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in the 68th issue of our heatbeat Research Newsletter, we present a recent review article. It examines how the thermal interaction between buried pipes and the surrounding ground is represented in simulation models of low-temperature thermal source networks (TSNs)—and shows why this seemingly minor modelling choice can change key results by more than 40%. In addition, this newsletter provides a brief overview of the most important developments and news regarding heatbeat and our Digital Twin.
In May, we had the opportunity to support a special initiative: Starting this summer semester, Nuremberg Tech (TH Nürnberg), in collaboration with AGFW, is offering a new master’s program in “District Heating Systems.” In the course “Introduction to District Heating and Introduction to the Simulation of Thermal Energy Systems,” we at heatbeat are responsible for the section on the simulation of thermal energy systems. To this end, we provide students with free access to our heatbeat Digital Twin and demonstrate how the design module can be used to simulate complex energy systems. We are pleased to be able to contribute to this important educational program in Nuremberg.
From the development process, we can highlight the following key features of the Digital Twin released in May:
To provide even better support to district heating network operators regarding the rollout and use of remotely readable meters, meter locations and meters can now be managed even more effectively and interactively within the Digital Twin
Working with building data across multiple variants has been significantly improved. In the building dashboard, you can now clearly specify whether changes should be applied specifically to the active network variant or made available as base data for all variants
We have also improved the selection and editing of pipe sections , making adjustments even easier
In addition, since May we have been providing our Insights customers—who have a live data connection to the Digital Twin—with a comprehensively improved and expanded measurement data report. At the start of each week, operators receive a review of the previous week’s operations. This allows them to gain even more insights from the collected measurement data.
In addition, in May we were represented at the Polis Convention in Düsseldorf, the Berlin Energy Days, and the 1st Heat Dialogue Forum in Mannheim. There, through many fruitful discussions, we gained both new ideas for the further development of our Digital Twin and valuable contacts, and we would like to thank the organizers in both cases for the excellent organization and all the participants.
As cities transition toward more efficient and sustainable energy systems, a new class of networks is gaining ground: thermal source networks (TSNs), also referred to as 5th generation district heating and cooling (5GDHC) or anergy networks. Unlike conventional district heating, TSNs operate at very low temperatures—typically between 5 and 40 °C—and rely on decentralised heat pumps and frequently uninsulated distribution pipes. This allows them to provide heating and cooling simultaneously and to integrate low-temperature sources such as geothermal energy, surface water, and waste heat. But it also makes one factor decisive that is often treated as a side effect: the thermal exchange between the pipes and the surrounding ground.
This is exactly where the review by Müller et al. comes in. Through a structured database search complemented by natural-language search and citation tracing (2010–present), the authors identify and classify 56 simulation studies according to how they represent the ground. The result is a clear picture: the majority of studies (43) rely on simplified assumptions, while only a few use transient ground representations (8) or detailed 2D/3D and multiphysics models (5). To make these approaches comparable, the authors develop a classification framework spanning six categories—from Category 0 (no pipe–ground interaction at all) through fixed-temperature models (Category 1a/1b) and transient 1D radial ground models (Category 2) up to high-fidelity 2D/3D multiphysics simulations (Categories 3 and 4).
The central finding is a sharp distinction between insulated and uninsulated networks. Insulated low-temperature networks, typical of 4th generation district heating, aim to minimise ground interaction and can therefore be represented adequately with simplified models. Uninsulated networks, which are characteristic of 5GDHC, behave fundamentally differently: they actively use the ground as a thermal source, sink, or buffer. In these networks, the buried pipes behave much like ground heat exchangers, exhibiting dynamic, transient heat exchange that simplified models simply cannot capture. The authors stress that labelling both network types merely as “low temperature” is misleading, as their operation and modelling needs are distinct.
To quantify the consequences of model choice, the authors ran controlled baseline simulations across categories under identical boundary conditions. The results are striking. In a one-year seasonal simulation, the static-ground models (Categories 1a and 1b) predicted annual heat losses of roughly 2,000 MWh, whereas the transient 1D ground model (Category 2) predicted only 1,170 MWh—a reduction of more than 40%. The reason is physical: as soil near the pipe warms during operation, the local temperature gradient and thus the heat loss rate decrease—an effect that static models cannot reproduce. Notably, adding pipe-wall thermal capacity (Category 1b) had almost no influence on the annual energy balance, confirming that it is the treatment of the ground, not the pipe wall, that drives the difference.
Equally important, the review reveals a recurring inconsistency in current practice. When TSNs incorporate geothermal sources such as borehole or ground heat exchangers, these sources are usually modelled in great detail—while the connected distribution pipes, despite exhibiting very similar ground-coupled behaviour, are simplified or treated as passive transport elements. For uninsulated pipes, this is a missed opportunity, because the same advanced ground models already used for geothermal components could be readily extended to the distribution network, improving both consistency and accuracy. Some advanced studies even report that up to 50% of the delivered thermal energy can originate from ground gains along an uninsulated network.
Based on the reviewed studies and their baseline comparison, the authors derive practical guidance for model selection: simplified models (Categories 0, 1a, 1b) are appropriate for insulated networks and first-level planning, while uninsulated networks—especially below operating temperatures of around 20 °C—require at least a transient ground model (Category 2 or higher) for reliable analysis. Category 2 offers a good balance between accuracy and computational effort for most optimisation and control tasks; more advanced 2D, 3D, or multiphysics models become relevant when asymmetric soil layering, close pipe spacing, or processes such as moisture transport, groundwater flow, and freezing play a decisive role. Throughout, the authors emphasise transparent documentation of boundary conditions, discretisation, and timesteps, and the use of sensitivity analyses whenever simplified ground assumptions inform decisions.
Finally, the review identifies a clear research gap: direct, field-based validation of pipe–ground heat exchange remains rare, as most validation efforts focus on substations or overall energy demand rather than the buried pipes themselves. Publicly accessible long-term datasets under realistic, ground-coupled conditions are needed to reduce uncertainty and enable more robust system optimisation. Overall, the work makes a compelling case that transient ground modelling should be treated as the baseline for credible simulation of uninsulated TSNs—these networks are no longer mere transport pipes, but multidimensional thermal components whose potential can only be unlocked with models that capture their dynamic behaviour.
As always, we recommend that you read the full article. The treatment of pipe–ground interaction is highly relevant for the planning and operation of low-temperature and 5th generation networks, where heat losses—and gains—depend strongly on local ground conditions. In our heatbeat Digital Twin, we account for the thermal behaviour of the network and its surroundings, allowing realistic assessments of losses, gains, and operational performance. Together with our engineering team, we can support you at any time in designing, simulating, and optimising thermal source networks and district heating systems.
The next issue of our newsletter will be published on July 1, 2026.
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