Background: The transition to a sustainable future challenges the current energy grids with the integration of variable, distributed renewable energy sources. On a technical level, multi-energy systems may provide the necessary flexibility to minimise the gap between demand and supply. Suitable methods and tools are necessary to derive relevant results and to support a transition to renewable energy sources. While several, dedicated tools to model grids and infrastructure of single-energy carriers exist, there are no tools capable of modelling multi-energy systems in detail. Thus, this paper presents the necessary aspects to consider when modelling grid-based multi-energy systems, presents three open source frameworks for modelling grid-based energy systems and points out the major challenges. Methodology: The current main aspects and challenges for modelling grid-based energy systems are derived from a literature review. Three open source multi-energy modelling frameworks (Calliope, oemof, urbs) are presented, and the extent to which they consider these aspects and how they tackle challenges is analysed. Grid-based MES modelling: We identified five general energy system modelling aspects (modelling scope, model formulation, spatial coverage, time horizon, data) and three aspects specific to modelling energy grids (level of detail, spatial resolution, temporal resolution). While the specific aspects mainly influence the representation of the technical parts of the energy system and the computational effort, the general aspects primarily relate to the system boundaries and scope of the model. For the evaluation of the modelling results, we identified several assessment criteria, including economic, energetic, exergetic and reliability. Each of the studied open source modelling frameworks provides generic capabilities to model energy converters, and the electricity, gas and district heat networks. However, the general and specific aspects present respective challenges. Relating to the general aspects, complexity of model formulation increases when including additional boundary conditions. The accuracy of the results is also dependent on data quality. Temporal and spatial resolutions are the major specific challenges for modelling the energy infrastructure. Conclusions: There is still a broad field of opportunities for researchers to contribute to grid-based energy system modelling. This encompasses especially the consideration of short-and long-term dynamics of renewable energy sources in planning models.

HyFlow is a grid-based multi-energy system (MES) modelling framework. It aims tomodel the status quo of current energy systems, future scenarios with a high share of fluctuatingenergy sources or additional consumers like electric vehicles, and to compare solution strategies ifcertain parts of the infrastructure are congested. In order to evaluate the congestion limits and thefeasibility and suitability of solution strategies (e.g., energy storage, sector coupling technologies,demand response (DR)), load flow calculations of all three main grid-bound energy carriers areimplemented in one single modelling framework. In addition to the implemented load flow models,it allows the interaction of these grids with the use of hybrid elements. This measure enables aproper assessment of future scenarios, not only for the infrastructure of one energy carrier, but forthe overall energy system. The calculation workflow of HyFlow, including the implemented loadflow calculations, as well as the implementation of the flexibility options, is described in detail inthe methodology section. To demonstrate the wide range of applicability of HyFlow with differentspatial ranges, two case studies referring to current research problems are presented: a city and aregion surrounding the mentioned city. The calculations for the mentioned case studies areperformed for three levels. A “status quo” level, a “high-stress” level with added fluctuatingenergy sources and consumers, and an “improvement” level, where flexibility options areintroduced to the system. The effect of the flexibility options on future energy grids is, therefore,analyzed and evaluated. A wide variety of evaluation criteria can be selected. For example, themaximum load of certain power lines, the self-sufficiency of the overall system, the total transportlosses or the total energy consumption.

In developed countries like Austria the renewable energy potential might outpace the demand. This requires primary energy efficiency measures as well as an energy system design that enables the integration of variable renewable energy sources. Municipal energy systems, which supply customers with heat and electricity, will play an important role in this task. The cumulative exergy consumption methodology considers resource consumption from the raw material to the final product. It includes the exergetic expenses for imported energy as well as for building the energy infrastructure. In this paper, we determine the exergy optimal energy system design of an exemplary municipal energy system by using cumulative exergy consumption minimisation. The results of a case study show that well a linked electricity and heat system using heat pumps, combined heat power plants and battery and thermal storages is necessary. This enables an efficient supply and also provides the necessary flexibilities for integrating variable renewable energy sources.

Im Rahmen der Thematik der Gestaltung eines smarten zukünftigen kommunalen Energieverbundsystems stehen die zukunftsgerechte Entwicklung der Infrastruktur der unterschiedlichen Energieträger (Wärme, Gas, Strom) sowie die CO 2 -neutrale Energiebereitstellung aus regionalen Ressourcen und eine mögliche Sektorkopplung im Fokus. Basierend auf einem zellularen Ansatz wird im Rahmen des "FFG -Smart Cities Demo"-Projektes "Smart Exergy Leoben" ein Modell entwickelt, welches zeigt, dass rund ein Drittel des elektrischen Gesamtenergieverbrauchs durch Ausbau von Photovoltaik in das örtliche Verteilernetz eingespeist werden kann, ohne im heute bestehenden Netz Überlastungen herbeizuführen. Das Modell zeigt, dass -wie zu erwarten ist -besonders in den Mittagsstunden beträchtliche Energieflüsse in die nächst höhere Netzebene rückgespeist werden. Soll der regionale Autarkiegrad eines solchen elektrischen Verteilernetzes erhöht werden, um Belastungen bzw. einen Ausbau der Netzebenen zu vermeiden, ist eine Integration von Speichern an strategischen Punkten in Betracht zu ziehen. Hierbei wird im gegenständlichen "FFG -Stadt der Zukunft"-Projekt "Move2Grid" die Sektorkopplung des elektrischen Netzes mit Elektromobilität untersucht. Dabei gilt es, zukünftig abzuklären, in wieweit Ladestationen, welche möglicherweise mit stationären Speichern ausgestattet sind, zur Lastglättung beitragen können und so möglichweise nötige Netzausbaumaßnahmen reduziert werden können.Schlüsselwörter: zellularer Ansatz; elektrisches Netz; Mittelspannung; Verteilernetz; Photovoltaik Application of the cellular approach to design energy systems for the future. In order to design a smart urban energy system in the future, the focus is on developing infrastructure for the different energy sources (heat, gas, and electricity), producing energy from renewable energy sources and forcing their hybridisation. Based on the cellular approach it is the goal of the project "Smart Exergy Leoben" to develop a model which shows, that it is possible to feed the grid with about 1/3 of the total electricity consumption produced by

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