Kyushu University Academic Staff Educational and Research Activities Database
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Koichi Watanabe Last modified date:2020.07.03

Graduate School

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Academic Degree
Ph.D (Engineering)
Field of Specialization
Fluid Dynamics, Wind Engineering
Research Interests
  • Wind Solar Tower Harnessing Solar Energy and Wind Energy
    keyword : Hybrid Power Generation, Wind Solar Tower
  • Development of High-Performance Wind-Lens Turbines
    keyword : Wind-Lens, Wind acceleration device, Power augmentation
  • Development of High Accuracy Wind Prediction Methods
    keyword : Wind Prediction, Numerical Simulation Method
Academic Activities
1. Koichi Watanabe, Sho Fukutomi, Yuji Ohya, Takanori Uchida, An Ignored Wind Generates More Electricity
A Solar Updraft Tower to a Wind Solar Tower, International Journal of Photoenergy, 10.1155/2020/4065359, 2020, 2020.01, A solar updraft tower is one of the wind power generation plants which utilizes solar energy. The purpose of this study was to ascertain whether the tower was also able to utilize crosswind energy. Wind tunnel experiments and numerical simulations were conducted simulating the crosswind. The results showed that suctioned updraft speed in the tower was proportional to the crosswind speed, and its conversion rate depended on the tower configuration. A diffuser-shaped tower with a vortex generator achieved to produce the updraft whose speed exceeded the crosswind speed. It was due to the low pressure created by the vortex atop the tower and to the diffuser effect. The crosswind utilization enables the simple power generation device to generate electricity during the night, and the hybrid utilization of renewable energies contributes to the increasing wind energy market..
2. Koichi Watanabe, Yuji Ohya, Takanori Uchida, Power output enhancement of a ducted wind turbine by stabilizing vortices around the duct, Energies, 10.3390/en12163171, 12, 16, 2019.08, A brimmed-diffuser augmented turbine (called a wind lens turbine: WLT) actively uses vortices around the brim to enhance its power output. However, the vortices are usually unstable and asymmetric. This study attempted to stabilize the vortices to enhance the power output of a WLT. Then, we investigated new approaches using vortex stabilization plates and polygonal brims in wind tunnel experiments and numerical simulations. Both approaches achieved a 1.5-3.8% increase in power output compared with a standard WLT. Our numerical simulations revealed a periodicity existing in a fluctuating vortex structure on the circular brim. Importantly, vortex stabilization plates and polygonal brims must be the same periodic scale to suppress the vortex fluctuation and stabilize the vortices effectively. In addition, a larger brim tended to enhance the stabilizing effects. We believe that this discovery provides an easy way to increase the power output of existing wind turbines. It is particularly important in light of advances in wind energy technology and the increasing wind energy market..
3. Ohya Yuji, Koichi Watanabe, A new approach toward power output enhancement using multirotor systems with shrouded wind turbines, Journal of Energy Resources Technology, Transactions of the ASME, 10.1115/1.4042235, 141, 5, 2019.05, A multirotor system (MRS) is defined as containing more than one rotor in a single structure. MRSs have a great potential as a wind turbine system, saving mass and cost, and showing scale ability. The shrouded wind turbine with brimmed diffuser-augmented wind turbines (B-DAWT) has demonstrated power augmentation for a given turbine diameter and wind speed by a factor of about 2-5 compared with a bare wind turbine. In the present research, B-DAWTs are used in a multirotor system. The power output performance of MRSs using two and three B-DAWTs in a variety of configurations has been investigated in the previous works. In the present study, the aerodynamics of an MRS with five B-DAWTs, spaced in close vicinity in the same vertical plane normal to a uniform flow, has been analyzed. Power output increases of up to 21% in average for a five-rotor MRS configuration are achieved in comparison to that for the stand-alone configuration. Thus, when B-DAWTs are employed as the unit of a MRS, the total power output is remarkably increased. As the number of units for an MRS is increased from two to five, the increase in power output becomes larger and larger. This is because that the gap flows between B-DAWTs in a MRS are accelerated and cause lowered pressure regions due to vortex interaction behind the brimmed diffusers. Thus, a MRS with more B-DAWTs can draw more wind into turbines showing higher power output..
4. Koichi Watanabe, Yuji Ohya, Multirotor Systems Using Three Shrouded Wind Turbines for Power Output Increase, Journal of Energy Resources Technology, Transactions of the ASME, 10.1115/1.4042971, 141, 5, 2019.05, Brimmed-diffuser augmented wind turbines (B-DAWTs) can significantly increase the performance of the rotor. Multirotor systems (MRSs) have a lot of merits such as significant saving mass and overall cost of the wind turbine system. In the present research, B-DAWTs are studied in a MRS. In wind tunnel experiments, the power output and aerodynamics of three B-DAWTs placed in close vicinity have been investigated. The results show a significant increase of up to 12% in total power output of the MRS with B-DAWTs compared to the sum of the stand-alone (SA) same turbines. The accelerated gap flows between B-DAWTs in a MRS cause lowered pressure regions due to vortex interaction behind the brimmed diffusers and draw more wind into turbines..
5. Power Output and Drag Characteristics of Multi Rotor Wind Turbine System Using Brimmed-Diffuser Augmented Wind Turbines (in Japanese).
6. Improvement of Power Output of Straight-Bladed Vertical Axis Wind Turbines with Surrounding Structures (in Japanese).
7. Koichi Watanabe, Yuji Ohya, Takanori Uchida, Tomoyuki Nagai, Numerical Prediction and Field Verification Test of Wind-Power Generation Potential in Nearshore Area Using a Moored Floating Platform, Journal of Flow Control, Measurement & Visualization, DOI:10.4236/jfcmv.2017.52002, Vol.5, No.2, pp.21-35, 2017.04.
8. Yuji Ohya, Jumpei Miyazaki, Uli Goeltenbott, Koichi Watanabe, Power Augmentation of Shrouded Wind Turbines in a Multi-Rotor System, The American Society of Mechanical Engineers, Journal of Energy Resources Technology, DOI:10.1115/1.4035754, Vol.139, Issue5, 051202, 2017.02.
9. Yuji Ohya, Takashi Karasudani, Tomoyuki Nagai, Koichi Watanabe, Wind Lens Technology and Its Application to Wind and Water Turbine and Beyond, Journal of Renewable Energy and Environmental Sustainability, DOI:10.1051/rees/2016022 , Vol.2, 2, 2017.01.
10. Yuji Ohya, Masaki Wataka, Koichi Watanabe, Takanori Uchida, Laboratory Experiment and Numerical Analysis of a New Type of Solar Tower Efficiently Generating a Thermal Updraft, Energies, DOI:10.3390/en9121077 , Vol.9, Issue12, 1077, 2016.12.
11. Koichi Watanabe, Shuhei Takahashi, Yuji Ohya, Application of a Diffuser Structure to Vertical-Axis Wind Turbines, Energies, DOI:10.3390/en9060406, Vol.9, Issue6, 406, 2016.05.
12. Application of a Wind Acceleration Device to Vertical Axis Wind Turbines (in Japanese).
1. The Combination Performance of Heat and Wind Created by a Wind Solar Tower.
2. The Combination Performance of Heat and Wind Created by a Wind Solar Tower.
3. Numerical Simulation of Thermal Updraft Generated by a Wind Solar Tower.
4. Koichi Watanabe, Yuji Ohya, Energy conversion efficiency of a wind solar tower, Wind Energy Science Conference 2019 (WESC2019) , 2019.06, A wind solar tower (called WST) is one of wind-power generation plants. WST is composed of four parts; a transparent thermal collector on the ground, a tall tower at the center of the collector, a vortex-generation plate on the top of the tower, and a wind turbine inside the tower. The tower utilizes an updraft generated in the tower to rotate its turbine. Two mechanisms below exist for generating the updraft;
i) Solar energy warms the air inside the collector. Then the warm air rises in the tower with its buoyancy.
ii) Wind energy generates a vortex above the tower. The low pressure of the vortex sucks up the air inside the tower.
Although the first mechanism has already used in a traditional solar tower, the second one is our new idea. Generally, renewable-energy power plants are expected to increase their outputs per unit area. WST has a chance of becoming the most powerful device, due to its advantage over the hybrid utilization of renewable energies. Then we have tried to optimize the tower shape for the further enhancement of its power output.
Our previous study showed that a diffuser-shaped tower with a semi-open angle of 4° was efficient. The diffuser-shaped tower demonstrated about 4-5 times power output as large as a cylindrical tower. Recently, we conducted some experiments using towers in several varieties of aspect ratios (AR = h/D). We supplied heat energy to the towers in laboratory experiments and wind energy to them in wind tunnel experiments respectively. In the case of heat utilization, the tower with larger aspect ratio tended to show the high heat energy conversion efficiency (=ηH). Similarly, in the case of wind utilization, the tower with a larger aspect ratio increased the wind energy conversion efficiency (=ηW). However, the tower with the vortex-generation plate achieved high ηW regardless of its aspect ratio. This result suggests that wind energy enables us to gain a large power output with a low tower. The low tower will be built at relatively low cost and contribute to the growth of renewables..
5. Koichi Watanabe, Yuji Ohya, Power output enhancement of a ducted wind turbine with various shapes of brim, Wind Energy Science Conference 2019 (WESC2019) , 2019.06, A brimmed-diffuser augmented turbine (called Wind Lens Turbine: WLT) utilizes vortices around the brim actively to enhance its power output. However, the vortices are usually unstable and asymmetric. This study attempted to stabilize the vortices and enhance the power output of WLT. Polygonal brims were investigated in wind tunnel experiments and numerical simulations. As a result, ideas achieved 2% increase of the power output compared with standard (round) WLTs. The numerical simulation unveiled a periodicity existed in the fluctuating vortex structure on a circular brim. Polygonal brims with the same periodic scale were able to suppress the vortex fluctuation and stabilize the vortices effectively. Besides, the larger brim tended to encourage the stabilizing effects..
6. Yuji Ohya, Koichi Watanabe, Multi-rotor systems in staggered arrangements using diffuser augmented wind turbines, Wind Energy Science Conference 2019 (WESC2019), 2019.06, Multi-rotor systems(MRSs) are defined as single structures containing more than one rotor. For more power output of MRSs, we examined staggered arrangements of MRSs with brimmed-diffuser augmented turbines (Wind Lens Turbine (WLT)). When the central turbine on staggered arrangement was moved in upwind direction, the output increase was close to a single unit’s value, while the total output of MRS did not change a lot. The changes on the individual power output increases were thought to be due to flow pattern changes of accelerated gap-flows among them. The results showed that staggered arrangements have a potential to make flows around the turbines more effective for their power output..
7. Influence of aspect ratio of Wind Solar Tower to its wind collection performance.
8. Performance of Wind Solar Tower that utilizes solar heat and wind simultaneously.
9. Field experiment on power output of multi rotor system using diffuser augmented wind turbines.
10. Research on the efficiency improvement of wind lens turbines for staggered arrangement.
11. Taiga Murakami, Takanori Uchida, Koichi Watanabe, Yuji Ohya, Researches on the efficiency of multi wind turbines with brimmed diffuser on the staggered arrangement, 10th Kyushu Univ.-KAIST Symposium on Aerospace Engineering, 2018.12.
12. Koichi Watanabe, Takanori Uchida, Yuji Ohya, Wind solar tower harnessing wind energy and solar energy, The 29th International Symposium on Transport Phenomena (ISTP29), 2018.11, Wind Solar Tower is a new power-generating device using renewable energies. A similar conventional device known as a solar chimney has utilized the solar energy alone. Besides, its generation efficiency was very low. Therefore, we largely improved the generation efficiency by arranging the shape of the tower from the cylinder type to the diffuser type. At the same time, we attached a vortex generation plate on the tower. The plate enabled the tower to utilize not only solar energy but also wind energy. The simultaneous utilization of renewable energies with one device is notable discovery. We tried the optimization of the tower shape by laboratory experiments (including wind tunnel ones) and numerical analysis. As a result, we found that the diffuser tower with 4° is the optimal shape, and we succeeded in wind-converging whose speed inside of the tower was faster than upper-wind speed, using the vortex generator. Numerical analysis contributed significantly to clarify the mechanisms. Eventually, we demonstrated the simultaneous utilization of the renewable energies in field experiments..
13. Koichi Watanabe, Yuji Ohya, Effective utilization of wind energies by wind solar tower, Grand Renewable Energy 2018 International Conference and Exhibition (GRE2018), 2018.06.
14. Yuji Ohya, Koichi Watanabe, A new wind turbine system using multiple rotors with brimmed diffusers (Multi-Lens Turbine), Grand Renewable Energy 2018 International Conference and Exhibition (GRE2018), 2018.06.
15. Yuji Ohya, Koichi Watanabe, Multi-Rotor System Using Brimmed-Diffuser Wind Turbines for Power Output Increase, 15th International Energy Conversion Engineering Conference, AIAA Propulsion and Energy Forum, 2017.07.
16. Koichi Watanabe, Yuji Ohya, A Wind Solar Tower Harnessing Sunshine and Wind Energies, Wind Energy Science Conference 2017 (WESC2017), 2017.06, [URL], In 1989, a pilot plant of a solar chimney was erected in Manzanares, Spain, to evaluate the feasibility of the solar tower as a new source of renewable energies. Since then, the solar tower was discussed in the press and academic research, but no commercial plant succeeded the development. A reason for this could be very low efficiency, i.e., very low power output compared with other renewable energy production systems. However, a solar tower that can generate electricity in a simple structure, and enables easy and less costly maintenance, has considerable advantages. A solar tower consists mainly of three components. The collector area is a glass roof, above ground with increasing height towards the center. Attached to the center of the collector is a vertical tower inside which a wind turbine is mounted at the lower entry into the tower. When solar radiation heats the ground through the glass roof, the uprising warm air is guided to the center into the tower.
To improve the efficiency in power production for a solar tower, we focused on the shape of the tower and modified the structure from a conventional cylindrical type to a diffuser-type tower. This modification is considered to combine two mechanisms. Firstly, the mechanism to “Generate Wind” which becomes effective when solar thermal energy is available, and secondly the mechanism to “Converge Wind” which becomes effective when wind passes the top end of the diffuser even when solar thermal energy is not available. For a new power generation system using both of wind and solar energies, we named “Wind Solar Tower (WST)”. By combining these two independent concepts which complement one and another, a profitable energy availability power generation system is expected, that is capable of a larger power generation compared to the original solar tower prototype and much larger capacity factor.
As a result of fluid dynamic shape optimization, a diffuser-shaped tower shows an increase in the updraft speed of a scaled model. A remarkable improvement in the power output of the internal wind turbine as much as 4-5 times compared to the cylindrical tower is obtained (Fig.1). Based on the two kinds of laboratory experiments, we are now making a field experiment using a prototype of WST with 10m tower height (Fig.2). We have elucidated that the thermal updraft speed is proportional to the root of tower height and temperature difference between the collector inside and the ambient aloft. In the present research, we propose a new renewable energy device that can harness both sunshine and wind energies. It is now showing a high potentiality, reaching 50 times in power output compared to the Manzanares. Furthermore, almost similar amount of power output by wind aloft over the tower is expected depending on wind condition of the site..
17. Yuji Ohya, Akio Munakata, Koichi Watanabe, Power Output Performance of Clustered, Diffuser Augmented Wind Turbines - Multi Rotor System Using Wind-Lens Turbines -, Wind Energy Science Conference 2017 (WESC2017), 2017.06, [URL].
18. Shuhei Takahashi, Yuji Ohya, Takashi Karasudani, Koichi Watanabe, Numerical and Experimental Studies of Airfoils Suitable for Vertical Axis Wind Turbines and an Application of Wind-Energy Collecting Structure for Higher Performance, The Fourth International Symposium on Computational Wind Engineering (CWE2006), 2006.07.
19. Koichi Watanabe, Yuji Ohya, Takashi Karasudani, Kimihiko Watanabe, Development of a Wind Turbine Generating High Output Power, The 5th Korea/Japan Joint Workshop on Aeronautics and Astronautics, 2004.07.
Membership in Academic Society
  • Japan Wind Energy Association
  • Japan Association for Wind Engineering
  • The Japan Society of Fluid Mechanics
  • The Japan Society for Aeronautical and Space Sciences
Educational Activities
Department of Aeronautics and Astronautics Graduate School of Engineering