To produce enough power to be worth the investment, home wind turbine blades need to adhere to a certain design, as well as height standards. Most of the horizontal-axis models are built with either two or three blades attached to a central rotor. By means of the air currents, the blades are pushed into a rotating movement which essentially converts the kinetic force of the wind into usable torque that turns a shaft.
Many have wondered whether such a thing as a wind turbine blade design that maximizes electricity gain even exists. Read on to find out.
What are Wind Turbine Blades?
Wind turbine blades are the lengthy, slim wings you see attached to the middle of a propeller, called a rotor. Although the aerodynamics of modern blades might be a bit complicated and theoretical, the fundamental force that drives the mechanism is lift. The blades rely on the wind to turn them by slowing the airflow down as it passes over one of their sides. This is achieved through an intricate, careful design because the windmill blades cannot be allowed to turn too slow or too fast. (Click here for a fuller list of the pros and cons of wind energy.)
If the turbine is too slow, too much wind passes through the blades without rotating them, whereas if the wind turbine blade design makes them spin too fast, the opposite is the case. In fact, as you may have already noticed from older propeller-types of planes when the rotor is turning too fast, it appears as a large disc. From an aerodynamic perspective, the blades might very well be a disc because the movement causes a considerable amount of drag force to act against their motion.
Consequently, there is an optimal tip speed ratio, namely the speed at which the rotor is turning relative to that of the air current spinning it. The latter is dependent on the number, design, and shape of the windmill blades. The tip speed ratio represents the best wind turbine speed that generates the maximum amount of electricity given certain atmospheric conditions.
Types of Windmill Blades
1. Flat windmill blades
These are the oldest type of design. They were used for thousands of years on all sorts of mills. However, with the onset of the 20th century and the introduction of the thin, curved airfoil, the flat form fell increasingly out of use. The main problem with this model is that the wind pushes against the blade, and the latter’s response is to push back in the same direction.
The rotation that comes out of this process is incredibly slow, not to mention the fact that, on the upturn, the blade actually pushes against the wind, which is the complete opposite of what you want to achieve. Because they rely on drag force to spin, many people refer to these as drag-based rotors.
The main advantage of this type of windmill blade design is the fact that they are easy and cheap to make for someone looking to tackle a wind turbine DIY project. Either metal or PVC can easily be cut in the same size and shape without much effort. Their biggest drawback is low efficiency in terms of power generation.
2. Curved windmill blades
The curved wind turbine blades are the most widespread and common type. They are almost identical in design to that of an airplane wing or airfoil. The difference from a flat type can be noticed if you take look at a cross-sectional view of the blade. From this perspective, you can see that the top part of the blade is curved, while the bottom one is flat. The reason for this is that they function on the same principle that airplane wings do, namely by harnessing lift forces.
The angle at which the curved side makes the air bend is crucial towards generating an optimal aerodynamic lift force. In simplistic terms, it has to have enough of an arc to make the wind move slower on top than it does underneath. Because faster-moving wind tends to rise, the curved surface will create a low-pressure pocket of air above it. This area essentially draws the blade in the direction of the downwind created by the arc.
Usually, the faster the wind, the more lift is produced, which is why these wind turbine blades are the most widespread kind in the world’s largest wind farms. You might be able to also do a curved turbine blade design yourself by cutting a standard-sized drainage pipe, but you have to pay close attention to the angle of the arc.
Improving the Performance of Curved Wind Turbine Blades
As we’ve mentioned before, there’s another crucial element to wind turbine blade design that impacts the torque-generating efficiency of the system and this is the angle of attack. Too efficient of a blade can rotate at such speeds that it becomes inefficient because too little air is passing through it. In order to prevent this, a balance must be achieved between the drag force that works against the rotation of the propeller, as well as the latter’s ability to generate lift and spin faster. Of course, the type of wind turbine will also affect which type of blade is most efficient.)
Basically, this balance can be achieved by means of the angle at which the wind hits the blade itself, the slant between the pitch of the blade and the direction of the wind. Modern wind turbine blades are designed with a slight twist along their length in order to achieve this. The angle of attack can vary anywhere between 10° and 20° from the base of the blade where it connects to the rotor towards the tip. This ensures that optimal lift and rotation is achieved.
The Winds of Change
The most powerful nations around the world are switching to renewable energy, wind being the most reliable and widespread kind, closely followed by solar and hydroelectric. This article covered all the fundamental aspects of wind turbine aerodynamics, giving you an insight into how they work.
For more specific information on DIY wind turbine blade design, you should first find out how tall your turbine is going to be, but also what length the blades should have for optimal efficiency.