Patent Granted for Device for Compact Energy Storage

Achievement date: 

Researchers at the NSF-funded Engineering Research Center (ERC) for Compact and Efficient Fluid Power (CCEFP) at the University of Minnesota have created a hydraulic flywheel accumulator (HFA) for compact energy storage in both hydro-pneumatic and rotating kinetic energy domains. The device has been granted a full utility patent.


The HFA’s unique energy domain coupling allows hydraulic system pressure to be directly controlled by modulating the method of storage. The ability to store energy in two modes separates hydraulic system pressure from of the state of charge, allowing the system pressure to be more actively controlled. The changing volume of hydraulic fluid in the device results in a variable flywheel inertia that creates the coupling between the energy domains.

The energy density of the HFA is theoretically more than an order of magnitude higher than that of conventional accumulators. This is due to the high energy density of rotating kinetic energy storage, which reaches 325 kilojoules per kilogram (kJ/kg) for high performance flywheels.


The HFA is a cylindrical piston-style accumulator rotating about its central axis and coupled to a pump and motor. Hydraulic fluid enters and exits the HFA at the center of one end of the cylinder. The opposing side of the piston is occupied by nitrogen gas at the pre-charged pressure.

Energy can be added or removed from the HFA in two ways, either through an applied torque or by adding or removing hydraulic fluid. Due to the centripetal acceleration and density of the hydraulic fluid, a parabolic pressure distribution is formed. When hydraulic fluid is added, the piston compresses the gas, increasing the pneumatic energy storage, and the moment of inertia increases. In the absence of an applied torque, the increase in inertia creates a decrease in the angular velocity due to conservation of angular momentum. If the quantity of hydraulic fluid in the HFA remains constant as torque is applied, the angular velocity increases, causing a decrease in the hydraulic system pressure.