Decarbonising the German steel industry requires not only mature low-carbon technologies, but also framework conditions that make their deployment feasible in practice. This study identifies the most critical barriers using a mixed-methods approach: 1) a systematic review of 42 publications and 2) semi-structured interviews with experts from all German primary steel producers. The review results in a comprehensive inventory of barriers associated with feasibility dimensions, which serves as a structured basis for the interviews. Experts weighted each barrier and highlighted the most important obstacles.The resulting ranking by practitioners differs markedly from rankings inferred from literature, suggesting that frequency of mention may be a weak indicator of practical relevance. Barriers with high consensus and importance relate to energy costs and the long-term availability of low-carbon electricity and hydrogen, regulatory certainty (EU ETS, CBAM, support schemes), international competition with global overcapacities, and the creation of lead markets for green steel. Socio-cultural barriers appear comparatively minor in this sectoral context.Methodologically, the study demonstrates a structured diagnostic approach: capturing the barrier landscape via literature, refining and prioritising through expert input, and translating top barriers into actionable indicators. The results provide policymakers and energy systems modellers with an empirically grounded, prioritised set of barriers for designing more realistic and feasible decarbonisation pathways for the steel industry.

The Middle East and North Africa region faces critical water scarcity and food security challenges that threaten economic development. Fertilizer use supports food self-sufficiency, but its production is highly water intensive. Supplying desalinated water to a decarbonized fertilizer plant offers an environmentally sustainable pathway. This study investigates co-locating a decarbonized fertilizer plant with a seawater desalination facility, optionally implementing minimum liquid discharge (MLD) to generate additional revenue through recovery of magnesium hydroxide and sodium chloride (NaCl). Three configurations were modeled: a conventional seawater reverse osmosis (SWRO)-based plant; and two MLD configurations using high-pressure RO (HPRO), osmotically-assisted RO (OARO), and crystallizers. Financial performance was assessed using a novel discounted and allocated levelized cost (DALC) method, internal rate of return (IRR), and net present value (NPV). In a Moroccan case study, the conventional configuration achieved the lowest DALC and energy consumption (0.70 USD/m3water and 3.8 KWhel/m3), with an IRR of 23.9 %. The first MLD configuration had higher costs (0.94 USD/m3water, 12.0 KWhel/m3) and a lower IRR (14.5 %), with water recovery limited to 71.4 % due to nonuse of magnesium crystallizer effluent (60.4 % in the conventional setup). Reusing this effluent in the second MLD configuration increased water recovery to 96.7 %, yet higher impurities at the NaCl crystallizer feed reduced the IRR to 9.7 %, which could be improved through financing strategies such as lowering capital costs to endorse the MLD-maximizing option. The findings emphasize advancing impurity removal methods and exploring innovative project financing strategies to enable financially and environmentally sustainable seawater desalination for decarbonized fertilizer production.

The use of fossil fuels is the main cause of greenhouse gas emissions and a major driver of climate change. Although direct electrification can replace fossil fuels in many applications, certain sectors will continue to require high-density energy sources in the future, such as long-distance aviation, maritime shipping, and some high-temperature processes in the industry. For these applications, electricity-based synthetic gases and fuels, produced from renewable energies through Power-to-X (PtX) processes, represent a sustainable solution. This paper presents a novel approach for the large-scale estimation of PtX production costs and potentials, considering regional solar and wind power conditions. At its core is the DLR_ITEMS GlobalFuel framework, which calculates local production costs using temporally and spatially resolved data. The framework uses linear optimization to design cost-optimal systems by dimensioning the components for maximal efficiency. It accommodates various converter and storage technologies, enabling high utilization rates even with fluctuating renewable electricity supply. The methodology supports the analysis of multiple pathways by integrating different technologies and assumptions, including wind power, photovoltaics or concentrated solar power, electrolysis, and synthesis. This analysis provides a spatially detailed overview of future PtX generation potential and its levelized production costs. Additionally, the inclusion of domestic demand for renewable energy (RE) and green fuels enables realistic estimates of the export potentials of individual countries, highlighting production and export hot spots. This work focuses on the cost potentials of Fischer–Tropsch fuel production in the Middle East and North Africa (MENA) region. The findings indicate that domestic demand in large territorial countries has a moderate impact on the increase in levelized production costs for export potential. In contrast, small countries often have to fully utilize their limited renewable energy resources for domestic supply of electricity and green fuels. The analysis identifies the most favorable fuel potentials in the MENA region, with cost-optimal ranges between 190 and 220 €/MWh and an estimated annual production potential of 3,400 TWh in 2030. Of this, approximately 3,100 TWh could be available for export markets.

Lowering the use of fossil fuels not only mitigates climate effects by decreasing the emission of greenhouse gases, but also reduces the release of harmful air pollutants into the atmosphere. Thus, the transition to a carbon-free energy system in the upcoming years could potentially have a major impact on lower air pollutant emissions, leading to better air quality and less harmful impacts on human health and ecosystems. Currently, emissions from power plants in the energy supply sector (e.g. coal or oil) contribute strongly to total air pollutant emissions in Europe. Among others, especially emissions of sulphur oxides (SOx), nitrogen oxides (NOx) and particulate matter (PM) are highly relevant regarding air quality issues. In order to be able to make informed statements about the impact of the European energy transition and the phase-out of fossil fuels on air quality, providing detailed information on the temporal and spatial character of air pollutant emissions in the future are required. However, the future projection of air pollutant emissions from power plants poses a major challenge because it is influenced by various factors like the pace of renewable energy rollout, power line capacities and the phase-out of fossil power plants. This work aims to provide estimates of NOx, SOx and PM emissions from power plants in Europe for the year 2030 and to analyse the temporal and spatial dynamics of these emissions in differing energy transition scenarios compared to current emission characteristics. The energy system model framework REMix is used to model activities of power plants in 2030. It considers the effects of power line capacities, renewable energy capacity increase, consumption patterns and the future power plant fleet of European countries in order to simulate power plant activities in high spatial and temporal resolution. The corresponding emission projections are based on current emission factors of power plants, e.g. from emission reports and information on installed flue gas cleaning systems, and are modelled considering the implementation of European emission standards for power plants in 2030. The results show that ambitious scenarios for the energy transition cause significant changes in the spatial and temporal occurrence of the considered air pollutant emissions compared to the current emission characteristics of power plants in Europe.

Scientific literature and the energy policies of many countries indicate that hydrogen and its derivatives, such as ammonia and synthetic hydrocarbons, are likely to play an important role in future energy systems and economies. Global plans indicate that import-export energy markets will likely continue to be part of future energy systems, but there has been limited literature on the specific evolution of international energy trade with regards to magnitudes, potential energy carriers, and influence of social and economic factors. Here we review and discuss various aspects of hydrogen’s potential for becoming a globally tradeable energy commodity in the context of the Frontiers in Energy Research Topic: “Export and import of electrolytic hydrogen using renewable energy and subsequent synthetic fuels between regions–assessment of technology routes, potentials, and strategies”. Technical challenges are likely to impact that hydrogen potential including the form of energy being transported, primary energy and raw material availability and costs, hydrogen generation, derivative synthesis, and transport infrastructure. Timing of infrastructure development is a key consideration due to the potential economic impacts of unusable infrastructure if export, import, and transport capabilities become available at different times. Additionally, we identify and review social and market considerations including hydrogen certification, water availability and use, ecologic considerations, social acceptance, other human factors, investment risk, and market development. Based on those considerations, we propose factors that would benefit nations and investors to consider as they contemplate investments in hydrogen systems and set the stage for further research contributions to this Frontiers in Energy Research Topic.

The practical feasibility of GHG mitigation pathways is increasingly acknowledged as essential to climate scenario development. However, energy system models (ESMs) still lack a structured and comprehensive approach for integrating feasibility considerations. This study proposes a conceptual framework that addresses this gap by guiding the integration of feasibility aspects into model-based scenario studies, with a specific focus on the industrial sector.At the heart of the proposed concept lies the Feasibility Loop—the core contribution of this work. It provides a structured, visual, and process-oriented approach to systematically link existing methods, indicators, and data sources across the entire modelling process. The loop identifies key steps in a model-supported feasibility assessment, clarifies how different types of methods contribute to these steps, and supports modellers in understanding where and how feasibility aspects can be meaningfully integrated.The framework is built around three distinct types of feasibility constraints—hard, quantitative soft, and qualitative soft—which serve as a conceptual bridge between assessment content, modelling tools, and interdisciplinary knowledge. Supporting components include a 5W1H-based structuring of the research context, a typology of feasibility-relevant indicator categories, and guidance for modelling requirements such as granularity and adaptability.Rather than prescribing a fixed workflow, the proposed concept serves as a flexible toolbox, enabling tailored application depending on available resources and research goals. Finally, it aims to improve the relevance, comparability, and transparency of scenario results and support more robust decision-making in the transformation of energy-intensive industry systems.

Reducing greenhouse gas emissions in the transport sector is among the hardest challenges in transforming energy systems to zero emissions. Transport energy demands are driven by an interplay of social behavioral, technical factors, political decisions and economic conditions, motivating detailed transport demand modeling.In Germany, transport energy supply – increasingly from electricity – is expected to challenge the energy supply infrastructure. Recent studies assume large shares of imported clean energy carriers and proclaim global renewable fuel import potentials. Simultaneously, sustainable biofuels’ impacts on required electricity supply infrastructure is yet not well understood.We assess the impact of climate ambition, indirect electrification shares and biofuel availability on energy supply infrastructure in 8 demand scenarios. Coupling the European energy system model REMix with the biofuel allocation model BENOPTex, we calculate cost-minimal energy supply infrastructure for each scenario. This high detail of integrated transport sector and biofuel modeling is novel to energy system analysis.We find that incorporating user preferences in sales decisions clearly narrows the range of transport energy demand. As the German renewable energy potential is exhausted, higher clean fuel demand is covered by imports. Still, the use of these fuels drives the required power grid expansion, and especially electrolysis and fuel production capacities. Biofuel availability may significantly reduce e-fuel demand reducing cost-optimal hydrogen production capacity in the medium term and necessary grid expansion within Germany beyond 2030.The model outcome is limited by assumptions on costs and availability of import options. Future work should further address modal shift transport scenarios.

Urea, a globally dominant synthetic nitrogen fertilizer, presents a complex challenge for India. While promoting agricultural productivity, its production-reliant on natural gas-is projected to drive a threefold increase in India's natural gas consumption by 2050. To meet ambitious climate targets while ensuring food security, India's existing urea plants must be decarbonized. This study conducted techno-economic modeling of "blue" and "green" urea production techniques for all 34 existing urea plants in India, incorporating technologies such as electrolyzers and carbon capture. Using a mixed-integer programming approach from a central planner's perspective, we evaluated key indicators of business-as-usual and decarbonization pathways for the sector under different scenarios. The results indicate that a high level of decarbonization is economically feasible under most scenarios, with the base scenario showing a potential adoption of over 93% green urea by 2050, thus reducing the sector's current natural gas consumption intensity of 645SCM t urea by 96%. This transition also results in a lower freshwater withdrawal intensity of approximately 4 m 3 t urea , which is below India's current average of 6.4m3 t urea . The levelized costs of urea for the decarbonization pathway are more robust against external factors, ranging from 398 to 487 USD 2026 t urea , depending on the scenario. However, these costs must compete with the internationally traded urea prices, which fluctuated between 202 and 925 USD t urea from 2019 to 2024, largely driven by natural gas prices. Low future natural gas prices could be a key barrier to achieving decarbonization and reducing the water intensity of urea. This study suggests that implementing a carbon tax could serve as an effective mitigation strategy in such cases. Future research should consider the integrated modeling of hydrogen and ammonia demands, which are relevant green fuels for the energy transition.

This study presents a novel energy system modelling approach for the analysis and comparison of global energy transition pathways for the decarbonisation of the electricity sector. The results of the International Energy Agency (IEA), and the Teske/DLR scenarios are each reproduced. Additionally, five new energy transition trajectories, called LUT, are presented. The research examines the feasibility of each scenario across nine major regions in 5-year intervals, from 2015 to 2050, under a uniform modelling environment with identical technical and financial assumptions. The main differences between the energy transition paths are identified across: (1) the average electricity generation costs; (2) energy diversity; (3) system flexibility; (4) energy security; and, (5) transition dynamics. All LUT and Teske/DLR scenarios are transitioned to zero CO2 emissions and a 100% renewable energy system by 2050 at the latest. Results reveal that the LUT scenarios are the least-cost pathways, while the Teske/DLR scenarios are centred around energy diversity with slightly higher LCOE of around 10–20%. The IEA shares similarities with the Teske/DLR scenarios in terms of energy diversity yet depends on the continued use of fossil fuels with carbon capture and storage, and nuclear power. The IEA scenario based on current governmental policies presents a worst-case situation regarding CO2 emissions reduction, climate change and overall system costs.

The transport sector is at the center of discussions on accelerating the energy transition due to its still increasing contribution to greenhouse gas emissions worldwide; therefore, the EU has set binding targets for the use of renewable energy in transport through the Renewable Energy Directive. To analyze the economic impact of these targets, we developed an optimization model that considers bio- and electricity-based fuel options, various transport sectors, and future policy requirements. Our study of the German transport sector found that imported alternative fuels play a key role in reducing fossil fuel usage. We also identify two technological and managerial obstacles: policymakers need to prioritize the rapid electrification of vehicles in the near future; and in the distant future, more attention is needed in research for new technologies in commercial transport. Although our findings are tailored to Germany, the employed approach can be transferred to other models and countries.

Green hydrogen and synthetic fuels are increasingly recognized as a key strategic element for the progress of the global energy transition. The Middle East and North Africa (MENA) region, with its large wind and solar potential, is well positioned to generate renewable energy at low cost for the production of green hydrogen and synthetic fuels, and is therefore considered as a potential future producer and exporter. Yet, while solar and wind energy potentials are essential, other factors are expected to play an equally important role for the development of green hydrogen and synthetic fuels (export) sectors. This includes, in particular, adequate industrial capacities and infrastructures. These preconditions vary from country to country, and while they have been often mentioned in the discussion on green hydrogen exports, they have only been examined to a limited extent. This paper employs a case study approach to assess the existing infrastructural and industrial conditions in Jordan, Morocco, and Oman for the development of a green hydrogen and downstream synthetic fuel (export) sector.

Research on 100% renewable energy systems is a relatively recent phenomenon. It was initiated in the mid-1970s, catalyzed by skyrocketing oil prices. Since the mid-2000s, it has quickly evolved into a prominent research field encompassing an expansive and growing number of research groups and organizations across the world. The main conclusion of most of these studies is that 100% renewables is feasible worldwide at low cost. Advanced concepts and methods now enable the field to chart realistic as well as cost- or resource-optimized and efficient transition pathways to a future without the use of fossil fuels. Such proposed pathways in turn, have helped spur 100% renewable energy policy targets and actions, leading to more research. In most transition pathways, solar energy and wind power increasingly emerge as the central pillars of a sustainable energy system combined with energy efficiency measures. Cost-optimization modeling and greater resource availability tend to lead to higher solar photovoltaic shares, while emphasis on energy supply diversification tends to point to higher wind power contributions. Recent research has focused on the challenges and opportunities regarding grid congestion, energy storage, sector coupling, electrification of transport and industry implying power-to-X and hydrogen-to-X, and the inclusion of natural and technical carbon dioxide removal (CDR) approaches. The result is a holistic vision of the transition towards a net-negative greenhouse gas emissions economy that can limit global warming to 1.5 degrees C with a clearly defined carbon budget in a sustainable and cost-effective manner based on 100% renewable energy-industry-CDR systems. Initially, the field encountered very strong skepticism. Therefore, this paper also includes a response to major critiques against 100% renewable energy systems, and also discusses the institutional inertia that hampers adoption by the International Energy Agency and the Intergovernmental Panel on Climate Change, as well as possible negative connections to community acceptance and energy justice. We conclude by discussing how this emergent research field can further progress to the benefit of society.

Germany 2050: For the first time Germany reached a balance between its sources of anthropogenic CO2 to the atmosphere and newly created anthropogenic sinks. This backcasting study presents a fictional future in which this goal was achieved by avoiding (∼645 Mt CO2), reducing (∼50 Mt CO2) and removing (∼60 Mt CO2) carbon emissions. This meant substantial transformation of the energy system, increasing energy efficiency, sector coupling, and electrification, energy storage solutions including synthetic energy carriers, sector‐specific solutions for industry, transport, and agriculture, as well as natural‐sink enhancement and technological carbon dioxide options. All of the above was necessary to achieve a net‐zero CO2 system for Germany by 2050.

Research on 100% renewable energy systems is a relatively recent phenomenon. It was initiated in the mid-1970s, catalyzed by skyrocketing oil prices. Since the mid-2000s, it has quickly evolved into a prominent research field encompassing an expansive and growing number of research groups and organizations across the world. The main conclusion of most of these studies is that 100% renewables is feasible worldwide at low cost. Advanced concepts and methods now enable the field to chart realistic as well as cost- or resource-optimized and efficient transition pathways to a future without the use of fossil fuels. Such proposed pathways in turn, have helped spur 100% renewable energy policy targets and actions, leading to more research. In most transition pathways, solar energy and wind power increasingly emerge as the central pillars of a sustainable energy system combined with energy efficiency measures. Cost-optimization modeling and greater resource availability tend to lead to higher solar photovoltaic shares, while emphasis on energy supply diversification tends to point to higher wind power contributions. Recent research has focused on the challenges and opportunities regarding grid congestion, energy storage, sector coupling, electrification of transport and industry implying power-to-X and hydrogen-to-X, and the inclusion of natural and technical carbon dioxide removal (CDR) approaches. The result is a holistic vision of the transition towards a net-negative greenhouse gas emissions economy that can limit global warming to 1.5°C with a clearly defined carbon budget in a sustainable and cost-effective manner based on 100% renewable energy-industry-CDR systems. Initially, the field encountered very strong skepticism. Therefore, this paper also includes a response to major critiques against 100% renewable energy systems, and also discusses the institutional inertia that hampers adoption by the International Energy Agency and the Intergovernmental Panel on Climate Change, as well as possible negative connections to community acceptance and energy justice. We conclude by discussing how this emergent research field can further progress to the benefit of society.

AbstractThis chapter summarizes all the calculated energy demands for the industry, service, transport, and building sectors. The supply side results for the OECM 1.5 °C scenario are documented. Electricity generation and the power generation required globally are provided by technology, together with the corresponding renewable and fossil energy shares. A detailed overview of the heat demand by sector, the heat temperature levels required for industrial process heat, and the OECM 1.5 °C heat supply trajectories by technology are presented, in both total generation and installed capacities. The calculated global final and primary energy demands, carbon intensities by source, and energy-related CO2 emissions by sector are given. Finally, the chapter provides the global carbon budgets by sector.

AbstractThis section summarizes the main findings of all parts of the research, with priority given to the most important findings to avoid the repetition of previous chapters. The key findings for the industry, services, buildings, and transport sectors, including the 12 sub-sectors analyzed, are provided and discussed. Policy recommendations for each sector and recommendations for the actions for governments, industries, the real economy, and financial institutions are offered.

We present an energy transition pathway constrained by a total CO2 budget of 7 Gt allocated to the German energy system after 2020, the Budget Scenario (BS). We apply a normative backcasting approach for scenario building based on historical data and assumptions from existing scenario studies. The modeling approach combines a comprehensive energy system model (ESM) with REMix-a cost optimization model for power and heat that explicitly incorporates sector coupling. To achieve the necessary CO2 reduction, the scenario focuses on electrifying all end use sectors until 2030, adding 1.5-2 million electric vehicles to the road per year. In buildings, 400,000-500,000 heat pumps would be installed annually by 2030, and the share of district heating would double until 2050. In the scenario, coal needs to be phased out by 2030. Wind and Photovoltaic (PV) capacities would need to more than double to 290 GW by 2030 and reach 500 GW by 2050. The BS results indicate that a significant acceleration of the energy transition is necessary before 2030 and that this higher pace must be maintained thereafter until 2050.

In which way, and in which sectors, will renewable energy be integrated in the German Energy System by 2030, 2040, and 2050? How can the resulting energy system be characterised following a −95% greenhouse gas emission reduction scenario? Which role will hydrogen play? To address these research questions, techno-economic energy system modelling was performed. Evaluation of the resulting operation of energy technologies was carried out from a system and a business point of view. Special consideration of gas technologies, such as hydrogen production, transport, and storage, was taken as a large-scale and long-term energy storage option and key enabler for the decarbonisation of the non-electric sectors. The broad set of results gives insight into the entangled interactions of the future energy technology portfolio and its operation within a coupled energy system. Amongst other energy demands, CO2 emissions, hydrogen production, and future power plant capacities are presented. One main conclusion is that integrating the first elements of a large-scale hydrogen infrastructure into the German energy system, already, by 2030 is necessary for ensuring the supply of upscaling demands across all sectors. Within the regulatory regime of 2020, it seems that this decision may come too late, which jeopardises the achievement of transition targets within the horizon 2050.

AbstractThis section focuses on technologies that provide heat, and especially process heat, with renewable energy and electrical systems. All the technologies described, except those that use high-temperature geothermal or concentrated solar heat (CSH) for process heat, are used for the OECM 1.5 °C pathways described in Chaps. 5, 6, 7, and 8. The authors have included geothermal and solar technologies to highlight the further technical options available and to underscore that more research is required in the area of renewable process heat.

AbstractBackground information is given on the Paris Climate Agreement and the role of nationally determined contributions and net-zero pledges. An overview of historical energy-related CO2 emissions since 1750 and how they relate to economic development, measured in gross domestic product (GDP), is provided, together with the cumulative energy-related CO2 emissions by region. The future energy demand if historical trends in energy efficiency and carbon intensity continue until 2050 is projected. The term ‘science-based target setting’ is defined, and how it relates to the carbon budget published in the Sixth Assessment Report of the IPCC is discussed. The energy-related CO2 emission pathway required to achieve the 1.5 °C target is outlined.