The future energy systems dominated by variable renewable energy sources require system flexibility for balancing fluctuating supply and demand. This study is motivated by the need to investigate how sector coupling between the power and district heating sectors can enhance flexibility. It is hypothesised that a partially disaggregated sector-coupling approach can efficiently capture interactions between energy generation, conversion, and storage technologies. A mathematical optimisation framework is developed to analyse cost-optimal and environmentally benign energy system transitions in Sweden up to 2050. The model accounts for the ten largest Swedish district heating systems integrated within the national power system. Results reveal that wind turbines, with a 56% share, supported by electricity storage dominate electricity generation in 2050. Electricity storage enables demand to be met with 7% lower installed power generation capacity. The resulting generation mix drives a shift in district heating supply, with the heat generation share of combined heat and power plants declining to 24% and that of heat pumps increasing to 61% by 2050. Seasonal thermal storage systems play an important role in this shift, supplying 11% of district heating demand. However, transitions towards low-temperature district heating reduce the seasonal storage share while further favouring heat pumps (up to 80% of heat generation). Increased availability of stable waste heat for direct district heating supply also diminishes the role of seasonal heat storage. Overall, the results highlight that district heating provides a flexibility service for the energy system, but multiple flexibility solutions are needed to fully exploit electricity oversupply. © 2026 The Author(s).

The transition to a low-carbon economy and sustainable development necessitates the widespread adoption of renewable energy resources. This study implemented a novel methodological approach that combines flexibility to evaluate the interaction of various system design parameters and system efficiency through our newly developed system use index. It investigates the interaction among key system parameters, such as storage capacity, hours of storage, penetration, curtailment, wind-solar mix, and balancing capacity needs, by evaluating a wide range of scenarios using hourly data from PVGIS and GWA, thereby offering deeper insights into issues of system design and performance. These scenarios demonstrate that diurnal storage of less than 0.5 average daily demand is sufficient to satisfy a penetration of 80–90 % of the annual demand, in combination with varying levels of curtailment associated with specific wind-solar mix. However, depending on the resource mix, achieving the final 10–20 % penetration requires applications other than diurnal storage. While some previous studies have observed similar trends in limited scenarios, this work provides broader validation by simulating extensive and diverse scenarios across all major design parameters, confirming that the observed trends are case-independent. More importantly, it links the above system design parameters with system efficiency and demonstrates, for the first time, that an optimal range of these parameters ensures an optimal system efficiency, which forms a multidimensional constraint that is difficult to implement in any technoeconomic modeling tool but provides important boundary conditions for policy and regulation. © 2025 Elsevier Ltd

Driven by cost reductions, improved performance, and advancements in smart technologies, large-scale PV and wind integration is accelerating. This study develops a comprehensive modeling approach to investigate the complex interaction among key system parameters, including the PV-wind mix, storage, curtailment, and balancing capacity, using PVGIS and Global Wind Atlas-derived data for Eritrea, addressing a gap often overlooked in most energy transition models that prioritize economic data over such interactions. The goal is to develop design and operational guidelines that address rising uncertainties in renewable-dominated grids to maximize renewable integration in future power systems. Different storage technologies are deployed to handle the daily and seasonal mismatches. The results show that diurnal storage with a 0.16 average daily demand enables 80% penetration except for a few cases. In all tested scenarios, penetration increases sharply at lower storage levels but gradually slows and stabilizes beyond a certain threshold. Beyond 80% penetration, adding diurnal storage yields minimal gains, as small increases in penetration require disproportionately large diurnal storage or excess generation. However, adding seasonal storage, with about 8 average daily demand at a RE-to-load ratio of 1.2, enables full demand coverage without additional balancing capacity. To optimize the different parameters, the system-use-index introduced in our previous work is adopted. This index provides a holistic measure of system performance by linking system efficiency with other parameters, such as storage and curtailment, thereby enhancing system flexibility and reliability. These insights contribute to the development of scalable, flexible, and sustainable future energy systems. © 2026 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license. http://creativecommons.org/licenses/by/4.0/

District heating and cooling systems can aid in decarbonisation and the provision of efficient heating and cooling in Europe. However, whereas these systems have achieved high penetration rates in colder climates of Northern, Central and Eastern Europe, they remain marginal in milder climates of Southern Europe. In terms of network design, district heating and cooling systems can be configured in different ways. In so-called warm networks, the required temperature for all the consumers is attained city-wide, and in so-called cold systems, the necessary temperature is achieved at the consumers' premises by ancillary equipment. The most cost-effective heating and cooling solution for urban areas requires investigation. This research models and compares cold and warm district energy systems with other heating and cooling solutions through a comprehensive case study executed in the city of Bilbao, Spain. The city is characterised by a mild climate and a high population density which is characteristic of many Southern European cities. The results show that district energy systems are economically advantageous compared to other low-carbon solutions, such as air-source heat pumps. However, these systems are not able to outcompete natural gas under current cost and taxation levels. Warm networks provide a cheaper source of heat compared to cold networks, but both network types lead to similar expenditures for combined heating and cooling supply. This paper, presents the study context and its results, and is complemented by an exhaustive detailed methodology document and a separate supplementary material repository. © 2024 The Authors

The global shift towards renewable energy necessitates careful planning and integration strategies, especially in regions like Eritrea, which have abundant solar and wind resources but limited grid infrastructure. This study explores strategies for maximizing direct renewable energy consumption by incorporating residential photovoltaic (PV) and wind energy into Eritrea's electricity grid. Our research offers a unique approach by proposing tailored grid expansion and management strategies to maximize renewable integration, specifically designed for the context of developing countries like Eritrea, addressing the specific challenges posed by limited infrastructure and data availability. By analyzing historical data and using simulation techniques, the study explored the ideal deployment of PV and battery storage systems to maximize penetration while minimizing curtailment, using a straightforward algorithm for PV injection, battery charging, and discharging for each hour of the year. Key findings reveal that imposing feed-in limit and integrating battery storage significantly reduce curtailment, with a feed-in limit of 0.4 to 0.5 kW/kWp and battery storage below 2 kWh/kWp yielding best results. The analysis also highlights the trade-off between installing additional PV capacity and battery capacity, especially at lower renewable capacity levels. In certain scenarios it is found that curtailment is preferred over storage, particularly at lower PV capacities. The study emphasizes the crucial role of storage utilization and balancing generators in maintaining grid stability during adverse weather and peak demand. These insights provide valuable guidance for policymakers and grid planners to advance sustainable energy strategies and achieve ambitious renewable energy targets in Eritrea. © 2024 The Author(s)

Wind power only received occasional attention since the introduction of electricity until the 1970s, when a revived interest in alternativeenergy sources spurred the development thread that led to today’s wind turbines. Although attention and financial support at thetime were directed toward government-funded MW-scale wind turbines, the small models developed in the late 1970s for the Danishmarket were ultimately the way forward. The wind industry has since matured, as evidenced by the lower specific power and highercapacity factors of recent turbine models and the similarity between their power curve shapes. Moreover, this study highlights two historicalaccomplishments by Europeans that are sometimes incorrectly credited to Americans: the first wind turbine to generate electricitywas built in 1883 by Austrian Josef Friedländer and the Danish Agricco (1919) became the first public grid-connected windturbine. © The Author(s) 2024

The ongoing transformation in European district heating systems fromthe usage of fossil-based technologies to non-fossil heat supplies issummarised by a collection of 70 possibilities linked to decarbonisation. These possibilities are exemplified by 284 implemented, planned, orproposed cases. The 70 possibilities for decarbonised district heatinginclude using heat, connecting customers, moving heat, storing heat, removing carbon dioxide, and supplying heat together with somefeatures for the entire value chain, to heat usage from heat generation orrecycling. This collection of 70 possibilities is neither complete nor doesit contain any recommendations for the possibilities or advocate forspecific possibilities.

The purpose of this project was to provide an extensive inventory ofdecarbonisation activities recently performed by district heating operators andother heat suppliers. These decarbonisation activities include the directsubstitution of heat obtained from the combustion of fossil fuels and indirectactions for obtaining more efficient district heating systems. These indirect actionsreduce costs and increase revenue, thereby improving the competitiveness ofdistrict heating. The time horizon, which is linked to the EU’s target for thereduction of greenhouse gas emissions by 55% compared to 1990 levels, is 2030. This inventory of early decarbonisation projects concerning district heatingsystems has revealed the following three key conclusions.

First, decarbonisation activities can be divided into substituting and supportingpossibilities. Substituting possibilities in heat supply include linear supply fromrenewables, heat recycling from processes that generate excess heat, and non-fossilways of meeting peak heat demands during very cold days. The linear heat supplyis based on geothermal heat, solar heat, and electricity supply. Heat recycling ispossible from various processes related to biorefineries, hydrogen supply, petrochemical plants, electricity distribution, district cooling, data centres, batteryfactories, food supply chains, and sewage waters. Heat storage can make heatdelivery more independent of heat supply and provide additional opportunities toreduce peak loads. Supporting possibilities mainly comprise activities forobtaining lower temperatures in heat distribution networks to increase profitabilitywhen using low-temperature heat sources. These activities are performed whenconnecting customers, moving heat, and using heat. Another supporting activity is the removal of biogenic carbon dioxide from the natural carbon cycle, although anappropriate international accounting system for its removal is still missing.

Second, the decarbonisation possibilities of district heating systems differ fromthose of traditional systems based on fossil fuels. The availability ofdecarbonisation possibilities for district heating depends on local conditions,whereas fossil fuels are transported from available global resources and are usedworldwide. Hereby, decarbonised district heating systems will not be as uniformas traditional systems based on fossil fuels. The local conditions lower the degrees of freedom for the implementation of substituting possibilities in existing buildingsand systems. Hence, it is important to adopt new methods for utilising the highestdegree of freedom possible in new buildings and systems.

Third, the common denominators for the 70 identified possibilities are degrees offreedom for decarbonisation, action plans for achieving lower heat distributiontemperatures, the use of heat pumps for upgrading low-temperature supplies tomeet high-temperature demands, smart digitalisation options, clear supplyresponsibilities, favourable institutional frameworks, and digital planning models. These seven common denominators are efficient tools for obtaining decarbonisedand more efficient district heating systems in the future. These redesigned and newsystems will be somewhat different than traditional systems, which have beenbased on a district heating technology that was originally elaborated for systemsbased on fossil fuels.

Den pågående omvandlingen av europeiska fjärrvärmesystem från användning av fossilbaserad teknik till icke-fossil värmeförsörjning sammanfattas med en utvald samling av 70 möjligheter kopplade till fossilfrihet. Dessa möjligheter exemplifieras med 284 genomförda, planerade eller föreslagna fall. De 70 möjligheterna för koldioxidfri fjärrvärme omfattar att använda värme, ansluta kunder, flytta värme, lagra värme, avskilja koldioxid och tillföra värme tillsammans med några aspekter för hela värdekedjan till värmeanvändning från värmeåtervinning eller värmegenerering. Uppsättningen av 70 möjligheter är varken komplett eller innehåller några rekommendationer för vilka möjligheter som bör användas. Syftet med detta projekt har varit att tillhandahålla en omfattande inventering av tidiga aktiviteter för att erhålla fossilfri fjärrvärme som nyligen utförts av fjärrvärmeföretag eller andra värmeaktörer. Dessa aktiviteter omfattar både direkt substitution av värme som tidigare erhållits från förbränning av fossila bränslen och stödjande indirekta åtgärder för att erhålla mer effektiva fjärrvärmesystem. Dessa stödjande åtgärder minskar kostnaderna eller ökar intäkterna som förbättrar fjärrvärmens konkurrenskraft. Tidshorisonten har varit 2030, kopplat till EU:s mål för minskning av växthusgasutsläppen med 55 % jämfört med 1990 års utsläpp. Denna inventering av tidiga projekt för fossilfri fjärrvärme har givit följande tre viktiga slutsatser. För det första, aktiviteter för fossilfri fjärrvärme kan delas in i ersättande och stödjande möjligheter. Ersättande möjligheter i värmeförsörjningen inkluderar linjär försörjning från förnybar energi, värmeåtervinning från processer som genererar restvärme och icke-fossila sätt att möta spetsbehov under mycket kalla dagar. Den linjära värmeförsörjningen baseras på geotermisk värme, solvärme och eltillförsel. Nya aktiviteter för värmeåtervinning är möjliga från många olika samhällsprocesser, såsom bioraffinaderier, vätgasförsörjning, petrokemiska anläggningar, eldistribution, fjärrkyla, datacenter, batterifabriker, livsmedelsförsörjning och avloppsvatten. Värmelager kan göra värmeleveransen mer oberoende av värmetillförseln, vilket också ger ytterligare möjligheter att minska spetsbelastningar. Stödjande möjligheter innehåller främst aktiviteter för att erhålla lägre temperaturer i värmedistributionsnät, vilket ökar lönsamheten vid användning av lågtempererade värmekällor. Dessa aktiviteter utförs när man använder värme, ansluter kunder och flyttar värme. En planerad stödaktivitet är också avskiljning av biogen koldioxid från det naturliga kolkretsloppet, även om ett lämpligt internationellt ersättningssystem för detta fortfarande saknas. För det andra, karaktären hos möjligheterna till fossilfritt skiljer sig från de traditionella erfarenheterna baserade på fossila bränslen. Tillgången på möjligheter till fossilfritt beror på lokala förhållanden, medan fossila bränslen transporterades från tillgängliga globala resurser, vilket gav full frihet att använda fossila bränslen var som helst i världen. Härigenom kommer fossilfria fjärrvärmesystem inte bli så 4likartade som traditionella fjärrvärmesystem var med fossila bränslen. De lokala förutsättningarna för fossilfri fjärrvärme ger något lägre frihetsgrader för implementering av ersättande möjligheter i befintliga byggnader eller system. Därför är det viktigt för framtiden att utnyttja den högre frihetsgrad som är möjlig i nya byggnader och system genom att använda nya metoder mm. För det tredje, de gemensamma nämnarna för de 70 identifierade möjligheterna är antal frihetsgrader för fossilfrihet, handlingsplaner för att erhålla lägre temperaturer i värmedistributionsnät, olika sätt att använda värmepumpar för att uppgradera låga framtemperaturer för att tillgodose högre temperaturbehov hos kunderna, möjliga smarta digitaliseringsalternativ, tydliga leveransansvar, gynnsamma institutionella ramar samt digitala planeringsverktyg. Dessa sju gemensamma nämnare är effektiva verktyg för att få mer effektiva fossilfria fjärrvärmesystem, eftersom den traditionella fjärrvärmetekniken en gång i tiden utformades för system baserade på användning av fossila bränslen. 

Wind power received sporadic interest after the breakthrough of electricity in the late 1800s, but was sidelined by other electricity generation technologies until the 1970s. A renewed interest in alternative energy sources during the 1970s spurred the development of wind turbines, which gradually evolved into the modern large wind turbines. Whereas significant attention and financial support were focused on government-funded multi-MW prototypes, the small wind turbines developed for the Danish market in the late 1970s proved to be the template for future development. © 2024 Elsevier Inc. All rights are reserved, including those for text and data mining, AI training, and similar technologies.