Design and Experimental Results of a Bench Top Flywheel-Accumulator for Compact Energy Storage

The energy density of conventional hydraulic accumulators is significantly lower than competing energy domains. In this paper, a novel solution to improve the energy density of hydraulic energy storage, the flywheel-accumulator, is presented. The flywheelaccumulator integrates rotating kinetic and pneumatic energy storage, where the hydraulic fluid is used to change the mass moment of inertia of the flywheel. Furthermore, due to the two unique methods of adding energy to the system, the hydraulic system pressure becomes independent of the quantity of energy stored, enabling a constant system pressure. A computational model of the flywheel-accumulator is presented, which demonstrates an increase in energy density of 10 times greater than a conventional accumulator. The concept is demonstrated through the design and testing of a bench top prototype system. The bench top prototype includes a transparent chamber, allowing high-speed video visualization of the dynamic behavior. INTRODUCTION / BACKGROUND Reducing fossil fuel based energy consumption has become a global priority due to the cost, environmental, social, and political impacts. Many opportunities exist in fluid power systems to improve efficiency including improved component design, switch-mode (digital) control, improved system control strategies, and energy regeneration with storage. Energy storage is becoming an increasing popular option in applications from construction equipment, such as excavators, to hybrid vehicles. When considering a hydraulic circuit with regenerative capabilities, such as the series hydraulic hybrid drive train illustrated in Figure 1, conventional hydraulic accumulators result in two major limitations. First, the energy density of advanced composite hydraulic accumulators approaches 6 kJ/kg , which is two orders of magnitude lower than modern electric batteries . The limited energy density prevents scaling hydraulic hybrid drive trains to smaller vehicles with limited volume and prevents technologies such as “plug-in” hydraulic hybrids. Second, when an accumulator is connected to the high-pressure rail of a hydraulic circuit, the pressure becomes a function of the quantity of energy stored. Thus for a 2:1 expansion ratio, the hydraulic components must be sized to provide the required power at half of the maximum operating pressure, resulting in heavy and bulky components. Figure 1. Series hydraulic hybrid drive train. Note that the pressure is determined by the accumulator state of charge. A literature review of approaches to improve energy density of hydraulic accumulators was previously presented . These approaches successfully isothermalized the compression and expansion of the nitrogen gas through increasing the surface area with a variety of mediums . A less conventional approach is the open-accumulator, which combines both pneumatic and hydraulic energy storage with the benefits of a significant increase in energy density and the hydraulic pressure becoming independent of the quantity of energy stored . Another unconventional approach is a strain energy accumulator, where energy is stored by using hydraulic fluid to deform an elastomer . A final method, the topic of this paper creates multi-energy domain storage by integrating rotating kinetic and pneumatic energy storage in a flywheel-accumulator. This paper presents the architecture and behavior of the flywheel-accumulator. First, the system is described through a basic steady-state analysis. Following the analytical analysis, the design of a low-energy bench top prototype is presented. Experimental methods, results, and a discussion follow. The paper closes with concluding remarks about future work for this promising technology. FLYWHEEL-ACCUMULATOR An alternative method to increase the energy storage density of hydraulic systems is to integrate pneumatic and rotational kinetic energy storage in a flywheelaccumulator. A piston style architecture of the flywheelaccumulator, as shown in Figure 2, is considered, however other architectures are also possible. The piston style flywheel-accumulator consists of a cylindrical pressure vessel with a piston separating compressed gas and hydraulic fluid with a hydraulic port at the center of the endcap. The flywheel accumulator rotates about the central axis and is coupled to a hydraulic pump/motor, either directly or through a gearbox. Figure 2. Piston-style flywheel-accumulator energy storage device. As in a conventional accumulator, when hydraulic fluid enters the flywheel-accumulator, the piston moves axially and the gas is compressed. Because the density of nitrogen is low, the centrifugal force has little influence on the gas pressure distribution. However, the centrifugal force does create a significant radial pressure gradient in the hydraulic fluid, due to the higher density. From a force-balance on a fluid element subject to centripetal acceleration, the pressure of the fluid as a function of the radius is :