The global energy system is undergoing a major transition, and in energy planning and decision-making across governments, industry and academia, models play a crucial role. Because of their policy relevance and contested nature, the transparency and open availability of energy models and data are of particular importance. Here we provide a practical how-to guide based on the collective experience of members of the Open Energy Modelling Initiative (Openmod). We discuss key steps to consider when opening code and data, including determining intellectual property ownership, choosing a licence and appropriate modelling languages, distributing code and data, and providing support and building communities. After illustrating these decisions with examples and lessons learned from the community, we conclude that even though individual researchers' choices are important, institutional changes are still also necessary for more openness and transparency in energy research

Energy system models have become indispensable tools for planning future energy systems by providing insights into different development trajectories. However, sustainable systems with high shares of renewable energy are characterized by growing cross-sectoral interdependencies and decentralized structures. To capture important properties of increasingly complex energy systems, sophisticated and flexible modelling tools are needed. At the same time, open science is becoming increasingly important in energy system modelling. This paper presents the Open Energy Modelling Framework (oemof) as a novel approach to energy system modelling, representation and analysis. The framework provides a toolbox to construct comprehensive energy system models and has been published open source under a free licence. Through collaborative development based on open processes, the framework supports a maximum level of participation, transparency and open science principles in energy system modelling. Based on a generic graph-based description of energy systems, it is well-suited to flexibly model complex cross-sectoral systems and incorporate various modelling approaches. This makes the framework a multi-purpose modelling environment for modelling and analyzing different systems at scales ranging from urban to transnational.

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Energy system models have become indispensable to shape future energy systems by providing insights into different trajectories. However, sustainable systems with high shares of renewable energy are characterised by growing crosssectoral interdependencies and decentralised structures. To capture important properties of increasingly complex energy systems, sophisticated and flexible modelling environments are needed. This paper presents the Open Energy Modelling Framework (oemof) as a novel approach in energy system modelling, representation and analysis. The framework forms a structured set of tools and sub-frameworks to construct comprehensive energy system models and has been published open source under a free licence. Using a collaborative development approach and extensive documentation on different levels, the framework seeks for a maximum level of transparency. Based on a generic graph based description of energy systems it is well suited to flexibly model complex crosssectoral systems ranging from a distributed or urban to a transnational scale.This makes the framework a multi-purpose modelling environment for strategic planning of future energy systems.

Background: The research field of energy system analysis is faced with the challenge of increasingly complex systems and their sustainable transition. The challenges are not only on a technical level but also connected to societal aspects. Energy system modelling plays a decisive role in this field, and model properties define how useful it is in regard to the existing challenges. For energy system models, evaluation methods exist, but we argue that many decisions upon properties are rather made on the model generator or framework level. Thus, this paper presents a qualitative approach to evaluate frameworks in a transparent and structured way regarding their suitability to tackle energy system modelling challenges. Methods: Current main challenges and framework properties that potentially contribute to tackle these challenges are derived from a literature review. The resulting contribution matrix and the described application procedure is then applied exemplarily in a case study in which the properties of the Open Energy Modelling Framework are checked for suitability to each challenge. Results: We identified complexity (1), scientific standards (2), utilisation (3), interdisciplinary modelling (4), and uncertainty (5) as the main challenges. We suggest three major property categories of frameworks with regard to their capability to tackle the challenges: open-source philosophy (1), collaborative modelling (2), and structural properties (3). General findings of the detailed mapping of challenges and properties are that an open-source approach is a precondition for complying with scientific standards and that approaches to tackle the challenges complexity and uncertainty counteract each other. More research in the field of complexity reduction within energy system models is needed. Furthermore, while framework properties can support to address problems of result communication and interdisciplinary modelling, an important part can only be addressed by communication and organisational structures, thus, on a behavioural and social level. Conclusions: We conclude that the relevance of energy system analysis tools needs to be reviewed critically. Their suitability for tackling the identified challenges deserves to be emphasised. The approach presented here is one contribution to improve current evaluation methods by adding this aspect.