The 'lift' and 'drag' profile for a large number of airfoil shapes have been experimentally measured and literature values are used in our calculations for power. Our software for the design of home built wind turbines has been developed and tested throughout the course each turbine project, validating the known airfoil data and giving rational efficiency values depending on the chord accuracy and smoothness.
Turbine performance is calculated by comparing the power output from the operating generator, to the calculated 'theoretical' power using our 'Blade Calculator' software. The value for theoretical power given by our software is the maximum power possible for all of the design dimensions. Blade efficiency is assumed to be 30% (0.30) for three accurately built blades. Any improvement to the design and construction technique are considered after blade efficiency is calculated using the data obtained during performance measurements.
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Wind Turbine Blade Manufacture
At a constant electrical resistance in your circuit, (generator windings and transmission lines), the power output produced from the wind turbine is generated with a change in voltage and current over the range of speeds. When the blade tip speed is at the correct ratio to the incoming wind speed (the correct TSR), airfoil lift is generated to boost efficiency in the power conversion process. Any rapid increases of voltage and current due to gusts are regulated using electronic circuitry to adjust a dumpload resistance in parallel with the system. This produces electricity suitable for domestic use. The excess energy is expended in a 'dumpload' resistor as heat.
The design of the generator determines the voltage and power produced, both of which increase with rotational speed in revolutions per minute (RPM). The generators specified RPM needs to closely match the turbines RPM at operating wind speeds to produce the power and voltage rated on the generators plate.
The power that can be converted into electricity from wind energy is proportional to the circular 'swept' area that the blades cover as they rotate. An increase in turbine power requires a greater swept area to be covered, therefore the square of the blade radius is proportional to area and also power produced. The average windspeed is adjusted to suit the location and can be useful to approximate the increases blade radius required for low wind speed.
Online Blade Calculator Software
Modifying the blades shape and twist will change the ratio between the tip speed of the blade and incoming wind speed (TSR, tip speed ratio) with a lesser number of blades able to spin at a higher TSR.
Adjusting the TSR in the appropriate range can more closely match the blades to the generator. Two bladed turbines operate in a TSR range between 8 and 10, with 3 bladed turbines spinning at a TSR between 6 to 7.
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Power output from the turbines electrical generator will change depending on the speed of rotation. Large commercial turbines operate at a constant speed and power fed into the electrical grid is 'synchronous' with the AC phase that travels in the transmission cables. This is not the case with small domestic wind turbines and the excess power needs to be managed.
Frequently asked questions about plan orders, blade design, electrical work and generator matching can be found on our Q&A pages.
Our original 'Bladecalc' software, written in visual basic 6 (VB6), remains hosted on this site and is available for download. Progressive improvements are incorporated into newer versions of the online 'Blade Calculator'. Accessibility options and other functions are added for cross-discipline users. We aim to eventually assist in all design aspects including hub and generator specifications.
Electrical Circuit Design:
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