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.

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.

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 lowtemperature 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.

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.

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.

Wind power only received occasional attention since the introduction of electricity until the 1970s, when a revived interest in alternative energy sources spurred the development thread that led to today's wind turbines. Although attention and financial support at the time were directed toward government-funded MW-scale wind turbines, the small models developed in the late 1970s for the Danish market were ultimately the way forward. The wind industry has since matured, as evidenced by the lower specific power and higher capacity factors of recent turbine models and the similarity between their power curve shapes. Moreover, this study highlights two historical accomplishments by Europeans that are sometimes incorrectly credited to Americans: the first wind turbine to generate electricity was built in 1883 by Austrian Josef Friedl & auml;nder and the Danish Agricco (1919) became the first public grid-connected wind turbine.

District heating is one of the technologies that can contribute to the decarbonisation of the European heat sector. Nonetheless, these infrastructures only deliver about a tenth of the heat demands in the continent. Therefore, it is essential to assess the expansion potential of these systems and to identify which areas should be target for further investigations, which calls for easy-to-use and straightforward methods such as Persson & Werner's network capital cost model. Pivotal parameters of the model are the effective width, a metric of trench length by land area, alongside the average pipe diameter and the linear heat density. This study has carried out an in-depth analysis of these crucial parameters with respect to both distribution and service pipes in a large Danish district heating network, which has allowed to explore the behaviour of effective width in a broad range of building densities and derive new equations for both effective width and average pipe diameter. The model has subsequently been validated in another large network in Denmark and several minor districts in the same country, showing the accuracy of the model on an aggregated level.

We review the development of wind turbines for generating electricity from the late 19th century to the present, summarizing some key characteristics. We trace the move from two and four blade wind turbines to the three blades common today. We establish that it was not the governmental-funded wind programs with its large-scale prototypes of the 1970–80s that developed into the commercial turbines of today. Instead, it was the small-scale Danish wind turbines, developed for an agricultural market, that developed into the commercial turbines of today. And we show that much of what we know today about wind turbine design was known by the 1930s and certainly well known by the late 1950s. This work is divided into two parts: the first part takes up the development from the first electricity producing wind turbines through to the 1960s and a second part on development from the 1970s onward.

Energy is essential to human survival, and with increasing concerns regarding the global warming caused by greenhouse emissions, the energy field has become a global focal point [...]

We review the development of wind turbines for generating electricity from the late 19th century to the present, summarizing some key characteristics. We trace the move from two to four blade wind turbines to the three blades common today. We establish that it was not the governmental-funded wind programs with its large-scale prototypes of the 1970–80s that developed into the commercial turbines of today. Instead it was the small-scale Danish wind turbines, developed for an agricultural market, that developed into the commercial turbines of today. And we show that much of what we know today about wind turbine design was known by the 1930s and certainly well known by the late 1950s. This work is divided into two parts: the first part takes up the development from the first electricity producing wind turbines through to the 1960s and a second part on development from the 1970s onward.

In this study, the sizing of primary side components for preparation of domestic hot water are analysed, both qualitatively and based on measurements of domestic hot water demand in one multi-family building with 268 apartments. The collected data spans a period of 18 days during the end of 2020 and is collected in 15-min, 1-min, and 6-s intervals. An overview of the historic development for the design of domestic hot water flow in Sweden is also presented. There is a long-standing argument in Sweden, that the current district heating guidelines result in an overdesign of the flow for domestic hot water. The consequence of this is oversizing heat exchangers and valves in the substations. This study assesses, qualitatively, the issues related to overdesigned primary side valves for preparation of domestic hot water. A revised design for domestic hot water flow for the Swedish context is also conceptualised. The study suggests that an improved design flow for domestic hot water in multi-family buildings can be derived based on empirical measurements. The 15-min intervals are observed to tone down information of peaks to a degree where it is unsuitable to use as basis for a new design flow. The 1-min data does appear to preserve information to a degree where it can be used to assess a design flow when related to data with a 6-s interval. The 6-s data is expected to constitute a resolution that may be less available, and in this study, it constitutes a representation of the real domestic hot water demands. However, to find a fitted curve to empirical data, for the design flow based on number of apartments per multi-family building, the population of datasets needs to be increased.

A simple model for accounting for tower mass when estimating the first-mode eigenfrequency of a semi-guy-wired tower has been derived. This extends previous work where an analytical model of the semi-guy-wired tower of a 200-kW vertical-axis wind turbine was developed. The model was primarily used to estimate the eigenfrequencies as a result of adding guy wires to a free-standing tower (thus creating a semi-guy-wired setup). However, a weakness with the model was that the tower mass was accounted for in a rough way that essentially ignored the guy wires, which gave a larger-than-necessary error. In this work, an effective top mass, that takes into account the tower mass and the constraints from the guy wires, is derived to achieve a higher accuracy when estimating the first-mode eigenfrequency. This, together with the earlier models, gives a more complete method to estimate the eigenfrequencies for a semi-guy-wired wind turbine.

Based on data from 2083 wind turbines installed in Sweden from 1988 onwards, the accuracy of the predictions of the annual energy production (AEP) from the project planning phases has been compared to the actual wind-index-corrected production. Both the electricity production and the predicted AEP come from Vindstat, a database that collects information directly from wind turbine owners. The mean error for all analyzed wind turbines was 13.0%, which means that, overall, the predicted AEP has been overestimated. There has been an improvement of accuracy with time with an overestimation of 8.2% for wind turbines installed in the 2010s, however, the continuous improvement seems to have stagnated around 2005 despite better data availability and continuous refinement of methods. Dividing the results by terrain, the error is larger for wind turbines in open and flat terrain than in forest areas, indicating that the reason behind the error is not the higher complexity of the forest terrain. Also, there is no apparent increase of error with wind farm size which could have been expected if wind farm blockage effect was a main reason for the overestimations. Besides inaccurate AEP predictions, a higher-than-expected performance decline due to inadequate maintenance of the wind turbines may be a reason behind the AEP overestimations. The main sources of error are insecurity regarding the source of AEP predictions and the omission of mid-life alterations of rated power.

Based on data from 1162 wind turbines, with a rated power of at least 1.8 MW, installed in Sweden after 2005, the accuracy of the annual energy production (AEP) predictions from the project planning phases has been compared to the wind-index-corrected production. Both the production and the predicted AEP data come from the database Vindstat, which collects information directly from wind turbine owners. The mean error was 7.1%, which means that, overall, the predicted AEP has been overestimated. The overestimation was higher for wind turbines situated in open terrain than in forest areas and was higher overall than that previously established for the British Isles and South Africa. Dividing the result over the installation year, the improvement which had been expected due to the continuous refinement of the methods and better data availability, was not observed over time. The major uncertainty comes from the predicted AEP as reported by wind turbine owners to the Vindstat database, which, for some cases, might not come from the wind energy calculation from the planning phase (i.e., the P50-value).

This paper presents the wind energy potential and wind characteristics for 25 wind sites in Eritrea, based on wind data from the years 2000–2005. The studied sites are distributed all over Eritrea, but can roughly be divided into three regions: coastal region, western lowlands, and central highlands. The coastal region sites have the highest potential for wind power. An uncertainty, due to extrapolating the wind speed from the 10-m measurements, should be noted. The year to year variations are typically small and, for the sites deemed as suitable for wind power, the seasonal variations are most prominent in the coastal region with a peak during the period November–March. Moreover, Weibull parameters, prevailing wind direction, and wind power density recalculated for 100 m above ground are presented for all 25 sites. Comparing the results to values from the web-based, large-scale dataset, the Global Wind Atlas (GWA), both mean wind speed and wind power density are typically higher for the measurements. The difference is especially large for the more complex-terrain central highland sites where GWA results are also likely to be more uncertain. The result of this study can be used to make preliminary assessments on possible power production potential at the given sites.

Models of inflow-turbulence noise and turbulent-boundary-layer trailing-edge noise are compared to earlier measurements of a 200-kW vertical axis wind turbine so that conclusions regarding the origin of the aerodynamic noise can be drawn. The measurement campaigns, which aimed at establishing the noise emission value and locating the aerodynamic noise sources with a microphone array, are here both compared to further modified versions of the trailing-edge and inflow-turbulence models respectively. Unlike the case for horizontal axis wind turbine, inflow-turbulence noise is deemed as the prevailing noise mechanism. Reducing the self-induced turbulence could then be an effective way of lowering the noise levels for vertical axis wind turbines. Also, looking at the directivity of the inflow-turbulence noise model which indicate most noise in the cross-wind directions, a deviation from the standard downwind measurement position for measuring noise emission is suggested for the VAWT case.

This paper summarizes wind turbines of Swedish origin, 50 kW and above. Both the large governmental-funded prototypes from the early 1980s and following attempts to build commercial turbines are covered. After the 1973 oil crisis, a development program for wind turbine technology was initiated in Sweden, culminating in the early 1980s with the 2 and 3-MW machines at Maglarp and Näsudden. However, government interest declined, and Sweden soon lost its position as one of the leading countries regarding wind turbine development. Nevertheless, several attempts to build commercial wind turbines in Sweden were made in the following decades. Most attempts have, like the earlier prototypes, used a two-bladed rotor, which has become synonymous with the Swedish wind turbine development line. The current ongoing Swedish endeavors primarily focus on the niche-concept of vertical axis wind turbines (VAWTs), which is a demonstration of how far from the broad commercial market of Sweden has moved. Thus far, none of the Swedish attempts have been commercially successful, and unlike countries like Denmark or Germany, Sweden currently has no large wind turbine producer. Suggested reasons include early government interventions focusing on two-bladed prototypes and political disinterest, with wind power grants cut in half by 1985, and the domestic industry not being favored in government policies for deploying wind power.

This paper summarizes and introduces all vertical axis wind turbine (VAWT) projects where 100 kW or larger turbines have been installed. The basis for the review is both existing literature and personal correspondence with people once involved in the different developments. By citing the most relevant work for each project, the paper will also work as an information hub, making information on these projects more accessible. Since the 1970s, there have been several VAWT projects with installed turbines of significant size, either as attempts to commercialize VAWTs, or as university led research projects, or as a combination of the two. Most have involved Darrieus turbines built in North America during the 1980s. However, H-rotors, which have always been a favored concept in Europe, have seen a revival during the 2010s. The reason VAWTs have never fully challenged the success of the horizontal axis wind turbine (HAWT) is too broad a question to answer here. However, the reasons some VAWT projects have failed are addressed in this paper. Besides the fact that many of the prototypes had terminal failures, most of the installed medium or large-scale VAWTs have to some extent had problems with metal fatigue and durability. Additionally, a lack of long-term interest from governmental or private funders, as well as the introduction of reliable HAWTs, was a recurring theme from those involved in VAWT development, regarding the reason VAWTs so far have failed to succeed.

Alongside the story of today’s commercially successful, propeller-type, horizontal axis wind turbine (HAWT), there is the lesser known story of the vertical axis wind turbine (VAWT). Once seen as a ...