Thrust 2: Compactness

Title

2C.2: Advanced Strain Energy Accumulator

Project Leader

Prof. Eric Barth (Vanderbilt)

Statement of Project Goals

The research objective of this work is to extend the current state of knowledge in the use of strain energy storing materials for the engineering design of compact energy storage devices. Specifically, this project seeks a low cost, low/no maintenance, high energy density accumulator primarily targeted toward a fluid powered automotive regenerative braking system (hydraulic hybrid). As opposed to the open accumulator concept (project 2C.1) requiring a compressor and a compressible fluid power subsystem, the strain energy accumulator will act as a traditional hydraulic accumulator linked only to a hydraulic drive system. This project will focus on extending the energy storing capabilities of accumulators for the specific purpose not of flow smoothing, but of storing large amounts of hydraulic energy with an energy density appropriate for applications such as regenerative braking in passenger vehicles. The envisioned high energy density accumulator will be appropriate for either series or parallel hydraulic hybrid vehicles. The metric for success of the project will be an experimental prototype capable of storing up to 200 kJ of energy (3500 lbs at 35 mph) at a peak power of 90 kW (35 mph to zero in 4.5 second) in a package of acceptable weight and volume for a compact to midsized passenger vehicle (accumulator system energy density >10 kJ/liter). This metric will enable implementation in a passenger vehicle for city driving. Additional significant benefits of this research potentially include solutions to more traditional accumulator problems including cost, pre-charge issues, and fluid contamination from gas diffusion through the bladder.

Project's Role in Support of the Strategic Plan

This project will contribute to the Center's goal of breaking the barrier of a lack of compact energy storage.  The task of designing new compact energy storage devices is central to the Center's vision of "significantly reducing energy consumption" by "enabling the migration of fluid power to passenger cars".  As identified in a recent NSF site visit, compact energy-dense storage solutions are critical to the success of this migration.  This project addresses the knowledge level of this goal (explore new energy density concepts) by seeking a design to provide the enabler (improve energy density of storage mechanisms) and ultimately the needed system capability (reduce size and weight of FP systems to work in passenger vehicles) for this important goal [4].  This project will be demonstrated on Test bed 3.

Description and Explanation of Research Approach

This project seeks to investigate, design and experimentally implement a compact energy storage accumulator via strain energy in materials not traditionally utilized in existing accumulators.  A control strategy and control laws for regulating power flow will be formulated and implemented.  Concerns regarding the efficiency of the hydraulic pump/motor will be out-of-scope and left to researchers in the efficiency thrust.

Hydraulic accumulators are energy storage devices commonly used to provide supplementary fluid power and absorb shock.  One particularly interesting recent application of these devices is regenerative braking.  Although a theoretically appealing concept, hydraulic regenerative braking is difficult to implement due to some major inherent weaknesses of conventional accumulators.  The primary weakness of spring piston accumulators that prohibits them from being used in hydraulic regenerative braking is their low gravimetric energy density.  Using linear analysis, spring steels and titanium alloys have a gravimetric energy density of around 1-1.5 kJ/kg [1].  Consequently, in order to store enough energy to bring a mid-sized 4-door sedan (mass=3500 lb (1590 kg)) to rest from 35 mph (15.65 m/s), the accumulator spring would have to weigh somewhere from 130 kg to 195 kg. In automotive manufacturing, where minimizing vehicle weight is vital, including such a heavy component would be largely impractical.

Gas bladder accumulators and piston accumulators with a gas pre-charge (PAGPs) use gas for energy storage and, therefore, are much lighter than their spring piston counterparts.  In these accumulators, a gas, separated by a bladder or a piston, occupies a certain volume of a container which is otherwise filled with an incompressible fluid.  As fluid is forced into this container, the gas inside the separated volume is compressed and energy is stored in the thermal domain (kinetic theory of gasses).  Such accumulators are subject to two serious drawbacks: 1) inefficiency due to heat losses, and 2) gas diffusion through the bladder into the hydraulic fluid.  The drawback of inefficiency via heat loss is mild, but the gas diffusion issues gives rise to high maintenance costs associated with "bleeding" the gas out of the fluid often.

With regard to inefficiency, if the energy stored in the compressed gas of such an accumulator is not retrieved soon, the heat flow from the gas to its immediate surrounding results in much less energy being retrieved.  Pourmovahed et al. showed that with as little as 50 seconds passing between gas compression and expansion, a piston-type gas accumulator's efficiency can fall to about 60% [1].  Since a vehicle remains immobile at a stop light for such a length of time or longer, this makes gas bladder and piston accumulators with a gas pre-charge less than ideal for hydraulic regenerative braking applications. Several methods to mitigate these heat losses have been proposed.  For PAGP, one promising method involves placing an elastomeric foam into the gas enclosure.  This foam serves the purpose of absorbing the generated heat during gas compression that would otherwise be transferred to the walls of the gas enclosure, and ultimately lost.  The foam is capable of collecting a large amount of this generated heat and returning it to the gas when the latter expands.  According to Pourmovahed, "the insertion of an appropriate amount of elastomeric foam into the gas enclosure...[can] virtually eliminate thermal loss" [2].  Incorporation of elastomeric foam has shown how accumulator efficiency can be vastly improved through slight modification.  However, this modification still does not solve the maintenance issues associated with gas diffusion.

The purpose of this research is to investigate a new method of energy storage in a hydraulic accumulator suited for use in hydraulic regenerative braking.  Unlike the use of foam, however, our approach departs from existing methods as opposed to modifying conventional technology.  The advocated technique involves using strain as the mechanism for energy storage, as in the case of spring piston actuators.  The difference from spring piston accumulators comes from the fact that an elastomer is chosen as the working material as opposed to a metal.  An elastomeric bag or bladder will be designed and tested for its capacity to store and return energy by stretching in response to a hydraulic fluid being pumped in and out of it.  This approach presents a new and unconventional method which aims to simultaneously avoid the susceptibility to heat losses and gas diffusion inherent to gas pre-charged accumulators, while attaining a higher gravimetric energy density than that of metallic spring piston accumulators.  This design fundamentally avoids the gas diffusion problem given that the pressure gradient between gas and hydraulic fluid is opposite of that of a gas charged accumulator.  Additionally, the design pursed will be advantageous due to low cost, relative simplicity and good manufacturability.

Material Selection
The selection of an appropriate energy storing material for the design of the high energy-density accumulator requires: 1) a high volumetric energy density, 2) a high gravimetric (or mass specific) energy density, 3) the ability to absorb and release the targeted power efficiently, 4) the ability to store the targeted energy efficiently for a duration on the order of minutes.  A promising candidate energy storing class of materials includes elastomers such as polyurethane, nitrile rubbers, polyisoprenes, and natural rubber.  Figure 1 below shows polyurethane as possessing an order a magnitude better volumetric energy density than steels (springs) and two orders of magnitude better gravimetric energy density than steels.  The high elongation percentage of polyurethane' (500% to 700%) allows for a straightforward accumulator design that directly stretches the energy storing material without utilizing a transformer to scale pressure and displacement.  Polyurethane also exhibits a fatigue strength of 5000 psi at 10,000,000 cycles.

Figure 1: Material property charts (Cambridge Engineering Selector, 2008) showing volumetric energy density, gravimetric energy density (top chart), fatigue strength at 10,000,000 cycles and elongation (bottom chart).

Scientific and Engineering Research Goals

• Scientific discovery: expand the field of knowledge of fundamental strain energy storage mechanisms in materials not traditionally considered for high energy density energy storage (such as elastomers and composites).
• Engineering discovery: utilize new fundamental models/understanding of high energy density energy storage for the design of a viable and cost effective hydraulic hybrid; ultimately expand the capabilities and application domain involving energy storage (contributing to the goal of hydraulic ubiquity)

References

[1]    M.F. Ashby, Materials Selection in Mechanical Design, Pergamon, Oxford, 1992.
[2]    Pourmovahed, A., Baum, S.A., Fronczak, F.J., and Beachley, N.H., 1988a. "Experimental Evaluation of Hydraulic Accumulator Efficiency With and Without Elastomeric Foam". Journal of Propulsion and Power, 4(2), March-April, pp. 188.
[3]    Pourmovahed, A., 1988b. "Energy Storage Capacity of Gas-Charged Hydraulic Accumulators". AIAA Thermophysics, Plasmadynamics and Lasers Conference, June 27-29, San Antonio, Texas. pp. 10..
[4]    K. Stelson, Center Overview & Infrastructure Presentation, NSF Site Visit, Minnesota, MN, February 20, 2008.