Publikationen

This paper analyses the potential of a novel hydraulic variable inertia flywheel to improve standby power systems when attached to directly grid-connected electrical machines. Conventional standby power systems often require several seconds to reach full operation after a grid outage. During this time, critical loads may be left without power, unless supported by an additional uninterruptible power supply system. By flange-mounting a hydraulic variable inertia flywheel to an existing grid-connected electrical machine, emergency power can be provided immediately without the need for power electronics. The ability of the hydraulic variable inertia f lywheel to discharge at a quasi-constant speed offers improved frequency and voltage stability. The performance of the hydraulic variable inertia flywheel is compared with traditional flywheels of equivalent inertia and energy content across the two most common machines. The simulation results identify the most appropriate machine- f lywheel configuration to complement standby power systems. Combining a hydraulic variable inertia f lywheel with an electrically excited synchronous machine provides the most stable voltage and frequency support. The application of the widely used induction machine is found to be not preferable. Beyond acting as supplementary technology for standby power systems, combining directly grid-connected electrical machines with flywheels also offers secondary and tertiary benefits.

Wind turbulence introduces variations and oscillations in the power output of wind turbines and vibrations of wind turbine components. Higher turbulence intensities lead to significant wind speed fluctuations, necessitating frequent aerodynamic regulation of the rotor in near-rated operating points. Reducing these fluctuations stabilizes the operation of the wind turbine, which reduces power fluctuations in the grid, and which reduces wear and tear in the wind turbine. This study investigates a hydraulic-pneumatic flywheel system designed to leverage these fluctuations by absorbing excess energy during gusts (charging) and releasing it during lulls (discharging), effectively smoothing the power output. The charging phase of this system introduces an additional electrical load by driving hydraulic pumps with electric motors, thereby increasing the generator torque. Simultaneously, fluid movement along the rotor blades induces Coriolis forces, generating a negative torque that slows down the rotor. This flywheel system application reduces rotor speed and pitch angle excursions during peak wind events by storing surplus energy. When the wind speeds drop below rated wind speed due to turbulence, the stored energy is released by transferring the fluid from the tip accumulators to storage tanks in the blade roots, reducing the flywheel inertia, and hence, releasing energy. This bidirectional functionality allows mitigating the downsides of turbulence. The objective of this study is to assess the performance of the flywheel system in stabilizing power output and to quantify the energy yield improvement, with a particular focus on the influence of turbulence intensity.

The ongoing transition towards inverter-based generation is significantly reducing conventional rotational inertia in power systems, leading to increased rates of change of frequency. To address this challenge, the Hydraulic Variable Inertia Flywheel (HVI-FW) represents a novel alternative for distributed and inherent inertia provision. This paper presents a design tool developed to optimize the geometric parameters of the novel flywheel, with respect to specific energy. A parameter study is conducted by varying key geometric dimensions, such as radius and height. The results demonstrate that the HVI-FW can achieve significantly higher specific energy than conventional flywheels, particularly for small radii, making it well suited for coupling with small, synchronously rotating electrical machines. The findings highlight the potential of the HVI-FW as an effective solution for providing distributed inertia in future low-inertia power systems.

The rise of wind power in bulk electric systems creates a growing need to model, understand, and characterize responses to various sources of disruption. Despite the numerous advantages of computer models, physical emulators have enormous benefits in characterizing dynamic magnetic electromechanical phenomena which are unexpected or difficult for models to process accurately. The development of an experimental apparatus for physical wind farm emulation is described. The control and operation of an induction motor-generator setup for mimicking wind generator dynamics are demonstrated. Recommendations for further application of this emulator are also given.

Researchers in the wind sector continuously aim to lower mechanical and structural loads on wind turbines to reduce the amount of material used in their components. One such innovative concept that is being investigated here is the integration of a flywheel system into rotor blades. In contrast to its intended aim of reducing the material usage and thereby CO2 emissions, the flywheel system in its implementation requires additional materials, which increases the carbon footprint considerably. Hence, to achieve a net CO2 saving, the CO2 expenditure of the additional components needs to be saved by the system as a minimum. The purpose of this work is to quantify the rise in CO2 emissions, caused by the installation of the flywheel system, relative to the original design. Thereby, the flywheel system’s minimal performance requirement for an exemplary wind turbine is defined.

Due to the energy transition, the future electric power system will face further challenges that affect the functionality of the electricity grid and therefore the security of supply. For this reason, this article examines the future frequency stabilisation in a 100% renewable electric power system. A focus is set on the provision of inertia and frequency containment reserve. Today, the frequency stabilisation in most power systems is based on synchronous generators. By using grid-forming frequency converters, a large potential of alternative frequency stabilisation reserves can be tapped. Consequently, frequency stabilisation is not a problem of existing capacities but whether and how these are utilised. Therefore, in this paper, a collaborative approach to realise frequency stabilisation is proposed. By distributing the required inertia and frequency containment reserve across all technologies that are able to provide it, the relative contribution of each individual provider is low. To cover the need for frequency containment reserve, each capable technology would have to provide less than 1% of its rated power. The inertia demand can be covered by the available capacities at a coverage ratio of 171% (excluding wind power) to 217% (all capacities). As a result, it is proposed that provision of frequency stabilisation is made mandatory for all capable technologies. The joint provision distributes the burden of frequency stabilisation across many participants and hence increases redundancy. It ensures the stability of future electricity grids, and at the same time, it reduces the technological and economic effort. The findings are presented for the example of the German electricity grid.

A novel variable inertia flywheel that uses the mass of a rotating hydraulic fluid is proposed in this paper. In contrast to variable inertia flywheels that use solid masses, this flywheel stands out for its simplicity. In contrast to many other common energy storages, it does not require any environmentally harmful, rare or expensive materials. The basic working principle and the equations that cover the hydraulic behaviour, the pneumatic behaviour and the energy from angular momentum are introduced. These equations are applied to the geometry of the proposed flywheel concept, which allows quantifying the pressures that act on the different mechanical flywheel components. These pressures, together with the centrifugal acceleration from rotation, lead to the loads that have to be withstood by the mechanical components. A simple method for quantifying these loads, and for dimensioning the mechanical components, allows deriving the stationary masses and inertias. A parameter study is conducted, in which different parameters of the flywheel are varied in order to find the maxima in energy density and specific energy. The results of this parameter study reveal that the proposed hydraulic variable inertia flywheel is a very simple and safe energy storage that could provide AC power systems with inertia and control power to support their frequency.

The utilization of renewable energy resources significantly increases in order to reduce the impact of climate change. Wind turbines are one of the most important renewable energy sources and have an important role to play in power generation. They do, however, have to serve the increasingly variable demands of the grid. Some of these demands cannot be satisfied with the standard control mechanisms of state-of-the-art wind turbines. A hydraulic-pneumatic flywheel in a wind turbine rotor is one mechanism which, in addition to its various grid services, can also reduce the mechanical loads on the structure of a wind turbine. However, the installation of such a flywheel into rotor blades increases the weight of the blades. This paper focusses on the development of a design method for reducing the additional mass of the flywheel. This method incorporates the piston accumulators of the flywheel in the blade support structure, which allows for the replacement of parts of the blade spar caps with composite material from the piston accumulators. This enables the flywheel to be installed into the rotor blades without making the wind turbine significantly heavier.

In this paper, atmospheric irrigation with wind turbines is proposed. This technology addresses the problem of water scarcity by enhancing the natural water circuit in the atmosphere with wind turbines. There are three different operating modes conceivable for this technology. In two of these the wind turbines interact with the ground in their near wake. The third operating mode is the one which is discussed in this paper, and it aims at transporting water potentially over long distances. The basic working principle, the utilized physical phenomena and the basic design of the technology are introduced. The equations governing the hydraulic and the hydrological effects are presented. The goal of this paper is to quantify the necessary power and the necessary amount of water when wind turbines humidify a certain volume of air in the atmosphere. For this purpose, the power and water demand are assessed, both in a generalized manner and for a realistic scenario. It is concluded that the proposed system can achieve the objective in most wind speed conditions. However, the required amount of water is substantial. Therefore, an alternative source of fresh water has to be found when the system is used on a comparably large scale.