This paper describes a novel approach for optimizing a district heating distribution network under various flow rate conditions. For district heating systems, the demand or the flow and pressure at each node varies over the time of year. The flow control that affects the operational cost can be based on the variable speed and the on/off control on serial pumping or pressure controlled valves. In the pipe system design, the topology, or the pipe layout, and the pipe diameter is optimized using genetic algorithms. Standardized methods are used for calculating the pipe thickness, supports, anchors and the thermal expansion loops. The interconnection between the pipe system and the pump station design is discussed. The objective is to minimize the total or life cycle cost (capital maintenance and operational cost), subject to ensuring demands or constraints at all points. The results are compared to classical methods where the pump station and the pipe system are designed separately and the improvements are discussed.

The problem is formulated by developing an objective function where the optimization parameters define the pump arrangement, pipe system topology, and pipe diameters. The pump station and pipe system optimization consist of selecting components from a pre-defined set of elements and is implemented with discrete decision variables. Optimization of pipe elements consists of optimizing the diameter, after the topology has been defined, and is implemented with discrete variables. Flow distribution and pressure analysis is performed. Thicknesses, pressure classes, supports, expansion loops and anchors are not part of the optimization parameters, but are determined during the evaluation of the objective function. Each time the objective function is evaluated, the pipe system is designed in a sub-optimization according to given loads. The pressure head constraints are used to design the pumping curves. The method is tested on a district heating system in Reykjavik, Iceland.

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