiler capacity). having a bio-oil boiler with a higher or lower capacity. The reason for these higher operating costs is due to the distribution of the heat loads among the boilers at the TVAB site. As the capacity of the new bio-oil boiler is raised from approximately 10 MW, the load of one of the two biomass boilers at the TVAB site is reduced until it reaches its operational minimum. At this point, the load of the second biomass boiler, which was running at full load, would have to be reduced in order to compensate a further reduction of the heat output requested from the TVAB site. In this situation, the operating costs would increase due to the low combined efficiency of the two biomass boilers running at minimum and partial load, until a further increase in the capacity of the new bio-oil boiler (around 13 MW) allows the shutdown of one of the two boilers, so that the other can return to operate at full load. Both Figures 6 and 7 shows that the minimum of the operating costs is around 0.85 SEK/s, which is found for a new bio-oil boiler capacity of 16 MW. Again this is due to the way in which the boilers at the TVAB site are used to balance the thermal loads in the DH network. When the capacity of the new bio-oil boiler is larger than 16 MW, the municipal waste boiler at the TVAB site has to reduce its load. Since TVAB is actually paid to burn the municipal waste, this load reduction causes a significant increase in the operating costs, as can be seen in Figure 7 from the Pareto fronts obtained with new bio-oil boiler capacities of 17 and 18 MW. The investment and operating costs of the optimal solutions in Figure 6 can be further analyzed to understand which terms provide the most significant contributions. The new boiler investment cost is of course a function of its capacity and goes from 60.2 MSEK for 7 MW, to 84 MSEK for 10 MW and up to 128.9 MSEK for 16 MW. It always is the larger share of the overall investment cost, while the investment for the piping layout is almost constant, around 33.1 MSEK, at least for the optimal solutions having a low operating cost. On the other hand, the solutions having a much higher operating cost and a new boiler capacity of 7 MW feature piping layouts with smaller pipe diameters, and in this case the piping investment costs could go down to 25.6 MSEK. Nearly all of the operating cost comes from the cost for heat generation, which rises from 0.834 to 0.867 SEK/s as the capacity of the new boiler is reduced from 16 to 7 MW. The pumping cost has a marginal influence (from 0.015 SEK/s at 16 MW to 0.03 SEK/s at 7 MW) until it grows significantly, up to 0.16 SEK/s, when the diameters of the pipes in the layout are reduced.