Alternating current power systems rely on rotating electric machines, such as generators and motors, whose rotational speed form the power system frequency. The consumption of electric energy, and the generation of renewable energy, are subject to fluctuations, leading to variations in the power system frequency. To cope with this variability, electrical energy needs to be stored – for example in batteries. However, some energy will be wasted in the conversion from electrical alternating current to direct current energy, and from electrical direct current energy to chemical energy. This necessary energy conversion also increases the complexity of the power system. Read More
An alternative solution could be the traditional flywheel: one of the oldest energy-storing devices ever invented. When driven to spin by a rotating electric machine, it accumulates rotational energy, which can be released by decelerating the wheel. This energy can then be fed into electric power systems, without any efficiency-draining conversions.
However, the traditional flywheel faces a major drawback. As they rotate, traditional flywheels can only exchange energy when their rotational speed is varied. This is because they have a constant rotational inertia, which means they continue rotating when forces act upon them. Hence, significant variations in the power system frequency are needed to store and release energy from traditional flywheels.
Alternatively, rotational energy could be transferred to a flywheel with variable inertia, which can be connected to a rotating generator or motor. This type of flywheel could store and release energy while maintaining a steady rotational speed, allowing it to maintain a constant power system frequency. Although the variable-inertia flywheel is not new, its complexity means that it has never gained much traction.
To overcome this drawback, Dr Clemens Jauch of Flensburg University of Applied Sciences, together with colleagues at HYDAC Technology, introduced a simple innovation to the flywheel’s design.
Their design replaces the flywheel’s traditionally solid mass with a rotating hydraulic fluid. Within the device, the non-compressible fluid is allowed to flow between a central cylinder, and the hollow walls of an outer cylinder – both rotating about the same central axis.
Both cylinders contain separate volumes of compressible gas: one enclosed by the central cylinder, and the other within the walls of the outer cylinder. When the flywheel is stationary, all its fluid is kept in the central cylinder; but as it rotates, this fluid experiences a centrifugal force, driving it into the outer cylinder.
In the process, the gas in the outer cylinder is compressed, causing it to gain potential energy, much like a spring. Eventually, this potential energy will become perfectly balanced with the fluid’s kinetic energy.
As the flywheel’s rotational energy decreases, this balance is maintained as the gas decompresses, driving the fluid back into the central cylinder. This redistribution of mass allows the flywheel to maintain a quasi-constant speed even when its rotational energy is varied.
Because the team’s flywheel can be manufactured without any rare or environmentally harmful materials, and contains hardly any movable mechanical components, the researchers hope that it will help renewable sources improve their efficiency.
