For sizing district heating pipes in nPro, there are two different methods: design based on load profiles and design based on the maximum heating load. This page explains how both methods work and the differences between them.

How are pipes for a district heating network designed in nPro?

The design of district heating networks in nPro is based on the maximum power that must be transmitted through the pipe. This maximum power is derived from the load profiles of the buildings served by the respective pipe. It is taken into account that the peak loads do not occur simultaneously. This can be considered either through the diversity factor or through the use of hourly resolved load profiles, which show that different building types do not have peak loads at the same time. The pipe design is then based on the maximum power and the maximum volumetric flow, which is crucial for pipe dimensioning. There are two different methods available in nPro: design based on load profiles and design based on the maximum heating load. The distinction between these two methods is important as they yield different results and thus different pipe designs.

Calculation of maximum power

To determine the maximum required power for each building, the load profiles for heating and cooling demands of the buildings must first be calculated. These are determined according to the user's specifications. The maximum required power is then derived from the peak of the load profiles, taking into account any decentralized systems such as decentralized heat pumps.

Diversity factor for network design

The diversity factor typically ranges between 0.5 and 1 and accounts for the fact that the peak demands of buildings do not occur simultaneously. A diversity factor of 1 represents the scenario where all buildings demand maximum power simultaneously. In nPro, there are several options for determining or calculating the diversity factor: heuristic profiles can be used, or a custom functional relationship can be defined via a profile. The diversity factor is then calculated for each network section depending on the number of buildings served by the pipe in question.

Design based on load profiles

In design based on load profiles, the hourly resolved load profiles of all buildings served by the pipe in question are summed. Prosumers who feed heat into the network also play a role. For heat injection from the return to the supply pipe, the injected heat is subtracted from the heat demand, reducing the heat demand that must be met by the energy center. For heat injection from the supply to the return pipe, the injected heat is added to the heat demand, increasing the required volumetric flow in the return (and supply) pipe. The net load profiles at a pipe section are converted into volumetric flows, considering the temperature spread (between supply and return) prevailing at each time. This is particularly relevant for changing temperature spreads throughout the year (e.g., with sliding supply and return temperatures). A higher transferable heat load can result in a lower volumetric flow at a high temperature difference compared to a lower load at a small temperature difference. Thus, the entire year must be analyzed on an hourly basis. The highest occurring volumetric flow determines the pipe dimension. The volumetric flow is then multiplied by the diversity factor to determine the design volumetric flow.

Design based on the maximum heating load

In the design based on the maximum heating load, all maximum heating loads of the buildings served by the pipe in question are summed. It is important to note that cooling loads in the form of decentralized injection are disregarded. The time of the highest cumulative heating load is then critical for the design. The previously calculated diversity factor is then applied to this value to determine the design power and volumetric flow.

Which method should be used?

In the design based on load profiles, the summation of hourly load profiles already accounts for the fact that peak loads do not occur simultaneously. This is particularly relevant for systems with different building types. Additionally, decentralized heat injection (from cooling loads) can reduce the net heat demand in a pipe section. In addition to these effects, the diversity factor is also applied. Therefore, the design method using load profiles is the less conservative design approach. On the other hand, the design using maximum heating loads is more conservative as it reduces the loads only by the diversity factor. It represents the traditional design approach.