Available online at www.sciencedirect.com ScienceDirect Energy Procedia 105 (2017) 973 – 979 The 8th International Conference on Applied Energy – ICAE2016 Cross-Axis-Wind-Turbine: A Complementary Design to Push the Limit of Wind Turbine Technology Wen-Tong Chonga,*, Kok-Hoe Wonga, Chin-Tsan Wangb, Mohammed Gwania,c, Yung-Jeh Chua,Wei-Chin Chiaa, Sin-Chew Poha a Deparment of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia. b Deparment of Mechanical and Electro-Mechanical Engineering, National Ilan University, Ilan 260, Taiwan, R.O.C. c Department of Physics, Kebbi State University of Science and Technology, Aliero, 1144, Kebbi State, Nigeria. Abstract Situations such as low wind speed, high turbulence and frequent wind-direction change can reduce the performance of horizontal axis wind turbine (HAWT). Certain vertical axis wind turbine (VAWT) designs have the ability to operate well in these harsh operating conditions but they possess low power coefficient generally. In order to tackle the mentioned problems, a novel cross-axis-wind-turbine (CAWT) is conceptualized to extract wind energy from both the horizontal and vertical directions of the on-coming winds to maximize the wind energy generation. The CAWT consists of three vertical blades and six horizontal blades arranged in cross axis orientation. Initial testing showed that maximum RPM generated by the CAWT is 166% higher than the VAWT under the same experimental conditions with well-improved starting behavior. Computational Fluid Dynamics (CFD) analysis was done to illustrate the flow field of the deflected and channeled air stream by omni-directional shroud. The air stream deflected upwards by the guide vane interacts with the horizontal blades. The CAWT is applicable in a wide variety of locations, creating significant opportunities for the use of wind energy devices and therefore alleviating dependencies on fossil fuel. © Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ©2017 2016The The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. keywords:cross-axis-wind-turbine, efficiency, renewable energy, urban energy system, wind energy, offshore wind turbine. 1. Introduction A wind turbine is a device that converts energy from the wind into electrical power. There are basically two types of wind turbine; the horizontal axis wind turbine (HAWT) and the vertical axis wind * Corresponding author. Tel.: +6012-723 5038; fax: +603- 7967 5317. E-mail address: [email protected], [email protected] (Wen-Tong Chong) 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.430 974 Wen-Tong Chong et al. / Energy Procedia 105 (2017) 973 – 979 turbine (VAWT). The HAWTs are very effective in generating electricity from the wind [1] but they are not without their problems, such as the need for yaw mechanisms, regular maintenance and repair for the transmission, additional expenses in reinforcing the tower structure supporting the heavy nacelle, maximizing the diameter of the rotor and the number of rotor blades, dangerous to surrounding animals and birds, high degree of noise and the rotor must face the wind direction for effective power extraction [2]. On the other hand, the VAWT is deemed more suitable to be used in urban areas [3]. The VAWT can be scaled down easily and still harness wind energy efficiently in urban areas due to its gearbox and generator situated at lower site and its rotor size can be increased or decreased horizontally without affecting its height. Yawing mechanism is not required by the VAWT as it can harness wind energy from all directions. This results to a lower manufacturing and maintenance costs because of the simpler structure due to elimination of yaw mechanisms. The VAWT might harness less wind energy than HAWT in steady wind but it is fairly efficient in capturing rapid changing wind such as gusts. The low operating rotational speed of VAWT ensures safe flight of birds and also produces low level of noise. Despite the VAWT general superiority in comparison with the HAWT, the VAWT also have its disadvantages such as the relatively lower efficiency (e.g. Savonius rotor) because the wind strikes on both sides of the rotor blade, i.e. one following the wind direction and the other which counters it, thereby neutralizing part of the available wind force. The other disadvantage of the VAWT is the inability of the rotor to start by itself (e.g. Darrieus rotor) [4]. Due to these various disadvantages of both of the VAWT and the HAWT, the main objective of the project is to overcome the drawbacks of both the VAWT and the HAWT by introducing cross-axis-windturbine (CAWT) that can overcome the disadvantages of each wind turbine type while being capable of a maximum exploitation of wind power irrespective of the direction of the wind blowing, without necessitating any type of orientation mechanism and furthermore provides a better self-starting capabilities. The CAWT took the same space as VAWT but consist of more lift force generation surfaces. The disadvantages in terms of on-coming wind directions for HAWT and VAWT are illustrated in Fig. 1. The HAWT shown in Fig. 1 relies on one horizontal wind direction, therefore requiring a yaw mechanism to rotate the wind turbine. Although the VAWT shown in Fig. 1 is an omni-directional wind energy device, it is just limited to one direction only with winds coming from the horizontal direction. As discussed, the wind conditions in urban areas require specially designed wind turbine to maximize the potentials of wind energy, hence the novel cross-axis-wind-turbine is proposed. Fig. 1. Comparison between wind direction for HAWT and VAWT. Wen-Tong Chong et al. / Energy Procedia 105 (2017) 973 – 979 2. Design concept The cross-axis-wind-turbine (CAWT) comprises of a supportive frame, a turbine rotor assembly mounted on the supportive frame and rotates on its vertical axis. For converting kinetic energy caused by the movement of the turbine rotor assembly to electrical energy and mechanical energy; an electric generator is connected to the turbine assembly. The CAWT has three main vertical blades that are connected to the six horizontal blades via specially designed connectors. This arrangement forms the cross-axis-wind-turbine. The significant advantage of the CAWT is that it can function with air flow that is omni-directional from the sides for the vertical axis wind turbine, and from the bottom of the turbine through the horizontal axis blade (see Fig. 2). The horizontal blades act as the radial arms of the CAWT, connecting the hub to the vertical blades. The vertical wind flow, either created by the building (see Fig. 3) or a guide-vane structure, interacts with the aerofoil-shaped arms. Connectors are used to couple horizontal airfoil blades to vertical airfoil blades. Each horizontal blade is arranged at an upward angle over horizontal plane. Incident horizontal wind from any direction can be harvested by vertical blades. Vertical air stream from the omni-directional shroud can be harvested by horizontal blades which improves self-starting capability of turbine at the same time produces aero-levitation force. The aero-levitation force reduces the bearing frictions in the generator, hence extending the lifespan of the wind turbine. Fig. 2. General arrangement of the CAWT (arrows indicates the wind directions). (a) (b) Fig. 3. (a) CAWT on top of building and; (b) CAWT with guide vane structure. 975 976 Wen-Tong Chong et al. / Energy Procedia 105 (2017) 973 – 979 3. Initial lab test and Computational Fluid Dynamics (CFD) simulation Initial testing was performed to demonstrate the capability of the CAWT compared to a conventional straight-bladed VAWT. The test was done on a mock-up roof-top where the CAWT and the VAWT are mounted at a height of 100 mm above the roof of the building as shown in Fig. 4. Two sets of tests were carried out for the CAWT and the conventional VAWT. Fig. 4 shows the comparison of rotational speed performance between the CAWT and the VAWT. The RPM of both of the wind turbines increases linearly with time until it reaches a maximum value of 609 rpm and 229 rpm at t = 223 s and t = 161 s for the CAWT and VAWT, respectively. From the initial test, the maximum RPM generated by the CAWT is 166% higher than the VAWT under the same experimental conditions. The increase in maximum RPM proved that the CAWT harnesses more wind energy than the conventional VAWT. (a) (b) Fig. 4. (a) Experimental set up. (b) Experimental test result (graph rotational speed against time). A preliminary CFD simulation was also carried out on the prospect of using a guide-vane shroud structure with the CAWT (as shown in Fig. 5). The concept of using the omni-directional shroud which its function is similar to the one used by Chong et al. [5] that forms the outer covering of the turbine but direct the wind from all directions to interact with the horizontal blades. The shroud consists of a series of deflectors that are shaped to capture the wind (free stream velocity = 8m/s) and channeling it upwards. The vector plot in Fig. 5 shows that the horizontal wind is deflected far above the guide vane. Thus, this deflected wind is expected to interact with the mentioned horizontal blades and provide extra power to the CAWT. Wen-Tong Chong et al. / Energy Procedia 105 (2017) 973 – 979 Fig. 5. CAWT model, particle streamlines and vector plots of CFD simulation (k-omega, SST) of a guide-vane structure for capturing, accelerating and deflecting the oncoming wind towards the bottom of the CAWT. 4. Potential applications and impacts The CAWT is proposed to be used in harvesting off-shore wind power. The HAWT, VAWT and CAWT in off-shore applications are illustrated in Fig.6. The proposed CAWT is able to float on water surface by its guide-vane serving as buoy. The CAWT is more stable when compared to HAWT as its center of gravity is expected to be located at lower site especially during rough sea profiles in bad weather. The CAWT power extraction performance at off-shore is expected to out-perform the VAWT also as higher torque is available from the guided wind produced by the omni-directional guide vane for which it is absent in most VAWT design. CAWT also surpasses HAWT in terms of total power extraction during rapid changing wind direction where CAWT is still able to continue harvesting energy while HAWT loses its function momentarily due to yaw error. The extra cost for modifying horizontal struts of the existing VAWT into airfoil blades and omni-directional guide vane installation are worthy for the sake of more power extraction in wind turbine farm. Detailed techno-economic cost-benefit analysis of CAWT will be done in future study. Fig. 6. Off-shore application of wind turbines showing HAWT, VAWT and the proposed CAWT. 977 978 Wen-Tong Chong et al. / Energy Procedia 105 (2017) 973 – 979 The CAWT system can be installed stand-alone or it can be integrated with building for power generation as shown in Fig. 7. The installation is easy and without wire trenching on the ground, and the electric power generated can be supplied to the user for any application. Such on-site power generation especially for urban areas and islands, represents a wide market, example, the islands country (Philippines & Indonesia). And it is suitable for location away from national grid line where potential installation locations include high-rise buildings in cities, fish farm, highlands and resorts. (a) (b) Fig. 7. (a) Artist’s impression of standalone CAWT along the beach; (b) CAWT integrated with the building structure 4. Conclusion The initial results from experiments showed that the proposed CAWT has the potential to give strong impact to the development of wind turbine industry. The maximum rotational speed of the proposed CAWT is 609 rpm which is 166% higher than the maximum rotational speed of the VAWT, 229 rpm. The increase in maximum rotational speed indicates that the horizontal blades introduced in CAWT contribute extra power generation by harnessing the deflected wind energy from the guide vane. CFD results show that the proposed omni-directional guide vane (located below CAWT) is able to deflect the horizontal wind effectively towards vertical direction to interact with the horizontal blades of CAWT. The advantages offered by this novel wind turbine compliment the strength of both HAWT and VAWT while diminishing their shortcomings as suggested in previous sections. It is expected that the study of CAWT will bring some insight into the research in wind turbine industry. Acknowledgement The authors would like to thank the University of Malaya for the research grants allocated (RU018G2016). Special appreciation is also credited to the Malaysian Ministry of Higher Education, MOHE for the Prototype Research Grant Scheme (PR005-2016). The authors would also like to thank the Centre for Research Grant Management Unit, University of Malaya for the Postgraduate Research Grant (PG1502016A) and the members in Renewable Energy and Green Technology Laboratory, University of Malaya. References [1] Pope K, Dincer I, Naterer G. Energy and exergy efficiency comparison of horizontal and vertical axis wind turbines. Renew Energ 2010;35:2102–2113. [2] Ahmed NA, Cameron M. The challenges and possible solutions of horizontal axis wind turbines as a clean energy solution for the future. Renew Sustain Energy Rev 2014;38:439–60. Wen-Tong Chong et al. / Energy Procedia 105 (2017) 973 – 979 [3] Chong WT, Pan KC, Poh SC, Fazlizan A, Oon CS, Badarudin A, et al. Performance investigation of a power augmented vertical axis wind turbine for urban highrise application. Renew Energ 2013;51:388-97 [4] Barker JR. Features to aid or enable self-starting of fixed pitch low solidity vertical axis wind turbines. J Wind Eng Ind Aerod 1983;15:369-380 [5] Chong WT, Fazlizan A, Poh SC, Pan KC, Hew WP, Hsiao FB. The design, simulation and testing of an urban vertical axis wind turbine with the omni-direction-guide-vane. Appl Energ 2013;112:601-9. Biography Dr. Chong Wen Tong, Associate Professor in University of Malaya, who focuses on renewable energy and green technology research. He has published more than 70 technical papers, filed 12 intellectual property rights and won prestigious awards and medals from international events. 979
