Rotor module
Rotor Module
Introduction
Why a vertical axis turbine?
Interest in vertical axis turbines has recently resurged due to their different advantages compared with horizontal axis turbines, which have long been favored as a research subject. The vertical axis turbine has unique pros and cons.
Tangential force is not generated constantly throughout the rotation and causes torque ripple, as the blades pass in and out of torque-generating regions. The torque fluctuation makes it more difficult 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 can be placed above the water surface and driven directly by the shaft. Only a single bearing is required underwater. The turbine rotates in the same direction independently of the flow direction. Due to its vertical design, multiple turbines can be stacked, and vertical axis turbines can be scaled easily. For this reason, bringing this technology to rural areas in developing countries without access to the electricity grid is gaining importance. It is feasible for electricity generation and a cheaper alternative to burning fossil fuels – however, the technology is still a work in progress and has not yet reached maturity.
Blade profile
Blade profile
Several blade profiles with their respective manufacturing processes were considered. NACA 00XX profiles manufactured as a turned part riveted with aluminium sheet metal were discarded, since the manufacturing process was too complicated for the individual production of the prototype.
A concept with helical blades, which was expected to deliver a steadier torque, was also examined – and discarded because the manufacturing process is very complex and expensive.
A concept with 60°-shifted half-length blades was intended to achieve the effect of helical blades with easy-to-manufacture profiles. However, the torque was not steadier, the starting torque was high and the additional cantilever arms produced too much resistance.
A different three-bladed concept with only one cantilever arm at the mid-section of each blade turned out less sturdy and efficient than the current prototype.
Symmetrical NACA 00XX blade profiles made of wood were used in the final prototype because of easy manufacturing, low production costs, availability on the international market and a well-tried design.
Connection type
Connection type
Different connection types were tested. Vertical screws parallel to the driveshaft perforated the blade profile and weakened it, compromising the structural integrity. Horizontal screws perpendicular to the blades proved much more durable. Cantilever arms are only present at both ends of the blade to reduce resistance; a mid-level arm only added resistance and unneeded sturdiness.
Production possibilities
Production possibilities
NACA 00XX blades can be produced in different ways. Since NACA profiles are common, they can be bought as extruded yard goods and cut to length – less optimal for customized blades. Pressing or casting is suited for series production. For the prototype, ground wood profiles are recommended. Turned parts riveted with aluminium sheet metal should also be considered for series production; with a standard profile, extruded yard goods are recommended.
Improvements
Findings from research
To optimize the efficiency of the turbine, several features and enhancements were researched. The work Parametric Characterization of an Experimental Vertical Axis Hydro Turbine (Rawlings, 2008, University of British Columbia) proved invaluable.
- Comparing different arm profiles, the NACA 00XX profile proved to be the superior choice.
- Arms connected to the ends of the blades also increase the efficiency.
- Angle of attack (AoA): 3° is optimal for TSR < 2.35 and 5° for TSR > 2.35. Changing the AoA between 3° and 5° increased the Ck value by over 21%.
- End plates on the blades (arms mounted at ¼ chord): NACA profiles were more efficient than circular end plates; end plates increased Ck by 12–16% depending on flow velocity (16% at 1.5 m/s, 12% at 2 m/s).
- Cambered blades have superior efficiency – even more so combined with an AoA of 5°.
- A shaft fairing decreased turbine efficiency by approximately 6%.
- A single-blade configuration increases its Ck with higher TSR; the three-blade configuration reaches its maximum at a TSR of 2.55.
Physical quantities
The basic physics
The tip speed ratio (TSR) is the ratio between the tangential speed of the blade tip and the flow velocity:
λ = ω · R / v
The rotational speed of the turbine depends on the tip speed ratio:
n = 60 · λ · v / (2π · R)
The water power is the total power passing through the turbine’s cross section A:
Pwater = ½ · ρ · A · v³
The extracted power is the product of a power coefficient Ck and the water power (a global Ck = 0.35 was used for the calculations):
P = Ck · Pwater
The resulting torque is:
M = P / ω
A local Ck that depends on the TSR can be determined experimentally; it is more accurate than the global Ck, especially at low TSR. According to RAGHEB, the optimal tip speed ratio of a carefully designed three-bladed rotor may be about 25–30% above the theoretical optimum, increasing rotational speed and power.
For the prototype, however, the generator predetermined the rotational speed: from n follow the angular velocity ω = 2π · n / 60 and the tip speed u = ω · R – and thus the optimal TSR for the Rotor with the generator used. Tip speed ratios depend on turbine design, airfoil profile and number of blades; literature on this subject is still limited.
Solidity describes the ratio of blade area to swept area (σ = N · c / (2 · R) for N blades of chord length c). In the spreadsheet “Rotor calculations” different solidities can be tested and the corresponding power and rpm curves plotted. Further papers are listed on the open source page.