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Hydrogen is often suggested as a universal fuel that can replace fossil fuels. This paper analyses the feasibility of direct hydrogen utilisation in all energy sectors in a 100% renewable energy system for Europe in 2050 using hour-by-hour energy system analysis. Our results show that using hydrogen for heating purposes has high costs and low energy efficiency. Hydrogen for electricity production is beneficial only in limited quantities to restrict biomass consumption, but increases the system costs due to losses. The transport sector results show that hydrogen is an expensive alternative to liquid e-fuels and electrified transport due to high infrastructure costs and respectively low energy efficiency. The industry sector may benefit from hydrogen to reduce biomass at a lower cost than in the other energy sectors, but electrification and e-methane may be more feasible. Seen from a systems perspective, hydrogen will play a key role in future renewable energy systems, but primarily as e-fuel feedstock rather than direct end-fuel in the hard-to-abate sectors.

With the Danish government's goals of decreasing 70% of CO2 emissions by 2030 and reaching a fully decarbonised society in the years after, this paper aims to identify the role of sustainable bioenergy in achieving this goal. The methodology and approach presented are relevant for other countries heading in the same direction. The focus is on strategies to further develop the sustainable biomass resources and conversion technologies within energy and transport paired with CCUS (carbon capture utilisation and storage) to coordinate with other sectors and achieve a fully decarbonised society. By using hourly energy system modelling and a Smart Energy Systems approach, it is possible to create a robust multiple technology strategy and keep the sustainable bioenergy levels. The results are presented as principles and guidelines on how to include the use of sustainable biomass in the individual country as an integrated part of global decarbonisation.

Today, coal is responsible for 40% of annual CO2 emissions. At the same time, global warming causes climate changes accompanied with catastrophic meteorological phenomena all over the world. After the 2015 Paris Agriment many countries set ambitious energy policy to reduce the annual greenhouse gas emission. The 2021 UN Climate Change Conference, COP26 - Glasgow, ended with the adoption of a less stringent resolution than some anticipated: countries only agreed to “phase down” rather than “phase out” coal. Is possible the realization of the Paris Agreement after COP-26? For achieving this ambitious targets in such conditions, the support of the multi-disciplinary scientific knowledge is needed. Since 2002 a series of SDEWES Conferences were founded. This paper presents an overview of published researches in special issues of leading journals dedicated to the series SDEWES Conferences, including also the papers in current special issue presented on Conferences held in 2020: 2nd LA SDEWES Conference - Buenos Aires, 1st AP SDEWES Conference - Gold Coast, 4th SEE SDEWES Conference - Sarajevo and 15th SDEWES Conference - Cologne. The focus is on five main fields: (1) energy system analysis ; (2) energy savings in the building sector ; (3) district heating ; (4) electrification of transport and (5) water-energy nexus. Undoubtedly, the researches presented in this special issue as well as in previous ones, will contribute to the achievement of the goals of the Paris Agreement in difficult conditions after COP26.

Following the 2015 Paris Agreement and other agreements which have been signed after, sustainable energy development is becoming the major goal in many countries. EU have set the targets to reduce the annual greenhouse gas emission by 40% in 2030 and 80% for 2050 compared with 1990 levels. For achieving this ambitious energy policy, three major technological changes are available: energy savings on the demand side, efficiency improvements in the energy production, and replacement of fossil fuels by various sources of renewable energy. Also, for realization cited three major technological changes the support of the multi-disciplinary scientific knowledge is needed. Therefore, from the beginning of the 21st century, since 2002, a series of Sustainable Development of Energy, Water and Environment Systems (SDEWES) Conferences were founded. This paper presents an overview of published researches in special issues of leading journals dedicated to the series SDEWES Conferences, including also the papers in current special issue. The focus is on five main fields: (1) energy policy and analysis of smart energy systems; (2) energy savings in the building sector; (3) co/polygeneration - integrated heating, cooling and electricity systems and district heating; (4) biomass for energy; and (5) measurement of energo-environmental sustainability of processes and systems. Researchers are focused on the development of methods, policies and technologies for both efficient production of energy, mainly from renewable sources, and consumption in industry, buildings and transport. Undoubtedly, the researches presented in the papers in this special issue as well as in previous ones, dedicated to SDEWES conferences, will contribute to the promotion of sustainable development and thus to the achievement of the goals of the Paris Agreement.

This paper presents a strategy for achieving a fully decarbonized Danish energy system (including transport and industry) in 2045. The strategy could also be relevant for most countries at a global level. The energy system analysis includes hour-by-hour computer simulations leading to the design of a Smart Energy System with the ability to balance all sectors of the complete energy system. In the analysis, issues such as international shipping and aviation, the sustainable use of biomass, and the exchange of electricity and gas with neighbouring countries are all considered. Moreover, the energy system is coordinated with other sectors to achieve a fully decarbonized society. Finally, the size of the employment impact of investing in decarbonizing the Danish economy is discussed.

This report introduces the purpose, functioning, and output of the IndustryPLAN tool. The tool is accompanied by a beta version of the IndustryPLAN tool. Due to the nature of being a beta-version, certain functionalities are not incorporated into the tool at this stage, and continued development will take place as the tool is being finalized. The tool is available upon request to the project coordinator of sEEnergies.

In order to support European-wide transition of heating systems, it is useful to categorise the types of transitions that are necessary. Coherent actions are needed at (supra-)national level to support transition aligned with the energy efficiency first principle and long-term development of a smart energy system. Owing to the decentralised nature of heating, transition must also reflect particular local circumstances. This article uses commonalities between countries to create a representative typology, which can suggest appropriate policies for transition. Following the energy efficiency first principle, transition should include supply-side and demand-side efficiency to ensure coherency and efficient use of resources. Their comparative analysis supports implementing the energy efficiency first principle locally, and a more coherent European strategy for the heating sector. Methodologically, 14 national heating strategies are considered which include current and future energy system developments, demand- and supply side energy efficiency, hectare-level thermal mapping and energy system analysis. Four heat sector types are proposed and discussed. These are (1) extant heat planning traditions, aiming for more efficiency and integration; (2) extant heating infrastructure, aiming to refurbish and upgrade both building stock and existing heating infrastructure; (3) existing gas infrastructure, requiring radical transition; (4) and those without strong historic heat planning traditions.

This paper investigates the impact of applying optimisation and simulation approaches for energy system modelling and scenario design in a municipal context. In doing so, the previous understanding of what constitutes “optimisation” and “simulation” is expanded with further distinctions and it is documented how the outlined modelling approaches are applied in the existing literature. In a practical comparison, it is tested how the choice of modelling approach influences the design of future energy system scenarios for a municipal case study. The energy system scenarios and results obtained from a proposed stepwise simulation approach are compared to an established multi-objective optimisation approach. The results show that for the investigated municipal case it is possible to obtain results with a simulation-based approach that are comparable to the results obtained from multi-objective optimisation. Ultimately, the choice of modelling approach is a complex issue in which modellers must consider the necessary degree of modelling freedom, stakeholder involvement, and available system knowledge. Modellers need to consider not only what tool to use, but also how it is used; a tool can be used for both optimisation and simulation, and both can be valid approaches for developing future energy system scenarios.

Grounded in the idea of meeting the needs of generations across time, sustainable development bears a close relationship to the way in which humanity consumes energy. Nevertheless, the historic notion that energy demand growth reflects improved living standards, economic development and prosperity are challenged when sustainability constraints and the impact on climate change are considered. As a result, a growing body of scientific research is exploring how energy demand can contribute to the energy transition instead of placing it in peril, by means of greater efficiency, digitalization, connectivity and a holistic approach to planning and management. Over the years, the Conference on Sustainable Development of Energy, Water and Environment Systems (SDEWES) has acted as a forum for scientific discourse in this field, shedding light on nuanced discussions and challenging siloed thinking. This special issue contains a selection of four papers that are focused on smart energy demand, demand response and decarbonization, presented at the 2021 SDEWES Conference (16th SDEWES Conference held in Dubrovnik, Croatia).

The transport sector contributes to approximately one third of Danish greenhouse gas (GHG) emissions and almost half of emissions from the energy sector. A unified Danish parliament agreed to reduce total emissions with 70% compared to 1990 levels by 2030. This paper estimates the potential for reducing the national transport sector GHG emissions in 2030 and proposes a pathway towards full decarbonisation in 2045 using a complex set of measures.Towards 2030, the major focus is on an extensive electrification for passenger cars, alongside the implementation of significant measures to achieve lower growth rates for kilometers travelled by car and aircraft. From 2030 onwards, a decisive focus is set on sector integration. Production of electrofuels proves to be a key measure to decarbonize aviation, shipping and long-distance road freight transport.The results show a reduction of GHG emissions of 41% in 2030 and full decarbonisation in 2045. The reduction is achieved without a significant increase of socio-economic costs. From 2030 to 2045, a substantial electrification of road transport and a focus of moving the need for mobility from roads towards rail and bicycles drives the full-decarbonisation together with the replacement of fossil fuels with electrofuels for aviation, shipping and heavy-duty road transport.

Trigeneration compressed air energy storage (TCAES) is one of the emerging solutions that will most likely find its market as a popular energy storage technology for sector coupling. The combination of a TCAES with an organic Rankine cycle (ORC) for waste heat recovery has also been found much effective for enhanced round trip efficiency and is thus to be preferred over conventional TCAES designs. The combined configuration is claimed to offer a very high coefficient of performance (COP) exceeding 1.5. This work aims to quantify realistic performance expectations from a combined TCAES-ORC subject to real operating conditions accompanied for instance by a wind turbine. The system is dynamically modeled, and its performance is analyzed regarding energetic/exergetic efficiency, environment, and economy. To make the investigations close to real-life conditions, a medium-sized 5 MW capacity TCAES-ORC unit is considered integrated with a wind farm for off-peak electricity utilization for storage and tri-generation of heat, cooling, and electricity when charging or discharging. The location of the use case is considered West Denmark, for which wind power production and pricing profiles are available. The results show that there is a considerable collapse in the cooling and power production when the system comes to low operating loads, while heat production potential is not significantly affected. The COP factor decreases from 1.5 in nominal mode to 1.26 in the off-design mode for a sample dynamic load, where the exergetic efficiency is reduced from 64% to 58%. With such a technical operation degradation, the levelized cost of storage (LCOS) is weakened from 141 €/MWh to 153.7 €/MWh, and the potential emission reduction will fall from 4163 to 3640 tonnes of equivalent CO2 per year.

Research into new advanced district heating concepts has increased since the first four generations of district heating were defined in 2014. This definition created a common framework for research and industry alike, and pointed to potential futures for district heating which could benefit from low-temperature heating in buildings. The fully developed fourth-generation district heating includes the cross-sectoral integration into the smart energy system. This paper defines four generations of district cooling to make a similar useful framework for district cooling. The first generation being pipeline refrigeration systems that were first introduced in the late 19th century, the second generation being mainly based on large compression chillers and cold water as distribution fluid, the third generation having a more diversified cold supply such as natural cooling, and the fourth generation combining cooling with other energy sectors sometimes into a renewable energy-based smart energy systems context, including combined heating and cooling.

In 2018 the Conference on Sustainable Development of Energy, Water and Environment Systems (SDEWES) was held in three different locations and periods, so as to offer to the scientific community working in the field several opportunities to share and discuss new ideas and disseminate results of research activities. The 1st Latin American SDEWES Conference was held in Rio de Janeiro (Brasil) in January 28–31 and attracted 180 scientists from 35 countries, with a high number of contributions mainly from Southern American and European academics and professionals, who reciprocally enriched about emerging issues in the field of sustainable development in these continents. Also, the participants ackowledged the inclusion in the programme of 3 invited lectures by eminent scientists and 5 Special Sessions focused on specific major topics. Then, the 3rd South East European Conference on SDEWES was held in Novi Sad (Serbia) from June 30 to July 4, which brought together 190 experts from around 30 countries and whose programme offered 3 invited speechs and 4 Special Sessions. Finally, the traditional Fall Conference on SDEWES was held in Palermo (Italy) from September 30 to October 4, and attracted 400 scientists from around 50 countries with 3 invited speakers and 9 Special Sessions

The present paper describes a quantitative method for preparing local heat supply strategies. Detailed spatial data on heat demand and supply are generated using combined top-down and bottom-up modelling for 14 member states of the European Union, which constitute 91% of its heat demand in buildings. Spatial analysis is used for zoning of heat supply into individual and collective heating. Continuous cost curves are used to model economically feasible district heating shares within prospective supply districts. Excess heat is appraised and allocated to prospective district heating systems by means of a two-stage network allocation process. Access to renewable energy sources such as geothermal, large-scale solar thermal, as well as sustainable biomass, is analysed. The result is a comprehensive and detailed set of heat supply strategies in a spatially discrete manner. The findings indicate that in the 14 European Union member states, up to 71% of building heat demand in urban areas can be met with district heating. Of this, up to 78% can be covered with excess heat, while the remainder can be covered with low enthalpy renewable energy sources. The conclusion shows the possibility of a largely de-carbonised heat sector as part of a smart energy system for Europe.

Germany has set ambitious policies for increasing renewable energy shares and decommissioning nuclear energy, but there are certain scientific gaps on how this transition should occur, especially when considering all energy sectors. The purpose of this study is to advance the knowledge of transitioning the German energy system to 100% renewable energy towards 2050. Taking into consideration renewable resource potentials, energy system costs and primary energy supply this study develops a path for transitioning the German energy system within the heating, industrial, transport and electricity sectors. The analysis demonstrates that it is possible to carry out this transition from a technical and economic perspective with some measures being vital for achieving this ambition in a cost-effective manner. The most significant challenge in this transition is regarding resource potentials where especially biomass resources are constrained and under pressure. Finally, the most influential measures for achieving the renewable transition are discussed.

This paper provides a perspective on the development of future district heating systems and technologies and their role in future smart energy systems. The reviewed papers elaborate on or otherwise contribute to the theoretical scientific understanding of how we can design and implement a suitable and least-cost transformation into a sustainable energy future. Focus is on the important role of the next generation of district heating and cooling technologies. The status of the scientific contributions demonstrates a high level of understanding of how to deal with the technical aspects. The primary current challenge seems to be the understanding of the implementation of these.

Large-scale heating infrastructures in the form of district heating have significant potentials to increase energy efficiency and integrate renewables in line with the Paris Agreement and EU targets. Such infrastructures face challenges due to high investment costs, monopoly situations, regulation, and ownership and are often not supported by status-quo market regimes. This paper investigates how Denmark, the United Kingdom, and the Netherlands achieved high market shares in their heat supply using large-scale infrastructures between 1950 and 1980. The analysis investigates the drivers, actors, organizations, ownership models, financing, policy, and regulation that was involved in implementing these large-scale infrastructures. The findings illustrate how global events such as the oil crisis in 1973 promoted the need for concerted action. The infrastructures were realized through significant government intervention, coordinated work including repurposing existing infrastructure and actors, deploying new regulations, subsidies, and business models. The conclusions reflect on contemporary heat transitions towards renewable energy supply and how historical lessons are relevant for socio-technical transitions today. New heating infrastructures in the form of district heating should be built according to the specific local conditions, through a combination of government support, new regulatory tools, appropriate business models for recirculating funds into new investments, and by engaging existing actors into developing the new supply systems. Achieving high market shares of large-scale heating infrastructures is not business as usual, but requires significant adjustments in all aspects of energy systems. Large-scale heating infrastructures in the form of district heating have significant potentials to increase energy efficiency and integrate renewables in line with the Paris Agreement and EU targets. Such infrastructures face challenges due to high investment costs, monopoly situations, regulation, and ownership and are often not supported by status-quo market regimes. This paper investigates how Denmark, the United Kingdom, and the Netherlands achieved high market shares in their heat supply using large-scale infrastructures between 1950 and 1980. The analysis investigates the drivers, actors, organizations, ownership models, financing, policy, and regulation that was involved in implementing these large-scale infrastructures. The findings illustrate how global events such as the oil crisis in 1973 promoted the need for concerted action. The infrastructures were realized through significant government intervention, coordinated work including repurposing existing infrastructure and actors, deploying new regulations, subsidies, and business models. The conclusions reflect on contemporary heat transitions towards renewable energy supply and how historical lessons are relevant for socio-technical transitions today. New heating infrastructures in the form of district heating should be built according to the specific local conditions, through a combination of government support, new regulatory tools, appropriate business models for recirculating funds into new investments, and by engaging existing actors into developing the new supply systems. Achieving high market shares of large-scale heating infrastructures is not business as usual, but requires significant adjustments in all aspects of energy systems.

In response to the present climate crisis, increased penetration of energy-efficient technologies in the transport sector is of paramount importance if Europe is to achieve its goal of carbon neutrality by 2050. Electrification thus offers great opportunities for increasing the overall energy efficiency of the transport sector. This study analyzes two different electrification scenarios for a 100 % renewable energy-based transport sector in the EU. The first scenario – ‘electrification and e-fuels’ makes use of electro fuels and battery electric vehicles for electrification, whereas in the second – ‘electrification +’ scenario, a deeper level of electrification is envisioned with the use of electric roads for heavy-duty transport.The results from these scenarios are compared with a traditional baseline development for 2050 in terms of final energy demand and transport systems costs. The results indicate that it is entirely possible to achieve carbon neutrality in the transport sector without incurring huge additional costs by going forward with a deep level of electrification.The annual transport system costs for the ‘electrification +’ scenario are comparable to the baseline scenario in 2050. This is possible due to a reduction in the overall energy demand by 26 % in the ‘electrification and e-fuels’ scenario and by 33 % in the ‘electrification +’ scenario as compared to the baseline. The costs related to vehicles increase slightly in the electrification scenarios, but the increased costs are balanced out by the reduction in fuel costs. The results also highlight that options like electric road systems provide an energy-efficient alternative to synthetic electro-fuels for heavy-duty trucks where battery electrification is limited and that these fuels should be reserved for hard to electrify sectors like aviation and shipping.

EnergyPLAN is an energy system analysis tool created for the study and research in the design of future sustainable energy solutions with a special focus on energy systems with high shares of renewable energy sources. The tool has been under development since 1999 and has formed the basis for a substantial number of PhD theses and several hundreds of research papers. EnergyPLAN is designed to exploit the synergies enabled from including the whole energy system, as expressed in the smart energy system concept. Thus, with EnergyPLAN, the user can take a holistic approach focusing on the analysis of the cross-sectoral interaction. Traditionally disparate demand sectors, such as buildings, industry and transport, are linked with supply technologies through electricity, gas, district heating and cooling grids. In this way, EnergyPLAN enables the analysis of the conversion of renewable electricity into other energy carriers, such as heat, hydrogen, green gases and electrofuels, as well as the implementation of energy efficiency improvements and energy conservation. This article describes the overall structure of EnergyPLAN and the essential algorithms and computational structure. EnergyPLAN is an energy system analysis tool created for the study and research in the design of future sustainable energy solutions with a special focus on energy systems with high shares of renewable energy sources. It has been under development since 1999 and has formed the basis for a substantial number of PhD theses and several hundreds of research papers. EnergyPLAN is designed to exploit the synergies enabled from including the whole energy system, as expressed in the smart energy system concept. Thus, with EnergyPLAN, the user can take a holistic approach focusing on the analysis of the cross-sectoral interaction. Traditionally disparate demand sectors, such as buildings, industry and transport, are linked with supply technologies through electricity, gas, district heating and cooling grids. In this way, EnergyPLAN enables the analysis of the conversion of renewable electricity into other energy carriers, such as heat, hydrogen, green gases and electrofuels, as well as the implementation of energy efficiency improvements and energy conservation. This article describes the overall structure of EnergyPLAN and the essential algorithms and computational structure.

The aim of this study is to identify the technical solution space for future fully renewable energy systems that stays within a sustainable biomass demand. In the transition towards non-fossil energy and material systems, biomass is an attractive source of carbon for those demands that also in the non-fossil systems depend on high density, carbon containing fuels and feedstocks. However, extensive land use is already a sustainability challenge and an increase in future demands threat to exceed global sustainable biomass potentials which according to an international expert consensus is around 10 – 30 GJ/person/year in 2050. Our analytical review of 16 scenarios from 8 independent studies of fully renewable energy system designs, and synthesis of 9 generic system designs, reveals the significance of the role of electrification and hydrogen integration for building a fully renewable energy system which respects the global biomass limitations. The biomass demand of different fully renewable energy system designs was found to lie in the range of 0 GJ/person/year for highly integrated, electrified, pure electrofuel scenarios with up to 25 GJ/person/year of hydrogen to above 200 GJ/person/year for poorly integrated, full bioenergy scenarios with no electrification or hydrogen integration. It was found that a high degree of system electrification and hydrogen integration of at least 15 GJ/person/year is required to stay within sustainable biomass limits.

There are three strategic levels for successful energy planning in cities: 1) Integration strategy for integrating energy planning into urban planning institutions; 2) Practice strategy for developing suitable energy planning practices in urban planning institutions, and 3) Vision strategy for the creation and integration of energy visions and scenarios required for long-term decarbonisation. The vision strategy is critical but not well researched and is the focus of this article. Using Strategic Energy Planning (SEP) as an analytical framework, the vision strategy of eight forerunner European cities are analysed. Some critical elements of SEP include the use of long-term targets, holistic energy system thinking, and retention of scenarios. The results indicate that the level of understanding and practice of the vision strategy is still deficient in the cities. Cities often use the practice of urban planning, which does not fit very well with energy planning, particularly with the vision strategy. The energy planning in the cities mostly focuses on shorter-term goals and actions, and they often abandon energy scenarios once extracted. However, through trial and error, some cities are finding ways forward. The article concludes with several recommendations, particularly that cities need to see scenarios as retainable long-term servants providing information desired by the planner, rather than serving as a guide to the planner.

EU is moving towards a climate neutrality goal in 2050 with heating of buildings posing a major challenge. This paper provides a deep understanding of the historical development, path dependency and current status of the EU-28 residential heat sectors to inform strategy and policy makers and to open up this black box. Data is combined for buildings, installed technologies, fuel consumption and energy supply for Member States from 1990 to 2015, to analyse the importance of large-scale infrastructures and supply chains. Primary energy supply for residential heating is mainly based on fossil fuels, 70% in 2015 with 69% imported. The building level technologies are dominated by non-condensing boilers and stoves. Primary and final energy consumption decreased in spite of an increase in the total occupied living area in most countries. Path-dependency effects are found in the residential heat supply in EU. The analysis show path-dependent trajectories are present in most Member States, especially regarding natural gas infrastructure. The period shows many options for decarbonisation are not used to the full potential, e.g., energy efficiency in buildings, district heating, heat pumps. Past experiences should be considered when developing new decarbonisation strategies in Member States and on the EU level.

In a world powered by intermittent renewable energy, electrolyzers will play a central role in converting electrical energy into chemical energy, thereby decoupling the production of transport fuels and chemicals from today’s fossil resources and decreasing the reliance on bioenergy. Solid oxide electrolysis cells (SOECs) offer two major advantages over alternative electrolysis technologies. First, their high operating temperatures result in favorable thermodynamics and reaction kinetics, enabling unrivaled conversion efficiencies. Second, SOECs can be thermally integrated with downstream chemical syntheses, such as the production of methanol, dimethyl ether, synthetic fuels, or ammonia. SOEC technology has witnessed tremendous improvements during the past 10 to 15 years and is approaching maturity, driven by advances at the cell, stack, and system levels. In a world powered by intermittent renewable energy, electrolyzers will play a central role in converting electrical energy into chemical energy, thereby decoupling the production of transport fuels and chemicals from today’s fossil resources and decreasing the reliance on bioenergy. Solid oxide electrolysis cells (SOECs) offer two major advantages over alternative electrolysis technologies. First, their high operating temperatures result in favorable thermodynamics and reaction kinetics, enabling unrivaled conversion efficiencies. Second, SOECs can be thermally integrated with downstream chemical syntheses, such as the production of methanol, dimethyl ether, synthetic fuels, or ammonia. SOEC technology has witnessed tremendous improvements during the past 10 to 15 years and is approaching maturity, driven by advances at the cell, stack, and system levels.

This short paper introduces the contents of selected papers from the 4th International Conference on Smart Energy Systems and 4th Generation District Heating – SES4DH. All of the papers elaborate on or contribute to the theoretical scientific understanding on how we can design and implement a suitable and least-cost transformation into a sustainable energy future with a focus on the important role of district heating and cooling technologies. This short paper introduces the contents of selected papers from the 4th International Conference on Smart Energy Systems and 4th Generation District Heating – SES4DH. All of the papers elaborate on or contribute to the theoretical scientific understanding on how we can design and implement a suitable and least-cost transformation into a sustainable energy future with a focus on the important role of district heating and cooling technologies.

Ever since 2002, when the first Conference on Sustainable Development of Energy, Water and Environment Systems (SDEWES) was held in Dubrovnik, the SDEWES conferences series has been providing a forum for world-wide scientists and those interested in learning about the sustainability of development, to present research progress and to discuss the state of the art, the future directions and priorities in the various areas of sustainable development. This includes the improvement and dissemination of knowledge on methods, policies and technologies for increasing the sustainability of development, taking into account its economic, environmental and social pillars, as well as methods for assessing and measuring sustainability of development, regarding energy, transport, water, food production and environment systems and their many combinations. SDEWES has maintained high publishing standard as nearly 1100 SDEWES papers are published in special issues of leading scientific journals including “Energy - The International Journal".In 2016, the two SDEWES conferences - the regional one, South East European (2nd SEE SDEWES, 15–18 June, Piran/Portorož, Slovenia) and the regular one (11th SDEWES, 4–8 September, Lisbon, Portugal) delivered 538 contributions and 10 special sessions devoted to various sustainability topics. The Energy journal has continued its cooperation with SDEWES launching a special issue dedicated to both 2016 Conferences (SDEWES 2016 SI). It includes 35 papers which focus on various low-carbon technologies in energy supply, buildings, transport, and industry, as well as address other key issues like demand response, co-benefits to other energy policies objectives, the role of declining costs for renewables and markets - all needed to facilitate energy sector transition. This editorial outlines SDEWES contribution, including of SDEWES 2016 SI, in shedding light on the essential elements of that transition.

Fourth-generation district heating (4GDH) has been used as a label or expression since 2008 to describe a transition path for decarbonization of the district heating sector and was defined in more detail in 2014. During recent years, several papers have been published on a concept called fifth generation district heating and cooling (5GDHC). This article identifies differences and similarities between 4GDH and 5GDHC regarding aims and abilities. The analysis shows that these two are common not only in the overarching aim of decarbonization but that they also to some extent share the five essential abilities first defined for 4GDH. The main driver for 5GDHC has been a strong focus on combined heating and cooling, using a collective network close to ambient temperature levels as common heat source or sink for building-level heat pumps. It is found that 5GDHC can be regarded as a promising technology with its own merits, yet a complementary technology that may coexist in parallel with other 4GDH technologies. However, the term “generation” implies a chronological succession, and the label 5GDHC does not seem compatible with the established labels 1GDH to 4GDH.

Energy systems in cities need to be decarbonized and are becoming more integrated via energy sector coupling. Today, cities often use simple methods to assess their low carbon targets, e.g., carbon calculators, and these methods use annualized carbon reduction potentials. For example, reductions from heat savings in buildings or fuel demand in transport. This is done because it is simple and fast. This paper describes a methodology that goes beyond carbon calculators and assesses highly renewable energy systems. The methodology is carried out for a case city&mdash, S&oslash, nderborg, Denmark. Using a national 100% renewable energy study and a suitable energy system analysis tool (EnergyPLAN), the method accounts for inter-sector coupling and energy system dynamics. The energy system is assessed by comparing the results from the analysis tool against numerous key sustainability factors for a Smart Energy System. The paper illustrates how the method delivers a sustainable 100% renewable Smart Energy System for S&oslash, nderborg, which can be part of the Danish energy system in 2050 based on local resources. The paper discusses the broader applicability of the method within strategic energy planning.

The environmental impact of the energy sector and the security and economics of energy supply and utilisation have been raising increasing concerns, stimulating the search for innovative solutions for a sustainable use of resources. This article provides an overview of published research in this area, with a focus on papers contributed in special issues of leading journals dedicated to the series of Conferences on Sustainable Development of Energy, Water and Environment Systems (SDEWES), including the articles in the current special issue. Based on this review, research trends are identified and achievements supporting the energy transition are highlighted. The studies that focused on regional or urban energy planning have aimed at (a) increasing penetration of renewable energy sources, (b) optimizing the mix of centralized and distributed technologies (c) introducing additional flexibility in the power sector and (d) identifying solutions for sustainable heating. Conversely, researches focused on single plants or users have targeted the development of technologies for efficient energy use in industry and buildings, with emphasis on multi-generation and waste heat utilisation. The review highlights that future scenarios based on sustainable energy systems are viable, although their implementation will require further efforts by researchers and investors and active involvement of consumers.

United Kingdom and the Nordic power market have plans to interlink directly through a sub-sea power transmission line in The North Sea. Such power market couplings have complicated implications for the interconnected energy systems and for different agents in the common power market. We analyse this case by modelling the hourly operation of the Nordic-UK power market coupling, considering the local district heating (DH) system in each country as well. According to the results, after the operation of the new interconnection between Norway and the UK (North Sea Link), the overall socio-economic benefits (social welfare) in the region will likely improve by 220–230 million euro per year, without considering the cost of the interconnector itself. The UK-Nordic market coupling enhances the flexibility of the UK power system in wind integration, irrespective of the share of wind in the Nordic countries. However, increasing wind capacity in the UK will diminish the expected economic benefits of the link. The merit order effect of wind integration in the UK will reduce the price gap between UK and Norway, and so the congestion income of the link in many hours a year when the link is congested from Norway towards the UK.

The design of future energy systems requires the efficient use of all available renewable resources. Biomass can complement variable renewable energy sources by ensuring energy system flexibility and providing a reliable feedstock to produce renewable fuels. We identify biomass gasification suitable to utilise the limited biomass resources efficiently. In this study, we inquire about its role in a 100% renewable energy system for Denmark and a net-zero energy system for Europe in the year 2050 using hourly energy system analysis. The results indicate bio-electrofuels, produced from biomass gasification and electricity, to enhance the utilisation of wind and electrolysis and reduce the energy system costs and fuels costs compared to CO2-electrofuels from carbon capture and utilisation. Despite the extensive biomass use, overall biomass consumption would be higher without biomass gasification. The production of electromethanol shows low biomass consumption and costs, while Fischer-Tropsch electrofuels may be an alternative for aviation. Syngas from biomass gasification can supplement biogas in stationary applications as power plants, district heat or industry, but future energy systems must meet a balance between producing transport fuels and syngas for stationary units. CO2-electrofuels are found complementary to bio-electrofuels depending on biomass availability and remaining non-fossil CO2 emitters.

The EU energy policy aims at creating a single European electricity market through market couplings and grid expansions. To analyse the implications of such power market couplings, we propose a market-based multi-region energy system model. The model simulates a multi-region power market (by applying market optimization and network theory), with detailed representation of each region as an energy system (by simulation of both heat and power sectors). We examine the impact of further integration of variable renewable energy (VRE) in Germany on the Nordic power market. The results indicate that the average electricity price slightly grows in the Nordic power market after Germany's Energy Transition (Energiewende). Hence, the economic surplus of Nordic consumers diminishes while Nordic producers improve their gain under new market conditions. Considering the gird congestion income, the overall system-level benefits (social welfare) will improve in the Nordic region after Germany's Energiewende. However, this gain is not equally distributed among different Nordic countries and across different stakeholders. Furthermore, the Energiewende slightly increases carbon emissions from power and district heating (DH) sectors, and reduces the flexibility in integration of VRE in some Nordic countries like Denmark. The direct interconnection of Norway and Germany through NordLink will contribute to the flexibility in wind integration in other Nordic countries, such as Denmark and Finland.

Jordan is a country faced with several environmental and energy related issues. It is the Worlds' fourth most water deprived country with a water consumption of only 145 m3 per capita annually, less than a third of the established severe water poverty line. Jordan is also a country rich in wind and solar potential but practically no utilization with 99% of the produced electricity coming from imported fossil fuels resulting in high CO2 emissions and a potential security of supply issue. The utilization of reverse osmosis desalination in a combination with brine operated pump storage units and wind and (PV) photovoltaic plants can tackle both issues. The desalination plants can produce the much needed water and act as a flexible demand to increase the penetration of intermittent renewables supported by the brine operated pump storage units. This paper presents six scenarios for the development of the Jordanian energy system until the year 2050. The results have shown that the demonstrated configuration can increase the share of intermittent renewables in the production of electricity up to 76% resulting in a high reduction of fuel consumption, CO2 emissions and costs. These analyses have been performed using the EnergyPLAN advanced energy system analyses tool.