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Wind power is seen as a vitally important component in the continued development of the sustainable energy mix that will take us away from the fossil fuel alternatives that are now reportedly nine times more expensive to use. Granted, while there are still teething issues with renewable storage the higher cost of living may well be with us for some time, so in this regard, I wanted to discuss whether it is possible to take advantage of these technologies on a smaller scale using wind-based microgeneration.
In a previous article we reviewed the Marlec Rutland FM910-4 Windcharger, a miniature wind turbine with some impressive specs and performance but in need of fine-tuning outside of its ideal operating conditions and where we still intend to mount it to an off-grid campervan project. In this regard, this article aims to address some common issues that the prospective off-grid enthusiast may face and any observations that will help optimise turbine performance for a broader range of wind conditions.
State of the market
As a general rule, domestic wind turbines do take more trial and error to set up than their equivalent solar systems. However, this is more a statement on just how well solar power works as a renewable energy source and wind is often overlooked as a result. This indifference has also been compounded by a market that is plagued with dubious performance information which further emphasises the need to understand the real-world characteristics of miniature wind turbines before making any decisions.
Typical horizontal and vertical axis wind turbine designs
There are generally two main types of wind turbine to consider, vertical and horizontal axis. Vertical axis turbines can be reliable and cost-effective due to their simplistic construction, where they only need one moving part but have a lower maximum efficiency due to increased drag. Horizontal axis turbines, on the other hand, are a lot more efficient, and as a result, are the most popular type used in commercial wind farms, but are more complex in their construction, where they require at least two moving parts.
This particular article will focus on the specific characteristics of an off-the-shelf horizontal axis wind turbine, although any observations made will likely apply to both types. However, if you are interested in wind power, vertical turbines can be more accessible to the undecided, where they are a lot easier to build from scratch and experiment with, which may well be something we explore in a later article.
It’s important to understand the difference between the stated output power and useable power when evaluating miniature wind turbines, as the quoted figures can be a bit misleading. Ultimately, the practicality of any miniature wind turbine is determined by its ability to convert mechanical energy into electrical energy that can be stored, but this is not always as intuitive as it sounds.
Performance graph does not show useable output power
In the case of the Furlmatic turbine, we observed that it needed to produce a minimum of around 13v before it could start charging a standard 12v leisure battery. This threshold means that while the turbine is able to start generating power at the low cut-in windspeed of 3 m/s, the usable power and subsequent useful windspeed was actually a lot higher. In this situation, it’s easy to see how large turbines, that can more easily produce higher voltages at lower wind speeds, would be more efficient.
Mean annual windspeed graph assumes ideal operating conditions
Already we can see how much the real-world performance of miniature wind turbines can differ from any initial expectations, so it’s important to know their limits. However, it is also worth noting that we have a particular interest in understanding how these turbines work outside of their ideal operating environments in order to make them more accessible. For example, during testing, we observed the Furlmatic turbine could spin easily and rapidly in most wind speeds, even during strong turbulence, yet the aerodynamic characteristics of the six-blade design meant that despite high average wind speeds, the ratio of wind to angular velocity, or the tip-speed ratio, did not produce enough voltage.
In theory then, if you could lower the threshold voltage needed to charge a battery and/or increase the tip-speed ratio of the turbine, the overall yield of the system should be notably higher. Given that the Furlmatic is capable of trickle charging a lead-acid battery without any intermediate electronics, it is possible therefore that we could use a 6v battery or even a 2v cell instead, which would greatly improve the yield in locations with a lower average wind speed, poor wind quality, or for mobile applications.
Aerodynamic characteristics can differ notably with blade number and overall design
In the specific case of the Furlmatic, where the blades are removable, we could also experiment with the aerodynamic characteristics of different blade numbers to see how the tip-speed ratio is proportional to the number used. This might lower the yield in unrealistic ideal conditions, but is a good compromise if both the average rotational velocity and subsequent output voltage of the turbine can be optimised with fewer blades, especially in the more common lower wind speed ranges.
In conclusion, it is possible to take advantage of wind-based renewables on a smaller scale, this could be for outbuildings, small offices and other off-grid projects but they do take more trial and error in cases where operating conditions aren’t ideal. From my experience, I would like to see miniature wind turbines designed more for a broader range of wind conditions and this is something I am keen to explore with the Kickstart Kamper project, alongside the potential for lower voltage battery storage systems.