Investigation of Drivetrain Dynamics of 5kW Small Wind Turbine

Bradley M Skellern

The economic viability of small wind turbines (SWTs) is often squandered upon, and the utility provided by them is largely considered limited and niche. This paper explores a potential for these turbines to become financially viable in certain circumstances by investigating if reducing the specific power (the ratio of nameplate generation capacity to swept area) will increase the total output power and what effect this will have on the structural fatigue of the turbine. In other words, if the blade length of a given SWT is to be increased, what will the effects be on the output power and total damage equivalent load (DEL) that the blades and turbine experience throughout its life cycle? Nrel’s FAST (Fatigue, structure, aerodynamics and turbulence) version 7 software was used to run over 65 unique simulations, each recording 6,000 points in time measuring 52 different dynamics including various structural fatigues, power generation characteristics and environmental conditions. The turbine used in this analysis is a 5kW Aerogenesis horizontal axis SWT, with an original radius of 2.5m. The turbine was proportionately remodeled to both a 3m and 3.5m radius, all of which were input into FAST. A significant limitation of the FAST software was that the controlled braking (automatic braking system in turbines that activates at high windspeeds) was not implemented into the code of the software. This made previous simulations highly inaccurate as the results showed the turbine power and structural damage increasing indefinitely as windspeed increased, so a Matlab script was developed to implement this feature. The results conclude that as the specific power of the turbine is lowered, the average output power is increased at roughly 1.5 times the rate that the blade is increased. The DEL increases at a higher rate than this, so the ratio of rotor power to DEL decreases as the specific power is lowered. However, when factoring in the increased strength of the blades, these numbers become roughly equal. The ratio also improves as wind speed increases. This paper concludes that the power output can be increased by lowering the specific power, however the damage equivalent load will increase at roughly the same rate and therefore the viability of low specific power SWTs will depend on the economics of the blade costs and levelized cost of electricity. This paper also presents an accurate simulation of the controlled braking system that can be used for future FAST simulations.

Investigation of Drivetrain Dynamics of 5kW Small Wind Turbine

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