sustainability

Review Comparing Patent and Scientific Literature in Airborne Wind Energy

Anny Key de Souza Mendonça *, Caroline Rodrigues Vaz, Álvaro Guillermo Rojas Lezana, Cristiane Alves Anacleto and Edson Pacheco Paladini Department of Industrial and Systems Engineering, Federal University of Santa Catarina, Florianópolis, 88040-900 Santa Catarina, Brazil; [email protected] (C.R.V.); [email protected] (Á.G.R.L.); [email protected] (C.A.A.); paladini@floripa.com.br (E.P.P.) * Correspondence: [email protected]; Tel.: +55-48-3721-7024

Academic Editor: Tomonobu Senjyu Received: 20 February 2017; Accepted: 25 May 2017; Published: 31 May 2017

Abstract: Airborne Wind Energy (AWE) is a renewable energy technology that uses wind power devices rather than traditional wind turbines that take advantage of the kinetic wind energy, and remain in the air due to aerodynamic forces. This article aims to compare the scientific literature with the patents on wind power with tethered airfoils, to obtain better insights into the literature of this area of knowledge. The method used in this study was a comparative bibliometric analysis, using the Web of Science and Derwent Innovations Index databases, and the Network Analysis Interface for Literature Review software and VosViewer. It was possible to verify the main authors, research centers and companies, countries and journals that publish on the subject; the most cited documents; the technological classes; and the networks of collaborations of this work. It was also possible to identify that researches on wind energy with tethered airfoils began their studies in the late 1970s with the first patent apparently dated from 1975 by the inventors Dai and Dai. The first scientific publication was in 1979 by authors Fletcher and Roberts, followed by Loyd in 1980. United States is the country that presented the highest number of patents and scientific papers. Both scientific papers and patents set up networks of collaboration; that is, important authors are interacting with others to establish cooperative partnerships.

Keywords: airborne wind energy; patents; literature review

1. Introduction In recent years, important themes related to social well-being have been discussed, and among these themes the world energy matrix has received special attention. Energy is fundamental for the social and economic development of a country, being fundamental for the provision of services such as water supply, food, health, education, employment and communication. Today, most of the energy consumed worldwide comes from fossil fuels (e.g., coal, oil and natural gas), which are not in conformity with new environmental legislation aimed at improving the quality of life of living things and the natural balance of the environment [1]. To address these problems, many countries are adopting energy policies focusing on the increasing of renewable technology deployment, such as hydroelectric and small hydropower plants, wind energy, solar energy, marine energy, biomass, and so on. In this study, the focus is on a new renewable energy technology: Airborne Wind Energy (AWE). This innovative technology for renewable energy exploration uses aerial devices rather than traditional wind turbines to make use of wind kinetic energy and it can remain in the air through aerodynamic forces [2–4]. Unlike wind turbines with towers, airborne wind power systems operate in flight and they are connected to a foundation by a cable, either to transmit the energy generated at the airfoil or to transmit mechanical energy to the

Sustainability 2017, 9, 915; doi:10.3390/su9060915 www.mdpi.com/journal/sustainability Sustainability 2017, 9, 915 2 of 22

Sustainability 2017, 9, 915 2 of 22 generator on the ground. The airfoils may be rigid or flexible wings and the energy is harvested by exploitingenergy to the the aerodynamic generator on liftthe and ground. drag The forces airfoils produced may be on rigid the airfoilor flexible by the wings wind and [3]. the In energy some cases, is tetheredharvested balloons by exploiting can also the be aerodynamic used [3]. lift and drag forces produced on the airfoil by the wind [3]. In Thesome advantages cases, tethered of this balloons innovative can also technology, be used [3]. when compared to other technologies established such asThe wind advantages turbines, are of lowerthis costsinnovative of construction, technology, installation when compared and transport, to other higher technologies probability of reachingestablished stronger such andas wind consistent turbines, winds are lower and highercosts of speedsconstruction, at high installation altitudes. and AWE transport, can reach higher 600 m aboveprobability the ground, of reaching and thereforestronger and can consistent exploit a winds greater and energy higher potential speeds at [5 high]. Wind altitudes. technology AWE can with towersreach cannot 600 m take above advantage the ground, of this and potential therefore due can to structural exploit a andgreater economic energy factors potential that limit[5]. Wind the use oftechnology wind towers with to atowers maximum cannot height takeof advantage approximately of this 125 potential m [6]. due to structural and economic factors that limit the use of wind towers to a maximum height of approximately 125 m [6]. The different approaches used in AWE technology can be seen in Figure1. These examples are The different approaches used in AWE technology can be seen in Figure 1. These examples are different because of the type of airfoil, how it holds suspended in the air, and whether the electric different because of the type of airfoil, how it holds suspended in the air, and whether the electric generation occurs on board or on the ground. Many technologies based on these concepts are being generation occurs on board or on the ground. Many technologies based on these concepts are being developed around the world. developed around the world. ApproachesApproaches dealing dealing with with aerodynamic aerodynamic elevation ca cann be be found found in in References References [7–13]. [7–13 They]. They make make useuse of airplaneof airplane wings wings or kitesor kites to generateto generate energy energy and and in the in formthe form of aerostatic of aerostatic elevation, elevation, which which utilizes structuresutilizes structures lighter than lighter air (such than as air balloons) (such as that balloons) may have that generatorsmay have ongenerators board [14 on] orboard positioned [14] or on thepositioned ground [9 on,11 ,the15]. ground In this case,[9,11,15]. the electricIn this case, energy the can electric be generated energy can by thebe generated airfoil and by transmitted the airfoil to theand ground transmitted by cable. to Electricthe ground energy by cable. can also Electric be generated energy can on thealso groundbe generated by a cable on the that ground transmits by a the mechanicalcable that energytransmits generated the mechanical by the energy airfoil. generated Technologies by the using airfoil. balloon-type Technologies airfoils using generate balloon-type energy inairfoils flight and generate transmit energy it to in the flight ground and transm throughit it a to cable the [ground15]. through a cable [15].

FigureFigure 1. From1. From left left to right, to right, the prototypes the prototypes of Airborne of Airborne Wind EnergyWind Energy generators: generators: a generator a generator positioned onpositioned the ground on with the TUground Delft’s with flexible TU Delft’s airfoil flexible [9], a generator airfoil [9], positioned a generator on positioned the ground on generator the ground with rigidgenerator airfoil with from rigid Ampyx airfoil Power from [Ampyx11], a GoogleX Power [11] system, a GoogleX with onboardsystem with generators onboard [generators12], balloon [12], with onboardballoon turbine with onboard from Altaeros turbine Energiesfrom Altaeros [15], Energies and generator [15], and positioned generator on positioned the floor withon the flexible floor with airfoil flexible airfoil from UFSCkite [16]. from UFSCkite [16].

The potential of AWE technology with tethered airfoils has been investigated for more than threeThe decades. potential Several of AWE authors technology refer to the with pioneering tethered work airfoils of hasLoyd been [2] in investigated 1980 as the precursor for more thanof threestudies decades. of AWE Several technology. authors Loyd refer showed to the pioneering that high workaerodynamic of Loyd forces [2] in can 1980 be as obtained the precursor when of studiesairfoils of are AWE operated technology. in high-speed Loyd showed maneuvers. that high Loyd aerodynamic was the first forces to investigate can be obtained and calculate when airfoils the arepower operated generation in high-speed potential maneuvers.with a high-speed Loyd tethered was the airfoil. first to He investigate described two and ways calculate of generating the power generationelectric energy potential by AWE with technology: a high-speed (i) tetheredlift mode airfoil. with generators He described on the two ground; ways ofand generating (ii) drag mode electric energywith onboard by AWE generators. technology: These (i) lift ways mode can withbe foun generatorsd in Crosswind on the kite ground; power and (for (ii) large-scale drag mode wind with onboardpower generators.production). These ways can be found in Crosswind kite power (for large-scale wind power production).Patent analysis has demonstrated that wind, geothermal, and hydro technologies represent onlyPatent 8% of analysis the total has amount demonstrated of granted that (patents) wind, des geothermal,pite the fact and that hydro they technologiesare widely recognized represent as only 8%effective of the total and amount sustainable of granted technologies (patents) to be despite used thein the fact next that future they are [17]. widely Patent recognized data have as greatly effective andattracted sustainable both technologiesresearchers and to bepractitioners. used in the Patent next future data can [17 ].be Patent used to data take have into greatlyaccount attracted the results both researchersof innovation and activities practitioners. [17,18]. Patent Patent data data can have be used emerged to take as intovaluable account sources the resultsof information of innovation for studying innovative dynamics. Ardito et al. [19] argue that the question of how breakthrough activities [17,18]. Patent data have emerged as valuable sources of information for studying innovative inventions come to exist is not new to the literature. dynamics. Ardito et al. [19] argue that the question of how breakthrough inventions come to exist is According to Albino et al. [17], patents do not portray the whole innovative portfolio. not new to the literature. Additionally, some innovations are not patentable and patents do not always represent the most suitable mechanism to protect innovations. The value of patents can vary across countries and the Sustainability 2017, 9, 915 3 of 22

According to Albino et al. [17], patents do not portray the whole innovative portfolio. Additionally, some innovations are not patentable and patents do not always represent the most suitable mechanism to protect innovations. The value of patents can vary across countries and the characteristics of appropriation regimes may affect the propensity to patent. Finally, not all patented inventions are actually implemented in market applications [20]. For example, Ardito et al. [19] attempt to fill in the literature gap on the types of invention that can become breakthroughs, by providing more insights into the ongoing debate about the role of nascent versus existing technologies. In another study, after analyzing 88,748 patents in the “Alternative energy production” and “Energy conservation” technological classes, Ardito [21] finds that when an organization’s inventive team, devoted to develop a new technology, is large or geographically dispersed in diverse countries, one can derive greater benefits from a wider search breadth. A mentioned earlier, this article aims to compare the scientific literature with patents on wind energy with the use of tethered airfoils in order to obtain better insights into the state of the art of this technology. This article consists of four main sections, including this introduction. Section2 presents the theoretical framework informing the study. Section3 describes the materials and methods used to analyze the two basic sources of information here proposed: scientific literature and the patents selected for the analysis. Finally, Section4 provides the conclusions of the study.

2. Theoretical Framework Airborne Wind Energy (AWE) or High Altitude Wind Energy (HAWE) is a branch of research in the relatively new wind energy field that uses aerial devices instead of conventional wind turbines to extract a portion of the wind’s kinetic energy, and convert this energy to electricity [22,23]. These devices called tethered airfoils, in some cases use wings like that of a paraglider or kitesurf or like a balloon or airplane wings. Several structures with tethered airfoils have been studied for harnessing the energy of the winds at high altitudes. They differ, for example, in relation to the type of wing, which can be rigid or flexible; as to the location of the electric generator, which can be on the ground or suspended in the air; in terms of the aerodynamic power being exploited, drag or lift; and finally, as to how to control the flight of the airfoil, which depends on the number of cables and the position of the actuators and sensors. This technology has begun to be intensively studied in recent years, although commercial products are not currently available, significant research and investments are currently being made. Aloys Van Gries [24] launched a 398-page book in 1921 and its main theme was “aircraft”. This book is available in German by Springer Publishing Company with the title “Flugzeugstatik” [Aircraft Statistics]. The book might give some clues to Aloys van Gries’ mind that later brought about the filing of his patent on the AWES scheme. In this book, there are robust analyses of biplane aircraft structures; especially for those interested in the field of biplane aircraft. In 1935, Aloys van Gries filed the patent on airborne wind energy, about 15 years following the publication of his major textbook on aircraft statics. Therefore, the first patent on Airborne Wind Energy can be attributed to Van Gries and it dates back to 1938 (GB489139) [24]. This patent is related to improvements in or relating to wind-driven power apparatuses. According to Van Gries [24], the energy generated by the wind wheel may also be led or transmitted mechanically straight to the ground by means of the guy-rope. In this invention, he provided a wind-driven power apparatus for utilizing high winds, wind-driven machines that are held by captive kites, characterized using a system of interconnected kites. This way the lifting power can be increased to a practically unlimited extent as any desired number of kites can be coupled together. Each kite must carry only that part of the guy-rope that connects it to next lower kite. The result is that the strength of the guy-rope, particularly towards the upper end, can be small whilst, with a single kite, because this large dead weight, it must be as strong as possible at the upper end. Sustainability 2017, 9, 915 4 of 22

It further follows that each kite of the system, as its lifting power exceeds the weight of the pertaining guy-rope, increases the total lifting power. Therefore, it is possible, by means of the wind-drivenSustainability 2017 machine,, 9, 915 to bring large weights to great heights and thus utilize the stronger and4 more of 22 uniform high winds. Then, a guy-rope is utilized for the power transmission or, if the change of wind windenergy energy into electricity into electricity is to be is effected to be effected on the kite on systemthe kite itself, system it is itself, formed it asis aformed cable foras conductinga cable for conductingthe electric currentthe electric being current produced being [24 produced]. [24]. In 1939, Hermann Hermann Honnef Honnef described described the the design design fo forr an an arrangement arrangement of of wind wind turbines turbines atop atop a towera tower approximately approximately 300 300 m in m height, in height, designed designed to ta toke take advantage advantage of the of faster the faster winds winds found found at high at altitudeshigh altitudes [25,26]. [25 Interest,26]. Interest in high in altitude high altitude wind en windergy increased energy increased in the 1970s in the and 1970s 1980s and as a 1980s response as a toresponse the oil to crises, the oil with crises, small with smallgroups groups and individu and individualal researchers researchers first first publishing publishing their their ideas ideas and concepts. For example, Hermann Oberth in 1977 presented a design for a “kite power plant”: a wind turbine liftedlifted upup to to 12,000 12,000 m m in in altitude altitude by by a balloon, a balloon, and and secured secured by a longby a tetherlong tether to the groundto the ground [27,28]. The[27,28]. Kite The Power Kite Station Power isStation a wind-driven is a wind-driven power station power atstation high altitudes.at high altitudes. Kambouris [29] [29] explains that as fossil fuel prices lowered from the crisis peaks, interest in these ideas waned over the 1980s and 1990s, with little publishedpublished during this time. Since Since the the mid mid 2000s, 2000s, there hashas beenbeen a a resurgence resurgence of interestof interest and and AWE AWE has begunhas begun to emerge to emerge as a recognized as a recognized field of research,field of withresearch, its own with association, its own association, conferences, conferences, and a considerable and a considerable body of academic body of academic publications. publications. An overviewoverview ofof the the state state of theof the art onart AWEon AWE technology technology can be can found be infound [23,30 in]. Figure[23,30].2 presentsFigure 2 presentsa world mapa world with map the 55 with research the 55 institutions research aroundinstitutions the worldaround actively the world engaged actively in Research engaged and in ResearchDevelopment and Development (R & D) in this (R area. & D) in this area.

Figure 2. InstitutionsInstitutions involved involved in in R R & & D D and and using using AWE technology in 2015 [[30].30].

These groupsgroups aimaim to to harness harness available available wind wind energy energy at higherat higher altitudes altitudes in relation in relation to current to current wind windtechnology. technology. In this In sense, this sense, this study this study can justify can justify its method its method of comparing of comparing the scientific the scientific literature literature with withthe existing the existing intellectual intellectual properties properties (i.e., patents)(i.e., patents) on the on subject. the subject.

3. Materials and Methods This research has a more theoretical nature in relationrelation to the topic addressed. As for its technical procedures, itit isis framedframed asas a a bibliographic bibliographic study, study, since since it it will will deal deal with with data data and and verifications verifications based based on workon work already already done done about about the researched the researched subject. subject. From theFrom point the of point view of of view the objectives, of the objectives, it is classified it is classified as exploratory and descriptive, as it will seek specific information and characteristics of what is being studied [31]. Literature review means for the researcher the first step in the quest to develop a work and build knowledge in each context. It also allows an introductory view on the development of a research project and takes back the accumulated scientific knowledge about the subject [32]. It allows the researcher to become even more familiar with the researched topic, enabling him to create new constructs and definitions. Sustainability 2017, 9, 915 5 of 22 as exploratory and descriptive, as it will seek specific information and characteristics of what is being studied [31]. Literature review means for the researcher the first step in the quest to develop a work and build knowledge in each context. It also allows an introductory view on the development of a research project and takes back the accumulated scientific knowledge about the subject [32]. It allows the researcher to become even more familiar with the researched topic, enabling him to create new constructsSustainability and 2017 definitions., 9, 915 5 of 22 In thisIn this context, context, a bibliometrica bibliometric analysis was was carried carried out, out, which which is a technique is a technique for mapping for mapping the the mainmain authors,authors, journals journals and and keywords keywords on a on given a given topic. topic. This technique This technique uses tools uses that toolsrely on that a rely on ascientifically-recognized scientifically-recognized methodological methodological theoretical theoretical basis, basis,which whichenable the enable use theof statistical use ofstatistical and and mathematicalmathematical methods methods to to map map information information from from bibliographic bibliographic records records of documents of documents stored stored in in databasesdatabases [33]. [33]. This This method method was was divided divided intointo th threeree stages, stages, which which are are presented presented in Figure in Figure 3 and3 and describeddescribed in the in following.the following.

FigureFigure 3. 3.Methodological Methodological procedures of of the the research. research.

The first step was to define the keywords and the database to be explored. The database to be The first step was to define the keywords and the database to be explored. The database to consulted was chosen from the Capes (Coordination for the Improvement of Higher Education be consulted was chosen from the Capes (Coordination for the Improvement of Higher Education Personnel) journal portal, Web of Science, because it is multidisciplinary and indexes only the most Personnel)cited journals journal in portal, their respective Web of Science,areas. It is because also an index it is multidisciplinary of citations, informing, and for indexes each article, only the the most citeddocuments journals in cited their in it respective and the documents areas. It that is alsocited an it. Today index it of has citations, more than informing, 9000 indexed for journals. each article, the documentsIn this research, cited documents in it and indexed the documents by the Science that citedCitation it. Index Today Extended it has more (SCI-Expanded) than 9000 and indexed journals.Conference In this Proceedings research, documents Citation Index-Science indexed by the(CPC ScienceIS) were Citation used. CAPES Index Periodicals Extended (SCI-Expanded)Portal does and Conferencenot provide access Proceedings to the Web Citation of Science’s Index-Science Book Citation (CPCIS) Index-Science were used. subbase, CAPES so the Periodicalsauthors chose Portal doesto not manually provide include access tothe the book Web “Airborne of Science’s Wind Book Energy” Citation (the Index-ScienceSpringer textbook subbase, “Airborne so the Wind authors choseEnergy” to manually from 2013, include with the 35 bookpeer-reviewed “Airborne book Wind chapters Energy” that (the cover Springer the entire textbook field and “Airborne has more Wind than 60,000 downloads). Energy” from 2013, with 35 peer-reviewed book chapters that cover the entire field and has more than The Patent database, chosen from the Derwent Innovations Index of the ISI Web of Knowledge, 60,000 downloads). is a powerful patent search tool that combines Derwent World Patents Index®, Patents Citation TheIndex™ Patent and database, Chemistry chosen Resource. from Derwent the Derwent Innovati Innovationsons Index Indexis updated of the weekly. ISI Web This of Knowledge,Patent ® is a powerfuldatabase patenthas more search than tool 16 thatmillion combines practical Derwent inventions World from Patents 1963 Indexto the ,present Patents Citationday. Patent Index ™ information is collected from 41 patent authorities around the world and is classified into three categories or sections: Chemical, Engineering and Electrical and Electronic. The keywords used for this research were validated by a specialist in the area, Table 1. Sustainability 2017, 9, 915 6 of 22 and Chemistry Resource. Derwent Innovations Index is updated weekly. This Patent database has more than 16 million practical inventions from 1963 to the present day. Patent information is collected from 41 patent authorities around the world and is classified into three categories or sections: Chemical, Engineering and Electrical and Electronic. The keywords used for this research were validated by a specialist in the area, Table1.

Table 1. Keywords.

Keywords Expression TS=(“Airborne Wind Energy”) OR TS=(“Airborne Wind Power”) OR TS=(“High Altitude Wind Energy”) OR TS=(“High Altitude Wind Power”) OR TS=(“Kite wind generator$”) OR TS=(“kite wind energy”) OR TS=(“Crosswind kite$”) OR TS=(“Airborne Wind Turbine$”) OR TS=(“Flying Electric Generator$”) OR TS=(“Kite power”) OR TS=(“Kite energy”) OR TS=(“Pumping kite$”) OR TS=(“Lighter-Than-Air Wind Energy System$”) OR TS=(“Kite model$”) OR TS=(“tethered undersea kite$”) OR TS=(“Kite-Based Wind Energy”) OR TS=(“kite wind power”) OR TS=(“Kite-Powered System$”) OR TS=(“Kite towed ship”) OR TS=(“crosswind towing”) OR SO=(“Airborne Wind Energy”) OR TS=(Parawing AND energy) OR (TS=(Kite) AND TS=(“ship propulsion”)) OR (TS=(“Wind keywords for literature review Power”) AND TS=(“flying kite$”)) OR (TS=(“kite”) AND TS=(“tracking control”)) OR (TS=(“kite”) AND TS=(“flight control”)) OR (TS=(“Kite generator”) NOT DO=(“10.1007/s00145-015-9206-4”)) OR (TS=(“Towing kite$” AND “wind energy”)) OR (TS=(“Kite system$”) AND TS=(“Wind energy”)) OR (TS=(“Kite system$”) AND TS=(“Power Generating”)) OR (TS=(“Power Kite$”) AND TS=(“Wind Energy”)) OR (TS=(“Tethered Airfoil$”) AND TS=(“Wind Energy”)) OR (TS=(“kite system”) AND TS=(wind) NOT DO=(10.1007/BF00123534)) OR (TS=(kite) AND AU=(“Creighton, Robert”)) OR (TS=(Laddermill) AND TS=(kite)) OR DO=(“10.2514/3.48003”) OR DO=(“10.1016/0167-6105(85)90015-7”) OR DO=(“10.1016/j.apenergy.2013.07.026”) OR DO=(“10.2514/1.31604”) OR DO=(“10.1002/rnc.1210”) TS=(“Airborne Wind Energy”) OR TS=(“Airborne Wind Power”) OR TS=(“High Altitude Wind Energy”) OR TS=(“High Altitude Wind Power”) OR TS=(“Kite wind generator$”) OR TS=(“kite wind energy”) OR TS=(“Crosswind kite$”) OR TS=(“Airborne Wind Turbine$”) OR TS=(“Flying Electric Generator$”) OR TS=(“Kite power”) OR TS=(“Kite energy”) OR TS=(“Pumping kite$”) OR TS=(“Lighter-Than-Air Wind Energy System$”) OR TS=(“Kite model$”) OR TS=(“tethered undersea kite$”) OR TS=(“Kite-Based Wind Energy”) OR TS=(“kite wind power”) OR TS=(“Kite-Powered System$”) OR TS=(“Kite towed ship”) OR TS=(“crosswind towing”) OR TS=(“Parawing AND energy”) keywords of the search for patents TS=(“Airborne Wind Energy”) OR TS=(“Airborne Wind Power”) OR TS=(“High Altitude Wind Energy”) OR TS=(“High Altitude Wind Power”) OR TS=(“Kite wind generator$”) OR TS=(“kite wind energy”) OR TS=(“Crosswind kite$”) OR TS=(“Airborne Wind Turbine$”) OR TS=(“Flying Electric Generator$”) OR TS=(“Kite power”) OR TS=(“Kite energy”) OR TS=(“Pumping kite$”) OR TS=(“Lighter-Than-Air Wind Energy System$”) OR TS=(“Kite model$”) OR TS=(“tethered undersea kite$”) OR TS=(“Kite-Based Wind Energy”) OR TS=(“kite wind power”) OR TS=(“Kite-Powered System$”) OR TS=(“Kite towed ship”) OR TS=(“crosswind towing”) OR TS=(“Parawing AND energy”) OR TS=(“Laddermill”) OR TS=(“flying kite$”) The research expressions used in the advanced search of the Web of Science are spelled out as follows: TS: topic; DO: DOI; SO: publication name; and the use of Boolians: AND, OR, NOT for search magnification. The search did not set a period, however, the access to Capes Periodicals Portal covers the time span between 1963 and 2017.

The second step was the choice of software for data analysis. The researchers opted for the Network Analysis Interface for Literature Review (NAILS) software, because it is a literature review tool that utilizes several customized statistical and network analysis functions to provide the user an overview of the Literature [34]. It has a unique interface with the Web of Science database, both in the analysis of the data of the scientific literature and of patents, comparing them. NAILS were used in this research for the quantification of inventors, classification of IPC codes, depositors, countries, patents cited and keywords. As for the network analysis, VosViewer is a software application used to build bibliometric networks based on data downloaded from bibliographic databases such as Web of Science and Scopus. The software enables the user to choose between the use of the total and the fractional counting method [35]. VosViewer was used in this study to generate the collaboration network of authors (literature review) and inventors (patents). The third step was to analyze the results obtained by the software of the scientific literature and the results of the patents. Sustainability 2017, 9, 915 7 of 22

4. Results and Discussion The survey was conducted in April 2017. We found a total of 239 scientific literature papers, whichSustainability were extracted 2017, 9, 915 from Web of Science Core Collection databases and 238 patent documents7 of 22 extracted from the Derwent Innovations Index published between 1980 and 2017. 4. Results and Discussion Table2 presents the results of the distribution by document type. As shown in Table1, the most representativeThe survey types was of documentsconducted in were April patents,2017. We proceedingsfound a total of papers, 239 scientific articles literature and book papers, chapters, accountingwhich forwere more extracted than 99%from of Web the totalof Science publication. Core Collection The 239 databases documents and were 238 patent written documents by 375 authors extracted from the Derwent Innovations Index published between 1980 and 2017. and co-authors. Together, they used 2532 references and 518 keywords. These documents were Table 2 presents the results of the distribution by document type. As shown in Table 1, the most producedrepresentative by 133 research types of institutions documents fromwere 26patents, countries proceedings and published papers, inarticles 113 journals. and book It chapters, was observed that Englishaccounting is the for dominantmore than language99% of the withtotal 235publication. records The (98%), 239 followeddocuments by were Chinese written with by 375 two and Germanauthors with and one co-authors. record. As Together, to the 238 they patent used documents2532 references found, and they518 keywords. were deposited These documents by 293 authors and co-authors,were produced and by belong 133 rese toarch 26 classes, institutions 21 countries from 26 countries and 166 and assignees. published in 113 journals. It was observed that English is the dominant language with 235 records (98%), followed by Chinese with two and German with one record. AsTable to the 2. 238Document patent documents type. found, they were deposited by 293 authors and co-authors, and belong to 26 classes, 21 countries and 166 assignees. Document Type Count (%) Cumulative (%) Table 2. Document type Patents 238 49.9 49.9 ProceedingsDocument paper Type Count 118 (%) 24.7 Cumulative 74.6 (%) ArticlePatents 238 81 49.9 17.0 49.9 91.6 BookProceedings Chapter paper 11835 24.7 7.3 74.6 99.0 Article; ProceedingsArticle paper81 4 17.0 0.8 91.6 99.8 ReviewBook Chapter 351 7.3 0.2 99.0 100.0 Total 477 100.0 Article; Proceedings paper 4 0.8 99.8 Review 1 0.2 100.0 When comparing the trends betweenTotal the scientific477 literature100.0 and the patents, it can be observed that the scientific literature represents in this research 50% of the publications and the patent When comparing the trends between the scientific literature and the patents, it can be observed applicationsthat the account scientific for literature 50%. Figure represents4 shows in this the distributionresearch 50% ofof publications the publications of scientific and the articlespatent and patentsapplications related to account the subject for 50%. within Figure the 4 period shows 1975–2017.the distribution It can of publications be observed of that scientific the scientific articles and literature with airbornepatents related wind energyto the subject beginning within in the the period late 1970s 1975–2017. with theIt can research be observed Electricity that the Generation scientific From Jet-Streamliterature Winds with by airborne Fletcher wind and Roberts energy [beginning36], and Crosswind in the late kite1970s power with bythe Loyd research [2] in Electricity 1980. The first patentGeneration located in From the Jet-Stream search of Winds this particular by Fletcher area and Roberts had been [36], filed and Crosswind by Dai and kite Dai power [37] by in Loyd 1975 with the title[2] “High-altitudein 1980. The first windpatent energylocated in combined the search solarof this generation particular area device, had been has filed balloon by Dai connected and Dai with [37] in 1975 with the title “High-altitude wind energy combined solar generation device, has balloon pulling rope that is connected with kite generating device, where pulling rope is fixedly connected connected with pulling rope that is connected with kite generating device, where pulling rope is with fixedfixedly block”. connected This with patent fixed was block”. included This in pate classnt Q54was andincluded X15. Bothin class publications Q54 and cameX15. Both out in the mid andpublications late 1970s; came and out it in is the noteworthy mid and late that 1970s; first and a patent it is noteworthy was obtained that first on thea patent subject was andobtained four years later aon scientific the subject publication and four years emerged. later a scientific publication emerged.

FigureFigure 4. Scientific 4. Scientific articles articles published published per per yearyear (left); Applications Applications for forpatents patents per year per year(right). (right). Sustainability 2017, 9, 915 8 of 22

Sustainability 2017, 9, 915 8 of 22

The 239The papers239 papers in thein the literature literature were were published published by by 375 375 authors authors from from 133 133 research research institutions institutions in in 26 countries26 countries (Figure (Figure5). The5). The most most prominent prominent authors authors werewere Fagiano Fagiano (32); (32); Diehl Diehl (21); (21); Adhikari, Adhikari, Olinger Olinger and Pandaand Panda (11); (11); Hably, Hably, Milanese, Milanese, Vermillion Vermillion andand OckelsOckels (10); (10); and and Schmehl Schmehl (9). (9). Fagiano, Fagiano, Milanese, Milanese, OckelsOckels and and Schmehl Schmehl are are also also the th onese ones who who figure figure onon the li listst of of most most cited cited authors. authors.

FigureFigure 5.5. AuthorsAuthors with with the the largest largest number number of documents of documents versus authors versus with authors the largest with thenumber largest of citations. number of citations. Fagiano, Lorenzo is a professor and researcher at Politecnico di Milano, Department of Control and Computing Engineering (DAUIN). He works in Systems and Control include robust Fagiano,constrained Lorenzo control, is Model a professor Predictive and researcherControl (MPC), at Politecnico efficient MPC di Milano, implementations Department and of Set Control and ComputingMembership Engineeringtheory for control (DAUIN). purposes, He with works applications in Systems to high-altitude and Control wind include energy robust generation. constrained control,In this Model search, Predictive he was considered Control (MPC), the most efficient active rese MPCarcher implementations with 32 publications. and Set He Membership is also inventor theory for controlof two purposes,international with patents, applications registered to high-altitudein nine countries, wind in energynine different generation. classes. In The this numbers search, heof was consideredthe Fagiano, the most L., Gerlero, active researcherI. and Milanese, with 32M. publications.patents are “WO2011121557-A2; He is also inventor WO2011121557-A3; of two international patents,CA2794344-A1; registered in EP2553262-A2; nine countries, US2013078097-A1; in nine different IT1399971-B; classes. The numbersJP2013527893-W; of the Fagiano, CN103038501-A; L., Gerlero, I. IN201208254-P4; RU2012146377-A; EP2553262-B1; JP5841124-B2; RU2576396-C2; US9366225-B2”, and Milanese, M. patents are “WO2011121557-A2; WO2011121557-A3; CA2794344-A1; EP2553262-A2; productive assignees were KITENERGY SRL and MODELWAY SRL. US2013078097-A1; IT1399971-B; JP2013527893-W; CN103038501-A; IN201208254-P4; RU2012146377-A; Milanese, Mario has carried out research in the areas of control systems engineering, EP2553262-B1;automotive JP5841124-B2; and aeronautical RU2576396-C2; engineering, mechatronics, US9366225-B2”, renewable productive energy assignees and space were power, KITENERGY wind SRL andpower MODELWAY and power systems SRL. and automation and control, and has 10 published articles with 120 Milanese,citations. He Mario is a hasprofessor carried and out researcher research inat thePolitecnico areas of controldi Torino, systems Turin, engineering,Italy. He maintains automotive and aeronauticalcollaboration with engineering, the researchers mechatronics, Fagiano, L., renewable Razza, V., energyBonansone, and M., space Canale, power, M., windPiga, D., power and and powerNovara, systems C. and automation and control, and has 10 published articles with 120 citations. He is a professorDiehl, and Moritz researcher studied at physics Politecnico and mathematics di Torino, Turin, at Heidelberg Italy. He and maintains Cambridge collaboration University from with the researchers1993 to Fagiano,1999 and obtained L., Razza, a Ph.D. V., Bonansone, degree from M., Heidel Canale,berg University M., Piga, D.,in 2001 and at Novara, the Interdisciplinary C. Center for Scientific Computing. From 2006 to 2013, he was a professor with the Department of Diehl, Moritz studied physics and mathematics at Heidelberg and Cambridge University from Electrical Engineering, KU Leuven University Belgium, and served as the Principal Investigator of 1993 to 1999 and obtained a Ph.D. degree from Heidelberg University in 2001 at the Interdisciplinary KU Leuven’s Optimization in Engineering Center OPTEC. Since 2013, he has been a professor at CenterUniversity for Scientific of Freiburg, Computing. Germany, From where 2006 he to 2013,heads hethe was Systems a professor Control with and the Optimization Department of ElectricalLaboratory, Engineering, in the KUDepartment Leuven Universityof Microsystems Belgium, Engineering and served (IMTEK). as the PrincipalHis research Investigator focuses on of KU Leuven’soptimization Optimization and control, in Engineering spanning from Center numerical OPTEC. method Since 2013,development he has beento applications a professor in different at University of Freiburg,branches Germany,of engineering, where with he a heads view to the emb Systemsedded and Control renewable and Optimizationenergy systems. Laboratory, He maintains in the Departmentcollaboration of Microsystems with Gillis, J; Engineering Goos, J; Geebelen, (IMTEK). K; Swevers, His research J., Zanon, focuses M., on Ahmad, optimization H; Vukov, and M., control, spanningAndersson, from numericalJ., Horn, G., method Meyers, developmentJ., Stuyts, J., Van todermeulen, applications W., inDriesen, different J., and branches Wagner, of A. engineering, These withauthors a view are to embeddedresearchers or and have renewable been researchers energy at systems. IMTEK. As He to maintains Frison, G.,collaboration Jorgensen, J.B., withthey are Gillis, J; collaborating researchers at DTU Compute—Department of Applied Mathematics and Computer Goos, J; Geebelen, K; Swevers, J., Zanon, M., Ahmad, H; Vukov, M., Andersson, J., Horn, G., Meyers, J., Stuyts, J., Vandermeulen, W., Driesen, J., and Wagner, A. These authors are researchers or have been researchers at IMTEK. As to Frison, G., Jorgensen, J.B., they are collaborating researchers at DTU Compute—Department of Applied Mathematics and Computer Science Technical University Sustainability 2017, 9, 915 9 of 22 of Denmark—andSustainability 2017, 9 Horn,, 915 G is a collaborator of Systems Engineer at Kitty Hawk in Mountain9 of 22View, California, USA. Science Technical University of Denmark—and Horn, G is a collaborator of Systems Engineer at Ockels, Wubbo Johannes was a Dutch physicist and an astronaut of the European Space Agency Kitty Hawk in Mountain View, California, USA. (ESA). InOckels, 1985 he Wubbo participated Johannes in was a flight a Dutch on physicist a space and shuttle an astronaut (STS-61-A), of the making European him Space the Agency first Dutch citizen(ESA). in space.In 1985 he After participated his astronaut in a flight career, on a space Ockels shuttle was (STS-61-A), professor ofmaking Aerospace him the forfirst Sustainable Dutch Engineeringcitizen in and space. Technology After his astronaut at the Delft career, University Ockels was of Technology.professor of Aerospace He obtained for hisSustainable MSc degree in physicsEngineering and mathematicsand Technology in at 1973 the Delft and subsequentlyUniversity of Technology. a PhD degree He obtained in the samehis MSc subjects degree inin 1978 fromphysics the University and mathematics of Groningen. in 1973 His and thesis subsequently was based a PhD on degree experimental in the same work subjects at the in Nuclear-physics 1978 from Acceleratorthe University Institute of (KVI)Groningen. in Groningen. His thesis was based on experimental work at the Nuclear-physics Schmehl,Accelerator RolandInstitute is(KVI) Associate in Groningen. Professor at Delft University of Technology at the Aerospace EngineeringSchmehl, faculty. Roland He has is Associate an extensive Professor background at Delftin University Computational of Technology Fluid Dynamics at the Aerospace in the fields Engineering faculty. He has an extensive background in Computational Fluid Dynamics in the fields of low emission combustion, liquid propellant space propulsion, airborne wind energy, kite power of low emission combustion, liquid propellant space propulsion, airborne wind energy, kite power generationgeneration and and airbag airbag deployment. deployment. HeHe isis activeactive rese researcherarcher in in the the exploring exploring of the of thepotential potential of kite of kite powerpower generation generation and and propulsion. propulsion. TheThe 238 238 patent patent documents documents were were applied applied by 293 in inventors;ventors; Vander Vander Lind Lind (38), (38), Hachtmann Hachtmann (16), (16), GoldsteinGoldstein (13) (13) and and Jensen Jensen (10) (10) stand stand out, out, as as shownshown in Figure Figure 6.6.

FigureFigure 6. 6.Inventors Inventors withwith the larges largestt number number of ofpatents. patents.

Damon Vander Lind has B.S., physics, computer science/electrical engineering (2003–2008) Damonfrom the VanderMassachusetts Lind has Institute B.S., physics,of Technology. computer Vander science/electrical Lind was Chief Engineer engineering at Makani (2003–2008) Power, from the MassachusettsInc. Makani Power Institute develops of wind Technology. energy extracti Vanderon Lindtechnology. was Chief The company Engineer was at founded Makani by Power, Saul Inc. MakaniGriffith Power in 2006 develops and is headquartered wind energy in extraction Mountain technology. View, California, The USA. company He now was works founded for Kitty by Saul GriffithHawk. in 2006He was and considered is headquartered in this research in Mountain the most active View, inventor California, with USA.38 patent He documents. now works for Kitty Hawk. HeHachtmann, was considered Brian holds in this a researchMaster of the Science most activedegree inventorin Mechanical with 38Engineering patent documents. (2006–2008) Hachtmann,from StanfordBrian University. holds He a Master is Ground of Science Stationdegree Team Leader, in Mechanical Mechanical Engineering Engineer at (2006–2008) Google, in from StanfordMountain University. View, California, He is Ground USA, Station who is responsible Team Leader, for Mechanical16 patents. Engineer at Google, in Mountain View, California,No bibliography USA, who was is found responsible about the for inventors 16 patents. Sergey Pavlovich Andreev, Sergey Yakovlevich Chernin and Evgeni Fedorovich Novikov. No bibliography was found about the inventors Sergey Pavlovich Andreev, Sergey Yakovlevich In relation to the sources of publication, Figure 7 presents the five prominent journals on AWE Chernintechnology: and Evgeni Airborne Fedorovich Wind Energy Novikov. (book), Renewable Energy, 2016 American Control Conference In(ACC), relation IEEE to Transactions the sources on of Control publication, Systems Figure Technology,7 presents and the2012 five American prominent Control journals Conference on AWE technology:(ACC). Airborne Wind Energy (book), Renewable Energy, 2016 American Control Conference (ACC), IEEE Transactions on Control Systems Technology, and 2012 American Control Conference (ACC). Sustainability 2017, 9, 915 10 of 22 Sustainability 2017, 9, 915 10 of 22

FigureFigure 7.7. TopTop journalsjournals or conference proceeding proceedingss and and books books on on airborne airborne wind wind energy. energy.

The book Airborne Wind Energy provides an overview of the AWE research field. It is the first The book Airborne Wind Energy provides an overview of the AWE research field. It is the first book that provides a consistent compilation of fundamental theories, current research and book that provides a consistent compilation of fundamental theories, current research and development development activities, and economic and regulatory issues. The book is divided into 35 chapters activities,[3,7–12,15,38–64] and economic that demonstrate and regulatory the relevance issues. The of AWE book and is divided the important into 35 role chapters that this [3,7 technology–12,15,38– 64] thatcan demonstrate play in the transition the relevance to a renewable of AWE energy and the economy. important role that this technology can play in the transitionThe toJournal a renewable Renewable energy Energy economy. is a multidisciplinary journal in renewable energy engineering andThe research Journal that Renewable seeks to Energypromote is and a multidisciplinary disseminate knowledge journal on in renewabletopics related energy to renewable engineering andenergy research technologies. that seeks to promote and disseminate knowledge on topics related to renewable energyThe technologies. American Control Conference (ACC) is the annual conference of the American Automatic ControlThe AmericanCouncil (AACC), Control the Conference U.S. national (ACC) member is the organization annual conference of the International of the American Federation Automatic for ControlAutomatic Council Control. (AACC), AACC the is U.S.an association national memberof the control-related organization groups of the Internationalof eight member Federation societies: for AutomaticAIAA (American Control. Institute AACC of is Aeronautics an association and of Astronautics), the control-related AIChE groups(American of eightInstitute member of Chemical societies: AIAAEngineers), (American ASCE Institute (American of Aeronautics Society of Civil and Astronautics),Engineers), ASME AIChE (American (American Society Institute of Mechanical of Chemical Engineers),Engineers), ASCE IEEE, ISA (American (International Society Society of Civil of Auto Engineers),mation), ASMESCS (Society (American for Computer Society Simulation), of Mechanical Engineers),and SIAM IEEE,(Society ISA for (International Industrial and Society Applied of Mathematics). Automation), The SCS ACC (Society is internationally for Computer recognized Simulation), andas SIAMa scientific (Society and for engineering Industrial andconference Applied dedicated Mathematics). to advancing The ACC control is internationally theory and recognized practice, as a scientificbringing together and engineering an international conference community dedicated of re tosearchers advancing and control practitioners theory andto discuss practice, the bringinglatest discoveries in automatic control. It covers systems theory and practice, including topics such as together an international community of researchers and practitioners to discuss the latest discoveries biological systems, vehicle dynamics and control, cooperative control, control of communication in automatic control. It covers systems theory and practice, including topics such as biological systems, networks, control of distributed parameter systems, uncertain systems, game theory, among others. vehicle dynamics and control, cooperative control, control of communication networks, control of The 2012 American Control Conference (ACC) was held in Montréal, Quebec, Canada, being the first distributed parameter systems, uncertain systems, game theory, among others. The 2012 American ACC outside the United States. The 2016 American Control Conference (ACC) was held in Boston, ControlMA, USA. Conference (ACC) was held in Montréal, Quebec, Canada, being the first ACC outside the UnitedThe States. IEEE The Transactions 2016 American on Control Control Systems Conference Technology (ACC) was publishes held in papers Boston, on MA, technological USA. advancesThe IEEE in Transactionscontrol engineering. on Control It aims Systems to fill Technology in research publishes gaps between papers ontheory technological and practice advances in incontrol control engineering engineering. systems. It aims to The fill inmaterials research publis gaps betweenhed by IEEE theory Transactions and practice on in Control control engineeringSystems systems.Technology The reveal materials new published knowledge, by exploratory IEEE Transactions developments on Control or practical Systems applications Technology in all reveal aspects new knowledge,of the technology exploratory that need developments to implement or practicalcontrol systems, applications from analysis in all aspects and design of the to technology simulation that needand tohardware. implement control systems, from analysis and design to simulation and hardware. Sustainability 2017, 9, 915 11 of 22 Sustainability 2017, 9, 915 11 of 22

AsAs to to the the analyses analyses of of technology technology classes,classes, they were were in in accordance accordance with with CPI CPI (Chemical (Chemical Patents Patents Index)—classificationIndex)—classification used used by by the the Derwent Derwent Innovations Innovations Index. Index. The The patents patents found found in this in studythis study belong tobelong classes to X15, classes Q54, W06,X15, Q54, Q25, andW06, P36. Q25, The and X15 P36. and The Q54 X15 classes and belongQ54 classes to the belong Green Technologyto the Green area, Technology area, in the Green power sources and energy generation theme, the Hydroelectric power in the Green power sources and energy generation theme, the Hydroelectric power inquiry, along with inquiry, along with dams, generators, mini/micro plants, pumped storage, and turbines/water dams, generators, mini/micro plants, pumped storage, and turbines/water wheels. The W06, Q24 and wheels. The W06, Q24 and Q25 classes are in the Green Technology area, in the Green Q25 classes are in the Green Technology area, in the Green Transportation, in the aircraft, pedal power Transportation, in the aircraft, pedal power and boat inquiry, along with electric propulsion and and boat inquiry, along with electric propulsion and wind power. wind power. PatentsPatents were were also also analyzed analyzed accordingaccording to the Intern Internationalational Patent Patent Classification Classification (IPC). (IPC). IPC IPC is the is the internationalinternational classification classification system, system, createdcreated by the the Strasbourg Strasbourg Agreement Agreement (1971), (1971), whose whose technological technological areasareas are are divided divided into into classesclasses AA andand H.H. Within each class, class, there there are are subclasses, subclasses, main main groups groups and and subgroups,subgroups, by by means means of aof hierarchical a hierarchical system. system. The The classes classes deposited deposited by theby 238the research238 research patents patents belong tobelong classes to A63, classes B, C10, A63, D07, B, C10, E, F,D07, G and E, F, H, G asand shown H, as shown in Figure in 8Figure. 8.

FigureFigure 8.8. TopTop InternationalInternational Patent Patent Classification Classification Codes-IPC. Codes-IPC.

Figure 8 shows that 45% of patents belong to class F and focus on mechanical engineering, lighting,Figure heating,8 shows and that weapons 45% of patents (subclasses belong F01, to F03, class F24 F and and focus F41, onrespectively). mechanical Thirty-six engineering, percent lighting, of heating,patents and belong weapons to class (subclasses B patents for F01, processing F03, F24 and and transport F41, respectively). operations in Thirty-six subclasses percent B01, B21, of patentsB23, belongB60, B62, to class B63, BB64, patents B65 and for processingB66. and transport operations in subclasses B01, B21, B23, B60, B62, B63, B64,The B65 institutions and B66. providing the publications in both the scientific literature and the patents are displayedThe institutions in Figures providing 9 and 10. theFigure publications 9 shows inthat both the thefive scientific most representative literature and institutions the patents areare displayedPolitecnico in FiguresTorino, 9 Katloliekeand 10. FigureUniversity9 shows Leuven, that theDelft five University most representative of Technology, institutions National are PolitecnicoUniversity Torino, Singapore, Katlolieke and Worcester University Polytech Leuven, Inst. Delft University of Technology, National University Singapore, and Worcester Polytech Inst. Sustainability 2017, 9, 915 12 of 22 Sustainability 2017, 9, 915 12 of 22

Figure 9. TopTop funding higher education institutions. Sustainability 2017, 9, 915 13 of 22

Figure 10 displays the five most representative assignees; GOOGLE INC (GOOG-C); ST PETERSBURG SCI CULTURE & SPORT (SPET-Soviet Institute); XI’AN DAYU OPTOELECTRONIC SCI & TECHNOLO (XIAN-Non-standard); GOLDSTEIN L (GOLD-Individual) and LI Y (LIYY-Individual). Figure 11 shows that the USA was the country with the highest number of papers in both scientific articles and published patents. This is because the patents come from Google Inc., a multinational online services and software company from the United States and the most visited site in the world. The company was founded by Larry Page and Sergey Brin, both of whom were doctoral students at Stanford University, California, USA, in 1996. Google.org acquired in 2013 the American company Makani Power (Mountain View, California, USA) founded in 2006 by Corwin Hardham, Don Montague and Saul Griffith. They founded this company to exploit the unexploited wind resources more efficiently. A statement posted on the Makani Power website said that this acquisition will provide Makani with the resources needed to accelerate research to make the cost of wind energy competitive with fossil fuels and to make the development of wind-powered airborne systems a commercial reality. In relation to the scientific publications, 47 articles were registered in the USA and they were written by the authors Fagiano, Loyd, Archer, and Goldstein.

Figure 10. Top productive assignees. Figure 10. Top productive assignees.

Figure 10 displays the five most representative assignees; GOOGLE INC (GOOG-C); ST PETERSBURG SCI CULTURE & SPORT (SPET-Soviet Institute); XI’AN DAYU OPTOELECTRONIC SCI & TECHNOLO (XIAN-Non-standard); GOLDSTEIN L (GOLD-Individual) and LI Y (LIYY-Individual). Figure 11 shows that the USA was the country with the highest number of papers in both scientific articles and published patents. This is because the patents come from Google Inc., a multinational online services and software company from the United States and the most visited site in the world.

Figure 11. Top countries in terms of literature review (left) and patents (right).

Table 3 aims to present the most relevant articles of the sample (literature review) and its most cited references literature review. The column sample references column refers to works cited, including articles, conferences and books. The sample articles column refers to the cited articles found in the search database. Note that 14 sample articles are cited in the references (highlighted in bold). It is possible to observe that Loyd’s scientific work has 98 citations, followed by the works of Canale, Fagiano and Milanese (2010) with 60 citations and the work of Ilzhoer, Houska and Diehl (2007) with 57 citations. However, it can be observed that the book “Airborne Wind Energy” by the editors Ahrens U; Diehl, M; Schmehl, R., 2013, was found in the most cited references, with 30 citations.

Sustainability 2017, 9, 915 13 of 22

Sustainability 2017, 9, 915 13 of 22

The company was founded by Larry Page and Sergey Brin, both of whom were doctoral students at Stanford University, California, USA, in 1996. Google.org acquired in 2013 the American company Makani Power (Mountain View, California, USA) founded in 2006 by Corwin Hardham, Don Montague and Saul Griffith. They founded this company to exploit the unexploited wind resources more efficiently. A statement posted on the Makani Power website said that this acquisition will provide Makani with the resources needed to accelerate research to make the cost of wind energy competitive with fossil fuels and to make the developmentFigure 10. Top of productive wind-powered assignees. airborne systems a commercial reality.

FigureFigure 11. 11.Top Top countries countries inin termsterms of literature review review (left) (left) and and patents patents (right). (right).

Table 3 aims to present the most relevant articles of the sample (literature review) and its most In relation to the scientific publications, 47 articles were registered in the USA and they were cited references literature review. The column sample references column refers to works cited, written by the authors Fagiano, Loyd, Archer, and Goldstein. including articles, conferences and books. The sample articles column refers to the cited articles foundTable in3 theaims search to present database. the most Note relevant that 14 sample articles articles of the sample are cited (literature in the references review) and(highlighted its most cited in referencesbold). It is literature possible review.to observe The that column Loyd’s sample scientific references work has column 98 citations, refers followed to works by cited, the works including of articles,Canale, conferences Fagiano and and Milanese books. (2010) The sample with 60 articles citations column and the refers work to of the Ilzhoer, cited articlesHouska foundand Diehl in the search(2007) database. with 57 citations. Note that However, 14 sample it can articles be obse arerved cited that in the the book references “Airborne (highlighted Wind Energy” in bold). by the It is possibleeditors toAhrens observe U; thatDiehl, Loyd’s M; Schmehl, scientific R., work 2013, has was 98 found citations, in the followed most cited by thereferences, works ofwith Canale, 30 Fagianocitations. and Milanese (2010) with 60 citations and the work of Ilzhoer, Houska and Diehl (2007) with 57 citations. However, it can be observed that the book “Airborne Wind Energy” by the editors Ahrens U; Diehl, M; Schmehl, R., 2013, was found in the most cited references, with 30 citations.

Table 3. Top sample literature review cited versus sample reference literature review cited.

Sample References Citation Sample Articles Citation Loyd, ML, 1980 98 Loyd, ML, 1980 98 Canale, M; Fagiano, L; Milanese, M, 2010 60 Canale, M; Fagiano, L; Milanese, M, 2010 60 Ilzhoer, A; Houska, B; Diehl, M, 2007 57 Ilzhoer, A; Houska, B; Diehl, M, 2007 57 Williams, P; Lansdorp, B; Ockels, W, 2008 55 Williams, P; Lansdorp, B; Ockels, W, 2008 55 Archer, CL; Caldeira, K, 2009 43 Archer, CL; Caldeira, K, 2009 43 Argatov, I; Rautakorpi, P; 39 Argatov, I; Rautakorpi, P; Silvennoinen, R, 2009 39 Silvennoinen, R, 2009 Baayen, JH; Ockels, WJ, 2012 37 Canale, M; Fagiano, L; Milanese, M, 2007 37 Canale, M; Fagiano, L; Milanese, M, 2007 37 Baayen, JH; Ockels, WJ, 2012 37 Fagiano, L; Milanese, M, 2012 34 Fagiano, L; Milanese, M, 2012 34 Fagiano, L; Milanese, M; Piga, D, 2010 33 Fagiano, L; Milanese, M; Piga, D, 2010 33 Sustainability 2017, 9, 915 14 of 22

Table 3. Cont.

Sample References Citation Sample Articles Citation Ahrens, U; Diehl, M; Schmehl, R, 2013 30 Canale, M; Fagiano, L; Milanese, M, 2009 28 Roberts, BW; Shepard, DH; Caldeira, K; Canale, M; Fagiano, L; Milanese, M, 2009 28 27 Cannon, ME; Eccles, DG; Grenier, AJ; Freidin, JF, 2007 Ockels, W. J., 2001 28 Fagiano, L; Milanese, M; Piga, D, 2012 26 Roberts, BW; Shepard, DH; Caldeira, K; Cannon, ME; Eccles, DG; Grenier, AJ; 27 Erhard, M; Strauch, H, 2013 25 Freidin, JF, 2007 Fagiano, L; Zgraggen, AU; Morari, M; Fagiano, L; Milanese, M; Piga, D, 2012 26 24 Khammash, M, 2014 Houska B; Diehl, M, 2007 26 Argatov, I; Silvennoinen, R, 2010 18 Erhard, M; Strauch, H, 2013 25 Terink, EJ; Breukels, J; Schmehl, R; Ockels, WJ, 2011 17 Fagiano, L; Huynh, K; Bamieh, B; Fagiano L, 2009 25 16 Khammash, M, 2014 Fagiano, L; Zgraggen, AU; Morari, M; 24 Jehle, C; Schmehl, R, 2014 16 Khammash, M, 2014 Houska, B; Diehl, M, 2006 22 Fletcher, CAJ; Roberts, BW, 1979 13

Loyd was the pioneering researcher on AWE technology. He described Crosswind Kite Power in the Journal of Energy in 1980, a concept to produce large-scale wind power through aerodynamically efficient kites. Based on aircraft, kites fly across the wind at high speed. He developed equations of motion and his calculations were validated by comparison with simple analytical models [2]. In 2010, Canale, Fagiano and Milanese provided simulations and experimental results of a new class of wind power generators, called KiteGen, in the article “High Altitude Wind Energy Generation Using Controlled Power Kites” published in IEEE Transaction Control Systems Technology. They investigated two different types of KiteGen through numerical simulations, the yo-yo configuration and the carousel configuration. For each configuration, a generator with the same characteristics was considered [65]. Ilzhoer, Houska and Diehl in 2007 investigated nonlinear model predictive control (NMPC) for control of power generating kites under changing wind conditions. The authors derived a realistic nonlinear model for a kite and compute energy optimal loops for different wind speeds, and they solved this NMPC problem numerically with the real-time iteration scheme using direct multiple shooting [66]. In 2008, Williams, P, Lansdorp, B, and Ockels, W. focused their study on the use of a light lifting body at the end of a tether to generate useful power. They studied two major configurations, the kite is used to tow a ground vehicle in the crosswind direction and the kite is flown to generate power using a ground generator. According to the authors, the numerical results illustrate that optimal power generation is most sensitive to the cycle time, tether length, and wind speed [67]. Archer, CL and Caldeira, K (2009), evaluated in the article Altitude Wind Power, the available wind power resource worldwide at altitudes between 500 and 12,000 m above ground. The authors analyzed twenty-eight years of wind data from the reanalysis by the National Centers for Environmental Prediction and the Department of Energy. These data were analyzed and interpolated to study geographical distributions and persistency of winds at all altitudes [68]. Figure 12 shows the 19 most cited patents. The top five patents are: US20120104763-A1 (32 citations), US20130221679-A1 (26 citations), US20110260462-A1 (24 citations), US6254034-B1 (23 citations) and US20100295303-A1 (19 citations). They are described below. Sustainability 2017, 9, 915 15 of 22 Sustainability 2017, 9, 915 15 of 22

Figure 12. TopTop patents cited.

US20120104763-A1 was published in 2012 by Vander Lind, entitled Kite configuration and US20120104763-A1 was published in 2012 by Vander Lind, entitled Kite configuration and flight flight strategy for high wind speeds [69]. It describes a crosswind kite system adapted to operate in strategy for high wind speeds [69]. It describes a crosswind kite system adapted to operate in an an alternate mode in high winds. The system may operate at reduced efficiency in high winds to alternate mode in high winds. The system may operate at reduced efficiency in high winds to moderate moderate loading on the system during those high winds. The system may use multi-element loading on the system during those high winds. The system may use multi-element airfoils that are airfoils that are actuated to reduce the coefficient of lift of the airfoils to moderate loading in high actuated to reduce the coefficient of lift of the airfoils to moderate loading in high wind conditions. wind conditions. Other flight aspects may be controlled, including flying the crosswind kite in side Other flight aspects may be controlled, including flying the crosswind kite in side slip to induce drag slip to induce drag which may lower overloading on the system. which may lower overloading on the system. US20130221679-A1 was published in 2013, by Damon Vander Lind, entitled Kite Ground US20130221679-A1 was published in 2013, by Damon Vander Lind, entitled Kite Ground Station Station and System Using Same, and deals with kite system with a ground station adapted for and System Using Same, and deals with kite system with a ground station adapted for airborne power airborne power generation [70]. The kite system may include a kite that comprises one or more generation [70]. The kite system may include a kite that comprises one or more airfoils which have airfoils which have mounted thereon the plurality of turbine driven generators. The turbine driven mounted thereon the plurality of turbine driven generators. The turbine driven generators may also generators may also function as motor driven propellers in a powered flight mode, which may be function as motor driven propellers in a powered flight mode, which may be used during takeoff, used during takeoff, and may include aspects of vertical takeoff and landing. A perch adapted to and may include aspects of vertical takeoff and landing. A perch adapted to facilitate the takeoff and facilitate the takeoff and landing may be used as part of the system. The perch may pivot such that landing may be used as part of the system. The perch may pivot such that the pivot is oriented towards the pivot is oriented towards the tension direction of the tether. the tension direction of the tether. US20110260462-A1 was published in 2011 by Damon Vander Lind, entitled Planform US20110260462-A1 was published in 2011 by Damon Vander Lind, entitled Planform Configuration for Stability of a Powered Kite and a System and Method for Use of Same [71]. It Configuration for Stability of a Powered Kite and a System and Method for Use of Same [71]. discusses system and method of power generation, wind-based flight, and takeoff and landing using It discusses system and method of power generation, wind-based flight, and takeoff and landing a tethered kite with a raised tail mounted rearward of the main wing or wings. The tail may be fully using a tethered kite with a raised tail mounted rearward of the main wing or wings. The tail may rotatable and may be rotate more than 90° from its nominal position during a traditional flight be fully rotatable and may be rotate more than 90◦ from its nominal position during a traditional paradigm. flight paradigm. US6254034-B1 was published in 1999 by Howard G. Carpenter, entitled Tethered aircraft US6254034-B1 was published in 1999 by Howard G. Carpenter, entitled Tethered aircraft system system for gathering energy from Wind [72]. It discussed the tethered aircraft of the system is blown for gathering energy from Wind [72]. It discussed the tethered aircraft of the system is blown by wind by wind to travel downwind at a controlled rate for maximal mechanical energy gathering from to travel downwind at a controlled rate for maximal mechanical energy gathering from wind whose wind whose velocity fluctuates and gusts. A cycle of travel is completed when the aircraft is velocity fluctuates and gusts. A cycle of travel is completed when the aircraft is travelled upwind to the travelled upwind to the site of the beginning of downwind travel in which downwind travel is site of the beginning of downwind travel in which downwind travel is recommenced. The downwind recommenced. The downwind travelling aircraft pulling unwinds its tether from an anchored travelling aircraft pulling unwinds its tether from an anchored windlass drum to spin the rotor of an windlass drum to spin the rotor of an interconnected electrical machine to convert the mechanical interconnected electrical machine to convert the mechanical energy to electricity. energy to electricity. US20100295303-A1 was published in 2009 by Damon Vander Lind, Becker Van Niekerk, Corwin Hardham, entitled Tethered system for power generation [73]. It discusses a system for Sustainability 2017, 9, 915 16 of 22

US20100295303-A1 was published in 2009 by Damon Vander Lind, Becker Van Niekerk, Corwin Hardham, entitled Tethered system for power generation [73]. It discusses a system for power Sustainability 2017, 9, 915 16 of 22 generation comprising a wing, a turbine, a tether, and a tether tension sensor. The wing is for generatingpower lift. generation The turbine comprising is coupled a wing, to a the turbine, wing a andtether, it isand used a tether for generatingtension sensor. power The wing from is rotationfor of a propellergenerating or for lift. generating The turbine thrust is coupled using to thethe propeller.wing and it Oneis used end for ofgenerating the tether power is coupled from rotation to the wing. The tetherof a tensionpropeller sensor or for generating is for determining thrust using the the tension propeller. of theOne tether. end of the tether is coupled to the wing. The tether tension sensor is for determining the tension of the tether. Figure 13 shows the frequency of common keywords that appeared in patents and articles related Figure 13 shows the frequency of common keywords that appeared in patents and articles to windrelated energy to wind with energy tethered with airfoils. tethered Weairfoils. noted We that,noted inthat, both in both searches, searches, the the words words wind wind energy, and airborneenergy, windand airborne energy wind are found.energy are This found. allowed This allowed us to validate us to validate the adherence the adherence of the of the keywords used inkeywords the selection used ofin researchthe selection articles of research and patentsarticles and to thosepatents found to those in found the Web in the of ScienceWeb of Scienceand Derwent Innovationand IndexDerwentdatabases. Innovation Index databases.

Figure 13.FigureTop 13. keywords Top keywords by authorsby authors of of literature literature review abstracts abstracts (top (top); and); andtop keywords top keywords appearing appearing in patentin abstractspatent abstracts (below (below). ).

In this study, it was possible to perform the authors network analysis, as shown in Figure 14 for Inliterature this study, review it was and possible Figure 15 tofor perform patents. Figure the authors 14 shows network the network analysis, of collaboration as shown among in Figure 14 for literatureauthors review of scientific and Figurearticles 15found for in patents. the literatu Figurere review 14 shows search. the It networkdisplays the of collaborationpresence of 57 among authorsclusters of scientific with 294 articles items foundand their in connection the literature with review555 links. search. It was Itobserved displays that the the presence clusters formed of 57 clusters agree with the information presented previously by the authors who stood out with the most with 294 items and their connection with 555 links. It was observed that the clusters formed agree with published works in the area and highest number of citations. The most representative clusters are in the informationblue, red, green, presented light green, previously dark green, by the pink, authors purple, whoand yellow. stood out with the most published works in the area andThe blue highest cluster number consists of of citations. 12 authors The who most work representativetogether. In this clusterscluster, Fagiano, are in blue,Milanese, red, green, light green,Canale, dark Novara, green, Razza, pink, and purple, Piga are and the yellow.most prominent figures. This cluster is directly linked to the Thebeige blue cluster cluster (nine consists authors) ofin 12which authors Zgraggen, who Morari work and together. Khammash In stand thiscluster, out and with Fagiano, the light Milanese, purple cluster (four authors) consisting of Argatov, Rautakorpi, Shafranov and Silvennoinen. Canale, Novara, Razza, and Piga are the most prominent figures. This cluster is directly linked to the The red cluster consists of 20 authors who form a collaboration network. The authors who are beige clustermost prominent (nine authors) in this in cluster which are Zgraggen, Diehl, Gros, Morari Zanon, and Houska, Khammash Horn, Swevers, stand out Geebelen and with and the light purple clusterVukov. (fourThis cluster authors) is linked consisting to three of other Argatov, cluste Rautakorpi,rs: the light blue Shafranov cluster (six and authors) Silvennoinen. in which TheAhmad, red cluster Coleman consists and Toal of stand 20 authorsout; the brown who formcluster a(six collaboration authors) in which network. Williams, The Lansdorp authors who are mostand prominent Ruiterkamp in stand this out; cluster and the are gray Diehl, cluster Gros, (two Zanon, authors) Houska, in which Erhard Horn, and Swevers, Strauch Geebelenare the and Vukov.most This prominent cluster isauthors. linked to three other clusters: the light blue cluster (six authors) in which The yellow cluster consists of 14 authors who are related to each other. These authors are Ahmad, Coleman and Toal stand out; the brown cluster (six authors) in which Williams, Lansdorp and Vermillion and Kolmanovscky, who are directly linked to the cluster of authors such as Cherubini, RuiterkampFontana, stand Papini out; and and Vertechy. the gray cluster (two authors) in which Erhard and Strauch are the most prominent authors. The yellow cluster consists of 14 authors who are related to each other. These authors are Vermillion and Kolmanovscky, who are directly linked to the cluster of authors such as Cherubini, Fontana, Papini and Vertechy. Sustainability 2017, 9, 915 17 of 22 Sustainability 2017, 9, 915 17 of 22 Sustainability 2017, 9, 915 17 of 22

Figure 14. Network analysis of the scientific literature. FigureFigure 14. 14.Network Network analysisanalysis of of the the scientific scientific literature. literature.

Figure 15. Network analysis of the patents. FigureFigure 15. 15.Network Network analysis of of the the patents. patents.

As for the green cluster, it consists of 15 prominent authors, such as Schmehl, Ockels and Fechner. This cluster is directly related to the dark green cluster (eight authors), consisting of Oliger, Li, Sustainability 2017, 9, 915 18 of 22

Demetriou, Tryggvason and Ghasemi. This cluster in turn is linked to the beige cluster (10 authors), including authors such as Costello, Bonvin, Jones, Francois and Lymperopoulos. On the other hand, one can observe that the light green cluster does not make any connections with any author collaboration network. This cluster consists of five authors: Panda, Adhikari, Poraj, Prasanna and Rathore. Another cluster that is not related to any author collaboration network is the purple cluster, consisting of 13 prominent authors, including Kolar, Krismer, Looser, Gammeter and Friedemann. Finally, the pink cluster consists of nine prominent authors, including Hably, Lozano, Bacha and Dumon. This cluster is not directly related to any other cluster of author collaboration network. Figure 15 shows the collaborative network among patent inventors. The figure displays the presence of 107 clusters with 284 items and their connection with 523 links. It is possible to observe that the cluster goes in the opposite direction of the information presented previously about the inventors who stood out with the greatest number of patents in the area. It becomes evident in Figure 15 that four clusters are not linked to the patent inventors’ collaboration network. These clusters are brown cluster (six inventors) with Kang, Kim, Seong; blue cluster (11 authors) featuring Sawatani, Otsuta, Ito, and Miyazaki; light blue cluster (10 authors) represented by Andreev, Chernin, Novikov, Mayboroda and Yarkovenko; and beige cluster (seven authors) with Choi, Hahm, Kim, Lee and Tae. On the other hand, the most representative clusters in the collaboration network are: red, purple, beige and green clusters. The red cluster is made up of 30 inventors, most notably: Vander Lind, Jensen, Hachtmann, Lind, Hardhan and Gilroysmith. This cluster is directly related to the purple cluster (two inventors and four patents) with Casey and Goessling. The beige cluster consists of 10 inventors who are represented by Li Y., Li Q., Yang, Wang and Tan. This cluster is directly related to the blue cluster consisting of 14 inventors, among them the most prominent ones are Zhang, Liu, Zhou, Bian and Fan. The green cluster is directly related to the dark purple clusters (four inventors), in which the most prominent inventors are Austin and Mercier. The light purple cluster (14 inventors) is represented by Pu, Zhang, Wang and Huang. These purple clusters are connected respectively. Finally, it can be noticed that the inventor Goldstein presents seven patents, which in turn makes him the most representative in his network (dark red cluster), however, it is important to note that he is not relating to other inventors. In this sense, we validate the data about the authors presented previously and the inventors considered representative and pioneers in wind energy with tethered airfoils.

5. Conclusions In this article, we offered a brief overview of the scientific and technological progress of research on wind energy with wired airfoils, focusing both in academic and industrial activities. A total of 477 documents (239 scientific papers and 238 patents) were collected from the Web of Science and the Derwent Innovations Index. Those documents form the sample of this research. By means of the bibliometric analysis, it was possible to identify that researches on wind energy with tethered airfoils began their activities in the late 1970s and the first patent found was in 1975 by inventors Dai, C. and Dai, J. The first scientific publication was in 1979 by authors Fletcher and Roberts, and in 1980 by Miles L. Loyd. The most representative and cited authors on the subject were Fagiano, Milanese, Ockels and Schmehl. Additionally, the inventors who submitted more patents were Vander Lind, Hachtmann, Goldstein and Jensen. They work for the company that has more patents in this sample, Google Inc. The most representative research institutions are Politecnico Torino, Katlolieke University Leuven, Delft University Technology, National University Singapore and Worcester Polytech Inst. The United States is the country that presented the highest number of patents and scientific works, since the Sustainability 2017, 9, 915 19 of 22 company with the most patents and the most productive authors of the subject are from this country or have already developed research in this country. Journals are the main source of scientific articles. The most important journals were Airborne Wind Energy (book), Renewable Energy, American Control Conference (ACC), IEEE Transactions on Control Systems Technology, and American Control Conference (ACC). The technology classes were in accordance with the CPI (Chemical Patents Index)—classification used by the Derwent Innovations Index. The patents for this research are in classes X15, Q54, W06, Q25, and P36, which are in the Green Technology area. Finally, we noticed that both scientific works and patents set up collaboration networksl that is, the authors interact with each other to establish collaborative partnerships.

Acknowledgments: The authors would like to thank the Coordination for the Improvement of Higher Education Personnel (CAPES) for funding this research project. Author Contributions: Cristiane Alves Anacleto and Edson Pacheco Paladini searched the Web of Science Database and refined the collected data. Álvaro Guillermo Rojas Lezana refined the data collected from the NAILS and GEPHI tools. Anny Key de Souza Mendonça and Caroline Rodrigues Vaz analyzed combinations and interpreted the data. All authors contributed to the closing of this article. Conflicts of Interest: The authors declare no conflict of interest.

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