Developing more efficient blades for small domestic wind turbines.
In the summer of 2024, I was approached by Sven Ruin and his team from Mälardalen University in Sweden. They were working on improving the efficiency of blades for small domestic wind turbines but had trouble finding a local CNC manufacturer due to holiday schedules and long lead times. After discovering CNC Craft online and seeing my experience with turbine projects and interest in open-source design, they contacted me to see if I could help. It was a challenge I couldn’t refuse.
The Blade Design
Their blade design is quite unique. Unlike traditional blades that taper towards the tip, these blades feature parallel edges with a large surface area at the tip, maximizing energy conversion while incorporating a conventional twist to achieve a tip speed ratio of 1:7.
Initially, the team wanted to use laminated larch for the blades due to its durability. However, the cost of creating large laminated blocks was prohibitive. After some discussion, we opted for Accoya, a softwood treated with acetic acid to enhance its durability and dimensional stability, making the blades more affordable without sacrificing quality.
From Design to Production
The designs were modelled by the team in Sweden and sent to me as .stl 3D files. I imported them into Blender, the open-source 3D modeling software, and used the BlenderCAM extension to generate CNC tool paths for machining the blades on my router.
These images are screenshots of the Blender 3D environment showing the tool paths generated for the blades
To prepare the blanks, they first had to be planed to match the overall size of the blades. Next, I created a fixture and secured it to the router bed to allow precise flipping of the blanks along the X-axis. After cutting the first side, I flipped each blank in the fixture and cut the second side. Once machined, each blade was removed, trimmed, and sanded. We produced four blades: three for a turbine and a fourth for durability testing. They have now been shipped to Sweden, and I’m eagerly awaiting the test results.
These images show the blade being cut, the fixture that allows the blanks to be flipped , and the blades in their final configuration.
Testing the Blades
The Swedish team plan to test the efficiency of the 1200mm turbine in a local wind tunnel. However since I already have a working wind turbine, I suggested they create a one-meter-long blade for real-life comparison with my existing blades, which they agreed to do. To facilitate meaningful comparison, I needed to install sensors to monitor wind speed and energy output.
Circuit diagram for the wind speed sensor
First, I 3D-printed a rotary anemometer incorporating a hall effect sensor and a rare earth magnet, this I then connected to a Wemos D1 Mini microcontroller. Using the Arduino IDE, I developed the code to capture wind speed data and upload it to a Thingspeak channel. Here is a link to the channel, and there is a link to the arduino code I used at the bottom of the page, although you should use it with caution.
Next, I needed a sensor to measure the turbine’s output current. For this, I used an Arduino Wifi 4, because it has a the analogue pins needed to collect variable data from the sensor, and connected it to an ACS 758 Hall effect current sensor. This in turn was connected it to the input of our water heater. Again the arduino code uploads the data to a Thingspeak channel. Here is a link to the channel, and here is another link to a channel that shows wind and current in the same graph. There is a link to the arduino code I used at the bottom of the page, although you should use it with caution.
Basic circuit diagram for the current sensor measuring input to the water heater
Both the wind sensor, and the current sensor have been installed, and once sufficient data has been collected, we’ll swap the blades and conduct a comparison. You can download the 3D model of the blade and the arduino code used to collect the data in a zip file from the button below.
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