Browsing by Author "Ward, Dawn"

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    Analysis of the effect of a series of back twist blade configurations for an active pitch-to-stall floating offshore wind turbine
    (American Society of Mechanical Engineers, 2020-04-08) Ward, Dawn; Collu, Maurizio; Sumner, Joy
    For a turbine mounted on a floating platform, extreme induced loads can be increased by up to 1.6 times those experienced by a turbine situated on a fixed base. If these loads cannot be reduced, towers must be strengthened which will result in increased costs and weight. These tower loads would be additionally exasperated for a pitch-to-feather controlled turbine by a phenomenon generally referred to as “negative damping,” if it were not avoided. Preventing negative damping from occurring on a pitch-to-feather controlled floating platform negatively affects rotor speed control and regulated power performance. However, minimizing the blade bending moment response can result in a reduction in the tower fore-aft moment response, which can increase the tower life. A variable-speed, variable pitch-to-stall (VSVP-S) floating semi-submersible wind turbine, which does not suffer from the negative damping and hence provides a more regulated power output, is presented. This incorporates a back twist blade profile such that the blade twist, starting at the root, initially twists toward stall and, at some pre-determined “initiation” point, changes direction to twist back toward feather until the tip. Wind frequency weighting was applied to the tower axial fatigue life trends of different blade profiles and a preferred blade back twist profile was identified. This had a back twist angle of −3 deg and started at 87.5% along the blade length and achieved a 5.1% increase in the tower fatigue life.
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    ItemOpen Access
    Blade Twists
    (Cranfield University, 2020-08-11 08:40) Ward, Dawn
    Blade back twists for NREL 5 MW HAWT wind turbine with variable speed variable pitch-to stall control.
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    Reducing tower fatigue through blade back twist and active pitch-to-stall control strategy for a semi-submersible floating offshore wind turbine
    (MDPI, 2019-05-18) Ward, Dawn; Collu, Maurizio; Sumner, Joy
    The necessity of producing more electricity from renewable sources has been driven predominantly by the need to prevent irreversible climate chance. Currently, industry is looking towards floating offshore wind turbine solutions to form part of their future renewable portfolio. However, wind turbine loads are often increased when mounted on a floating rather than fixed platform. Negative damping must also be avoided to prevent tower oscillations. By presenting a turbine actively pitching-to-stall, the impact on the tower fore–aft bending moment of a blade with back twist towards feather as it approaches the tip was explored, utilizing the time domain FAST v8 simulation tool. The turbine was coupled to a floating semisubmersible platform, as this type of floater suffers from increased fore–aft oscillations of the tower, and therefore could benefit from this alternative control approach. Correlation between the responses of the blade’s flapwise bending moment and the tower base’s fore–aft moment was observed with this back-twisted pitch-to-stall blade. Negative damping was also avoided by utilizing a pitch-to-stall control strategy. At 13 and 18 m/s mean turbulent winds, a 20% and 5.8% increase in the tower axial fatigue life was achieved, respectively. Overall, it was shown that the proposed approach seems to be effective in diminishing detrimental oscillations of the power output and in enhancing the tower axial fatigue life.
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    Reducing tower fatigue through modelling and analysis of pitch-to-stall, back twist blade, for floating offshore wind turbines.
    (2020-04) Ward, Dawn; Sumner, Joy; Cullu, Maurizio
    The negative effects of anthropogenic climate change has led to an increase in electricity generated from renewable energy sources. Within this, offshore wind is currently one of the fastest growing markets, and industry is now looking towards the role that floating offshore wind turbines (FOWT) may play in the future. However, FOWT are often subjected to increased tower loads, up to 1.6 times those experienced by their fixed-to-seabed counterparts. Reducing these loads would help to decrease the costs of this fledgling technology, by diminishing the tower strengthening requirements. In the present work, as a possible technological solution to reduce the above mentioned loads, a variable speed, variable pitch-to-stall (VSVP-S) control strategy, is proposed for wind turbines. The 5 MW turbine is applied to both a semi-submersible and a spar floating platform, as these floaters both suffer from increased tower fore-aft oscillations. The VSVP-S control configuration avoids negative damping by design, allowing higher controller gain settings and hence a more regulated power output, than a pitch-to-feather controlled floating scenario. The FOWTs are further altered to incorporate back twisted blades, to decrease the blade bending moment response. This was seen to decrease the tower fore-aft moment range, creating an increase in the turbine tower life expectancy, when compared to their respective feather base models. To predict the overall tower axial fatigue life, the frequency of anticipated wind speeds are required. The likely occurrence of the different wind speed cases was accounted for through a weighted analysis. This took account of the distribution of probability of occurrence at three mean turbulent wind speeds: 8, 13 and 18 m/s. FOWT analysis highlighted trends in terms of back twist angle initiation point and magnitude were similar for both VSVP-S models (i.e. semi-submersible and a spar floating platforms). When a back twist angle to -6° at the tip was imposed, starting at 75% along the blade length, increases in the tower axial fatigue life of 10.2% for the semi-submersible and 18.8% for the spar were achieved, with the VSVP-S controlled turbines, compared to their feather controlled counterparts.