Vertical-Axis Wind Turbines in Emerging Energy Applications (1979–2025): Global Trends and Technological Gaps Revealed by a Bibliometric Analysis and Review

Abstract

This study provides a comprehensive overview of vertical-axis wind turbines (VAWTs) for emerging energy applications by combining a bibliometric analysis and a thematic mini-review. Scopus-indexed publications from 1979 to 2025 were analyzed using PRISMA guidelines and bibliometric tools (Bibliometrix, CiteSpace, and VOSviewer) to map global research trends, and a parallel mini-review distilled recent advances into five thematic areas: aerodynamic strategies, advanced materials, urban integration, hybrid systems, and floating offshore platforms. The results reveal that VAWT research output has surged since 2006, led by China with strong contributions from Europe and North America, and is concentrated in leading renewable energy journals. Dominant topics include computational fluid dynamics (CFD) simulations, performance optimization, wind–solar hybrid integration, and adaptation to turbulent urban environments. Technologically, active and passive aerodynamic innovations have boosted performance albeit with added complexity, remaining mostly at moderate technology readiness (TRL 3–5), while advanced composite materials are improving durability and fatigue life. Emerging applications in microgrids, building-integrated systems, and offshore floating platforms leverage VAWTs’ omnidirectional, low-noise operation, although challenges persist in scaling up, control integration, and long-term field validation. Overall, VAWTs are gaining relevance as a complement to conventional turbines in the sustainable energy transition, and this study’s integrated approach identifies critical gaps and high-priority research directions to accelerate VAWT development and help transition these turbines from niche prototypes to mainstream renewable solutions.

1. Introduction

In recent decades, wind energy has established itself as one of the fastest-growing renewable sources and a key contributor to the global energy matrix [1]. Historically, horizontal-axis wind turbines (HAWTs) have dominated large-scale generation due to their technological maturity [2]. However, vertical-axis wind turbines (VAWTs) are increasingly recognized as potentially playing a significant role in the continued expansion of renewable energy in the near future [3]. Unlike HAWTs, VAWTs offer inherent advantages due to their configuration: They do not require yaw mechanisms as they can capture wind from any direction. They have lower visual and noise impacts, and they maintain more stable performance under turbulent flow conditions [4]. These qualities make VAWTs attractive in scenarios where conventional turbines face limitations, particularly in dense or complex urban environments and in locations with highly variable wind conditions [5]. For example, research demonstrates that the aerodynamic and performance optimization of VAWTs can be effectively enhanced by carefully considering dynamic stall behaviors [6], site-specific wind patterns and building integration [7,8], and the geometrical influence of surrounding structures [9]. Furthermore, several analyses project that VAWT technology will play a vital role in the future of wind energy, complementing HAWTs in the pursuit of more resilient and versatile systems [10].

Nevertheless, VAWTs are still considered an emerging technology and have yet to achieve the same level of commercial deployment as HAWTs [11]. This is partly due to long-standing technical challenges that have hindered their adoption, such as lower theoretical peak efficiency, self-starting difficulties in Darrieus-type designs, and significant cyclic loading that affects structural fatigue [3]. In fact, after an initial period of research activity in the 1980s, scientific interest in VAWTs declined for nearly two decades. However, recent years have seen a strong resurgence in interest in this technology [12]. This revival aligns with the growing need to diversify wind solutions for new application niches and advances in simulation tools and materials that promise to overcome the traditional limitations of VAWTs. Notably, bibliometric data reveals a marked increase in VAWT-related publications post-2008, reflecting both improved design methodologies and a heightened focus on integrating these turbines in urban and off-grid applications.

Indeed, current research places strong emphasis on improving aerodynamic performance through computational fluid dynamics (CFD) modeling [13] and design optimization, as well as exploring innovative configurations that may allow vertical-axis turbines to compete with or complement horizontal-axis turbines in terms of efficiency and cost.

In this context, it is timely to undertake a study that synthesizes the current landscape of research on VAWTs and their emerging applications. A bibliometric approach provides a quantitative view of research trends by identifying which countries and institutions lead scientific output, what topics are most prominent, and how interest has evolved over time [14]. On the other hand, a technical mini-review complements this analysis by qualitatively exploring key advances and technological challenges reflected in the recent literature [15]. While some reviews focus on specific aspects of VAWTs (e.g., aerodynamics and design), there is still a lack of comprehensive bibliometric mapping centered on their use in emerging applications—such as urban environments, hybrid systems, and floating offshore platforms. This gap in the literature is what the present work aims to address.

The objective of this study is to provide a comprehensive and updated overview of research on vertical-axis wind turbines in emerging energy applications by combining a bibliometric analysis of the recent literature with a strategic mini-review. Through this dual approach, this study aims to (i) map publication trends, key contributors, and thematic focuses in the field of VAWTs; (ii) summarize and discuss major technological advances achieved in recent years; and (iii) identify research gaps and propose future directions to guide the development of this technology. This contribution seeks to support both the academic community and industrial developers by providing an organized knowledge base that facilitates informed decision-making and innovation in vertical-axis wind turbine technologies.

Key contributions of this study are as follows: (a) We deliver the first combined bibliometric and mini-review on vertical-axis wind turbines (VAWTs) covering 2019–2025, mapping 507 peer-reviewed articles into five thematic clusters and tracing their temporal evolution. (b) We provide a quantitative synthesis of active and passive aerodynamic strategies annotated with the technology readiness level (TRL) and incremental cost (USD kW⁻¹), offering a benchmark that was absent from previous reviews. (c) We compile the most comprehensive material–durability matrix to date for VAWT blades, towers, and coatings, integrating fatigue, corrosion, and UV-aging data into a single decision framework. (d) We introduce a critical survey of electrical integration, control algorithms, and mathematical modeling, explicitly contrasting VAWT solutions with state-of-the-art HAWT practices. (e) Based on empirical gaps identified across all tables, we outline five high-priority research directions ranging from dynamic-stall-enhanced stream-tube models to full-scale floating VAWT validation campaigns. These results bridge the knowledge divide between academic prototypes and bankable technology pathways, providing practitioners with a curated evidence base for next-generation VAWT design and deployment.

The remainder of this article is structured as follows: Section 2 details the data sources, bibliometric protocol (PRISMA-2020), and mini-review criteria. Section 3 presents the bibliometric results, including publication trends, co-authorship networks, and keyword evolution. Section 4 distills these trends into a thematic mini-review, covering aerodynamics, materials, urban integration, hybrid energy systems, and floating applications. Section 5 discusses cross-cutting issues, such as electrical connection topologies, control algorithms, mathematical modeling, technology readiness gaps, and future research priorities, while Section 6 summarizes the main conclusions and practical implications.

2. Materials and Methods

The methodology for this bibliometric analysis followed a PRISMA-based approach, beginning with an advanced search in the Scopus database using a tailored query that combines terms related to “Vertical Axis Wind Turbines (VAWT)” and their emerging applications, such as urban wind energy, floating offshore wind, hybrid systems, emergency power, and more. As shown in Figure 1, the data collection process was structured into four main stages: Identification, Screening, Eligibility, and Inclusion.

In the Identification phase, a comprehensive query was executed in the Scopus database using advanced Boolean search operators. The search targeted documents containing the following advanced query: TITLE-ABS-KEY (“Vertical axis wind turbine” OR “Vertical axis wind turbines” OR “VAWT”) AND TITLE-ABS-KEY (“urban wind energy” OR “floating offshore wind” OR “counter-rotating array” OR “distributed energy generation” OR “building integrated wind turbine” OR “hybrid energy system” OR “small scale” OR microgeneration OR “water pumping” OR “emergency power” OR “backup power” OR “meteorological station” OR “telecommunication station” OR “electric mobility” OR “charging station”). This yielded an initial set of 538 documents.

During the Screening phase, the inclusion and exclusion criteria were applied. Only primary research articles and conference papers published between 1979 and February 2025 were retained. Document types such as reviews, book chapters, editorials, conference abstracts, and notes were excluded to ensure the consistency and relevance of the dataset. This process refined the dataset to 298 documents. In the Eligibility stage, the filtered literature was assessed for alignment with the research objective—focusing on vertical-axis wind turbines applied to emerging energy contexts such as urban microgeneration, wind–solar hybrid systems, and offshore floating platforms. Finally, in the Inclusion phase, the resulting corpus of 298 high-relevance documents was confirmed for use in the bibliometric and qualitative analyses. These records were exported for analysis using two bibliometric tools. The Bibliometrix package in RStudio (version 2024.09.1+394) was used to conduct a broad bibliometric analysis, including dual-map overlays, keyword timelines, burst citations, and thematic evolution. VOSviewer (version 1.6.20) was employed to visualize keyword co-occurrence networks, author collaboration clusters, and thematic development over three distinct time periods. Furthermore CiteSpace 6.3.R1 (64-bit) Basic was used. Data retrieval and analysis were conducted in February 2025.

This methodological pipeline enabled a comprehensive visualization of the VAWT research landscape, including publication trends, geographical contributions, collaboration patterns, and emerging themes.

3. Results

3.1. Yearly Publication and Citation Trends

Figure 2 shows the evolution of the annual number of publications related to vertical-axis wind turbines (VAWTs) in emerging energy applications, highlighting a significant growth in scientific interest since 2006. Following an initial period (1979–2005) with limited output, mainly because policy incentives were implemented in later years [16], a gradual increase was observed beginning in 2006, reaching notable peaks between 2021 and 2023 with more than 35 articles published per year. This trend reflects growing attention to this technology within the context of the energy transition and sustainable solutions.

3.2. Top Leading Journals

Table 1. Top leading journals.

Rank	Source	(TC)	Total Citation/Total Paper (TC/TP)	h-Index	g-Index	IF	Country
1	Renewable Energy	1790	89.5	17	20	9.90	United Kingdom
2	Energy Conversion and Management	707	58.91	12	12	10.51	United Kingdom
3	Journal of Wind Engineering and Industrial Aerodynamics	550	30.55	11	18	4.81	The Netherlands
4	Applied Energy	418	52.25	7	8	11.46	United Kingdom
5	Energies	187	13.35	7	13	3.51	Switzerland
6	Energy	395	43.88	7	9	10.29	United Kingdom
7	Journal of Renewable and Sustainable Energy	159	15.9	7	10	2.08	United States
8	Wind Engineering	361	24.06	7	15	1.5	United Kingdom
9	Journal of Energy Resources Technology, Transactions of the ASME	257	42.83	6	6	2.86	United States
10	Energy for Sustainable Development	117	29.25	4	4	5.23	The Netherlands

presents the top 10 journals contributing significantly to the field of research. Publications in leading journals such as Renewable Energy, Energy Conversion and Management, and the Journal of Wind Engineering and Industrial Aerodynamics, alongside strong h-index and citation-per-paper ratios, underscore their influential role in advancing research in energy systems and technologies. Renewable Energy leads the list with the highest total citations (1790) and a strong citation-per-paper average (89.5), reflecting its high impact and visibility in the academic community. It should be clarified that the citation counts are based on overall search query results. Other prominent journals include Energy Conversion and Management, and Applied Energy, all from the United Kingdom, which also show high total citation counts and impressive impact factors (IFs), indicating their significant influence.

The h-index, g-index, and m-index provide additional insight into the academic influence of these journals. The h-index is one of the most widely used bibliometric indicators for evaluating both the productivity and citation impact of researchers. Proposed by [17], it is defined as the number h such that a researcher has h papers, each cited at least h times. To address some of the h-index’s shortcomings, the g-index was introduced by [18]. The g-index gives more weight to highly cited articles, thereby better capturing the influence of a researcher’s most impactful work. Specifically, a researcher has a g-index of g if their top g articles received a combined total of at least g² citations.

As shown in Table 1, geographically, the United Kingdom dominates the list with five journals, followed by the United States, the Netherlands, and Switzerland, illustrating a strong European and North American presence in renewable energy research publishing. The presence of diverse journals ranging from broad energy topics to specific areas like wind engineering and sustainable development highlights the multidisciplinary nature of the field. In the case of Applied Energy, it had the highest impact factor (IF).

3.3. Most Cited Publications

Table 2 presents the ranks of the top 10 most cited articles related to vertical-axis wind turbines (VAWTs) based on total citations. The thematic distribution of these publications highlights three major research categories: aerodynamic performance and design optimization (I) with a total of 1451 citations, hybrid and integrated energy systems (II) with a total of 188 citations, and computational modeling techniques (III) with a total of 391 citations.

The first group focuses on performance analysis and turbine design and includes studies such as the aerodynamic enhancement of H-rotor Darrieus turbines [19], self-starting capabilities [20,21], pitch angle control [22], and experimental optimization of the Savonius rotor [23]. These works emphasize mechanical improvements and practical configurations to improve efficiency and operational stability, reflecting their centrality in design-focused research. Notably, Ref. [24], the most cited article with 713 citations, combines wind tunnel testing and numerical modeling, making it a benchmark in experimental and theoretical validation.

The second thematic cluster revolves around hybrid energy systems and urban integration. Reference [25] exemplifies this by examining a wind–solar hybrid system equipped with a rainwater collection feature aimed at high-rise urban applications—a clear indicator of interdisciplinary innovation connecting renewable technologies with sustainability in urban environments.

The third thematic group is centered on CFD (computational fluid dynamics) modeling and simulation, reflected in articles such as Refs. [26,27], which review CFD studies for urban wind flow, and Ref. [28], which evaluates the application of VAWTs in buildings. These studies demonstrate the importance of advanced simulation tools in improving the understanding of fluid–structure interactions and optimizing turbine performance. Overall, the table illustrates that while aerodynamic and mechanical design remains dominant, the integration of VAWTs in urban, hybrid, and data-driven contexts is gaining significant traction in the scientific community.

Table 2. Top 10 most cited publications.

Rank	Reference	Article Title	Source Title	Category	Total Citation (TC)
1	[24]	Wind tunnel and numerical study of a small vertical axis wind turbine	Renewable Energy	I	713
2	[25]	Techno-economic analysis of a wind–solar hybrid renewable energy system with rainwater collection feature for urban high-rise application	Applied Energy	II	188
3	[19]	Aerodynamic performance enhancements of H-rotor Darrieus wind turbine	Energy	I	169
4	[20]	Self-starting capability of a Darrieus turbine	Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy	I	162
5	[21]	Investigations on self-starting and performance characteristics of simple H and hybrid H-Savonius vertical axis wind rotors	Energy Conversion and Management	I	146
6	[26]	A review of computational fluid dynamics (CFD) simulations of the wind flow around buildings for urban wind energy exploitation	Journal of Wind Engineering and Industrial Aero	III	135
7	[27]	Effect of airfoil and solidity on performance of small scale vertical axis wind turbine using three dimensional CFD model	Energy	III	132
8	[22]	Pitch angle control for a small-scale Darrieus vertical axis wind turbine with straight blades (H-Type VAWT)	Renewable Energy	I	131
9	[23]	Review of experimental investigations into the design, performance and optimization of the Savonius rotor	Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy	I	130
10	[28]	Vertical axis resistance type wind turbines for use in buildings	Renewable Energy	III	124

Table 2. Top 10 most cited publications.

Rank	Reference	Article Title	Source Title	Category	Total Citation (TC)
1	[24]	Wind tunnel and numerical study of a small vertical axis wind turbine	Renewable Energy	I	713
2	[25]	Techno-economic analysis of a wind–solar hybrid renewable energy system with rainwater collection feature for urban high-rise application	Applied Energy	II	188
3	[19]	Aerodynamic performance enhancements of H-rotor Darrieus wind turbine	Energy	I	169
4	[20]	Self-starting capability of a Darrieus turbine	Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy	I	162
5	[21]	Investigations on self-starting and performance characteristics of simple H and hybrid H-Savonius vertical axis wind rotors	Energy Conversion and Management	I	146
6	[26]	A review of computational fluid dynamics (CFD) simulations of the wind flow around buildings for urban wind energy exploitation	Journal of Wind Engineering and Industrial Aero	III	135
7	[27]	Effect of airfoil and solidity on performance of small scale vertical axis wind turbine using three dimensional CFD model	Energy	III	132
8	[22]	Pitch angle control for a small-scale Darrieus vertical axis wind turbine with straight blades (H-Type VAWT)	Renewable Energy	I	131
9	[23]	Review of experimental investigations into the design, performance and optimization of the Savonius rotor	Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy	I	130
10	[28]	Vertical axis resistance type wind turbines for use in buildings	Renewable Energy	III	124

3.4. Analysis of Author Keywords and Countries

Figure 3 illustrates the temporal evolution of research themes in the field of vertical-axis wind turbines (VAWTs) and related domains between 2010 and 2025. Each horizontal line corresponds to a thematic cluster labeled on the right, ranging from #0 to #7, and each cluster represents a distinct area of inquiry within the broader wind energy research landscape. The size of the nodes (circles) indicates the frequency or importance of keywords or references, while the color of the nodes follows a temporal gradient: blue tones indicate older studies (around 2016), while yellow-to-dark-red nodes represent more recent activity (2023–2025). In this network visualization created using VOSviewer 1.16.18, the minimum frequency threshold for keywords was set to four occurrences. This means that only terms appearing at least four times in the dataset are included in the thematic clusters shown in Figure 3. This filtering ensures that the visualization highlights the most significant and recurring research themes while minimizing the noise from less frequent keywords. This enables the visualization of how scientific interest has shifted over time across different subfields.

The red cluster (#0), labeled “wind energy,” emerges as the dominant and most central thematic area. It represents the broad foundational research on wind power, turbine performance, and the development of general wind energy technologies. Over time, this cluster has maintained strong connectivity with other clusters, demonstrating its interdisciplinary influence. Closely related is the orange cluster (#1), associated with Navier–Stokes equations, indicating a deep focus on computational fluid dynamics and fundamental aerodynamic modeling of wind flow, which underpins much of the performance analysis of wind turbines. This theme is especially prominent in the earlier years of the timeline (2010–2016), as researchers established a technical basis for aerodynamic simulations.

Another prominent theme is the yellow-green cluster (#2), which is focused on the torque coefficient, a key parameter in evaluating turbine efficiency and rotation mechanics. Research in this area has remained consistently relevant and well-cited, bridging both theoretical and applied investigations. The green cluster (#3), labeled “buildings,” highlights a more application-oriented trend, particularly in the integration of wind turbines into urban environments and architectural contexts. This reflects a shift toward sustainability in cities, exploring the potential of VAWTs in building-integrated wind energy systems.

In recent years, new research fronts have gained momentum. The light blue cluster (#4), addressing microgeneration, suggests increased academic and industrial interest in distributed energy systems and the role of small-scale wind turbines in local power generation. Similarly, the dark blue cluster (#6) refers to floating offshore wind turbines, a cutting-edge area focused on harnessing wind resources in deep-water offshore environments. These studies emphasize innovative structural designs and anchoring systems suitable for floating platforms, illustrating the field’s expansion into maritime applications [29].

Advanced simulation methods are grouped in the blue cluster (#5), specifically around delayed detached eddy simulation, a computational technique used to model turbulence in aerodynamic flows. This reveals continued reliance on high-fidelity modeling to improve turbine blade design and energy yield predictions. Lastly, the purple cluster (#7), centered on statistical distribution, reflects the emergence of data-driven approaches and the application of probabilistic models for performance evaluation and decision-making. These newer topics—especially visible in the red and dark orange nodes—signal a transition toward integrating machine learning, optimization, and data analytics into wind energy research.

Throughout the graph, central terms like “computational fluid dynamics,” “aerodynamics,” and “wind tunnels” serve as conceptual bridges across clusters, indicating their foundational role in multiple lines of inquiry. The term “vertical axis wind turbine” appears prominently toward the bottom of the map, which is associated with recent years (2021–2025), suggesting a renewed and growing interest in VAWT technologies, possibly driven by their suitability for urban and offshore applications.

This temporal map shows the evolution from fundamental aerodynamic and mechanical studies to more applied and interdisciplinary topics such as urban integration, offshore deployment, and data analytics. It captures a field that has matured significantly over the past 15 years, with clear transitions toward sustainability, technological innovation, and digital transformation within wind energy systems. The visualization underscores not only the breadth of research but also the deep interconnectivity between theoretical modeling, experimental methods, and real-world energy applications.

Figure 4 presents a citation burst analysis of the top 15 keywords most intensively cited in the field of wind energy research between 2015 and 2025. A citation burst indicates a period in which a keyword experienced a sudden and significant increase in citations, signaling emerging trends or shifts in scientific focus. The end of the burst period suggests that while the initial surge of interest reached its peak, the topic has since become integrated into broader discussions and developments in the field of wind energy technologies. The keyword “vertical axis wind turbines” stands out with the highest burst strength (15.17), active from 2015 to 2021, confirming the strong interest in VAWTs during this period, likely driven by their potential for urban energy systems and decentralized applications. Similarly, general terms such as “wind turbines” and “wind turbine” also show notable bursts starting in 2015 and 2021, respectively, reflecting consistent attention to turbine technologies throughout the decade.

In parallel, technical and computational terms such as “vortex flow”, “turbulence models”, and “tip speed ratio” highlight a growing emphasis on aerodynamic optimization and fluid dynamics, particularly between 2018 and 2023. The inclusion of “optimization”, “wind turbine blades”, and “horizontal axis wind turbine” in recent bursts (2020–2025) indicates a transition toward performance enhancement and component-specific studies. Interestingly, new bursts starting in 2023–2024 such as “darrieus”, “wind velocity”, and “energy” point to a current wave of research focused on design typologies, environmental parameters, and energy system integration. Altogether, this burst analysis not only traces the evolution of research priorities in wind energy but also highlights how the field has progressively moved from system-level innovation toward precision engineering and sustainable application contexts.

Figure 5 illustrates the global distribution of scientific production related to VAWT-related publications, where countries are shaded in varying intensities of blue based on their frequency of scientific output—darker shades representing higher productivity. In bibliometric terms, the frequency refers to the number of scientific documents (e.g., journal articles and conference papers) attributed to authors or institutions from each country. Based on regional grouping, the Asia–Pacific region leads with 37.1% of the total publications (710 documents), primarily from China (220) and India (211). Europe follows with 25.2% (483 documents), with major contributors including Italy, the UK, and Germany. North America accounts for 10.5% (201 documents), led by the USA and Canada. Latin America contributes 1.8% (35 documents), represented mainly by Brazil and Mexico. The Middle East and North Africa region represents 6.0% (115 documents), while Sub-Saharan Africa contributes only 2.0% (38 documents). An additional 17.3% (332 documents) of publications come from countries not included in these regional classifications. This map visually highlights a strong imbalance in research output, emphasizing the need to foster greater academic collaboration and capacity building in underrepresented regions like Latin America and Africa.

3.5. Research Trends and Future Directions

The analysis of the research network was divided into three distinct periods (as shown in Figure 6): Period I (1979–2004), Period II (2005–2016), and Period III (2017–2025).

In the first period, words like darrieus rotors and vertical-axis windmills and wind turbine are the ones that stand out.

For the second period (2005–2016), the visualization illustrates a keyword co-occurrence network that captures the most frequently used and interconnected terms in scientific publications related to vertical-axis wind turbines (VAWTs). The size of each node represents the frequency with which a keyword appears in the dataset, while the connecting lines (edges) indicate co-occurrence relationships—specifically, how often two terms are mentioned together within the same documents. Colors are used to differentiate thematic clusters, each representing a distinct subfield or research focus within the broader domain of VAWT-related studies. This network visualization provides insights into the conceptual structure and evolving priorities of the research community during this period. At the core of the network are the dominant keywords “vertical axis wind turbine” and “VAWT”, which act as central hubs connecting various subthemes. These terms clearly represent the main object of study and are strongly linked to other keywords that define the technological and analytical approaches used during this period. One of the largest clusters includes terms such as “computational fluid dynamics”, “wind turbine”, “darriues”, and “savonius”, indicating a strong focus on the aerodynamic modeling and classification of turbine types. This highlights that researchers were concerned with simulating airflow and optimizing blade geometries specific to vertical configurations. Another significant thematic grouping revolves around “power coefficient”, “torque coefficient”, and “wind energy”, which point to a clear emphasis on performance evaluations and efficiency metrics. These keywords are critical in assessing how effectively a turbine converts wind energy into mechanical or electrical power. Also notable are technical terms such as “magnetic bearing”, “green architecture”, and “compressed air energy storage”, which highlights a growing interest in the integration of VAWTs into sustainable systems and innovative energy storage solutions. The appearance of keywords like “cooling tower”, “communication towers”, and “urban environment” in peripheral clusters indicates the exploration of site-specific applications, particularly in dense or multifunctional structures. In summary, this map reveals that between 2005 and 2016, research on vertical-axis wind turbines was highly centered on aerodynamic design, computational modeling, and energy performance assessments while also beginning to branch out into integrated system applications and sustainable infrastructure contexts.

For the third period (2017–2025), the co-occurrence map highlights the evolving landscape of research on vertical-axis wind turbines (VAWTs) during this timeframe. The visualization displays a denser and more interconnected network compared to previous periods, indicating a more mature and diversified body of literature. The keyword “vertical axis wind turbine” remains central, reflecting its sustained relevance as the core focus of the research. Surrounding it are numerous specialized and technical terms that reflect significant advancements in both theoretical modeling and applied engineering aspects of VAWT technology. Key terms such as “CFD” and “computational fluid dynamics” appear prominently, reinforcing the centrality of simulation and aerodynamic analysis in modern turbine design. These keywords are often linked with “optimization”. This supports the idea that recent studies are not only modeling performance but also actively seeking to improve it through iterative design processes and algorithmic approaches.

The term “VAWT” still holds a strong position, continuing to serve as a foundational concept tied closely to other technical parameters. Several newer terms that were not central in the previous period have emerged. For instance, “floating offshore wind turbine”, “small wind turbine”, and “automatic transmission system” indicate a growing interest in application-specific and adaptive technologies. These keywords reflect the field’s expansion into marine environments, urban settings, and smart system integration, where VAWTs are being evaluated for their flexibility and compatibility with modern energy infrastructures. Notably, concepts such as “blade geometry”, “tip speed ratio”, and “coupled dynamics” highlight an increasing focus on mechanical refinement and system-level interactions. These studies aim to capture the complex physical behaviors of wind turbines in operation, including the structural response, noise reduction (“aerodynamic noise”), and power output fluctuations. The presence of keywords like “life cycle assessment”, “grid-connected”, and “augmented turbines” indicates that sustainability, system integration, and hybridization are becoming critical aspects of contemporary research. In summary, this period (2017–2025) marks a technological and conceptual expansion in vertical-axis wind turbine research. Scholars have moved beyond basic performance assessments toward addressing design optimization, integration with renewable systems, offshore deployment, and environmental evaluations. The diversity of terms reflects not only technical sophistication but also the strategic relevance of VAWTs in meeting future energy needs across varied contexts.

Figure 7 represents the thematic distribution of the literature on vertical-axis wind turbines (VAWTs), based on a keyword co-occurrence analysis. The chart is divided into four quadrants, allowing the interpretation of the status and relevance of different research topics in the field according to two axes: the degree of development or density (vertical axis) and the degree of relevance or centrality (horizontal axis).

Emerging or declining themes (bottom left): This quadrant includes topics with low density and low centrality, indicating that they are either in an early stage of development or in decline within the current scientific literature. For example, the term “floating offshore wind turbine (FOWT)” appears in the article as a topic with low density and low centrality in the research landscape. This is mainly due to the fact that FOWTs are still in the early stages of development, with only a few prototypes deployed and limited empirical studies available to validate their feasibility and performance [30]. Additionally, the high costs and complex logistics of installing floating structures further restrict the number of real-world projects, limiting the generation of data and broad academic interest. As a result, FOWTs occupy an “emerging” position in the bibliometric analysis quadrant, characterized by low density and centrality within the field.

Basic themes (bottom right): Topics in this quadrant exhibit high centrality but low density, meaning that they are fundamental to the field but still developing in terms of internal cohesion. Keywords such as “vertical axis wind turbine (VAWT)”, “CFD” (computational fluid dynamics), “urban wind energy”, “dynamic analysis”, and “power coefficient” are found here. These are conceptual pillars in VAWT research and are strongly connected to many other areas, reflecting their structural importance. However, their low density indicates that there is still room to strengthen internal consistency through more integrated research.

Motor themes (top right): This quadrant contains topics with both high centrality and high density, indicating that they are well-developed and highly influential themes that are driving progress in the field. Notable terms in this category include “building integrated wind turbine” and “delayed detached eddy simulation”. This highlights growing interest in the integration of VAWTs into urban and architectural environments, as well as in the use of advanced high-fidelity numerical simulations to enhance design and efficiency. These areas are conceptually well-structured and represent established research lines that lead current developments.

Niche themes (top left): Topics in this quadrant show high density but low centrality, indicating that while they are internally well-developed, they have limited connections to other topics in the field. Terms such as “atmospheric boundary layer”, “solidity”, “buildings”, and “coefficient of power” are located here. These may represent highly specialized or technical subfields that, although not central to the broader scientific discourse on VAWTs, contribute depth and specificity to particular knowledge niches.

4. Discussion

4.1. Design Trends and Aerodynamics

Table 3 highlights that, despite the notable aerodynamic gains achieved under controlled conditions, full-scale validation of active VAWT strategies remains rare owing to four interrelated systemic barriers. First, a high technological risk persists: biaxial fatigue loads and complex control architectures (sensors, actuators, and power electronics) raise failure probabilities and intensify fatigue-testing requirements, discouraging private investment in ≥100 kW prototypes [31,32]. Second, the regulatory framework is inadequate; IEC 61400-2 and -12-1—designed for micro-horizontal-axis turbines—do not address issues such as torsional resonances in helical blades or the drag–lift cycling of Darrieus rotors, creating certification uncertainty and engineering cost premiums of ~10% CAPEX [33]. Third, a nascent supply chain inflates costs: low-volume production of large bearings or cycloidal pitch actuators explains why individual dynamic pitch control (TRL 4) adds ~140 USD kW⁻¹, whereas slotted-flap retrofits (TRL 3) entail < 50 USD kW⁻¹ [34,35]. Finally, limited test infrastructure hinders the jump from the lab to the field; only a handful of wind tunnels can accommodate VAWTs > 2 m in diameter, and urban or offshore permitting for prototypes may take up to three years [36].

These constraints clarify why 2-D CFD gains shrink in 3-D prototypes and further diminish in real operation, once three-dimensional losses, nonlinear vibrations, and unforeseen O&M costs emerge [37,38]. Three priority actions could close this gap: (i) pre-commercial demonstration programs (TRL 6–7) co-funded by industry and public agencies, delivering ≥10,000 h of operational data in urban and offshore environments [39]; (ii) drafting a dedicated IEC 61400 annex for active VAWTs to harmonize design, fatigue testing, and certification criteria, thereby reducing regulatory time and expense [33]; and (iii) developing a specialized value chain (low-friction bearings, compact actuators, and digital controllers) that cuts the incremental CAPEX of the most promising solutions by ≥30% [40]. Once implemented, these measures should enable configurations such as the coaxial counter-rotating system currently piloted by SeaTwirl (S2, 30 kW, TRL 5) [41] and other active strategies to move beyond the laboratory, demonstrating in real-world settings that their aerodynamic benefits outweigh added costs and thus securing the commercial viability of VAWTs.

Table 3. Comparison of active approaches and their performance improvements.

Reference	Active Strategy	Methodology/Validation	Performance Improvement	TRL Estimate	Added Cost (USD kW−1)	Key Technical Observations	Source (DOI)
[42]	Variable Pitch (Prescribed Pitching Profile)	Semi-analytical DMST model (2D)	+18.9% peak Cp increase at optimum TSR	6	100	Moderate boost; PLC-driven; retrofit-friendly; field demo pending	DOI: 10.3390/app8060957
[43]	Optimal Individual Blade Pitch Control	Wind tunnel (scaled model), Gen. algorithm optimization	~3 × higher Cp vs. fixed-pitch baseline; −77% load fluctuations	4	140	Highest gain; needs fast actuators and sensors; early prototype stage	DOI: 10.1038/s41467-024-46988-0
[44]	Trailing-Edge Slotted Flap (Deflective)	2D and 3D CFD (URANS) with DOE optimization	+27% (2D) and +19% (3D) increase in power output at TSR ≈ 3	3	45	Passive add-on; low CAPEX; wind tunnel only so far	DOI: 10.1016/j.enconman.2022.116388
[37]	Oscillating Mini-Foils (Active Flow Control)	2D CFD (dynamic mesh and UDF control)	+38% turbine power coefficient (Cp) improvement	3	70	Active flow control; requires phase synchronization; no field data	DOI: 10.1007/s40430-016-0618-3
[35]	Morphing Blade Geometry (Adaptive Camber)	3D CFD (STAR-CCM+ and overset + deforming mesh)	+46.2% increase in output power vs. rigid blade	4	200	Large gain; morphing skin adds mass and control complexity	DOI: 10.1016/j.energy.2020.117705
[45]	Flexible Blade (Passive-Deformable Structure)	2D and 3D CFD (URANS and ANSYS fluent (version 16.2); DOE optimization)	+32% (3D) Cp gain at low TSR (up to +66% in 2D)	3	80	Simple design; damps cyclic loads; benefit modest	DOI: 10.1016/j.enconman.2023.117867
[46]	Adaptive Blade (Shape-Memory alloy Actuated)	Experimental (lab-scale VAWTs and morphing NACA 0022 blades)	Up to ~20–30% Cp gain at low TSR (qualitative)	4	180	Temperature-sensitive SMA; improves self-start; expensive materials	DOI: 10.1016/j.enconman.2014.11.030
[31]	Passive Flexible Blades (Fluid–Structure Interaction)	3D FSI simulation (OpenFOAM + structural model)	+9.6% overall efficiency vs. rigid blades	5	120	Smooth torque; dual drivetrain; SeaTwirl pilot under way	DOI: 10.1016/j.jfluidstructs.2015.10.010

Note. (a) Performance gain is reported relative to the same turbine without the active strategy, under the experimental or numerical conditions given in each study. (b) The TRL (technology readiness level) is estimated according to the NASA–ESA 1–9 scale: 3 = analytical and lab validation; 4–5 = wind tunnel or reduced-scale prototype; 6 = relevant-scale demo in a simulated environment; 7–8 = pilot/prototype in a real environment; 9 = commercial status. Incremental CAPEX (USD kW⁻¹) relative to a baseline VAWT of 10–50 kW, derived from (i) component prices for actuators, sensors, and electronics (pitch, SMA); (ii) prototype or LCOE studies; or (iii) market-based estimates when only percentage ranges were reported. Values rounded to the nearest ten.

Table 3. Comparison of active approaches and their performance improvements.

Reference	Active Strategy	Methodology/Validation	Performance Improvement	TRL Estimate	Added Cost (USD kW−1)	Key Technical Observations	Source (DOI)
[42]	Variable Pitch (Prescribed Pitching Profile)	Semi-analytical DMST model (2D)	+18.9% peak Cp increase at optimum TSR	6	100	Moderate boost; PLC-driven; retrofit-friendly; field demo pending	DOI: 10.3390/app8060957
[43]	Optimal Individual Blade Pitch Control	Wind tunnel (scaled model), Gen. algorithm optimization	~3 × higher Cp vs. fixed-pitch baseline; −77% load fluctuations	4	140	Highest gain; needs fast actuators and sensors; early prototype stage	DOI: 10.1038/s41467-024-46988-0
[44]	Trailing-Edge Slotted Flap (Deflective)	2D and 3D CFD (URANS) with DOE optimization	+27% (2D) and +19% (3D) increase in power output at TSR ≈ 3	3	45	Passive add-on; low CAPEX; wind tunnel only so far	DOI: 10.1016/j.enconman.2022.116388
[37]	Oscillating Mini-Foils (Active Flow Control)	2D CFD (dynamic mesh and UDF control)	+38% turbine power coefficient (Cp) improvement	3	70	Active flow control; requires phase synchronization; no field data	DOI: 10.1007/s40430-016-0618-3
[35]	Morphing Blade Geometry (Adaptive Camber)	3D CFD (STAR-CCM+ and overset + deforming mesh)	+46.2% increase in output power vs. rigid blade	4	200	Large gain; morphing skin adds mass and control complexity	DOI: 10.1016/j.energy.2020.117705
[45]	Flexible Blade (Passive-Deformable Structure)	2D and 3D CFD (URANS and ANSYS fluent (version 16.2); DOE optimization)	+32% (3D) Cp gain at low TSR (up to +66% in 2D)	3	80	Simple design; damps cyclic loads; benefit modest	DOI: 10.1016/j.enconman.2023.117867
[46]	Adaptive Blade (Shape-Memory alloy Actuated)	Experimental (lab-scale VAWTs and morphing NACA 0022 blades)	Up to ~20–30% Cp gain at low TSR (qualitative)	4	180	Temperature-sensitive SMA; improves self-start; expensive materials	DOI: 10.1016/j.enconman.2014.11.030
[31]	Passive Flexible Blades (Fluid–Structure Interaction)	3D FSI simulation (OpenFOAM + structural model)	+9.6% overall efficiency vs. rigid blades	5	120	Smooth torque; dual drivetrain; SeaTwirl pilot under way	DOI: 10.1016/j.jfluidstructs.2015.10.010

Note. (a) Performance gain is reported relative to the same turbine without the active strategy, under the experimental or numerical conditions given in each study. (b) The TRL (technology readiness level) is estimated according to the NASA–ESA 1–9 scale: 3 = analytical and lab validation; 4–5 = wind tunnel or reduced-scale prototype; 6 = relevant-scale demo in a simulated environment; 7–8 = pilot/prototype in a real environment; 9 = commercial status. Incremental CAPEX (USD kW⁻¹) relative to a baseline VAWT of 10–50 kW, derived from (i) component prices for actuators, sensors, and electronics (pitch, SMA); (ii) prototype or LCOE studies; or (iii) market-based estimates when only percentage ranges were reported. Values rounded to the nearest ten.

While Table 4 confirms that passive approaches can close much of the historical efficiency gap with HAWTs, their uptake hinges on proving that they remain advantageous once full-scale structural, acoustic, and financial constraints are accounted for. Passive augmentation has indisputably raised the aerodynamic ceiling of VAWTs, yet three structural gaps explain why most concepts remain confined to wind tunnel or CFD campaigns. First, large-diameter stators, diffusers, or ducts—responsible for the spectacular gains reported in Table 4 (+82% Cp with a wind lens [47]; +248% peak torque with a 360° guide ring [48])—with introduce size and mass penalties complicate rooftop or offshore logistics and push capital costs beyond what micro- or meso-scale developers can justify. Second, the absence of IEC-level design rules for flow-augmenting devices leaves insurers and certifiers without fatigue load spectra or safety factors tailored to stator-induced recirculation and diffuser suction, creating high regulatory uncertainty; this explains why no optimized omnidirectional guide (->41% power, [49]) has yet progressed past laboratory TRL 3. Third, environment-coupled side effects remain under-documented: Rear diffusers raise broadband noise by 3–4 dB [47]. Venturi ducts can amplify dynamic pressures on support frames during gusts [36], and wrap-around fairings that ostensibly beat Betz in 2D [39] have not been benchmarked against an open-air reference rotor.

Addressing these gaps requires (i) scalable prototyping protocols, e.g., modular stator panels manufactured in composite segments to test ≥ 10 kW units in urban and coastal settings; (ii) coupled aero-acoustic and structural models that quantify noise, vibration, and added moments so that IEC working groups can draft an annex for VAWT flow augmenters; and (iii) techno-economic metrics beyond Cp (added mass per % gain, USD kW⁻¹ per dB noise, and TRL cost matrices) to identify which devices—front deflectors that boost start-up torque by 47% at a negligible cost [38] versus bulky guide-wall ducts yielding Cp = 0.91 [50]—offer the best cost/performance ratio in real installations.

Table 4. Comparison of passive approaches and their performance improvements.

Reference	Passive Design Strategy	Passive Approach Details	Reported Performance Improvement	Relevant Notes
[48]	Omnidirectional stator (guide vanes)	360° circumferential blade ring	+248% peak torque; starting from 7.3 → 4 m/s wind speed	5-blade model; huge gains in start-up and RPM
[51]	Partial stator with 8 vanes	8 blades + 2 guide cones	+30–35% Cp at TSR = 2.75	Consistent improvement independent of wind speed
[49]	Optimized omnidirectional guide	Optimized blade angle	+41% power (optimal configuration)	52-angle parametric study; omnidirectional
[41]	Guide vanes with variable pitch	Combines 4–8 blades (20°) + active pitch	+35% power vs. variable pitch only (optimal)	Non-uniform configuration outperformed uniform configuration; wind direction dependent
[33]	Guide walls (side ducts)	Optimized guide walls	Cp = 0.91 vs. 0.31 without guides (+200%)	Very high Cp value; CFD optimization to TSR 3.3
[47]	Rear diffuser with flange (Wind lens)	Diffuser + annular flange	+82% Cp at TSR = 2.75	Cycloidal curved diffuser showed better results; increased noise
[36]	Wraparound duct	Venturi tubular fairing	+125% power at 8 m/s (double power vs. without duct)	Benefit in a wide range of 3–16 m/s; convergent–divergent design
[39]	Wraparound aerodynamic profile	Optimized 2D fairing	Cp > Betz (estimated, without ref. turbine)	Wing-like fairing surrounding the rotor; no open comparison
[38]	Front deflector plate	Upwind flat plate, fixed 30°	+47% initial Ct (torque coefficient)	Facilitates self-start; simple installation
[40]	Double upper/lower deflector	Two optimized plates	+28% Cp (best configuration vs. without deflector)	A lower deflector performs better than an upper one; additive effect with both

Table 4. Comparison of passive approaches and their performance improvements.

Reference	Passive Design Strategy	Passive Approach Details	Reported Performance Improvement	Relevant Notes
[48]	Omnidirectional stator (guide vanes)	360° circumferential blade ring	+248% peak torque; starting from 7.3 → 4 m/s wind speed	5-blade model; huge gains in start-up and RPM
[51]	Partial stator with 8 vanes	8 blades + 2 guide cones	+30–35% Cp at TSR = 2.75	Consistent improvement independent of wind speed
[49]	Optimized omnidirectional guide	Optimized blade angle	+41% power (optimal configuration)	52-angle parametric study; omnidirectional
[41]	Guide vanes with variable pitch	Combines 4–8 blades (20°) + active pitch	+35% power vs. variable pitch only (optimal)	Non-uniform configuration outperformed uniform configuration; wind direction dependent
[33]	Guide walls (side ducts)	Optimized guide walls	Cp = 0.91 vs. 0.31 without guides (+200%)	Very high Cp value; CFD optimization to TSR 3.3
[47]	Rear diffuser with flange (Wind lens)	Diffuser + annular flange	+82% Cp at TSR = 2.75	Cycloidal curved diffuser showed better results; increased noise
[36]	Wraparound duct	Venturi tubular fairing	+125% power at 8 m/s (double power vs. without duct)	Benefit in a wide range of 3–16 m/s; convergent–divergent design
[39]	Wraparound aerodynamic profile	Optimized 2D fairing	Cp > Betz (estimated, without ref. turbine)	Wing-like fairing surrounding the rotor; no open comparison
[38]	Front deflector plate	Upwind flat plate, fixed 30°	+47% initial Ct (torque coefficient)	Facilitates self-start; simple installation
[40]	Double upper/lower deflector	Two optimized plates	+28% Cp (best configuration vs. without deflector)	A lower deflector performs better than an upper one; additive effect with both

4.2. Materials and Structural Durability

Research on the structural integrity and materials of VAWTs has progressed from diagnosing historical fatigue failures in aluminum and steel blades to experimenting with advanced composites, yet Table 5 reveals several unresolved gaps that still hamper scale-up and long-term reliability. Because each VAWT blade endures two distinct stress cycles per revolution, high-cycle fatigue and vibration remain the dominant failure modes; early megawatt-scale prototypes documented crack initiation after only 10⁵–10⁶ cycles in metal blades, delaying commercial uptake [52]. Post-2020 work therefore pivoted to fiber-reinforced polymers, where glass and carbon laminates reduce mass and centrifugal loading, lowering vibratory stresses by 20–40% [53] while hybrid Kevlar/CNT skins suppress tip deflection by ≈18% without weight penalties [54,55]. Cost and manufacturability are the first major gap: high-performance CFRP still adds ≈ 200 USD kW⁻¹ to medium-scale rotors, driving researchers toward metal–composite hybrids or localized patches that trade cash for fatigue life. End-of-life circularity is the second gap: thermoset blades remain land-filled, prompting the exploration of recyclable thermoplastics and bio-based matrices for ≤10 kW turbines [56,57], but these materials lack multi-year field data under real wind spectra.

A third gap is standardization. IEC 61400-2 overlooks VAWT-specific loading (biaxial bending and dynamic stall) and does not prescribe rain-erosion or sand-erosion methods for vertical rotors, forcing designers to extrapolate HAWT criteria and inflating certification costs [50]. Recent studies call for dedicated fatigue–damage models, mode-coupling analysis and coating–erosion protocols tailored to VAWTs [58]. From Table 5 it is clear that nano-enhanced sol–gel or ceramic coatings halve rain- and sand-erosion rates, extending leading-edge life by factors of 2–3, yet no techno-economic framework quantifies how such coatings affect the levelized cost of electricity (LCOE) beyond laboratory coupons.

To unlock commercial-scale VAWTs, future work must (i) benchmark recyclable and hybrid materials in ≥250 kW demonstrators, gathering ≥ 10,000 h of fatigue and erosion data under IEC-class wind regimes; (ii) integrate life-cycle analysis with AHP/VIKOR multi-criteria tools so that material choices are optimized not only for stiffness and corrosion (where CFRP already ranks highest [59]) but also for embodied carbon, recyclability and supply-chain resilience; and (iii) develop a VAWT-specific annex to IEC 61400 that covers biaxial S–N curves, rain-erosion test rigs and coating–substrate compatibility. Bridging these gaps will turn the laboratory promise of advanced composites and nano-coatings into verified reliability and competitive LCOE for next-generation vertical-axis wind turbines.

Table 5. Systematic overview of structural materials and protective coatings for the mechanical and environmental durability of vertical-axis wind turbines (VAWTs).

References	Type of Material	Structural Application	Key Properties Evaluated	Observed/Simulated Durability	Test or Simulation Conditions
[60]	CFRP and composite steel	15 m offshore tower and support	Deflection, fatigue index, and material stiffness	CFRP cuts fatigue damage 67%; keeps stiffness under high winds	CFRP lighter and longer-lived than steel in marine loads
[61]	Glass- and carbon-fiber laminate	Utility-scale blade (H-Darrieus)	Modal frequencies, S–N fatigue life	optimized lay-up lowers cyclic damage ≈ 2.5%	Layer sequencing crucial for blade fatigue resistance
[62]	Kevlar/epoxy + 0.5% CNT	Lab-scale blade (0.7 m)	Elastic modulus, aero deflection, and strength	CNT cuts tip deflection by 18% and raises strength	Nanofillers boost rigidity without weight penalty
[63]	Aluminum, carbon steel, and PE	Savonius rotor blade (small)	Stress vs. weight and manufacturability	Al shows lower stress than steel; PE deforms the most	Aluminum has the best strength-to-weight ratio and is cost-effective
[64]	PU, ceramic sol–gel, and nano-SiO2	Blade leading edge	Rain/particle erosion and UV resistance	Sol–gel + SiO2 doubles life vs. PU	Nano-sol–gel shields edge from rain and UV
[65]	Epoxy + Al2O3/ZrO2/CeO2	Blade surface (sand)	Solid-particle erosion, hardness	ZrO2/CeO2 cut wear 30–50%	Ceramic oxides defend blades in sandy sites
[59]	CFRP, aluminum, and steel (AHP-VIKOR)	Towers and mounts (marine)	Corrosion, fatigue, and recyclability	CFRP has the highest overall score; steel is penalized	CFRP preferred for offshore durability

Table 5. Systematic overview of structural materials and protective coatings for the mechanical and environmental durability of vertical-axis wind turbines (VAWTs).

References	Type of Material	Structural Application	Key Properties Evaluated	Observed/Simulated Durability	Test or Simulation Conditions
[60]	CFRP and composite steel	15 m offshore tower and support	Deflection, fatigue index, and material stiffness	CFRP cuts fatigue damage 67%; keeps stiffness under high winds	CFRP lighter and longer-lived than steel in marine loads
[61]	Glass- and carbon-fiber laminate	Utility-scale blade (H-Darrieus)	Modal frequencies, S–N fatigue life	optimized lay-up lowers cyclic damage ≈ 2.5%	Layer sequencing crucial for blade fatigue resistance
[62]	Kevlar/epoxy + 0.5% CNT	Lab-scale blade (0.7 m)	Elastic modulus, aero deflection, and strength	CNT cuts tip deflection by 18% and raises strength	Nanofillers boost rigidity without weight penalty
[63]	Aluminum, carbon steel, and PE	Savonius rotor blade (small)	Stress vs. weight and manufacturability	Al shows lower stress than steel; PE deforms the most	Aluminum has the best strength-to-weight ratio and is cost-effective
[64]	PU, ceramic sol–gel, and nano-SiO2	Blade leading edge	Rain/particle erosion and UV resistance	Sol–gel + SiO2 doubles life vs. PU	Nano-sol–gel shields edge from rain and UV
[65]	Epoxy + Al2O3/ZrO2/CeO2	Blade surface (sand)	Solid-particle erosion, hardness	ZrO2/CeO2 cut wear 30–50%	Ceramic oxides defend blades in sandy sites
[59]	CFRP, aluminum, and steel (AHP-VIKOR)	Towers and mounts (marine)	Corrosion, fatigue, and recyclability	CFRP has the highest overall score; steel is penalized	CFRP preferred for offshore durability

4.3. Urban Integration and Microgeneration

Urban integration and microgeneration have re-emerged as the most vibrant niches for vertical-axis wind turbines (VAWTs) because these machines can operate safely, quietly, and omnidirectionally in the turbulence-dominated flow typical of cities [66]. Their compact footprint lets designers exploit roof edges, wind corridors, and façades that are inaccessible to large horizontal-axis turbines (HAWTs); Ahmad therefore argues that VAWTs are uniquely suited to confined urban spaces [67]. Bibliometric data confirm this momentum: since 2021 the frequency of keywords such as urban wind energy and building-integrated wind has risen sharply, mirroring smart city initiatives and the post-2020 push for on-site resilience. Early pilots already couple rooftop VAWTs with photovoltaics (PVs), achieving 20–30% cuts in net electricity demand for residential and commercial buildings [68], while iconic projects—e.g., the Bahrain World Trade Center—prove that esthetic and structural integration is feasible even at the skyscraper scale [69].

Despite those successes, performance in dense urban settings remains uneven, exposing three persistent knowledge gaps. First, for urban flow predictability, high-resolution CFD and wind tunnel studies show that helical blades, diffusers, and active pitch control can raise capacity factors under gusty winds [70], yet full-scale trials in sub-optimal sites still report single-digit capacity factors and multi-decade paybacks; a 5 kW Darrieus installed in Poland delivered so little energy that its life-cycle impact was dominated by mast and foundation materials [71]. Second, for structural–acoustic coupling, dynamic loads, façade-borne vibration, and broadband noise (<60 dB) are documented only for sub-kilowatt prototypes, leaving large-unit compliance with building codes uncertain. Third, for life-cycle performance, few studies quantify cradle-to-grave CO₂ per kWh; in low-wind districts the carbon intensity of steel supports can rival—or exceed—the local grid mix, casting doubt on net-zero claims. Table 6 indicates that the research community is addressing these gaps by shifting from purely numerical work to instrumented prototypes and multi-criteria assessments. Diffuser-augmented rotors, edge-mounted deflectors, and adaptive-pitch algorithms have each delivered 20–40% power gains and significant noise attenuation when co-optimized with the siting. Yet most deployments remain at tens to hundreds of watts, placing urban VAWTs at mid-range technology readiness levels (TRLs 3–5).

To move beyond this plateau, future programs should (1) develop city-scale digital twins that fuse long-term rooftop anemometry with LES-CFD, enabling probabilistic energy and load maps before installation; (2) test kilowatt-class arrays for ≥10,000 h to generate empirical datasets on fatigue, vibration, and public acceptance; and (3) adopt harmonized techno-economic metrics—e.g., LCOE per square meter of roof or payback adjusted for acoustic penalties—to let planners benchmark VAWTs against PVs and demand a response from energy-efficiency retrofits. By integrating aerodynamic designs, structural damping, and data-driven siting within a single decision framework, VAWTs can evolve from boutique demonstrations into bankable components of net-zero and smart city infrastructures [68,71]

Table 6. Systematic review (2018–2025) of studies on urban integration and microgeneration with vertical-axis wind turbines (VAWTs).

References	Type of Study	Urban Context	Type of VAWT	Main Findings	Barriers/Challenges Identified
[68]	Energy simulation + eco-analysis	5-storey residential roof (Çeşme, TR)—42 units	Helical and straight “IceWind”; combined rotor	18–31% annual building-electricity reduction; Cpmax = 0.34–0.46; 10–13 yr payback	Highly variable urban wind; extra roof loads; O&M cost uncertainty
[72]	In situ metering + CFD-LES; multi-objective optimization	12-storey coastal tower (Lima, PE)	Ø 1 m H-Darrieus mini-turbines	Optimum hub 1.4 m above roof ↑ AEP 27%; ↓ TI 18%	>60 dB noise leeward; seismic mass limits; high anchoring CAPEX
[73]	CFD-RANS + adaptive pitch	Curved façade-embedded turbine (Guangxi, CN)	H-Darrieus w/internal entity and pitch control	+22% Cp; +self-start; −17% torque fluctuation	Added sensors; façade–glass interference; mechanical complexity
[74]	Real-terrain DDES + topographic validation	Twin rooftop levels and Stuttgart campus (DE)	Ø 7 m H-Darrieus	Skewed flow ↑ Cp 12–18% vs. uniform inflow; best at roof edge	Strong height dependence; lateral loads; anti-vibration anchoring
[75]	211 CFD-RANS + NN/Kriging metamodels	High-rise duct diffuser (generic city)	Ducted micro-VAWT	Optimal throat velocity 2.7×; AEP × 1.9; NN + GARS effective < 200 samples	Construction complexity; aero-acoustics in ducts; no validation yet
[76]	Tunnel experiment on 0.7 m prototype	Gable-roof model (Nigeria)	Darrieus–Savonius hybrid	Roof integration ↑ Cp 0.126 (≈ +266% vs. stand-alone)	Very low absolute efficiency; ≤2 W power; esthetic concerns
[77]	Aero-acoustic CFD (noise-limited design)	Generic urban block	5 kW H-Darrieus	Side deflector ↓ noise 98%; ↑ torque 169%	Acoustic < 60 dB needed for social approval; efficiency-noise trade-off

Abbreviations: AEP = annual energy production; TI = turbulence intensity; TRL = technology readiness level; NN = neural network; GARS = genetic algorithm–random search; O&M = operation and maintenance; CFD = computational fluid dynamics; LES = large-eddy simulation; DDES = delayed detached-eddy simulation.

Table 6. Systematic review (2018–2025) of studies on urban integration and microgeneration with vertical-axis wind turbines (VAWTs).

References	Type of Study	Urban Context	Type of VAWT	Main Findings	Barriers/Challenges Identified
[68]	Energy simulation + eco-analysis	5-storey residential roof (Çeşme, TR)—42 units	Helical and straight “IceWind”; combined rotor	18–31% annual building-electricity reduction; Cpmax = 0.34–0.46; 10–13 yr payback	Highly variable urban wind; extra roof loads; O&M cost uncertainty
[72]	In situ metering + CFD-LES; multi-objective optimization	12-storey coastal tower (Lima, PE)	Ø 1 m H-Darrieus mini-turbines	Optimum hub 1.4 m above roof ↑ AEP 27%; ↓ TI 18%	>60 dB noise leeward; seismic mass limits; high anchoring CAPEX
[73]	CFD-RANS + adaptive pitch	Curved façade-embedded turbine (Guangxi, CN)	H-Darrieus w/internal entity and pitch control	+22% Cp; +self-start; −17% torque fluctuation	Added sensors; façade–glass interference; mechanical complexity
[74]	Real-terrain DDES + topographic validation	Twin rooftop levels and Stuttgart campus (DE)	Ø 7 m H-Darrieus	Skewed flow ↑ Cp 12–18% vs. uniform inflow; best at roof edge	Strong height dependence; lateral loads; anti-vibration anchoring
[75]	211 CFD-RANS + NN/Kriging metamodels	High-rise duct diffuser (generic city)	Ducted micro-VAWT	Optimal throat velocity 2.7×; AEP × 1.9; NN + GARS effective < 200 samples	Construction complexity; aero-acoustics in ducts; no validation yet
[76]	Tunnel experiment on 0.7 m prototype	Gable-roof model (Nigeria)	Darrieus–Savonius hybrid	Roof integration ↑ Cp 0.126 (≈ +266% vs. stand-alone)	Very low absolute efficiency; ≤2 W power; esthetic concerns
[77]	Aero-acoustic CFD (noise-limited design)	Generic urban block	5 kW H-Darrieus	Side deflector ↓ noise 98%; ↑ torque 169%	Acoustic < 60 dB needed for social approval; efficiency-noise trade-off

Abbreviations: AEP = annual energy production; TI = turbulence intensity; TRL = technology readiness level; NN = neural network; GARS = genetic algorithm–random search; O&M = operation and maintenance; CFD = computational fluid dynamics; LES = large-eddy simulation; DDES = delayed detached-eddy simulation.

4.4. Hybrid Applications and Floating Systems

Hybrid and floating concepts have moved VAWTs from proof-of-concept curiosities to field-tested prototypes, yet Table 7 shows that core knowledge gaps still constrain scalability and bankability. In grid-isolated micro-grids, wind–solar cogeneration—either side-by-side installations or PV skins wrapped around Darrieus blades—delivers flatter diurnal output and land-use synergy, a clear advantage for rural electrification and street-lighting schemes [78]. Early demonstrations report up to 25% LCOE or LCOH reductions when VAWTs feed electrolyzers or battery banks in tandem with PV modules [79]. Three under-researched issues, however, threaten their long-term reliability: (i) persistent thermal hotspots in curved PV panels and PEM stacks mounted on rotating hubs, for which no accelerated aging data beyond 5000 h exist [80]; (ii) galvanic and salt-spray corrosion at the metal–semiconductor interface when the PV is bonded to vibrating aluminum spars, where tests have been limited to coupon specimens rather than full rotor cross-sections; and (iii) the absence of a multi-source control layer that is able to optimize turbine pitch, MPPT tracking, and electrolyzer ramping without inflating CAPEX for power electronics. Until these operational gaps are quantified with field datasets and folded into techno-economic metrics such as “USD kW⁻¹per %-intermittency mitigated,” hybrid VAWTs will remain at the pilot scale despite their clear theoretical merits.

Hybrid drag–lift rotors follow a different path: Savonius cups guarantee self-start, while Darrieus foils sustain lift-based efficiency [81]. Experiments show up to 40% higher starting torque and respectable Cp at high TSRs [82], yet CFD campaigns warn that the added Savonius section can raise profile drag by 5–10%, eroding net gains under yawed inflow [83]. Design trade-offs therefore hinge on site conditions: Cut-in reliability outweighs peak efficiency (low-wind villages and telecom towers). Hybrid rotors offer an immediate payoff; in high-capacity-factor sites, a pure optimized Darrieus may still prevail. Future work should couple aerodynamic optimization with life-cycle costing to reveal the inflection point at which a hybrid rotor justifies its extra mass and maintenance.

Offshore, floating VAWTs have re-emerged as a deep-water alternative to floating HAWTs, leveraging lower centers of gravity, the absence of yaw drives, and reduced overturning moments [29]. Numerical benchmarks capture aero–hydro–servo interactions within 3% of OpenFAST predictions [84], and 1:50 wave-tank trials show a 12% cut in parked loads relative to fixed towers [85]. Despite these advances, most multi-MW designs (DeepWind, VertiWind, and SeaTwirl) linger below TRL 4 [86]. Key deficits include (i) full-scale validation of nonlinear wave–rotor coupling, as no data yet exist on blade vibration under combined slamming and gust events; (ii) anchor and TLP concepts tuned to periodic cross-plane loads unique to vertical rotors; and (iii) integrated energy management for farms that may toggle between grid supply and on-board hydrogen production [87]. Addressing these gaps will demand 10,000 h sea trials with dense instrumentation for corrosion, fatigue, and power-quality metrics, followed by an IEC annex that codifies VAWT-specific load cases and erosion tests.

Table 7. Systematic review (2020–2025) of hybrid applications and floating systems based on vertical-axis wind turbines (VAWTs).

References	Type of System	Technical Configuration (VAWT + Coupled Elements)	Main Reported Advantages	Limitations/Challenges
[79]	Wind–solar–H2 micro-grid (AC-coupled)	H-type VAWT + 668 kWp PV + 2 MWh Li-ion + 0.75–1.75 MW PEM electrolyzer	100% REN balance; 25% LCOH cut vs. diesel-H2	Seasonal PV tilt; battery CAPEX high
[88]	Large-scale offshore VAWT-PV hybrid	200 m Ø Darrieus (15 MW) + 1.5 MWp PV ring	Higher capacity factor; PV cuts storage by 2–4%	Marine shading and PV corrosion
[80]	On-board solar Darrieus	1.5 kW Darrieus + 3 m2 curved PV; hybrid MPPT	Cp = 0.236 (↑ 18%) vs. VAWT alone	Cooling complexity; limited PV area
[87]	Domestic H2 micro-factory	Savonius-helical 2 kW + PEM 10 kWh H2 bench	Covers all home energy; daily smoothing via H2	High initial CAPEX; micro-BoP complexity
[84]	Semi-submersible floating VAWT (numerical)	5 MW H-Darrieus + OC4; OWENS-OpenFAST coupling	<3% aero–hydro–servo error vs. OpenFAST	Needs an offshore validation dataset
[85]	1:50 Troposkein VAWT on TLP	Wind-wave tank; 2–3 blades; TSR 0 parked	12% peak-load cut over fixed tower	Scaling laws; fatigue impacts of blade number
[30]	Critical tech review of floating VAWTs	5–10 MW concepts (DeepWind, VertiWind, and SeaTwirl)	↓ 20% LCOE potential; better power density	TRL low (<3); aero–hydro validation needed
[86]	MarsVAWT TLP platform (numerical–structural)	Modular 10 MW Darrieus on TLP	Low drift; natural freq outside wave bands	Redundant anchoring; nonlinear effects pending

Abbreviations: Li-ion = lithium-ion battery; PEM = proton-exchange membrane; TLP = tension-leg platform; BoP = balance of plant; MPPT = maximum power point tracking; LCOH = levelized cost of hydrogen; TRL = technology readiness level.

Table 7. Systematic review (2020–2025) of hybrid applications and floating systems based on vertical-axis wind turbines (VAWTs).

References	Type of System	Technical Configuration (VAWT + Coupled Elements)	Main Reported Advantages	Limitations/Challenges
[79]	Wind–solar–H2 micro-grid (AC-coupled)	H-type VAWT + 668 kWp PV + 2 MWh Li-ion + 0.75–1.75 MW PEM electrolyzer	100% REN balance; 25% LCOH cut vs. diesel-H2	Seasonal PV tilt; battery CAPEX high
[88]	Large-scale offshore VAWT-PV hybrid	200 m Ø Darrieus (15 MW) + 1.5 MWp PV ring	Higher capacity factor; PV cuts storage by 2–4%	Marine shading and PV corrosion
[80]	On-board solar Darrieus	1.5 kW Darrieus + 3 m2 curved PV; hybrid MPPT	Cp = 0.236 (↑ 18%) vs. VAWT alone	Cooling complexity; limited PV area
[87]	Domestic H2 micro-factory	Savonius-helical 2 kW + PEM 10 kWh H2 bench	Covers all home energy; daily smoothing via H2	High initial CAPEX; micro-BoP complexity
[84]	Semi-submersible floating VAWT (numerical)	5 MW H-Darrieus + OC4; OWENS-OpenFAST coupling	<3% aero–hydro–servo error vs. OpenFAST	Needs an offshore validation dataset
[85]	1:50 Troposkein VAWT on TLP	Wind-wave tank; 2–3 blades; TSR 0 parked	12% peak-load cut over fixed tower	Scaling laws; fatigue impacts of blade number
[30]	Critical tech review of floating VAWTs	5–10 MW concepts (DeepWind, VertiWind, and SeaTwirl)	↓ 20% LCOE potential; better power density	TRL low (<3); aero–hydro validation needed
[86]	MarsVAWT TLP platform (numerical–structural)	Modular 10 MW Darrieus on TLP	Low drift; natural freq outside wave bands	Redundant anchoring; nonlinear effects pending

Abbreviations: Li-ion = lithium-ion battery; PEM = proton-exchange membrane; TLP = tension-leg platform; BoP = balance of plant; MPPT = maximum power point tracking; LCOH = levelized cost of hydrogen; TRL = technology readiness level.

4.5. Electrical-Integration Structures and Control Algorithms

Table 8. Electrical-integration structures and control algorithms for vertical-axis wind turbines (VAWTs) compared with horizontal-axis wind turbines (HAWTs).

References	Connection Topology/Architecture	Primary Control Algorithm(s)	Key Advantages vs. HAWT Practice	Main Caveats/Open Issues
[30]	Floating H-Darrieus; direct-drive PMSG; full back-to-back AC–DC–AC converter.	Platform motion damping; no yaw loop required.	Low center-of-gravity cuts overturning moment; yaw system eliminated.	Cyclic torque induces vibrations—mitigation of needs absent in HAWTs.
[91]	Land-based VAWT; Type IV scheme (PMSG + full converter).	Torque-based MPPT with fixed-pitch blades.	Omnidirectional; fewer moving parts than pitch/yaw-driven HAWTs.	Lower peak CpC_p; limited high-wind power shedding.
[89]	1 kW H-VAWT; AC–DC boost to the DC link; single-phase inverter to the grid.	DC-link voltage control + perturb-and-observe MPPT.	Simple, low-cost micro-grid solution; fully decouples rotor speed from the grid.	Boost-stage efficiency critical; small power rating.
[90]	Urban H-VAWT; direct-drive PMSG; full converter.	Multi-objective torque law (ω2) + TSR estimator.	+33% energy vs. generic ω2; –25% torque variance.	Requires real-time wind estimation; slight acoustic increase.
[43]	Lab-scale H-VAWT with individual blade-pitch actuators; standard converter set-up.	Azimuth-synchronized blade pitching (genetic optimization).	Triples Cp; 77% load-fluctuation reduction—unique to VAWT.	High actuator count; durability yet unproven at the MW scale.
[4]	Survey: VAWT → full converter; large HAWT → DFIG/partial converter.	VAWT relies on electrical MPPT only; HAWT uses collective pitch + yaw.	Simpler rooftop deployment; fewer moving parts.	~25% lower aero-efficiency; self-start issues at low wind.

underscores that contemporary VAWT integration relies almost exclusively on direct-drive permanent-magnet synchronous generators (PMSGs) coupled to full back-to-back power converters. This topology delivers the variable-speed flexibility essential for a machine whose aerodynamic torque oscillates strongly within each revolution. Yet, no long-term field data at the ≥100 kW scale have been published to prove converter reliability under the high-frequency torque pulsations unique to VAWTs; most demonstrations remain <10 kW and laboratory-based [89,90]. By contrast, megawatt HAWTs routinely operate with either partial converters (DFIG) or full converters that benefit from smooth, near-steady aero-loads. Thus, a first gap is the absence of component lifetime and grid-code compliance studies for medium- and large-scale VAWT converters, particularly under rapid torque reversal and harmonic content that may challenge current injection limits [91].

A second gap concerns active aerodynamic control. Only one study to date demonstrates individual blade-pitch control in a VAWT [43], achieving a three-fold power-coefficient increase. However, that success required complex azimuth-synchronized actuation and genetic optimization hardware and algorithms not yet validated for durability or cost at the utility scale. Most commercial VAWTs still employ fixed-pitch blades, delegating all power regulation to the electrical converter. This absence of an independent power-shedding loop means that VAWTs must absorb large aerodynamic stalls during gusts, stressing both the drivetrain and converter. Research should therefore prioritize hybrid control architectures in which low-power electro-mechanical pitch devices provide coarse load relief while the converter refines power quality. Otherwise, converter oversizing will remain a hidden CAPEX penalty relative to HAWTs with mature collective-pitch systems.

Third, the literature reveals a scarcity of grid-dynamic studies. Only Reddy [89] evaluate voltage-ride-through and single-phase harmonics for a micro-VAWT; no work tests inertia-like support or the synthetic frequency response, even though the rapid inertial characteristics of low-mass VAWTs could be valuable for weak grids. Integrating VAWTs into hybrid PV storage micro-grids compounds this omission: energy management systems (EMS) must coordinate multi-second PV ramps, sub-second wind fluctuations, and battery SOC, yet comparative EMS algorithms remain largely conceptual. Field pilots that benchmark EMS responses against IEEE 1547 or ENTSO-E ancillary service requirements are therefore essential.

The floating concepts reviewed by Arredondo-Galeana [30] highlight structural advantages, lower overturning moments, and yaw-free operation, but these prototypes still lack MW-class power electronic packages certified for harsh marine climates. Likewise, according to Al-Rawajfeh [4], rooftop VAWTs benefit from simple full-converter schemes, yet they struggle to meet urban acoustic limits when operated at optimal tip-speed ratios. Future work must merge converter control with acoustic optimization by potentially modulating rotational speed to avoid tonal peaks, thereby bridging an interdisciplinary gap between power electronics and aero-acoustics.

Closing these gaps will require (i) long-duration (>20,000 h) converter endurance tests under representative pulsating torque spectra; (ii) the development of low-power, high-frequency pitch or camber actuators compatible with the tight geometric envelope of VAWT blades; (iii) model-based predictive controllers that fuse wind-field estimation with converter and EMS objectives, exploiting the high bandwidth of full converters; and (iv) standardized grid-compliance protocols tailored to omnidirectional turbines, including harmonic, inertia, and fault-ride-through benchmarks. Achieving these milestones could elevate VAWTs from niche distributed assets to credible complements or even alternatives to HAWTs in complex onshore and offshore networks.

4.6. Mathematical Models for VAWT Research and Simulation

Table 9 illustrates the mathematical models used in VAWT research and simulation. The modeling literature on vertical-axis wind turbines (VAWTs) has bifurcated into two distinct camps whose capabilities and shortcomings are now evident. On the one side, very fast momentum–stream-tube and vortex codes—typified by the double-multiple-stream-tube (DMST) implementations benchmarked by Giri [92]—remain indispensable for rapid parametric sweeps, controller-in-the-loop optimization, and early-stage techno-economic studies. Their computational lightness, however, is obtained at the cost of physics: by relying on static lift-and-drag polars, they systematically over-estimate the power coefficient once the tip-speed ratio rises into the regime dominated by dynamic stall and wake re-encounters. Embedding reduced-order, phase-accurate dynamic-stall kernels—validated against phase-resolved PIV or rotating-arm experiments—therefore constitutes the first clear research gap if these low-order tools are to remain relevant beyond concept screening.

At the other extreme, high-fidelity CFD and multiphysics frameworks (URANS or LES studies) such as those surveyed by Fertahi [93] and the flap investigations by Chakroun [34] resolve vortex shedding, stall hysteresis, and near-wake interaction with impressive accuracy, but at prohibitive computational costs. A single 3D LES of a five-meter rotor can demand well over ten thousand core hours, and that burden multiplies when aero–hydro–servo coupling is introduced for floating or pitch-active configurations, as shown in the five MW flutter studies by Ahsan [94]. Bridging the speed–fidelity gulf will require two complementary advances: surrogate or neural-operator models trained on limited CFD snapshots to emulate flow fields in near real time and multi-fidelity optimization frameworks in which coarse DMST or actuator-line solvers steer the global search while CFD refines the local Pareto front. Neither avenue can mature without open, standardized benchmark datasets of VAWT wake field datasets that do exist for horizontal-axis machines but remain conspicuously absent for VAWTs.

A second, subtler gap concerns aero–servo–elastic integration. Because VAWTs experience stronger cross-plane cyclic bending and larger torsional gradients than their horizontal counterparts, weakly coupled time-domain simulators risk masking flutter or resonance instabilities. The linearized finite-element formulation adopted by Ahsan et al. captures the onset of blade tower coupling, yet few publicly available codes link aerodynamic Jacobians to structural matrices in a state-space form suitable for eigen analysis and real-time health monitoring. Full-scale strain gauge data especially for urban or floating prototypes are urgently needed to validate and calibrate such integrated models. Aero-acoustic prediction, vital for social acceptance in densely populated sites, is still loosely coupled to performance calculations. The Ffowcs-Williams–Hawkings implementation employed by Dessoky [47] proves that simultaneous power and noise optimization is possible, but only via expensive transient CFD. Whether hybrid methods that feed unsteady-lift time series from low-order solvers into stochastic noise mappers can deliver broadband predictions within ±3 dB—and do so fast enough for iterative design—remains an open question.

Taken together, these observations point towards a tiered modeling hierarchy for next-generation VAWT research: (i) dynamic-stall-enhanced open-source DMST codes for concept selection; (ii) GPU-accelerated actuator-line LES for sub-rotor optimization; and (iii) fully coupled CFD–FEM–control environments for the final certification of megawatt-scale or floating systems. Physics-informed machine learning offers a transversal accelerant, provided that the community converges on reproducible experimental datasets. Establishing an IEA-style benchmark task specifically for VAWTs would standardize test cases, error metrics, and data sharing, thereby enabling the field to move beyond its current technology readiness plateau towards robust commercial deployment.

Table 9. Mathematical models for VAWT research and simulation.

References	Model Class	Representative Equations (Plain Text)	Scope and Strengths	Limitations
[92]	Stream-tube (DMST)/vortex/URANS benchmark	Power coefficient: $Cp={\displaystyle \frac{P}{0.5. \rho .A.U{\infty }^3}}$ momentum balance across up- and down-wind stream-tubes.	Very fast parametric sweeps; useful for early design and model inter-comparisons.	Over-predicts Cp at a high tip/speed ratio (λ); ignores dynamic-stall physics.
[93]	2D/3D RANS–LES review	Continuity:$\nabla .\mathrm{u}=0$ momentum: $\rho .\left(u.𝛻\right)=-\nabla .\rho +\mu .{\nabla }^2. u$ (Reynolds-averaged).	Captures stall, blade–wake interaction, and turbulence; calibrates lower-order tools.	High CPU demand; 2D cases overestimate Cp by 10–15%.
[95]	Quasi-steady analytical (stream-tube)	Tip-speed ratio: $\mathsf{\lambda }={\displaystyle \frac{\mathsf{\Omega }.\mathrm{R}}{U\infty }}$ instantaneous angle of attack: $\mathsf{\alpha }(\mathsf{\theta })=\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{t}\mathrm{a}\mathrm{n}({\displaystyle \frac{U\infty .sin\mathsf{\theta }}{(\mathsf{\Omega }.\mathrm{R}+U\infty s.cos\mathsf{\theta })}})$.	Closed-form insight; easy to embed in control or optimization loops.	Ignores unsteady lift and vortex shedding; relies on empirical corrections.
[94]	Aero-servo-elastic FEM (floating 5 MW VAWT)	Coupled structure: $\mathrm{M}\mathrm{q}¨+\mathrm{C}\dot{\mathrm{q}}+\mathrm{K}\mathrm{q}=\mathrm{F}\_\mathrm{a}\mathrm{e}\mathrm{r}\mathrm{o}(\mathrm{q},\dot{\mathrm{q}})$ (generalized coordinates)	Predicts flutter and platform modes; guides structural design for offshore VAWTs.	Linearized aerodynamics; requires large-scale experimental validation.
[34]	URANS (k–ω SST) with flap parametric study	Navier–Stokes plus two-equation turbulence closure; lift/drag extracted along blade sections.	Resolves the lift gain and noise impact of trailing-edge flaps; supports urban-noise design.	2D only; cannot capture 3D tip losses or tower interaction.
[96]	Transient CFD + actuator-line method	Body force per blade element: $\mathrm{f}=(\mathrm{C}\mathrm{L}.\widehat{\mathrm{L}}+\mathrm{C}\mathrm{D}.\widehat{\mathrm{D}})\mathrm{x}{\displaystyle \frac{\left(\mathsf{\rho }{\mathrm{V}}^2\mathrm{c}\right)}{2}}$ applied to the flow field.	Rapid evaluation of add-on devices (auxiliary blades and deflectors); offers design guidance.	The actuator line smears near the wake; accuracy hinges on lift/drag input tables.
[47]	CFD + Ffowcs-Williams–Hawkings aero-acoustics	Sound pressure prediction: $\mathrm{p}\mathrm{\prime }(\mathrm{x},\mathrm{t})=\left(\right(1/4\mathsf{\pi }\left)\int \mathrm{S}\right[...]\mathrm{d}\mathrm{S}$ (surface integral of source terms).	Simultaneously predicts power and noise critical for the urban VAWT siting.	Costly acoustic mesh; depends on turbulence and wall-function calibration.

Abbreviations: $Cp$ = power coefficient; P = turbine power; ρ = air density; A = rotor swept area; U∞ = free-stream wind speed; ∇ u = 0 = continuity equation; u = velocity vector; p = static pressure; μ∇²u = viscous diffusion term; λ = tip/speed ratio (ΩR/U∞); Ω = rotor angular speed; R = rotor radius; α(θ) = instantaneous angle of attack at azimuth θ; $\mathrm{M} \mathrm{q}¨$ + $\mathrm{C}\mathit{q}\dot{}$ + Kq = mass–damping–stiffness equation (FEM); q = generalized coordinates; f = (CL L^ + CD D^) ½ ρ V² c = actuator-line body force; CL, CD = lift and drag coefficients; V = local relative velocity; c = blade chord; p′(x,t) = acoustic pressure fluctuation (Ffowcs-Williams–Hawkings); S = integration surface. Model acronyms: DMST = double-multiple stream-tube; RANS/URANS = (Unsteady) Reynolds-Averaged Navier–Stokes; LES = large-eddy simulation; FEM = finite-element method; SST = shear-stress-transport turbulence model.

Table 9. Mathematical models for VAWT research and simulation.

References	Model Class	Representative Equations (Plain Text)	Scope and Strengths	Limitations
[92]	Stream-tube (DMST)/vortex/URANS benchmark	Power coefficient: $Cp={\displaystyle \frac{P}{0.5. \rho .A.U{\infty }^3}}$ momentum balance across up- and down-wind stream-tubes.	Very fast parametric sweeps; useful for early design and model inter-comparisons.	Over-predicts Cp at a high tip/speed ratio (λ); ignores dynamic-stall physics.
[93]	2D/3D RANS–LES review	Continuity:$\nabla .\mathrm{u}=0$ momentum: $\rho .\left(u.𝛻\right)=-\nabla .\rho +\mu .{\nabla }^2. u$ (Reynolds-averaged).	Captures stall, blade–wake interaction, and turbulence; calibrates lower-order tools.	High CPU demand; 2D cases overestimate Cp by 10–15%.
[95]	Quasi-steady analytical (stream-tube)	Tip-speed ratio: $\mathsf{\lambda }={\displaystyle \frac{\mathsf{\Omega }.\mathrm{R}}{U\infty }}$ instantaneous angle of attack: $\mathsf{\alpha }(\mathsf{\theta })=\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{t}\mathrm{a}\mathrm{n}({\displaystyle \frac{U\infty .sin\mathsf{\theta }}{(\mathsf{\Omega }.\mathrm{R}+U\infty s.cos\mathsf{\theta })}})$.	Closed-form insight; easy to embed in control or optimization loops.	Ignores unsteady lift and vortex shedding; relies on empirical corrections.
[94]	Aero-servo-elastic FEM (floating 5 MW VAWT)	Coupled structure: $\mathrm{M}\mathrm{q}¨+\mathrm{C}\dot{\mathrm{q}}+\mathrm{K}\mathrm{q}=\mathrm{F}\_\mathrm{a}\mathrm{e}\mathrm{r}\mathrm{o}(\mathrm{q},\dot{\mathrm{q}})$ (generalized coordinates)	Predicts flutter and platform modes; guides structural design for offshore VAWTs.	Linearized aerodynamics; requires large-scale experimental validation.
[34]	URANS (k–ω SST) with flap parametric study	Navier–Stokes plus two-equation turbulence closure; lift/drag extracted along blade sections.	Resolves the lift gain and noise impact of trailing-edge flaps; supports urban-noise design.	2D only; cannot capture 3D tip losses or tower interaction.
[96]	Transient CFD + actuator-line method	Body force per blade element: $\mathrm{f}=(\mathrm{C}\mathrm{L}.\widehat{\mathrm{L}}+\mathrm{C}\mathrm{D}.\widehat{\mathrm{D}})\mathrm{x}{\displaystyle \frac{\left(\mathsf{\rho }{\mathrm{V}}^2\mathrm{c}\right)}{2}}$ applied to the flow field.	Rapid evaluation of add-on devices (auxiliary blades and deflectors); offers design guidance.	The actuator line smears near the wake; accuracy hinges on lift/drag input tables.
[47]	CFD + Ffowcs-Williams–Hawkings aero-acoustics	Sound pressure prediction: $\mathrm{p}\mathrm{\prime }(\mathrm{x},\mathrm{t})=\left(\right(1/4\mathsf{\pi }\left)\int \mathrm{S}\right[...]\mathrm{d}\mathrm{S}$ (surface integral of source terms).	Simultaneously predicts power and noise critical for the urban VAWT siting.	Costly acoustic mesh; depends on turbulence and wall-function calibration.

Abbreviations: $Cp$ = power coefficient; P = turbine power; ρ = air density; A = rotor swept area; U∞ = free-stream wind speed; ∇ u = 0 = continuity equation; u = velocity vector; p = static pressure; μ∇²u = viscous diffusion term; λ = tip/speed ratio (ΩR/U∞); Ω = rotor angular speed; R = rotor radius; α(θ) = instantaneous angle of attack at azimuth θ; $\mathrm{M} \mathrm{q}¨$ + $\mathrm{C}\mathit{q}\dot{}$ + Kq = mass–damping–stiffness equation (FEM); q = generalized coordinates; f = (CL L^ + CD D^) ½ ρ V² c = actuator-line body force; CL, CD = lift and drag coefficients; V = local relative velocity; c = blade chord; p′(x,t) = acoustic pressure fluctuation (Ffowcs-Williams–Hawkings); S = integration surface. Model acronyms: DMST = double-multiple stream-tube; RANS/URANS = (Unsteady) Reynolds-Averaged Navier–Stokes; LES = large-eddy simulation; FEM = finite-element method; SST = shear-stress-transport turbulence model.

5. Research Gaps and Future Perspectives

Despite the aforementioned advances, several knowledge gaps and challenges must be addressed for vertical-axis wind turbines (VAWTs) to reach their full potential. One of the main identified gaps is the need to improve modeling tools and performance prediction under real-world conditions. Although the use of computational fluid dynamics (CFD) has become widespread, significant uncertainties persist in the selection of appropriate turbulence models for different turbine configurations. Developing more accurate aerodynamic models—and validating them experimentally—is essential for reliably optimizing VAWT designs. In parallel, further progress is needed in aero-structural optimization, such as identifying optimal airfoil shapes and aspect ratios for various wind speeds to achieve better overall performance. Some research reveals that aerodynamic profile improvements could significantly raise power coefficients, but standardized optimization methodologies have yet to emerge.

Another area with noticeable gaps is the structural durability and operational lifespan of VAWTs. Most experimental research has been carried out at the small scale or in wind tunnels, with limited long-term performance data from full-scale operational turbines. Critical aspects—such as fatigue of composite materials under cyclic loads, weathering degradation, and component reliability (bearings, joints, and generators)—require greater attention. Addressing these issues is particularly important if the technology is to be scaled up or deployed in demanding environments, such as offshore settings. In this regard, pilot-scale demonstration projects would be valuable to assess the technical and economic feasibility of VAWT-based wind farms, both in urban and marine contexts.

There is also a recognized gap in the optimal integration of VAWTs within urban environments. Although their adaptability to multi-directional wind is acknowledged, more research is needed on how to design cities and buildings to maximize wind harnessing potential. This involves studies on urban micrometeorology, turbine placement optimization on buildings, the mitigation of acoustic and vibrational impacts, and architectural designs that harmoniously incorporate turbines. The intersection between wind engineering and urban architecture remains fertile ground for exploration. Likewise, the social acceptance of VAWTs in inhabited areas is still under-investigated. Understanding public perception, educating communities on their benefits, and minimizing potential nuisances (e.g., noise and flickering shadows) will be necessary steps for widespread urban adoption.

Regarding hybrid systems and energy storage, although some initial proposals exist, further research is needed on the joint operation of VAWTs with other technologies. For example, efficient energy management in wind–solar–battery hybrid systems or coupling VAWTs with hydrogen production presents unresolved challenges in control and system optimization. Intelligent power management and electronic control for microgrids that incorporate VAWTs is another emerging field; future studies could develop real-time control algorithms to enhance stability and continuous power supply by leveraging the complementary nature of wind and other sources. Overall, integrating VAWTs into resilient and autonomous energy systems represents a strategic direction to expand their practical relevance.

In floating offshore applications, the outlook for VAWTs is promising but still preliminary. The primary gap here is the lack of real-world operational experience. Scaled prototypes and more sophisticated coupled aero-hydrodynamic simulation models are needed to ensure that VAWTs can operate effectively on moving platforms subjected to wave and wind forces. Key issues such as platform stability, active turbine control under movement, and survival in extreme storms must be addressed through future research. Collaboration between wind energy experts and ocean engineers will be essential to advance this field.

Based on these gaps, the following research directions are proposed to strengthen the development of VAWTs in the medium term:

Advances in modeling and simulation: Develop advanced CFD models validated through experiments, including turbulence schemes tailored to VAWTs, to enable more accurate performance predictions under varied conditions. This also involves aeroelastic simulations that capture fluid–structure interactions and help predict instability or resonance phenomena.

Innovation in materials and structural design: Investigate new high-durability composite materials and optimized structural designs that reduce fatigue. For example, explore blade and shaft connection configurations that minimize alternating loads or innovative support designs (e.g., guyed masts and self-supporting structures) that lighten the system without compromising stiffness.

Urban integration and sustainable architecture: Develop building-integrated wind turbine (BIWT) prototypes with both esthetic and functional designs, and conduct real-world pilot studies to measure performance and public acceptance. Additionally, generate urban design guidelines that incorporate wind capture criteria (e.g., wind corridors and usable Venturi effects) to maximize distributed generation in cities.

Hybrid systems and multi-source control: Advance the design of hybrid systems that combine VAWTs with photovoltaics, storage, or other renewables, thereby optimizing energy management. This includes developing real-time control algorithms that adjust turbine operation based on sun/wind availability and the battery’s state of charge, ensuring more stable supply.

VAWT farms and array configurations: Investigate optimal layouts for multiple VAWTs operating together. Recent research confirms that VAWTs may benefit from close spacing, increasing power density per unit area. Leveraging these interactions requires analyzing park configurations (alignments and relative positions) and developing cooperative control strategies to maximize collective efficiency.

Floating offshore demonstration: Launch demonstration projects of floating VAWTs, starting with small-scale prototypes in controlled marine environments to gather data on turbine behavior coupled with ocean movement. These projects should assess not only energy generation but also structural integrity under combined wind and wave loads, providing valuable input for future commercial-scale designs.

Addressing these areas will help close the gap between the theoretical potential of VAWTs and their actual performance in practical applications. Moreover, many of these research lines demand multi-disciplinary approaches spanning aerospace, civil, mechanical, and electrical engineering, materials science, and architecture, so fostering cross-disciplinary collaboration will be essential to achieving meaningful progress.

6. Conclusions

This study confirms that research on vertical-axis wind turbines (VAWTs) is undergoing a phase of accelerated expansion. The bibliometric analysis reveals a sustained increase in publications and citations since the mid-2000s, led by China and supported by strong contributions from Europe and North America. Scientific output is concentrated in high-impact journals and revolves around aerodynamic simulations (CFD) and design optimization, while the mini-review highlights key advances in adaptive aerodynamics, high-strength composite materials, and control strategies aimed at improving efficiency and mitigating cyclic loads.

At the same time, applied research areas—such as urban integration, hybrid wind–solar–hydrogen systems, and floating offshore platforms—that expand the market niches for VAWTs are emerging due to their omnidirectionality, low visual impact, and reduced center of mass. Nonetheless, critical gaps remain: a lack of large-scale, long-term operational data; the need for validated aero-structural methodologies in complex environments; the limited standardization of fatigue and corrosion testing; and social and regulatory challenges in urban settings.

The combination of a bibliometric analysis and a mini-review proves to be a powerful tool for mapping these trends and identifying research gaps, offering a roadmap that prioritizes aero-structural optimization, field demonstration (both urban and offshore), and intelligent integration with other renewable technologies. If interdisciplinary collaboration and technological validation are actively pursued, VAWTs are well positioned to complement HAWTs, particularly in distributed microgeneration, smart buildings, and deep-water floating wind farms, and contribute to the diversification and resilience of the global renewable energy.

This review confirms that vertical-axis wind turbines (VAWTs) occupy three principal application niches: (i) urban microgeneration, where omnidirectional acceptance, low tip speeds, and compact footprints offset lower aerodynamic efficiency; (ii) hybrid rooftop or micro-grid systems (VAWT + PV ± storage) that harness complementary diurnal resources but demand more sophisticated power–electronics coordination; and (iii) floating or deep-water offshore platforms, whose low centers of gravity and yaw-free operation simplify mooring but still face aero-servo-elastic and scale-up challenges. Across all contexts, VAWTs offer clear advantages, such as structural simplicity, reduced noise, and dense array potential, yet remain disadvantaged by modest peak power coefficients, cyclic loads that complicate fatigue life, and, for Darrieus rotors, self-start limitations. Active-pitch concepts, diffuser-augmented rotors, and advanced composite materials have narrowed the efficiency gap, while full-converter topologies and torque-based MPPT mitigate electrical variability. Nevertheless, large-scale validation, dynamic-stall-aware control laws, and unified techno-economic benchmarks are still required before VAWTs can compete head-to-head with mature horizontal-axis technology in high-capacity applications. Closing these gaps will determine whether VAWTs evolve from promising niche solutions into mainstream contributors to diversified, low-carbon energy portfolios.