4. Results and Discussion
The survey was conducted in April 2017. We found a total of 239 scientific literature papers, which were extracted from Web of Science Core Collection databases and 238 patent documents extracted from the Derwent Innovations Index published between 1980 and 2017.
presents the results of the distribution by document type. As shown in Table 1
, the most representative types of documents were patents, proceedings papers, articles and book chapters, accounting for more than 99% of the total publication. The 239 documents were written by 375 authors and co-authors. Together, they used 2532 references and 518 keywords. These documents were produced by 133 research institutions from 26 countries and 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. As to the 238 patent documents found, they were deposited by 293 authors and co-authors, and belong to 26 classes, 21 countries and 166 assignees.
When comparing the trends between the scientific literature and the patents, it can be observed that the scientific literature represents in this research 50% of the publications and the patent applications account for 50%. Figure 4
shows the distribution of publications of scientific articles and patents related to the subject within the period 1975–2017. It can be observed that the scientific literature with airborne wind energy beginning in the late 1970s with the research Electricity Generation From Jet-Stream Winds by Fletcher and Roberts [36
], and Crosswind kite power by Loyd [2
] in 1980. The first patent located in the search of this particular area had been filed by Dai and Dai [37
] in 1975 with the title “High-altitude wind energy combined solar generation device, has balloon connected with pulling rope that is connected with kite generating device, where pulling rope is fixedly connected with fixed block”. This patent was included in class Q54 and X15. Both publications came out in the mid and late 1970s; and it is noteworthy that first a patent was obtained on the subject and four years later a scientific publication emerged.
The 239 papers in the literature were published by 375 authors from 133 research institutions in 26 countries (Figure 5
). The most prominent authors were Fagiano (32); Diehl (21); Adhikari, Olinger and Panda (11); Hably, Milanese, Vermillion and Ockels (10); and Schmehl (9). Fagiano, Milanese, Ockels and Schmehl are also the ones who figure on the list of most cited authors.
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 constrained control, Model Predictive Control (MPC), efficient MPC implementations and Set Membership theory for control purposes, with applications to high-altitude wind energy generation. In this search, he was considered the most active researcher with 32 publications. He is also inventor of two international patents, registered in nine countries, in nine different classes. The numbers of the Fagiano, L., Gerlero, I. and Milanese, M. patents are “WO2011121557-A2; WO2011121557-A3; CA2794344-A1; EP2553262-A2; US2013078097-A1; IT1399971-B; JP2013527893-W; CN103038501-A; IN201208254-P4; RU2012146377-A; EP2553262-B1; JP5841124-B2; RU2576396-C2; US9366225-B2”, productive assignees were KITENERGY SRL and MODELWAY SRL.
Milanese, Mario has carried out research in the areas of control systems engineering, automotive and aeronautical engineering, mechatronics, renewable energy and space power, wind power and power systems and automation and control, and has 10 published articles with 120 citations. He is a professor and researcher at Politecnico di Torino, Turin, Italy. He maintains collaboration with the researchers Fagiano, L., Razza, V., Bonansone, M., Canale, M., Piga, D., and Novara, C.
Diehl, Moritz studied physics and mathematics at Heidelberg and Cambridge University from 1993 to 1999 and obtained a Ph.D. degree from Heidelberg University in 2001 at the Interdisciplinary Center for Scientific Computing. From 2006 to 2013, he was a professor with the Department of Electrical Engineering, KU Leuven University Belgium, and served as the Principal Investigator of KU Leuven’s Optimization in Engineering Center OPTEC. Since 2013, he has been a professor at University of Freiburg, Germany, where he heads the Systems Control and Optimization Laboratory, in the Department of Microsystems Engineering (IMTEK). His research focuses on optimization and control, spanning from numerical method development to applications in different branches of engineering, with a view to embedded and renewable energy systems. He maintains collaboration with Gillis, J; 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 of Denmark—and Horn, G is a collaborator of Systems Engineer at Kitty Hawk in Mountain View, California, USA.
Ockels, Wubbo Johannes was a Dutch physicist and an astronaut of the European Space Agency (ESA). In 1985 he participated in a flight on a space shuttle (STS-61-A), making him the first Dutch citizen in space. After his astronaut career, Ockels was professor of Aerospace for Sustainable Engineering and Technology at the Delft University of Technology. He obtained his MSc degree in physics and mathematics in 1973 and subsequently a PhD degree in the same subjects in 1978 from the University of Groningen. His thesis was based on experimental work at the Nuclear-physics Accelerator Institute (KVI) in Groningen.
Schmehl, Roland is Associate Professor at Delft University of Technology at the Aerospace 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 generation and airbag deployment. He is active researcher in the exploring of the potential of kite power generation and propulsion.
The 238 patent documents were applied by 293 inventors; Vander Lind (38), Hachtmann (16), Goldstein (13) and Jensen (10) stand out, as shown in Figure 6
Damon Vander Lind has B.S., physics, computer science/electrical engineering (2003–2008) from the Massachusetts Institute of Technology. Vander Lind was Chief Engineer at Makani Power, Inc. Makani Power develops wind energy extraction technology. The company was founded by Saul Griffith in 2006 and is headquartered in Mountain View, California, USA. He now works for Kitty Hawk. He was considered in this research the most active inventor with 38 patent documents.
Hachtmann, Brian holds a Master of Science degree in Mechanical Engineering (2006–2008) from Stanford University. He is Ground Station Team Leader, Mechanical Engineer at Google, in Mountain View, California, USA, who is responsible for 16 patents.
No bibliography was found about the inventors Sergey Pavlovich Andreev, Sergey Yakovlevich Chernin and Evgeni Fedorovich Novikov.
In relation to the sources of publication, Figure 7
presents the five prominent journals on AWE technology: Airborne Wind Energy (book), Renewable Energy, 2016 American Control Conference (ACC), IEEE Transactions on Control Systems Technology, and 2012 American Control Conference (ACC).
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 development activities, and economic and regulatory issues. The book is divided into 35 chapters [3
] that demonstrate the relevance of AWE and the important role that this technology can play in the transition to a renewable energy economy.
The Journal Renewable Energy is a multidisciplinary journal in renewable energy engineering and research that seeks to promote and disseminate knowledge on topics related to renewable energy technologies.
The American Control Conference (ACC) is the annual conference of the American Automatic Control Council (AACC), the U.S. national member organization of the International Federation for Automatic Control. AACC is an association of the control-related groups of eight member societies: AIAA (American Institute of Aeronautics and Astronautics), AIChE (American Institute of Chemical Engineers), ASCE (American Society of Civil Engineers), ASME (American Society of Mechanical Engineers), IEEE, ISA (International Society of Automation), SCS (Society for Computer Simulation), and SIAM (Society for Industrial and Applied Mathematics). The ACC is internationally recognized as a scientific and engineering conference dedicated to advancing control theory and practice, bringing together an international community of researchers and practitioners to discuss the latest discoveries in automatic control. It covers systems theory and practice, including topics such as biological systems, vehicle dynamics and control, cooperative control, control of communication networks, control of distributed parameter systems, uncertain systems, game theory, among others. The 2012 American Control Conference (ACC) was held in Montréal, Quebec, Canada, being the first ACC outside the United States. The 2016 American Control Conference (ACC) was held in Boston, MA, USA.
The IEEE Transactions on Control Systems Technology publishes papers on technological advances in control engineering. It aims to fill in research gaps between theory and practice in control engineering systems. The materials published by IEEE Transactions on Control Systems Technology reveal new knowledge, exploratory developments or practical applications in all aspects of the technology that need to implement control systems, from analysis and design to simulation and hardware.
As to the analyses of technology classes, they were in accordance with CPI (Chemical Patents Index)—classification used by the Derwent Innovations Index. The patents found in this study belong to classes X15, Q54, W06, Q25, and P36. The X15 and Q54 classes belong to the Green Technology area, in the Green power sources and energy generation theme, the Hydroelectric power inquiry, along with dams, generators, mini/micro plants, pumped storage, and turbines/water wheels. The W06, Q24 and Q25 classes are in the Green Technology area, in the Green Transportation, in the aircraft, pedal power and boat inquiry, along with electric propulsion and wind power.
Patents were also analyzed according to the International Patent Classification (IPC). IPC is the international classification system, created by the Strasbourg Agreement (1971), whose technological areas are divided into classes A and H. Within each class, there are subclasses, main groups and subgroups, by means of a hierarchical system. The classes deposited by the 238 research patents belong to classes A63, B, C10, D07, E, F, G and H, as shown in Figure 8
shows that 45% of patents belong to class F and focus on mechanical engineering, lighting, heating, and weapons (subclasses F01, F03, F24 and F41, respectively). Thirty-six percent of patents belong to class B patents for processing and transport operations in subclasses B01, B21, B23, B60, B62, B63, B64, B65 and B66.
The institutions providing the publications in both the scientific literature and the patents are displayed in Figure 9
and Figure 10
. Figure 9
shows that the five most representative institutions are Politecnico Torino, Katlolieke University Leuven, Delft University of Technology, National University Singapore, and Worcester Polytech Inst.
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).
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.
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.
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
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.
US20120104763-A1 was published in 2012 by Vander Lind, entitled Kite configuration and flight strategy for high wind speeds [69
]. It describes a crosswind kite system adapted to operate in an alternate mode in high winds. The system may operate at reduced efficiency in high winds to moderate loading on the system during those high winds. The system may use multi-element airfoils that are actuated to reduce the coefficient of lift of the airfoils to moderate loading in high wind conditions. Other flight aspects may be controlled, including flying the crosswind kite in side slip to induce drag which may lower overloading on the system.
US20130221679-A1 was published in 2013, by Damon Vander Lind, entitled Kite Ground Station and System Using Same, and deals with kite system with a ground station adapted for airborne power generation [70
]. The kite system may include a kite that comprises one or more airfoils which have mounted thereon the plurality of turbine driven generators. The turbine driven generators may also function as motor driven propellers in a powered flight mode, which may be used during takeoff, and may include aspects of vertical takeoff and landing. A perch adapted to facilitate the takeoff and landing may be used as part of the system. The perch may pivot such that the pivot is oriented towards the tension direction of the tether.
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 discusses system and method of power generation, wind-based flight, and takeoff and landing using a tethered kite with a raised tail mounted rearward of the main wing or wings. The tail may be fully rotatable and may be rotate more than 90° from its nominal position during a traditional flight paradigm.
US6254034-B1 was published in 1999 by Howard G. Carpenter, entitled Tethered aircraft system for gathering energy from Wind [72
]. It discussed the tethered aircraft of the system is blown by wind to travel downwind at a controlled rate for maximal mechanical energy gathering from wind whose velocity fluctuates and gusts. A cycle of travel is completed when the aircraft is travelled upwind to the site of the beginning of downwind travel in which downwind travel is recommenced. The downwind travelling aircraft pulling unwinds its tether from an anchored windlass drum to spin the rotor of an interconnected electrical machine to convert the mechanical 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 power generation comprising a wing, a turbine, a tether, and a tether tension sensor. The wing is for generating lift. The turbine is coupled to the wing and it is used for generating power from rotation of a propeller or for generating thrust using the propeller. One end of the tether is coupled to the wing. The tether tension sensor is for determining the tension of the tether.
shows the frequency of common keywords that appeared in patents and articles related to wind energy with tethered airfoils. We noted that, in both searches, the words wind energy, and airborne wind energy are found. This allowed us to validate the adherence of the keywords used in the selection of research articles and patents to those found in the Web of Science
and Derwent Innovation Index
In this study, it was possible to perform the authors network analysis, as shown in Figure 14
for literature review and Figure 15
for patents. Figure 14
shows the network of collaboration among authors of scientific articles found in the literature review search. It displays the presence of 57 clusters with 294 items and their connection with 555 links. It was observed that the clusters formed agree with the information presented previously by the authors who stood out with the most published works in the area and highest number of citations. The most representative clusters are in blue, red, green, light green, dark green, pink, purple, and yellow.
The blue cluster consists of 12 authors who work together. In this cluster, Fagiano, Milanese, Canale, Novara, Razza, and Piga are the most prominent figures. This cluster is directly linked to the beige cluster (nine authors) in which Zgraggen, Morari and Khammash stand out and with the light purple cluster (four authors) consisting of Argatov, Rautakorpi, Shafranov and Silvennoinen.
The red cluster consists of 20 authors who form a collaboration network. The authors who are most prominent in this cluster are Diehl, Gros, Zanon, Houska, Horn, Swevers, Geebelen and Vukov. This cluster is linked to three other clusters: the light blue cluster (six authors) in which Ahmad, Coleman and Toal stand out; the brown cluster (six authors) in which Williams, Lansdorp and Ruiterkamp stand out; and 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.
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, 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.
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.