Wind turbine system

Prototyping

Twisted Savonius V1

CAD of twisted Savonius prototype

V1 of twisted Savonius prototype

We attempted to prototype a twisted Savonius turbine to allow a more continuous application of force when the wind turbine spins. Ideally, both sails would be slightly offset to each other to allow airflow from one airfoil to the other. Such an offset can be seen in the following picture that inspired our design.

With our next prototype, we seek to combine a twisted Savonius turbine with a Gorlov turbine (Rajdeep Nath Dr. John Rajan; How to Design and Fabricate a Vertical Axis Wind Turbine: Design, Analysis, and Fabrication using Gorlov and Savonius Blades (p. 75)). We can 3D print the blades but might want to scale down the prototype to decrease printing time.

Current System

view and comment on system diagram

Equipment manuals

Aurora 3kW inverter - data sheet
Aurora wind interface box/rectifier - manual
TCMG-3kW Taechang N.E.T. Generator - online specs
3kW Discharger Breaker -
3kW Dynamic Braking Resistor - instruction manual
Anemometer
Wind direction meter

Anemometer - wind speed measuring

Anemometer data logger prototype on porch

We are using a Vortex wind sensor. One revolution per second equals 2.5 mph. Since our anemometer has a relay (a mechanical switch), it creates a switch bounce. Therefore, we need a debounce circuit. Please refer to Electrical Control Unit for the repository and more information.

Research and background knowledge

Research papers, articles, media

Aerodynamics of vertical-axis wind turbines in full-scale and laboratory-scale experiments

Araya, D. B. (2016). Aerodynamics of vertical-axis wind turbines in full-scale and laboratory-scale experiments (Doctoral dissertation, California Institute of Technology).

Analysis of Different Blade Architectures on small VAWT Performance

Battisti, L., Brighenti, A., Benini, E., & Castelli, M. R. (2016, September). Analysis of different blade architectures on small VAWT performance. In Journal of Physics: Conference Series (Vol. 753, No. 6, p. 062009). IOP Publishing. https://iopscience.iop.org/article/10.1088/1742-6596/753/6/062009/pdf

General wind turbine knowledge

A lot of these insights are from the Wind Energy Coursera course.

Wind turbine terminology

Drivetrain of the nacelle

• rotor - generates aerodynamic torque
• nacelle - converts torque into electrical power
• tower - hold nacelle and rotor blades
• foundation - hold the whole turbine in place
• VAWT - vertical axis wind turbine
• HAWT - horizontal axis wind turbine

Forces acting on VAWT and HAWT blades

Forces acting on blade

If the vertical axis turbine has blades that use lift to create a net force that spins the turbine, the forces acting on the blade are equivalent to forces on horizontal axis turbines. The aerodynamic forces on the blades are described in the two image to the right.

Vector diagram of the aerodynamic forces acting on a VAWT airfoil. Here U is the freestream velocity, ω is the angular velocity of the turbine, R is the turbine radius, Urel is the relative freestream velocity as seen by the turbine blade, and Ft(lift) and Ft(drag) are the tangential components of the lift and drag forces, i.e., Flift and Fdrag, respectively.

Wind energy extraction

Mechanical power of a wind turbine

The mechanical power of a turbine can be calculated with the equation to the right. The Betz limit on the power of a fast-spinning wind turbine (59%) describes the maximum energy that a turbine can extract from the wind. Depending on the turbine design, the Betz power coefficient changes. Turbines that are based on lift (not drag) have generally speaking higher coefficients. Please refer to the different Betz based roughly categorized by turbine types to the left.

Betz coefficients for different turbines

Rotating lift-based machines - horizontal axis wind turbine, a vertical axis wind turbine with blades (Gorlov or Darrieus)

Rotating drag-based machines - vertical axis turbines - similar to cup anemometer; Savonius (efficiency 10%-15%) could be nice for aesthetically pleasing turbine

Flying lift-based machines - pulls up sail that pulls on a cord that spins spindel

Machines using flow-induced vibrations

Towers types

Different options for tower construction

The tower needs to withstand high torque. Metal ropes can be used to keep the top of the tower in place. Furthermore, the foundation needs to be solid and deep.

General tower types:

• tubular towers
• lattice towers
• tripod towers

Financing (for large-scale projects)

Our revenue is the production of net electricity in Wh (Annual Energy Production): Capacity of Farm x 8760h x capacity factor 0.25

1 - Simple payback time (SPT)

Simplistic Calculation

• estimate of annual production in Wh
• annual revenue - production x energy sale price
• annual operating costs

2 - Net present value (NPV)

(revenue - costs) - original investment
if less than 0, probably not profitable

excel has NPV function

3 - Levelised cost of Energy (LCoE)

goal is to find cost per MWh that can be used for comparison:
(Capital Investment + Operational Costs + Decommissioning Cost)/ MWh

Statistical Analysis of Wind Speeds and Turbulence

• Navier-Stokes equation (evolution equation) to calculate wind speeds in space
• statistical analysis, spectra, turbulence intensity is obtained by mean wind speed and standard deviation of wind speeds
• every wavelength (after Fourier transform) can be thought of as a length scale
• wind vector (u, v, w)
• integral length scale; from time series, we compute the auto-correlation function
• turbulence spectra
• averaging periods of 10 min are commonly used; 30 min period for turbulence studies; the larger integral time scale, the larger should be averaging period; sampling frequencies should be much smaller than the integral time scale
• you have to detrend the time series to get rid of high-frequency fluctuation