• Energy Research

As wind turbines increase in size and the demands for lifetime also increases, new methods of load reduction needs to be examined. One method is to make the yaw system of the turbine soft/flexible and hence dampen the loads to the system. This paper presents work previous done on this subject with foucus on hydraulic yaw systems. By utilizing the HAWC2 aeroelastic code and an extended model of the NREL 5MW turbine studies shows that a significant reduction in fatigue loads to the yaw system and rotor shaft is possible by the soft yaw drive concept.

In offshore winch drive applications, determining the required number of pistons in digital displacement motors is critical for minimizing torque ripples. Digital displacement motors have shown promise for improving energy efficiency for offshore operations, such as placing equipment on the seabed or mineral drilling. However, they are known for exhibiting significant torque ripples, which can affect load-handling precision. This paper estimates the required number of pistons for realizing a digital hydraulic winch drive based on information from a commercial winch. The proposed drive employs full-stroke displacement strategies at high speeds and partial-stroke at low speeds. By simulating steady-state operations, this study correlates torque output with position oscillations. The results show that 37 pistons are required to keep position oscillations below a benchmark threshold of 10 mm throughout the drive’s operating range to avoid hindering the drive’s performance. However, such a high piston count could result in high costs due to the large, expensive valves required for partial-stroke operations. Therefore, this paper suggests an alternative drive topology for future research, which could potentially reduce the number of pistons that are operated with partial strokes.

With the increasing size of wind turbines and with increasing lifetime demands, new methods for load reduction in the turbines need to be examined. One method is to make the yaw system of the turbine flexible, thereby dampening the loads to the system. This paper presents a hydraulic soft yaw concept and investigates the effect this has on critical loads in the turbine. To analyze the system, a novel friction model is developed and implemented for the yaw system using the NREL 5-MW turbine in the aerodynamic code FAST. Based on this model, the influence of friction is investigated and the results on load reduction for three different design load cases are presented and discussed. From the investigation, it is found that load reductions of up to 40% on fatigue loads and 37% on ultimate loads are possible when considering the yaw system.

In marine applications, a cyclic varying pitch (CVP) propeller is a propeller in which the propeller blade can be cyclic-pitched. This cyclic pitching of the propeller blades is used to adapt to the local flow conditions in the non-uniform wake field that the propeller operates in, behind the ship hull. This has the potential to improve the performance of the propulsion system relative to a propeller which has fixed pitch for each revolution. The potential performance improvements include increasing the propulsion efficiency and reducing the cavitation, pressure pulses, vibrations and noise problems. However, the CVP propeller is not on the market today, and several challenges have to be addressed before the CVP propeller may be realized. One of these challenges is how to design the individual cyclic pitch mechanism for the propeller. However, before the cyclic pitch mechanism can be designed, it is necessary to know the requirements for it, such as the required pitching power and torque. The focus of the current paper is therefore to present a model for the propeller, by which it is possible to determine the loads acting on the CVP propeller blades during the cyclic pitching, and hence the actuator force/torque and power requirements. To illustrate the usefulness of the model, an example is presented, in which the loads on a CVP propeller are determined, together with the requirements for the individual cyclic pitch mechanism. The efficiency results presented are, however, not representative of the efficiency improvement that may be obtained, as neither the propeller nor the pitch trajectory has been optimised. The results do, however, serve to show the benefit and validity of the model.

Wind turbines have become a significant part of the global power production and are still increasing in capacity. Pitch systems are an important part of modern wind turbines where they are used to apply aerodynamic braking for power regulation and emergency shutdowns. Studies have shown that the pitch system is responsible for up to 20% of the total down time of a wind turbine. Reducing the down time is an important factor for decreasing the total cost of energy of wind energy in order to make wind energy more competitive. Due to this, attention has come to condition monitoring and fault detection of such systems as an attempt to increase the reliability and availability, hereby the reducing the turbine downtime. Some methods for fault detection and condition monitoring of fluid power systems do exists, though not many are used in today’s pitch systems. This paper gives an overview of fault detection and condition monitoring methods of fluid power systems similar to fluid power pitch systems in wind turbines and discuss their applicability in relation to pitch systems. The purpose is to give an overview of which methods that exist and to find areas where new methods need to be developed or existing need to be modified. The paper goes through the most important components of a pitch system and discuss the existing methods related to each type of component. Furthermore, it is considered if existing methods can be used for fluid power pitch systems for wind turbine.

Within the research field of harvesting the energy of ocean waves, fluid power has been identified as a crucial technology in the Power Take-Off (PTO) design, due to the high torque densities required in Wave Energy Converters (WECs). The PTO is the technology converting the captured wave motion into electricity. However, conventional fluid power systems are characterized by offering poor efficiencies, rendering current PTO designs inefficient. This paper investigates the feasibility of a fluid power system based on implementing the force control of hydraulic cylinders by switching between a few fixed system pressures. The proposed design is optimized at multiple levels, as evaluating the feasibility of a solution highly depends on finding the optimum trade-off between e.g. harvested wave energy and losses in the PTO system.

Offshore winch drives require high energy efficiency and control precision, making digital displacement motors an attractive solution due to their high efficiency and potential controllability. However, the response time and the realized energy efficiency of these motors are heavily dependent on the chosen displacement control strategy, especially at low-speed operation. This paper considers various displacement control strategies to investigate whether digital displacement motors are a viable solution for offshore winch drive applications. The motor specifications are derived based on the requirements of a commercial offshore winch drive system. The analysis reveals that various displacement control strategies should be used across the drive's operational speed range to ensure both satisfactory performance and high efficiency. Full-stroke and partial-stroke strategies are optimal for speeds above 28 rpm and 20 rpm, respectively, but unsuited for lower-speed operation. For speeds below 20 rpm, an improved sequential-stroke strategy is therefore presented. The proposed displacement control strategy provides instantaneous motor response and maintains high energy efficiency, although its robustness is slightly reduced at higher operating speeds above 20 rpm.

This paper describes the development and application of a signal-based fault detection method for identifying gas leakage in hydraulic accumulators used in wind turbines [...]