2009 Vertical Wind Energy Engineering Ian Duffett 009723628 Jeff Perry 200211837 Blaine Stockwood 009224597 Jeremy Wiseman - 200336428 PRELIMINARY PROJECT EVALUATION OF TWISTED SAVONIUS WIND TURBINE Initial project update of twisted savonius wind turbine to be completed by Vertical Wind Energy Engineering.
Table of Contents 1 PROBLEM DEFINITION... 1 2 BACKGROUND... 1 2.1 Wind Energy and Wind Power... 1 2.2 Wind Turbines... 1 2.2.1 Horizontal Axis Wind Turbines (HAWT)... 2 2.2.2 Vertical Axis Wind Turbines (VAWT)... 2 3 DESIGN PLAN... 5 3.1 Modelling... 5 4 FULL SCALE CONCEPT... 8 4.1 Blade... 8 4.2 Electric Generator... 8 4.3 Support Structure... 8 5 PROTOTYPE FABRICATION & TESTING... 8 5.1 Fabrication... 8 5.2 Prototype Setup and Testing... 9 6 APPENDIX A: PROJECT PROPOSAL... 11 7 APPENDIX B: PROJECT SCHEDULE... 14
1 PROBLEM DEFINITION The twisted savonius concept combines the advantages of a savonius wind turbine with the twisted design of a helical darrieus. While the proposed concept is not experimentally proven, it is theorized that such a turbine will be self starting and its helical shape will create an even torque distribution throughout a complete revolution. On suggestion of the project supervisor, this project will focus on optimizing this concept by modeling and evaluating the effect of foil design, pitch, rotation and shape of the foil face on performance. The project proposal can be found in Appendix A. Once optimized, a scaled prototype will be fabricated and tested under various wind tunnel conditions to develop a performance matrix. 2 BACKGROUND 2.1 Wind Energy and Wind Power The conversion of wind energy into various other useful forms such as electricity is known as wind power. Wind energy has historically been used directly to propel sailing ships or converted into mechanical energy for pumping water or grinding grain, but the principle application of wind power today is the generation of electricity. Large scale wind farms are typically connected to the local electric power transmission network with smaller turbines being used to provide electricity to isolated locations. Wind energy is an ample and renewable source of green energy. The wide spread distribution suitable wind patterns and the declining cost of producing wind energy makes it a viable energy alternative. The main drawback to wind generated power is that wind is erratic and somewhat unpredictable causing intermittent wind energy input. Wind energy is highly variable making it difficult to manage within commercial electricity generation. Producing irregular wind energy can require energy storage solutions, increasing cost. Wind energy, as a power source, is favoured by many environmentalists as an alternative to fossil fuels as it is plentiful, renewable, widely distributed, clean, and produces lower greenhouse gas emissions. Although the construction of wind farms is not universally welcomed due to the negative visual impact and the effect on wildlife, it remains one of the largest forms of green energy used in the world today. 2.2 Wind Turbines A wind turbine is a rotating machine which converts the kinetic energy of wind into mechanical energy which in turn can be converted into electricity. There are two basic types of wind turbines determined by the orientation of axis on which the turbine rotates. Horizontal turbines are the more commonly used, however the use of vertical axis turbines is increasing. 1
2.2.1 Horizontal Axis Wind Turbines (HAWT) The main rotor shaft of and electrical generator of a HAWT are located at the top of the tower and the blades must be pointed toward the wind. While this may be accomplished by simple wind vanes on smaller turbines, larger models require wind sensor and a motorized pivot. The turbine blades are usually located upwind to avoid the turbulence created by the tower and are made from a stiff material to prevent the blades from making contact with the tower when bending under high winds. Despite the problems of turbulence, downstream machines, while less common have been built as they have the benefit of being self-aligning to the wind, and the blades may bend which reduces the swept area, decreasing wind resistance. HAWT Advantages Variable blade pitch, which gives the turbine blades the optimum angle of attack, so the turbine collects the maximum amount of wind energy for the time of day and season. The tall tower base allows access to stronger wind in sites with wind shear. High efficiency, since the blades always move perpendicularly to the wind, receiving power through the whole rotation. In contrast, all vertical axis wind turbines, and most proposed airborne wind turbine designs, involve various types of reciprocating actions, requiring airfoil surfaces to backtrack against the wind for part of the cycle. Backtracking against the wind leads to inherently lower efficiency. HAWT Disadvantages The tall towers and blades up to 90 meters long are difficult to transport. Transportation can now cost 20% of equipment costs. Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators. Massive tower construction is required to support the heavy blades, gearbox, and generator. Reflections from tall HAWTs may affect side lobes of radar installations creating signal clutter, although filtering can suppress it. Their height makes them obtrusively visible across large areas, disrupting the appearance of the landscape and sometimes creating local opposition. Downwind variants suffer from fatigue and structural failure caused by turbulence when a blade passes through the tower's wind shadow HAWTs require an additional control mechanism to turn the blades toward the wind. 2.2.2 Vertical Axis Wind Turbines (VAWT) Figure 1 - Three Bladed HAWT The main rotor shaft of vertical axis wind turbines are arranged vertically giving them the key advantage of not having to be aligned with the wind. This type of arrangement is highly advantageous on sites where the wind direction is highly variable as VAWTs can utilize wind from varying directions. 2
The generator and gearbox of a VAWT can be placed near or at ground level, eliminating the need to be supported by a tower. This also makes them more accessible for maintenance. It can be difficult to mount vertical axis turbines on towers, and as such are often installed on a base such as the ground or a building rooftop. However, at this elevation, wind speeds are lower decreasing the amount of available wind energy. In addition, air flow near the ground can be highly turbulent which will cause vibration within the turbine leading to bearing wear. If wear is significant, this will inevitably increase maintenance costs or shorten service life. Buildings generally redirect wind over the roof which can double the wind speed, increasing the power potential of roof mounted turbines. Maximum wind energy and minimum turbulence can usually be achieved atop a tower approximately 50% of the building height. Major concerns of VAWTs are a pulsating torque created by some models and drag forces experienced as the blades rotate into the wind. This pulsating torque is infamous for its negative effect on wildlife and as such, efforts are being made to eliminate the pulsating torque and to develop more wildlifefriendly designs. 2.2.2.1 VAWT Subtypes While there are numerical concepts for vertical axis wind turbines, most are variations on the following three designs. Darrieus Wind Turbine Darrieus turbines have good efficiency but produce large torque ripple and cyclic stress on the tower contributing to poor reliability. Also, they generally require some external power source to start turning, because the starting torque is very low. The torque ripple is reduced by using three or more blades which results in a higher solidity for the rotor. Solidity is measured by blade area over the rotor area. Newer Darrieus-type turbines are not held up by guy-wires but have an external superstructure connected to the top bearing. The blades of a Darrieus turbine can be canted into a helix. Since the wind pulls each blade around on both the windward and Figure 2 - Darrieus Wind Turbine leeward sides of the turbine, this feature spreads the torque evenly over the entire revolution, thus preventing destructive pulsations. The skewed leading edges reduce resistance to rotation; by providing a second turbine above the first, with oppositely directed helices, the axial wind-forces cancel, thereby minimizing wear on the shaft bearings. 1 1 http://en.wikipedia.org/wiki/darrieus_wind_turbine 3
GiroMills and Cycloturbines A subtype of Darrieus turbine with straight, as opposed to curved, blades. The cycloturbine variety has variable pitch to reduce the torque pulsation and is self-starting. The advantages of variable pitch are: high starting torque; a wide, relatively flat torque curve; a lower blade speed ratio; a higher coefficient of performance; more efficient operation in turbulent winds; and a lower blade speed ratio which lowers blade bending stresses. Straight, V, or curved blades may be used. 2 Figure 3 - GiroMill Turbine Savonius Wind Turbine These are drag-type devices with two (or more) scoops that are used in anemometers, Flettner vents (commonly seen on bus and van roofs), and in some high-reliability low-efficiency power turbines. They are always selfstarting if there are at least three scoops. They sometimes have long helical scoops to give a smooth torque. 3 VAWT Advantages A massive tower structure is less frequently used, as VAWTs are more frequently mounted with the lower bearing near the ground. Designs without yaw mechanisms are possible with fixed pitch rotor designs. A VAWT can be located nearer the ground, making it easier to maintain the moving parts. VAWTs have lower wind startup speeds than HAWTs. VAWTs may be built at locations where taller structures are prohibited. VAWTs situated close to the ground can take advantage of locations where hilltops, ridgelines, and passes funnel the wind and increase wind velocity. VAWTs may have a lower noise signature. VAWT Disadvantages Figure 4 - Savonius Wind Turbine Most VAWTs produce energy at only 50% of the efficiency of HAWTs in large part because of the additional drag that they have as their blades rotate into the wind. Versions that reduce drag produce more energy, especially those that funnel wind into the collector area. A VAWT that uses guy wires to hold it in place puts stress on the bottom bearing as all the weight of the rotor is on the bearing. Guy wires attached to the top bearing increase downward thrust in wind gusts. 2 http://en.wikipedia.org/wiki/giromill#giromills 3 http://en.wikipedia.org/wiki/savonius_wind_turbine 4
While VAWTs' internal equipment is located on the ground, it is also located under the weight of the structure above it, which can make changing out parts nearly impossible without dismantling the structure if not designed properly. 3 DESIGN PLAN The twisted savonius wind turbine design is based on complex fluid-structure interaction. It is in this regard that computational fluid dynamic (CFD) analysis is necessary to model and simulate the fluid interaction with varying design parameters. 3.1 Modelling Through modelling varying savonius blade designs within SolidWorks 2008, Vertical Wave Energy Engineering will develop an optimal prototype concept. Design parameters which are critical to the fluid-structure interaction include: Angle of Twist, α (Figure 5) Blade Radius (Figure 6) Blade Arc (Figure 7 & 8) Swept Area (Figure 9) Bottom Plane α Top Plane Figure 5 - Angle of Twist 5
r Figure 6 - Blade Radius Figure 7 - Circular Blade Arc Figure 3 Figure 8 - Elliptical Arc 6
Figure 9 - Swept Area Each design variation will undergo CFD analysis in COSMOSFloWorks (SolidWorks add-in) to determine the optimal design parameters, providing the highest efficiency. The CFD analysis will involve placing the wind turbine within a stream of air which has a predetermined velocity at atmospheric pressure. The torque generated on the shaft will be evaluated under varying wind conditions. The design which generates the highest torque will therefore have the potential to extract maximum energy from the working fluid. The maximum power available from the wind is governed by 3 Where: P = Power (Watts), ρ = fluid density (kg/m 3 ) A = swept area (m 2 ) = fluid velocity (m/s) Therefore, it is important to maximize the swept area available to capture the wind energy (shown in Figure 5 in red shading), and minimize the drag which acts to oppose the desired direction of rotation (shown in Figure 5 in green shading). Although wind tunnel size is an obvious restriction in terms of maximum area available for prototype testing, the prototype can easily be scaled to determine the expected energy output of the full size device based simply on larger available area to capture wind energy. Expected energy output from a wind turbine is estimated as Where: C p = coefficient of performance (estimated as 0.35 for a good design) N g = generator efficiency (80% or more for a permanent magnet generator) N b = gearbox/bearings efficiency (varies, good design may be as high as 95%) 7
4 FULL SCALE CONCEPT 4.1 Blade The blades of the twisted savonius wind turbine are used for converting the power of the wind into torque on a rotating shaft to produce output energy. The blade s unique design catches the wind from all directions forcing it to rotate. The blades are manufactured from PC-ABS thermoplastic (Polycarbonate-Acrylonitrile Butadiene Styrene). PC offers superior mechanical properties and heat resistance while ABS provides excellent feature definition and surface appeal. Both the PC and ABS contribute to a high impact strength product. The blades will be modular in construction for ease of installation and transport. The number and size of modular sections depends on overall turbine design. The blade sections will also be interlocking and keyed to centre turbine shaft. 4.2 Electric Generator The electrical generator is the device that converts the savonius wind turbine s torque (mechanical energy) into electrical energy. This is done by using electromagnetic induction, which is created by changing the magnetic field within the generator, which in turn induces a current in a surrounding wire coil circuit. The strength of the current generated relies on wind speed, gage of the coil wire, number of coil turns, number of coils, and efficiency of generators bearings. 4.3 Support Structure Depending on the size of the savonius wind turbine, the support structure changes. Small turbines which can be mounted to roof structure will be supported internally, while larger land based turbines will be supported either internally or externally by means of an aluminum tube structure. 5 PROTOTYPE FABRICATION & TESTING The prototype s design and construction is focused on turbine performance evaluation, not on electrical generators or support structure. This will allow for improved analysis of the turbine performance without introducing unknown variables such as generator efficiency and any line losses. 5.1 Fabrication Foil prototype will be constructed in sections using PC-ABS thermoplastic, the same as the full scale turbine. The prototype blade will be fabricated using the rapid prototyping machine Stratasys FDM1650. The FDM1650 uses Fused Deposition Modeling (FDM) to turn computer-aided design (CAD) geometry into foil sections that will be used for wind tunnel testing. The PC-ABS thermoplastic material is heated by the FDM head so it comes out in a semi-liquid state. The successive layers fuse together and solidify to build up an accurate, three-dimensional model of the foil design. The overall tolerance is +/- 0.005" in the X, Y, and Z axes. The Stratasys FDM1650 prototype machine used to fabricate the foils is shown in Figure 10. The Stratasys FDM1650 has a prototype build size of 254mm x 241mm x 254 mm (9.5" x 9.5" x 10") with an achievable accuracy of ±.127 mm (±.005 in). 8
Figure 10 - Stratasys FDM1650 Aluminum shafts shall be press-fitted into the top and bottom sections of the foil with low resistance bearings connected at the supporting ends of the shaft. 5.2 Prototype Setup and Testing The twisted savonius wind turbine will be installed centered and vertically within the wind tunnel with both ends of the shaft extruding through the bottom and top of the tunnel. The shaft shall have low resistance bearings connected at both ends to a support base. Instrumentation will be utilized on the top end of the shaft to record forces and power output. The testing facility is a horizontal open-circuit facility. It contains a rectangular 20.0 x 0.93 x 1.04 meter test section. This section has a windspeed range of 0 to 20 m/s. It was chosen as the test facility because of its physical location, size and speed range. The windows permitted visual observation of the testing. The instrumentation associated with obtaining turbine torque and turbine rotational speed shall be supplied by Memorial University. Performance evaluation of the savonius wind turbine drives the blade design. The blade is designed to take full advantage of the power of the wind, transferring this power into torque on a rotating shaft to produce output energy. Performance evaluation starts first during the design stage using fluid modelling software. When a blade design is selected, the second stage of performance evaluation will be conducted using the wind tunnel at Memorial University. Similarly to the performance evaluation conducted using Solidworks, the wind tunnel testing will capture the turbines shaft torque at various wind speeds. An eddy current brake and a permanent magnet dc motor setup will be implemented to record foil loading and power output during testing. 9
An eddy current brake, like a conventional friction brake, is responsible for slowing an object. Unlike friction brakes, which apply pressure on two separate objects, eddy current brakes slow an object by creating eddy currents through electromagnetic induction which create resistance. When testing begins, the turbine shall be rotated by the air motor at a very low, but constant, rotational speed to obtain the torque of the system caused by the friction in the bearings. The test matrix for the wind turbine tests shall be broken up into 2 sections, Static and Dynamic testing. All tests runs shall have data recorded for a minimum of 2 minutes once a steady-state condition is achieved. Static testing - The wind turbine shall be locked at particular angles starting at 0 degrees relative to the flow and the torque produced by the turbine will be recorded. This shall be repeated at 45 degree segments at nominal tunnel speeds of 5 and 10 m/s. Dynamic testing - The wind turbine shall be free to rotate under the load provided by the wind travelling through the tunnel at 5 to 10 m/s. Twisted Savonius Testing Matrix Test Number End Plate Wind Tunnel Speed (m/s) Test Condition 1 A 5 Static 2 A 10 Static 3 A 5 Dynamic 4 A 10 Dynamic 5 B 5 Static 6 B 10 Static 7 B 5 Dynamic 8 B 10 Dynamic 9 C 5 Static 10 C 10 Static 11 C 5 Dynamic 12 C 10 Dynamic Please note that the testing matrix is subject to change based on project analysis. 10
6 APPENDIX A: PROJECT PROPOSAL 11
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7 APPENDIX B: PROJECT SCHEDULE 14
ID Task Name Duration Start Finish uary February March April Jan 4 Jan 11 Jan 18 Jan 25 Feb 1 Feb 8 Feb 15 Feb 22 Mar 1 Mar 8 Mar 15 Mar 22 Mar 29 Apr 5 Apr 12 1 Wind Energy Research 8 days Mon 1/12/09 Wed 1/21/09 2 Concept Selection 1 day Thu 1/22/09 Thu 1/22/09 3 Preliminary Design 3 days? Fri 1/23/09 Tue 1/27/09 4 1st Meeting With Supervisor To Review Concept 1 day? Tue 1/27/09 Tue 1/27/09 Selection 5 Finalize Concept Selection 2 days? Tue 1/27/09 Wed 1/28/09 6 Design & Simulations 11 days? Wed 1/28/09 Wed 2/11/09 7 Foil Designs 10 days? Wed 1/28/09 Tue 2/10/09 8 Foil Simulations 10 days? Wed 1/28/09 Tue 2/10/09 9 Foil Selection Based On Simulations 1 day? Tue 2/10/09 Tue 2/10/09 10 Develop SDL Files On Selected Foil Design 2 days? Tue 2/10/09 Wed 2/11/09 11 Fabrication 11 days? Wed 2/11/09 Wed 2/25/09 12 Foil 11 days? Wed 2/11/09 Wed 2/25/09 13 Support Structure For Testing 11 days? Wed 2/11/09 Wed 2/25/09 14 Testing 27 days? Thu 2/26/09 Fri 4/3/09 15 Foil Outfitting 5 days? Thu 2/26/09 Wed 3/4/09 16 Testing Instrumentation Setup 5 days? Thu 2/26/09 Wed 3/4/09 17 Wind Tunnel Setup 2 days? Thu 3/5/09 Fri 3/6/09 18 Foil Testing Matrix 10 days? Mon 3/9/09 Fri 3/20/09 19 Analysis Of Testing Data For Reporting 10 days? Mon 3/23/09 Fri 4/3/09 20 Reports & Presentations 46 days? Mon 2/2/09 Mon 4/6/09 21 Report 1 - Project Update & Status 1 day? Mon 2/2/09 Mon 2/2/09 22 Interim Presentation 1 day? Wed 2/18/09 Wed 2/18/09 23 Report 2 1 day? Mon 3/2/09 Mon 3/2/09 24 Final Presentation 1 day? Mon 4/6/09 Mon 4/6/09 25 Final Report & Submission Of Logbooks 1 day? Mon 4/6/09 Mon 4/6/09 Project: Wind Schedule Date: Mon 2/2/09 Task Progress Milestone Summary Rolled Up Task Rolled Up Milestone Rolled Up Progress Split External Tasks Project Summary Group By Summary Deadline Page 1