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.

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.