The end of September saw a major milestone completed on the wind turbine project – we finished our wind tunnel testing at the NASA-Ames research center! The experience was fantastic and so far we have been blown away by the quality and quantity of data we collected. While it is still a bit too soon to publish our complete test results here (we still have a lot of analysis to do), I nevertheless wanted to get started by explaining our test set-up and posting a few photos from the tunnel.

Test Set-up

Since the beginning of this project we have focused on two vertical-axis turbine designs: Savonius and Lenz (see photos below). The major purpose behind our wind tunnel testing was to characterize the performance of these turbines, allowing us to select the most promising design and move forward with developing the alternator that will generate the electricity. In order to characterize the turbines, we needed to collect data on how they perform in different wind speeds while under different loads.

The end result of this testing will be a series of power curves (see chart below) describing the mechanical energy generated by the turbine as a function of wind speed and turbine speed (rpm). We also explored three different angles of attack for the turbine blades on both turbine designs to see how they impacted turbine performance.

The theory behind our turbine test is this simple equation:

Power = Torque x Rotational Speed

When the wind blows it hits the turbine, causing it to spin. In spinning, the turbine converts the energy contained in the wind into mechanical power. In order for us to figure out just how much wind energy is converted into mechanical power we needed to place the turbine in a flow of wind with a known speed and then apply a known torque load to the turbine while measuring the effect of the load on the turbine’s rotational speed. Referring to the equation above, we would supply a known torque and measure the rotational speed, thereby allowing us to calculate power.

The way we mechanically applied the torque load was relatively straightforward. We coupled a DC motor to the bottom of the turbine shaft and tried to turn the motor opposite the direction of the turbine rotation (see image below). Since the torque produced by a DC motor is a function of current, we could apply a known load to the turbine by applying a known current to the motor.

This test set-up, while elegant, nevertheless proved problematic as the motor we sized proved unable to resist the amount of torque the turbine was generating and we quickly overheated the motor. Another way of applying torque to the turbine was needed, fast. Thankfully, the NASA-Ames facility is full of advanced testing labs, many of them working on rotating equipment. The day after we identified our problem we had a torque cell in-hand and connected to the turbine shaft (see image below).

This torque cell contained a strain gage mounted on a shaft that output a voltage based on how much stress was applied to the shaft. One end of the shaft was attached to the turbine and the other end had a mountain bike disc brake attached to it that could apply a drag load that would slow the turbine and provide the strain that the gage would measure. This set-up allowed us to apply our known torque load as desired and the testing went on as planned. The only downside was that we were unable to maintain a steady torque and were forced to take our readings dynamically as the torque applied and turbine speed varied. This scenario was less than ideal, but we still managed to collect a copious amount of data that should allow us to compensate for any dynamic and inertial effects.

Ultimately we collected all the data we wanted and, at first blush, the results look great! My next blog post will focus on that data and our resulting analysis. Many thanks to Malcolm Knapp, Jeremy Kimmel, Sarah Felix, and Charlie Sellers who all devoted many days to the wind tunnel testing. Other Engineers Without Borders volunteers that played an important role are Jerry Pugh, Matt McLean, and Ann Torres. Finally, none of this would have happened without the help of Jose Navarette, Nili Gold, and Farid (all of whom work at the NASA-Ames facility), the technicians running the tunnel, and the generous donation of the facility by the US Army Aeroflightdynamics Directorate (which leases the tunnel from NASA).

To see more images of our wind tunnel tests, check out our Wind Tunnel album and watch the video below: