James D. Van de Ven

OPEN ACCUMULATOR CONCEPT FOR COMPACT FLUID POWER ENERGY STORAGE

Energy storage devices for fluid power applications that are significantly more compact than existing ones will enable energy regeneration for many applications, including fluid power hybrid vehicles and construction equipment. The current approach to hydraulic energy storage makes use of a compressed gas enclosed in a closed chamber. As the system must contain the expanded gas and the hydraulic oil displaced, the optimal energy density occurs at a modest expansion ratio resulting in a small energy density. By allowing intake and exhaust of compressed and expanded air from and to the atmosphere, a potential order of magnitude increase in energy density is available in the new open accumulator approach. Potential methods for realizing the new configuration are described. Analysis and simulation case studies illustrate both the advantages and challenges of the new approach.Copyright

High Speed Rotary Pulse Width Modulated On/Off Valve

A key enabling technology to effective on/off valve based control of hydraulic systems is the high speed on/off valve. High speed valves improve system efficiency for a given PWM frequency, offer faster control bandwidth, and produce smaller output pressure ripples. Current valves rely on the linear translation of a spool or poppet to meter flow. The valve spool must reverse direction twice per PWM cycle. This constant acceleration and deceleration of the spool requires a power input proportional to the PWM frequency cubed. As a result, current linear valves are severely limited in their switching frequencies. In this paper, we present a novel fluid driven PWM on/off valve design that is based on a unidirectional rotary spool. The spool is rotated by capturing momentum from the fluid flow through the valve. The on/off functionality of our design is achieved via helical barriers that protrude from the surface of a cylindrical spool. As the spool rotates, the helical barriers selectively channel the flow to the application (on) or to tank (off). The duty ratio is controlled by altering the axial position of the spool. Since the spool no longer accelerates or decelerates during operation, the power input to drive the valve must only compensate for viscous friction, which is proportional to the PWM frequency squared. We predict that our current design, sized for a nominal flow rate of 40l/m, can achieve a PWM frequency of 84Hz. This paper presents our valve concept, design equations, and an analysis of predicted performance. A simulation of our design is also presented.Copyright

Laser Transmission Welding of Thermoplastics—Part I: Temperature and Pressure Modeling

This paper discusses the development of a model of laser transmission welding that can be used as an analytical design tool. Currently the majority of laser transmission welding (LTW) applications rely on trial and error to develop appropriate process parameters. A more rigorous design approach is not commonly used primarily due to the complexity of laser welding, where small material or process parameter changes can greatly affect the weld quality. The model developed in this paper also enables optimizing operating parameters while providing monetary and time saving benefits. The model is created from first principles of heat transfer and utilizes contact conduction that is a function of temperature and pressure, Gaussian laser distribution, and many material properties that vary with temperature including the absorption coefficient. The model is demonstrated through a design example of a joint between two polyvinyl chloride parts. The model is then validated with samples welded with a diode laser system using the operating parameters developed in a design example. Using the weld width as the primary output, the error between the model and the experimental results is 4.3%, demonstrating the accuracy of the model.

Design, Modeling, and Validation of a High-Speed Rotary Pulse-Width-Modulation On/Off Hydraulic Valve

Efficient high-speed on/off valves are an enabling technology for applying digital control techniques such as pulse-width-modulation (PWM) to hydraulic systems. Virtually variable displacement pumps (VVDPs) are one application where variable displacement functionality is attained using a fixed-displacement pump paired with an on/off valve and an accumulator. High-speed valves increase system bandwidth and reduce output pressure ripple by enabling higher switching frequencies. In addition to fast switching, on/off valves should also have small pressure drop and low actuation power to be effective in these applications. In this paper, a new unidirectional rotary valve designed for PWM is proposed. The valve is unique in utilizing the hydraulic fluid flowing through it as a power source for rotation. An unoptimized prototype capable of high flow rate (40 lpm), high speed (2.8 ms transition time at 100 Hz PWM frequency), and low pressure drop (0.62 MPa), while consuming little actuation power (<0.5% full power or 30 W, scavenged from fluid stream), has been constructed and experimentally validated. This paper describes the valve design, analyzes its performance and losses, and develops mathematical models that can be used for design and simulation. The models are validated using experimental data from a proof-of-concept prototype. The valve efficiency is quantified and suggestions for improving the efficiency in future valves are provided. [DOI: 10.1115/1.4006621]

Increasing Hydraulic Energy Storage Capacity: Flywheel-Accumulator

AbstractThe energy storage density of hydraulic accumulators is significantly lower than energy storage devices in other energy domains. As a novel solution to improve the energy density of hydraulic systems, a flywheel-accumulator is presented. Energy is stored in the flywheel-accumulator by compressing a gas, increasing the moment of inertia of the flywheel by adding hydraulic fluid, and by increasing the angular velocity of the flywheel. Through a numerical model of the energy flows in the system, the energy storage of the flywheel-accumulator was demonstrated to be approximately 10 times greater than a conventional accumulator. Furthermore, the flywheel-accumulator allows the hydraulic system pressure to be independent of the quantity of energy stored. The integral flywheel-accumulator presents numerous future research challenges, yet offers the potential to transform and enable numerous applications including plug-in hydraulic hybrid vehicles.Abstract The energy storage density of hydraulic accumulators is significantly lower than energy storage devices in other energy domains. As a novel solution to improve the energy density of hydraulic systems, a flywheel-accumulator is presented. Energy is stored in the flywheel-accumulator by compressing a gas, increasing the moment of inertia of the flywheel by adding hydraulic fluid, and by increasing the angular velocity of the flywheel. Through a numerical model of the energy flows in the system, the energy storage of the flywheel-accumulator was demonstrated to be approximately 10 times greater than a conventional accumulator. Furthermore, the flywheel-accumulator allows the hydraulic system pressure to be independent of the quantity of energy stored. The integral flywheel-accumulator presents numerous future research challenges, yet offers the potential to transform and enable numerous applications including plug-in hydraulic hybrid vehicles.

Laser Transmission Welding of Thermoplastics—Part II: Experimental Model Validation

Two laser transmission welding experiments involving polyvinyl chloride are presented that aim to validate a previously presented welding model while helping to further understand the relationship between welding parameters and weld quality. While numerous previous research papers have presented the results of laser welding experiments, there exists minimal work validating models of the welding process. The first experiment explores the interaction of laser power and welding velocity while the second experiment explores the influence of clamping pressure. Using the weld width as the primary model output, the agreement between the welding experiments and the model have an average error of 5.6%. This finding strongly supports the validity of the model presented in Part I of this two paper set (Van de Ven and Erdman, 2007, ASME J. Manuf. Sci. Eng., 129, pp. 849-858). Additional information was gained regarding the operating window for laser transmission welding and the thermal decomposition of polyvinyl chloride. Clamping pressure was found to provide a small, but not statistically significant, influence on the visual appearance, weld width, and weld strength.

Bridging Gaps in Laser Transmission Welding of Thermoplastics

Gaps at part interfaces pose a major challenge for laser transmission welding (LTW) of thermoplastics due to the reliance on contact conduction between the absorptive and transmissive parts. In industrial applications, gaps between parts can occur for a variety of tolerance and process control reasons. Previous experimental and modeling work in LTW has focused on gap-free joints, with little attention to bridging a gap with thermal expansion of the absorbing material. A two-dimensional comprehensive numerical model simulated bridging gaps in LTW. Using the model, operating parameters were selected for welding across a 12.7 μm gap and a 25.4 μm gap by creating sufficient thermal strain to bridge the gap and form a weld. Using these operating parameters, PVC samples were welded in a T-joint geometry with a designed gap. The quality of the welds was assessed visually, by destructive force testing and by measuring the weld size to quantify the weld strength. All the experimental samples, for the two gap sizes, bridged the gap and formed welds. The average weld strength of the 12.7 μm gap samples was 16.1 MPa, while the 25.4 μm gap samples had an average strength of 10.0 MPa. Gaps were successfully bridged with LTW by using a two-dimensional model to design the operating parameters. To achieve higher modeling accuracy, a three-dimensional model might better simulate the thermal diffusion in the direction of laser travel.

Phase-shift high-speed valve for switch-mode control

Hydraulic applications requiring a variation in the speed or torque of actuators have historically used throttling valve control or a variable displacement pump or motor. An alternative method is switch-mode control that uses a high-speed valve to rapidly switch between efficient on and off states, allowing any hydraulic actuator to have virtually variable displacement. An existing barrier to switch-mode control is a fast and efficient high-speed valve. A novel high-speed valve concept is proposed that uses a phase shift between two tiers of continuously rotating valve spools to achieve a pulse-width modulated flow with any desired duty ratio. An analysis of the major forms of energy loss, including throttling, compressibility, viscous friction, and internal leakage, is performed on a disk spool architecture. This analysis also explores the use of a hydrodynamic thrust bearing to maintain valve clearance. A nonoptimized design example of a phase-shift valve operating at 100 Hz switching frequency at 10 l/min demonstrates an efficiency of 73% at a duty ratio of 1 and 64% at 0.75 duty ratio. Numerous opportunities exist for improving this efficiency including design changes and formal optimization. The phase-shift valve has the potential to enable switch-mode hydraulic circuits. The valve has numerous benefits over existing technology yet requires further refinement to realize its full potential.

Design of an Interrupted-Plate Heat Exchanger Used in a Liquid-Piston Compression Chamber for Compressed Air Energy Storage

In the Compressed Air Energy Storage (CAES) approach, air is compressed to high pressure, stored, and expanded to output work when needed. The temperature of air tends to rise during compression, and the rise in the air internal energy is wasted during the later storage period as the compressed air cools back to ambient temperature.The present study focuses on designing an interrupted-plate heat exchanger used in a liquid-piston compression chamber for CAES. The exchanger features layers of thin plates stacked in an interrupted pattern. Twenty-seven exchangers featuring different combinations of shape parameters are analyzed. The exchangers are modeled as porous media. As such, for each exchanger shape, a Representative Elementary Volume (REV), which represents a unit cell of the exchanger, is developed. The flow through the REV is simulated with periodic velocity and thermal boundary conditions, using the commercial CFD software ANSYS FLUENT. Simulations of the REVs for the various exchangers characterize the various shape parameter effects on values of pressure drop and heat transfer coefficient between solid surfaces and fluid. For an experimental validation of the numerical solution, two different exchanger models made by rapid prototyping, are tested for pressure drop and heat transfer. Good agreement is found between numerical and experimental results. Nusselt number vs. Reynolds number relations are developed on the basis of pore size and on hydraulic diameter.To analyze performance of exchangers with different shapes, a simplified zero-dimensional thermodynamic model for the compression chamber with the inserted heat exchange elements is developed. This model, valuable for system optimization and control simulations, is a set of ordinary differential equations. They are solved numerically for each exchanger insert shape to determine the geometries of best compression efficiency.Copyright

On fluid compressibility in switch-mode hydraulic circuits-part I: Modeling and analysis

Fluid compressibility has a major influence on the efficiency of switch-mode hydraulic circuits due to the release of energy stored in fluid compression during each switching cycle and the increased flow rate through the high-speed valve during transition events. Multiple models existing in the literature for fluid bulk modulus, the inverse of the compressibility, are reviewed and compared with regards to their applicability to a switch-mode circuit. In this work, a computational model is constructed of the primary energy losses in a generic switch-mode hydraulic circuit with emphasis on losses created by fluid compressibility. The model is used in a computational experiment where the system pressure, switched volume, and fraction of air entrained in the hydraulic fluid are varied through multiple levels. The computational experiments resulted in switch-mode circuit volumetric efficiencies that ranged from 51% to 95%. The dominant energy loss is due to throttling through the ports of the high-speed valve during valve transition events. The throttling losses increase with the fraction of entrained air and the volume of fluid experiencing pressure fluctuations, with a smaller overall influence seen as a result of the system pressure. The results of the computational experiment indicate that to achieve high efficiency in switch-mode hydraulic circuits, it is critical to minimize both the entrained air in the hydraulic fluid and the fluid volume between the high-speed valve and the pump, motor, or actuator. These computational results are compared to experimental results in part II of this two part paper series.