A Nonlinear Optimization Method for the Topological Design of District Heating Systems

In this paper, a method is presented to find optimal topology, pipe sizing, and operational parameters of a district heating system. The current high costs of district heating systems set limits for building new networks regarding the minimum heat demand density. Potential for cost reduction is offered by optimized routing with ideal pipe sizing as well as optimal operating parameters. Nonlinear optimization approaches with a simultaneous calculation of these parameters are limited to a small district heating network with few customers due to scaling computational costs. Therefore, to reduce the computational cost scaling with bigger district heating systems, the optimization problem is subdivided into two iterative subproblems. First, an initial sizing of the district heating pipes is assumed. Second, optimal hydraulic parameters and optimal pipe sizing are determined. In the subsequent optimization, the corresponding thermal solution is calculated. The solution of these two subproblems is iterated until the network converges to an optimal solution. As a proof of concept, a small-scale district heating system is optimized. With this small-scale example the correct functioning of the proposed iterative algorithm is intuitively shown.