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Modifying A Miniature Wind Turbine for An Off-Grid Campervan

In a previous article we took a first look at the Marlec Rutland Furlmatic 910-4 miniature wind turbine in an attempt to broaden our understanding of off-grid renewables. The ultimate goal in doing so was to see how feasible it would be to install wind power on our off-grid campervan project and how well it might perform in differing locations, wind speeds and overall conditions.

The challenge

During the initial testing, I was very impressed with the mechanical performance of the turbine as it responded very well to a broad range of wind speeds and turbulence. However, I was ultimately disappointed by its electrical performance, which did not yield well in what I would call typical wind conditions, as it struggled to reach a voltage that would allow it to charge our 12v leisure batteries.

Stated performance graph

Stated performance graph does not account for battery voltages

We concluded this was likely caused by a number of factors that are not necessarily unique to the Furlmatic turbine. For one, we observed that while the mechanical start-up wind speed of the turbine was very good, its terminal rotational velocity and therefore output voltage was limited even in firm winds. We determined that this was likely an aerodynamic issue characterised by the tip-speed ratio, otherwise known as the relationship between the rotational velocity of the turbine and wind speed. The required charging threshold voltage was also too high when we were using the recommended 12v leisure batteries, which meant that a lot of potential energy was not captured by the charge controller below a certain windspeed. Other issues included the need to mount the turbine to the campervan, which in its default state is quite bulky and heavier than initially expected.

Potential solutions

These are all challenges we need to overcome in order to fine-tune our wind turbine for our intended application. Luckily, our Furlmatic turbine is flexible enough that we can apply some hacks to the way it operates. In the last article, we floated two solutions that might help us with these problems. Firstly, we could improve the tip-speed ratio of the turbine so it spins faster in lower windspeeds or, lower the charging threshold voltage to capture more energy at these lower wind speeds.

attempting to improve aerodynamic drag

Before and after: attempting to improve aerodynamic drag and tip-speed ratio

The problem with the latter is that most off-grid equipment is designed for 12v, including the Marlec charge controller and our existing storage batteries, so we may need a new bank of 6v batteries and/or a 12v boost circuit to make this workable. By comparison, it would be a lot easier to improve the tip-speed ratio of the turbine by removing some of the blades. This would likely compromise our peak output power and our start-up windspeed but should improve overall yield in more typical wind conditions and with a two-blade design in particular, would allow us to lay it flat on its side for storage.

Testing

With this determined as the best option, I removed four of the blades from the turbine and attempted to recreate the conditions of our original test to compare the performance. Despite patches of very light or turbulent wind during testing, it was immediately apparent that a lot less effort was required for the turbine to get up to a higher rotational speed in lower winds. As predicted though, due to the reduced amount of torque, the start-up windspeed was marginally higher. Interestingly though, and for the same reasons, I found that the flywheel effect of the heavy rotor made the rotational acceleration slower but ultimately helped maintain inertia once up to speed.

charge controller output

Despite low windspeeds and high turbulence, the charge controller output was more consistent

Lastly, in terms of actual power yield, it was immediately noticeable how much more consistent the turbine was at pushing current into the battery, despite poor test conditions. In one instance while measuring the output, I found it necessary to apply the brakes as it was peaking over 20v in light gusts with no load!

The downside to this arrangement is that the turbine does look slightly less heroic with two blades and the peak output will probably be lower, but given its improved performance, this is an acceptable compromise. It will also be a lot easier to store, and more reliable as a renewable source in the changing conditions that it will be used.

If the turbine were needed for a more static application, I would also have liked to compare some of the other solutions, including a three-blade setup, as I suspect this would be the sweet spot of the Furlmatic turbine. However, I can now look forward to attempting to mount this as a collapsible part of my renewable collection on the campervan so stay tuned!

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A keen maker and electronic engineer with a passion for the environment, renewables, alternative transport and anything off-grid. Man with a van and founder of the Kickstart Kamper sustainable campervan project. Grassroots Education Sustainability Ambassador. Compulsive tea-drinker. BrightSpark 2017. BEng.
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