Recently interest in vertical axis turbines has resurged due to its different perks in comparison to its cousin the horizontal axis turbines. In the past horizontal axis turbines have always been favored as a research subject. The vertical axis turbine has unique pros and cons.
Tangential force is not generated constant through the whole rotation and causes torque ripple due to the blades passing in and out of torque-generating regions. The torque fluctuation increases the difficulty to manage generator function and structural integrity. The torque needed to start the turbine is also higher than for horizontal axis turbines. However the vertical axis turbine still holds many advantages.
The generator may be stored above the water surface and can be driven directly by the shaft. Only a single bearing is required underwater. The turbine also rotates in the same direction independently from the flow direction. Due to its vertical design it is possible to stack multiple turbines. Vertical axis turbines can be scaled easily. For this reason a trend to bring this technology to rural areas that do not have access to the electrical grid in developing countries is gaining importance. This technology is feasible for electricity generation and a cheaper alternative to the conventional combustion of fossil fuels. However this technology is still a work in progress and has not reached its maturity.
Blade profile and connection type
Several different blade profiles with their respective manufacturing process were considered for the vertical axis turbine. NACA 00XX manufactured as a turned part riveted with an Aluminium sheet metal were discarded since its manufacturing process was too complicated for the individual production of the prototype.
A concept with helical blade profiles which was thought to have a steadier torque was also examined. This idea was discarded since the manufacturing process is very complex and expensive.
A concept with 60 ° shifted half-length blade profiles was also examined. The idea was to gain the effects of helical blade profiles with easy-to-manufacture blade profiles.
The torque however was not steadier and the initial torque was also high. The additional cantilever arms produced too much resistance. This concept was therefore also discarded.
A different 3 bladed concept with only one cantilever arm attached to the mid-section of each blade turned out less sturdy and efficient than the current version of the prototype and was therefore also discarded.
Symmetrical NACA 00XX blade profiles made of wood were used in the final prototype because of easy manufacturing, low productions costs, availability in the international market and the fact that it is a well tried and tested design.
Different concepts for the connection type were tested. Vertical screws parallel to the driveshaft perforated the blade profile weakening it in the process. The structural integrity of the blades was compromised. Horizontal screws perpendicular to the blades proved to be much more durable for the blades. Cantilever arms are only present at both ends of the blade to reduce resistance. A mid-level cantilever arm only produced more resistance decreasing efficiency and generating unneeded sturdiness.
The NACA 00XX blades can be manufactured with different production possibilities. Since NACA blades are so common it is possible to buy them as extruded yard goods and later cut into the needed length. This production possibility however can be less optimal if customized blades are being used. Pressing or casting the blades is suited for series production. For the prototype grinded wood profiles are recommended. The blades manufactured as turned part riveted with an Aluminium sheet metal should also be considered for series production. For series production with standard blade profile, extruded yard goods are recommended.
To optimize the efficiency of the turbine multiple features and enhancements of the turbine were researched and tested. The research work PARAMETRIC CHARACTERIZATION OF AN EXPERIMENTAL VERTICAL AXIS HYDRO TURBINE from Rawlings 2008 from the University of British Columbia proved to be invaluable.
Rawlings compared different arm profiles and came to the conclusion that the NACA 00XX profile is the superior choice.
The arms connected to the ends of the blades also increase the efficiency of the turbine.
Rawlings also inspected the influence of the angle of attack (AoA). AoA = 3° is optimal for TSR < 2.35 and AoA = 5° for TSR > 2.35. Changing the AoA between 3° and 5° increased the Ck value by over 21%.
Rawlings tested using end plates on the blades to inspect the possibility of increasing efficiency when cantilever arms were mounted at the ¼ chord positions.
NACA profiles were shown to be more efficient than circular end plates. The end plates increased the Ck value between 12% and 16% depending on the flow velocity. (16% for 1.5 m/s and 12% for 2 m/s.)
Rawlings also examined the symmetry of the blade profiles and came to the conclusion that cambered blades have superior efficiency. The combination of cambered blades and an angle of attack of 5° increased the efficiency even more.
Rawlings investigated the effects of a shaft fairing and it was shown that the turbine efficiency decreased by approximately 6%.
It´s interesting to note that Rawlings found that a single blade configuration increases its Ck value with higher TSR. The 3 blade configuration reaches its maximum at a TSR of 2.55.
In the following chapter the basic physical quantities will be described. The tip speed ratio (TSR) is defined as . It is the ratio between the tangential speed of the tip of the blade and the actual velocity of the fluid. The revolutions per minute of the turbine depends on the tip speed ratio and is defined as. Water power is the total energy that passes through the turbines cross section and is defined as. Power extracted (based on Ck) is the multiplication of a global Ck and the water power:
A global Ck = 0.35 was used for this table. The torque extraction based on the power extracted with a global Ck is defined as. A local Ck that depends on the TSR can be determined experimentally. The local Ck is more accurate than the global Ck, especially for lower TSRs. The same formulas can be applied with the local Ck to calculate more accurate values:
The following table includes the values for the Rotor and Rawlings prototype in comparison. It´s important to note that Rawlings turbine is larger.
RAGHEB calculated the optimal. For a 3 bladed rotor. According to RAGHEB if the blade profile is designed with care, the optimal tip speed ratios may be about 25-30 percent above these optimal values. These highly efficient rotor blade designs increase the rotational speed of the blade rotor therefore generating more power.
For the prototype however the generator predetermined the frequency. Angular velocity is defined as . With predetermined by the generator and a tip speed is calculated. Thus the optimal TSR for the rotor with the generator used is. Tip speed ratios depend on the particular turbine design used, the rotor airfoil profile used, as well as the number of used blades. Since the technology has only been recently researched upon again, there is very limited literature on this subject. Solidity is defined as . On the Excel sheet ROTOR CALCULATIONS different solidities may be tested and its corresponding power and rpm curves can be plotted. Figure 10 shows the dependency of TSR and solidity. For more detailed information there are multiple papers and other research work that have been compiled during research.