Design and Validation of a High Energy Density Elastic Accumulator Using Polyurethane

Hydraulic accumulators (HAs) have been used successfully in regenerative braking systems by companies such as Ford and Eaton Corp. to increase fuel efficiency of heavy vehicles by as much as 25-35%. However, the relatively low gravimetric and volumetric energy densities of conventional HAs prohibit their use in average-sized passenger vehicles. In an attempt to address this problem, an elastomer will be used to construct a HA that will use strain as the primary energy storing mechanism. By using strain in the composition material, as opposed to compression of a precharged gas, this accumulator should virtually eliminate heat losses due to extended holding times. Because its gravimetric and volumetric elastic energy storage density values are among the highest of any material, polyurethane was the elastomer chosen as the constituent material. Using a curable type of polyurethane, an α-prototype is currently being manufactured to provide empirical data for validation. INTRODUCTION 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. A conventional braking system uses friction between brake pads and a brake disk to slow a vehicle down. This method results in energy being wasted as heat. In contrast, regenerative braking harnesses the kinetic energy of a vehicle during braking, instead of letting it dissipate in the form of heat. This collected energy can then be used in vehicle acceleration, thereby increasing the fuel efficiency. Hydraulic regenerative braking (HRB), specifically, decelerates wheel rotation by having the wheels pump a fluid into a device which resists this fluid’s flow and uses its power to harness energy. Although a theoretically appealing concept, hydraulic regenerative braking is difficult to implement due to some major inherent weaknesses of conventional accumulators. For example, spring piston accumulators use a spring attached to a piston to partition off a certain amount of volume in a container. As working fluid enters the container, it pushes on the piston, forcing the attached spring to contract. The contracting spring stores energy and provides a resistance to the entering fluid. The stored energy is retrieved when the pressure acting on the piston is reduced, allowing the spring to expand and push the piston towards its original position, thereby pushing the fluid back out of the container. The flow returns energy through fluid power. The primary weakness of these types of accumulators that prohibits them from being used in HRB 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 midsized 4-door sedan (mass=3500 lb (1590 kg)) to rest from 35 mph (15.65 m/s) in 2 seconds, 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. In compression, the gas serves a twofold purpose. First, it exerts a pressure which opposes and slows additional entry of fluid into the container. Additionally, the compressed gas stores energy from incoming fluid. Energy stored in this manner can be retrieved when the pressure exhorted on the volume the gas is contained in is lowered. When this occurs, the gas expands displacing some of the working fluid in the process, thereby returning energy through fluid power. There are several reasons that these two forms of accumulators are not suitable for use in HRB. For gas bladder accumulators there is the problem of gas diffusion across the bladder. This introduces gas bubbles into the working fluid which must be periodically removed. Additionally, both PAGPs and gas bladder accumulators are susceptible to large heat losses. When the gases in these accumulators are compressed, they heat up considerably. 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 (i.e., much lower efficiency). 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% [2]. Since it’s quite likely that a vehicle may remain immobile for around one minute, this makes gas bladder and piston accumulators with a gas pre-charge impractical for HRB 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” [3]. Incorporation of elastomeric foam has shown how accumulator efficiency can be vastly improved through slight modification, making this technology a prominent candidate for use in HRB. The purpose of this work is to propose another method of hydraulic accumulation suited for use in HRB. Unlike the use of foam, however, the proposed 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 will be tested on 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 avoid the susceptibility to heat losses inherent to gas pre-charged accumulators without foam, while attaining a higher gravimetric energy density than that of metallic springs. Additionally, the proposed design will be advantageous due to low cost, relative simplicity and good manufacturability. PRESCRIBED TARGET METRICS In order to ensure that the new accumulator design will be suitable for implementation in HRB, rough performance criteria were developed. The following target metrics were calculated to serve as guidelines during the design process: 1. Capable of storing 195 kJ of energy at a peak power of 195 kW 2. Volumetric energy density of 5 MJ/m or above 3. Gravimetric energy density of 5kJ/m or above The 195 kJ storage capacity requirement was arrived at by using the classical mechanics equation for kinetic energy, Eq. (1), where Ek is kinetic energy in J, m is mass in kg and v is velocity in m/s.