# Design Optimization of a District Heating Network Expansion, a Case Study for the Town of Kiruna
## 1 Introduction
The economic objective function included the terms for heat exchanger costs (depending on user demand), new piping costs (depending on diameter and length), pumping costs and the revenue from the heat sold to the consumers. Two scenarios were analyzed, the first considering a 10-year period in net present value calculations, while in the second a 5-year period was considered together with lower piping costsand increased connection fees. In both scenarios it was possible to identify profitable and unprofitable consumers. Thermal losses in delivering the hot water to the consumers were not considered by the model, and a fixed temperature drop was set in the heat exchangers, although it has been shown that the supply water temperature is strongly correlated with these two aspects [1]. [PERSON] et al. [8] investigated the pressure drops per unit length that are currently recommended in the design of DH networks (30-70 Pa/m in China and 100 Pa/m in most of European countries). A tree-shaped DH network in the Hebei province (China) was used as a case study, considering the demand for five outdoor temperatures combined with four operating strategies. The objective of the optimization problem was to minimize the sum of several annualized cost terms, among which the heat distribution cost (related to pumping and heat losses) and the pipe investment cost (related to pipe diameter, length, material, insulation thickness). The results showed that the pressure drops for unit length corresponding to the minimum annualized total costs were lower for the control strategies in which the mass flow rate in the primary side was kept constant (30-50 Pa/m) than for those in which the mass flow rate in the primary side was varied as a function of heat demand (60-100 Pa/m). [PERSON] et al. [9] used a genetic algorithm to compare the difference between distributed variable speed pumps and a conventional central circulating pump, in a district heating and cooling system, the objective being the minimization of the annualized total costs. The case study was the system for Binjiang business center in Changsha (China), which has a tree-shaped piping structure connecting 16 stations. The optimal pipe diameters found by the algorithm were almost the same in both cases, except for two pipes that had a small diameter when variable speed pumps were used. The option of optimizing piping layout was not considered in that work. In the present paper a methodology was proposed to deal with design optimization problems regarding new piping layouts for DH networks that already have a complex meshed structure. As with a series of previous works by the same group of authors, the town of Kiruna, located in the very north of Sweden, was used as a test case. The DH network of this town, which has a mature meshed structure and several heat production sites, has been the starting point for developing a general methodology (outlined in [10]) for modelling and optimizing DH networks of similar complexity using MATLAB and Simulink. A detailed analysis of network behaviour was performed in [11], and local temperature and pressure maps together with the pattern of hot water flow distribution were shown in the results. An optimization of network operation, minimizing the costs due to heat production and distribution along the network, was presented in [12], and the results specified for different levels of user demand the amounts of the different fuels that should be burnt in the boilers of the different heat production sites. The original contribution of this paper is a further step consisting in the implementation of a multi-objective evolutionary algorithm for the design optimization of the structure of a DH network when a new network is to be built from scratch or an existing one has to be expanded.