Performance Comparison of Control Schemes for Variable-Speed Wind Turbines

We analyze the performance of different control schemes when applied to the regulation problem of a variable-speed representative wind turbine. In particular, we formulate and compare a wind-scheduled PID, a LQR controller and a novel adaptive non-linear model predictive controller, equipped with observers of the tower states and wind. The simulations include gusts and turbulent winds of varying intensity in nominal as well as off-design operating conditions. The experiments highlight the possible advantages of model-based non-linear control strategies

'R&D and export performance: exploring heterogeneity along the export intensity distribution'

This study analyses the relationship between firm-level innovative effort as measured by R&D expenditures and export intensity. We apply quantile regression techniques to a sample of Italian firms to verify whether R&D expenditures’ effect varies along the conditional distribution of export intensity, after controlling for censoring and endogeneity issues. Empirical findings suggest that the effect of R&D expenditures on export intensity is positive and that firms taking most advantage from R&D activity are in the right tail of the export intensity distribution (from the 70th quantile onwards), that is, those exporting 50% of their sales or more. Overall, the results prove robust to several specification checks and suggest not only that firms’ innovative efforts help explaining heterogeneity in export intensity performance, but also that its positive effect differs across the export to sales ratio distribution. This implies that innovation policy measures might be more effective for firms characterised by a relatively high export intensive margin

Optimal Shutdown Management

The paper presents a novel approach for the synthesis of the open-loop pitch profile during emergency shutdowns. The problem is of interest in the design of wind turbines, as such maneuvers often generate design driving loads on some of the machine components. The pitch profile synthesis is formulated as a constrained optimal control problem, solved numerically using a direct single shooting approach. A cost function expressing a compromise between load reduction and rotor overspeed is minimized with respect to the unknown blade pitch profile. Constraints may include a load reduction not-to-exceed the next dominating loads, a not-to-be-exceeded maximum rotor speed, and a maximum achievable blade pitch rate. Cost function and constraints are computed over a possibly large number of operating conditions, defined so as to cover as well as possible the operating situations encountered in the lifetime of the machine. All such conditions are simulated by using a high-fidelity aeroservoelastic model of the wind turbine, ensuring the accuracy of the evaluation of all relevant parameters. The paper demonstrates the capabilities of the novel proposed formulation, by optimizing the pitch profile of a multi-MW wind turbine. Results show that the procedure can reliably identify optimal pitch profiles that reduce design-driving loads, in a fully automated way

And Yet they Co-Move! Public Capital and Productivity in OECD.

In this paper we add to the debate on the public capital-productivity link by applying very recent developments in the panel time series literature that take into account cross sectional correlation in non-stationary panels. In particular we evaluate the productive effect of public capital by estimating various production functions on a panel of 21 OECD countries over the period 1975-2002. Our results suggest that public capital has a positive long run impact on output, with elasticities that range between 0.05-0.15, depending on model specification. These findings are robust to the existence of spillover effects from public capital investments in other countries and to the inclusion of other productivity determinants, like human capital, the stock of patents and R&D capital. Finally, we do not find any important effect of public capital on GDP in the short run: this suggests that public infrastructure investments might not be a powerful countercyclical policy instrument

Articulated blade tip devices for load alleviation on wind turbines

This paper investigates the load alleviation capabilities of an articulated tip device, where the outermost portion of the blade can rotate with respect to the rest of the blade. Passive, semi-passive and active solutions are developed for the tip rotation. In the passive and semi-passive configurations tip pitching is mainly driven by aerodynamic loads, while for the active case the rotation is obtained with an actuator commanded by a feedback control law. Each configuration is analyzed and tested using a high-fidelity aeroservoelastic simulation environment, by considering standard operative conditions as well as fault situations. The potential benefits of the proposed blade tip concepts are discussed in terms of performance and robustness

Detection of rotor imbalance, including root cause, severity and location

This paper presents a new way of detecting imbalances on wind turbine rotors, by using a harmonic analysis of the rotor response in the fixed frame. The method is capable of distinguishing among different root causes of the imbalance. In addition, the imbalance severity and location, i.e. the affected blade, can be identified. The automatic classification of the imbalance problem is obtained by using a neural network. The performance of the method is illustrated with the help of different fault scenarios, within a high-fidelity simulation environment

Cyclic Pitch Control for the Reduction of Ultimate Loads on Wind Turbines

In this paper we study the use of individual blade pitch control as a way to reduce ultimate loads. This load alleviation strategy exploits the fact that cyclic pitching of the blades induces in general a reduction of the average loading of a wind turbine, at least for some components as the main bearing, the yaw bearing, or the tower. When ultimate loads are generated during shutdowns, the effect of the use of cyclic pitch results in reduced peak loads. In fact, as the machine starts from a less stressed condition, the response to an extreme gust or other event will result in reduced loading on its components. This form of load mitigation can be seen as a preventative load mitigation strategy: the effect on load reduction is obtained without the need to detect and react to an extreme event, but by simply unloading the machine so that, in case an extreme event happens, the result will be less severe. The effect of peak load mitigation by preventative cyclic pitch is investigated with reference to a multi-MW wind turbine, by using high-fidelity aeroelastic simulations in a variety of operating conditions

Load mitigation for wind turbines by a passive aeroelastic device

This paper conducts a preliminary investigation of a novel passive concept for the mitigation of loads on wind turbines. The device, which can be implemented as a flap or a pitching blade tip, moves passively in response to blade vibrations, opposing them, thereby yielding an attenuation of loads. In comparison to active load mitigation devices, such as active flaps, this solution has the advantage of not requiring sensors nor actuators, resulting in a particularly simple implementation, with potential benefits in manufacturing and maintenance costs, as well as in reliability and availability.The paper first describes the novel passive device, here implemented by means of a flap, highlighting its main characteristics. A proof of concept of the new idea is then given by a simulation study conducted with the combination of a sectional model of the flap and an aeroservoelastic multibody model of the rest of the machine. Results, obtained for a 10. MW wind turbine, indicate the ability of the passive flap in attenuating blade vibrations in a significant frequency range, which in turn yield a reduced fatigue damage to the structure without noticeable effects in terms of power production and ultimate loads

Integrated design optimization of wind turbines with noise emission constraints

This study integrates aeroacoustic noise emission models within a wind turbine design procedure to include overall sound pressure levels as design constraints. The proposed approach aims at the minimization of the cost of energy from wind, while ensuring the compliance with noise emission limits. The reference 3.35 MW onshore wind turbine developed within the international cooperation IEA Wind Task 37 is redesigned to reduce its noise emissions above and below rated wind speed, considering both single- and multi-objective design criteria. Results obtained with the proposed noise-constrained redesign methodology are compared with the simpler approach of reducing the tip speed without altering the blade shape. Results show that, while the simplistic approach causes a drop of −2.8% in annual energy production and a +2.5% increase in cost of energy, an optimized configuration fulfills the noise requirement without incurring into significant energy penalties

Combined preliminary–detailed design of wind turbines

This paper is concerned with the holistic optimization of wind turbines. A multi-disciplinary optimization procedure is presented that marries the overall sizing of the machine in terms of rotor diameter and tower height (often termed “preliminary design”) with the detailed sizing of its aerodynamic and structural components. The proposed combined preliminary–detailed approach sizes the overall machine while taking into full account the subtle and complicated couplings that arise due to the mutual effects of aerodynamic and structural choices. Since controls play a central role in dictating performance and loads, control laws are also updated accordingly during optimization. As part of the approach, rotor and tower are sized simultaneously, even in this case capturing the mutual effects of one component over the other due to the tip clearance constraint. The procedure, here driven by detailed models of the cost of energy, results in a complete aero-structural design of the machine, including its associated control laws. The proposed methods are tested on the redesign of two wind turbines, a 2.2 MW onshore machine and a large 10 MW offshore one. In both cases, the optimization leads to significant changes with respect to the initial baseline configurations, with noticeable reductions in the cost of energy. The novel procedures are also exercised on the design of low-induction rotors for both considered wind turbines, showing that they are typically not competitive with conventional high-efficiency rotors