Tidal current power is one of the proven marine renewable energy with high cost at the early stage of commercialisation. This article presents the production of a tidal current turbine using 3D printing technology. Marine renewable energy includes tidal power, wave power, tidal barrage, ocean thermal energy conversion (OTEC) and salinity gradient power. Tidal power is predictable based on the tidal cycle to determine the flow speed at a particular location at a certain time. The high predictable characteristics of tidal current leads this type of ocean energy to be competitive compared to the others. Meanwhile, tidal current power does not require to change the environment by flooding upstream as tidal barrage. Conversion of a wind turbine to be a tidal turbine in shipyard is easier to harness the ocean water with higher density compared to wind flow. Experiences for offshore wind power and tidal current power are exchangeable with lower risk in construction and installation. Conceptual design of tidal device involves the construction of physical model with high workmanship. Numerous locations are identified to have potential in Asia, including the Straits of Malacca [1]. [caption id="attachment_32955" align="alignright" width="191"]fig-1 Fig. 1: Traditional manufacturing method for a blade[/caption] Previous studies of the team include the seabed scour of tidal turbine, feed-in-tariff, wake model and carbon nanotube. Bohai, in northern China, has traditional strengths with regard to the offshore oil and gas industry and ocean engineering manufacturers, which are currently being investigated for the potential of marine renewable energy. Research on the single turbine, arrays and field installation have received great attention all over the world such as Bahaj works on the array [2], Adcock works on the site selection [3], Whittaker’s work on the field test [4] and Lam’s work in the Straits of Malacca [5-17]. The complexity of physical models can lead to high manufacturing cost in production by using the traditional method (see Fig. 1). The complicated geometry of blades requires longer time, more manpower hours and more material wastage during the manufacturing process. A new manufacturing process is required to produce the turbine. [caption id="attachment_32953" align="alignright" width="300"]wei-haur-lam Fig 2: The authors holding the printed Savonius turbine[/caption] 3D printing technology is an additive manufacturing technology enabling to produce a physical model of tidal current turbine through building up layers by layers, as shown in Fig.2. By using 3D printing technology, the printing cost does not increase with the complexity of geometry, as long as the model can be printed. The RepRap project is pushing forward the development of 3D printing technology as the first general-purpose self-replicating manufacturing machine [4]. Fundamental knowledge of the 3D printer is recommended to read the easy-to-understand Dummies book [5]. The qualities of 3D printed models vary depending on the kind of materials used. Tymark et al. investigated the mechanical properties of butadiene styrene (ABS) and polylactic acid (PLA). The results showed the average tensile strengths of 28.5MPa for ABS and 56.6MPa for PLA with average elastic moduli of 1807MPa for ABS and 3368 MPa for PLA [6]. The printed tidal current turbine is expected to be mechanically functional in tensile application.

3D printer and material selection

[caption id="attachment_32957" align="alignright" width="195"]fig-3 Fig. 3: 3D printer[/caption] A folding Savonius turbine was printed by using the 3D printer. The dimension of the turbine is 150mm in height and 100mm in diameter with two blades. A DIY printer is used by connecting the main board to the power supply and the display screen, as shown in Fig. 3. The step motor is connected to the bars, allowing the printing within a designated area. The injecting head is attached to the bar to enable the injection of melted material. The injecting head is able to move in three directions as an ‘x, y and z co-ordinate’ system. The motion of each direction is driven by each step motor, with a limit switch to keep the motion in a designated range. The printing platform is a board on which the model stands during the printing process. After switching on the electricity power, the printing platform is warmed and the injecting head is heated to melt the filament for printing purpose. The 3D motion system brings the injecting head to the programmed location for printing. The melted material from the injecting head is accumulated layer by layer. 3D printing machines are mainly categorised based on the printing materials. The common materials are thermoplastic, nylon, gypsum, titanium alloy, rubber etc. Thermoplastic is the most popular material in model printing. Thermoplastic is accumulated during the printing process and this technology is called fused deposition modelling (FDM). ABS and PLA are two kinds of thermoplastic plastic used in FDM. PLA filament is used as the material in the physical modelling, as this material is more environmentally friendly with high biodegradable ability. The tidal current turbine was initially modelled in solid modelling and translated into gcode as discussed below. The turbine was printed physically and then hydrodynamic characteristics and mechanical functionality were tested.

Computer modelling and physical printing

[caption id="attachment_32984" align="alignright" width="173"]savonius-turbine1 CLICK TO ENLARGE Fig. 4 (left): Computational model of a Savonius turbine; Fig. 5 (middle): Computational model of inner blades; Fig. 6 (bottom): Computational model of outer blades[/caption] The Savonius turbine was initially virtually constructed to produce the computational model as shown in Fig. 4. AutoCAD was used in the construction of the model and exported as a stl file. This turbine was designed to harness tidal current power under the circumstance of large velocity range. The blades of the turbine consist of inner blades and outer blades, as shown in Figs. 5 and 6. The foldable feature of blades reduces the costs of turbine during transportation. The 3D model is in stl format and translated later into gcode format for physical printing, using Cura software. The model can be displayed in Cura as shown in Fig. 7. In addition, Cura is able to move, rotate and scale the 3D model before printing. The parameters of the 3D printer is designed and set in Cura, which includes the basic and advanced parameters. The basic parameters include layer height, shell thickness, bottom/top thickness, fill density, print speed, support type and platform adhesion type. Advanced parameters have four aspects including the machine, quality, speed and cool. Default settings can be used with minor adjustments on a case-by-case basis. The combination of the settings determines the printing quality and the printing time. Finally, the model and the settings are saved in a gcode with a time estimated by the system, as shown in Fig. 10. [caption id="attachment_32967" align="alignright" width="199"]fig-7-the-model-in-cura Fig.7: The model in Cura[/caption] The file is transferred through the SD card to print the turbine according the codes. The printing is started from layers to layers from the bottom to the top Finally, the completed model is shown (see main image, top). The moving injecting head puts the material in the designated place. Print speed decides on the height of layer, the speed of injecting head and quality requirement. Generally, higher print speed causes lower print quality. It is vital to balance print speed against print quality. Print speed is required when print quality is ensured. [caption id="attachment_32971" align="alignright" width="146"]fig-13-curling Fig. 8: Curling[/caption] Attentions should be taken to prevent curling during 3D printing (Fig. 8). The building board should be perfectly flat and at horizontal level perpendicular to the injecting head. The starting layer connected directly to the building board is ensured to stick the board firmly in order to build up the model from bottom. Two methods can be used including brim and raft.

Hydro dynamical and mechanical functionality tests

The printed blades are attached to the bearing with 8mm in inner diameter and 16mm outer diameter. A handle is printed to hold the turbine in the functionality test in the purpose-built recirculating flume as shown in Fig. 9 The test proved that the 3D printed turbine has hydrodynamical characteristics to rotate and mechanical strength to sustain the rotation as the same scaled conventional turbine. [caption id="attachment_32968" align="alignright" width="163"]fig-14 Fig. 9 Inserting bearing to enable rotation of turbine[/caption] In conclusion, a tidal current turbine has been printed using a DIY 3D printer with PLA material. Bearing was inserted to allow the free rotation of the turbine under the incoming water flow. Printed turbine has sufficient structural strength to withstand the hydrodynamic loading due to strong flowing current. The hydrodynamic capability of turbine has been shown with smooth rotation under the incoming flow in a purposed-built recirculating flume. The study concluded that 3D printing technology is able to produce a cost-effective experimental model with hydrodynamic performance and structural strength like the conventional-made turbine. Mechanical functionality of the 3D printed turbine has been proven. Authors: Wei-Haur Lam (1 2) and Yanbo Ma (1, 2) (1) State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University, People’s Republic of China (2) Collaborat Innovat Ctr Adv Ship & Deep Sea Explor, Shanghai 200240, Peoples R China Acknowledgement: The authors would like Tianjin University, University of Malaya and alma mater Queen’s University Belfast for all the support received. References: [1] HY Chong, WH Lam (2013) 'Ocean renewable energy in Malaysia: The potential of the Straits of Malacca', Renewable & Sustainable Energy Reviews, 23 (2013)169 - 178. [2] A. S. Bahaj, LE Myers, 'Shaping array design of marine current energy converters through scaled experimental analysis', Energy, 59, 83-94, 2013. [3] T. Adcock, S. Draper, 'Power extraction from tidal channels - Multiple tidal constituents, compound tides and overtides', Renewable Energy, 63, 797-806, 2014. [4] P. Jeffcoate, T. Whittaker, C. Boake, B. Elsaesser, 'Field tests of multiple 1/10 scale tidal turbines in steady flows', Renewable Energy, 87, 240-252, 2016. [5] L Chen, WH Lam (2014) 'Methods for Predicting Seabed Scour around Marine Current Turbine', Renewable & Sustainable Energy Reviews, 29, 683–692. [6] XL Lim, WH Lam, R Hashim (2015). 'Feasibility of Marine Renewable Energy to the Feed-in Tariff System in Malaysia'. Renewable and Sustainable Energy Reviews, 49, 708-719. [7] L Chen, WH Lam (2015) 'A review of survivability and remedial actions of tidal current turbines', Renewable & Sustainable Energy Reviews, 43, 891–900. [8] WH Lam, L Chen, R Hashim (2015) 'Analytical wake model of tidal current turbine', Energy, 79, 512–521. [9] L Chen, WH Lam (2014) 'Slipstream between marine current turbine and seabed', Energy (68) 801-810. [10] WH Lam, CB Roy (2014) 'Insights into Ocean Health Index for marine renewable energy', Renewable & Sustainable Energy Reviews (33) 26-33. [11] WH Lam, L Chen (2014) 'Equations used to predict the velocity distribution within a wake from horizontal axis tidal current turbine', Ocean Engineering (79) 35-42. [12] XL Lim, WH Lam (2014) 'Public Acceptance on Marine Renewable Energy in Malaysia', Energy Policy (65) 16-26. [13] L Chen, WH Lam (2014) 'Methods for Predicting Seabed Scour around Marine Current Turbine', Renewable & Sustainable Energy Reviews, 29, 683–692. [14] XL Lim, WH Lam (2014) 'Review on Clean Development Mechanism (CDM) Implementation in Malaysia'. Renewable & Sustainable Energy Reviews, 29, 276–285. [15] WH Lam, A Bhatia (2013) 'Folding tidal turbine- An innovative concept towards the new era of turbines', Renewable & Sustainable Energy Reviews, 28, 463–473. [16] KW Ng, WH Lam (2013) 'A Review on Potential Applications of Carbon Nanotubes in Marine Current Turbine', Renewable & Sustainable Energy Reviews, 28, 331–339. [17] AS Sakmania, WH Lam, R Hashim, HY Chong (2013).' Site Selection for Tidal Turbine Installation in the Strait of Malacca', Renewable & Sustainable Energy Reviews, 21 (2013) 590 - 602. [18] B. M. Tymrak, M. Kreiger, J.M. Pearce, 'Mechanical properties of components fabricated with open-source 3-D printers under realistic environmental conditions', Material & Design, 58, 242-246, 2014. [19] RepRap Machines http://reprap.org/wiki/RepRap_Machines 15-6-2016 [20] K. K. Hausman, R. Horne, 3D Printing For Dummies paperback, 1st Ed., USA: John Willey & Sons, 2014.