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Several power take-off (PTO) architectures for wave-powered reverse osmosis (RO) desalination of seawater are introduced and compared based on the annual average freshwater production and the size of the components, which strongly relate to the costs of the system. The set of architectures compared includes a novel series-type PTO architecture not previously considered. These seawater hydraulic PTO architectures are composed of a WEC-driven pump, an RO module, an intake charge pump driven by an electric motor, and a hydraulic motor driving an electric generator for electric power production. This study is performed using an efficient two-way coupled steady-state model for the average performance of the system in a given sea state, including freshwater permeate production, electric power production, and electric power consumption. A multi-objective design problem is formulated for the purposes of this comparative study, with the objectives of maximizing annual freshwater production, minimizing the displacement of the WEC-driven pump, and minimizing the installed RO membrane area. This establishes a framework for comparison in the absence of a mature techno-economic model. The requirement that the system produces enough electric power to meet its consumption is applied as a constraint on the operation of the system. The oscillating wave surge converter Oyster 1 is assumed as the WEC. Weights on performance of the system in a given sea state are based on historical data from Humboldt Bay, CA. This study finds that (1) architectures in a series configuration allow for a reduction in the WEC-driven pump size of 59–92% compared to prior work, (2) varying the displacement of the WEC-driven pump between sea conditions does not provide any significant advantage in performance, and (3) varying the active RO membrane area between sea condition offers improvements between 7% and 41% in each design objective. [ABSTRACT FROM AUTHOR]

Previous work comparing power take-off (PTO) architectures for ocean wave-powered reverse osmosis suggests that variable displacement in the wave energy converter (WEC)-driven pump does not offer a significant performance advantage. A limitation of that study is that the WEC was subject to a constant load within a given sea state ("Coulomb damping") and did not account for controlled, moment-to-moment variation of the PTO load enabled by a variable displacement pump. This study explores the potential performance advantage of a variable PTO load over Coulomb damping. Model predictive control is used to provide optimal load control with constraints on the PTO load. The constraints include minimum and maximum loads and a limit on the rate of load adjustment. Parameter studies on these constraints enable conclusions about PTO design requirements in addition to providing an estimated performance advantage over Coulomb damping. Numerical simulation of the Oyster 1 WEC is carried out with performance weighted by historical sea state data from Humboldt Bay, CA. The results show a performance advantage of up to 20% higher yearly-average power absorption over Coulomb damping. Additionally, the parameter studies suggest that the PTO load should be adjustable down to at least 25% of the maximum load and should be adjustable between the minimum and maximum loads within a few seconds. [ABSTRACT FROM AUTHOR]

Reed valves are a type of check valve commonly found in a wide range of applications including air compressors, internal combustion engines, musical instruments, and even the human heart. While reed valves have been studied extensively in these applications, published research on the modeling and application of reed valves in hydraulic systems is sparse. Because the spring and mass components of a reed valve are contained in a single element, it is light and compact compared to traditional disk, poppet, or ball style check valves. These advantages make reed valves promising for use in high-frequency applications such as piston pumps, switch-mode hydraulics, and digital hydraulics. Furthermore, the small size and fast response of reed valves provide an opportunity to design pumps capable of operating at higher speeds and with lower dead volumes, thus increasing efficiency and power density. In this paper, a modeling technique for reed valves is presented and validated in a hydraulic piston pump test bed. Excellent agreement between modeled and experimentally measured reed valve opening is demonstrated. Across the range of experimental conditions, the model predicts the pump delivery with an error typically less than 1% with a maximum error of 2.2%. [ABSTRACT FROM AUTHOR]

High power density, good controllability, and low cost are the most appealing characteristics that make hydraulic systems the best choice for many applications. Current state-of-the-art hydraulic variable displacement pumps show high efficiency at high displacement while they have poor efficiencies at low displacement. This paper proposes a novel alternating flow (AF) variable displacement hydraulic pump to: 1) eliminate metering losses by acting as a high-bandwidth pump for displacement control; 2) achieve high efficiency across a wide range of operating conditions and displacements; and 3) allow multiple units to be easily common-shaft mounted for a compact multi-actuator displacement control system from a single prime mover. A dynamic model using first principles describes the cylinder pressure, flows between pairs of cylinders, and net inlet and outlet flows as a function of the pumps phase-shift angle. The model captures hydraulic check valve dynamics, the effective bulk modulus, leakage flows, and viscous friction. Piston kinematics and dynamics are discussed and energy loss models are presented. A first prototype of the AF pump was constructed from two inline triplex pumps that were modified so that three piston pairs were created. Experiment tests were conducted and used to validate the model. After model validation, simulation results from the model can offer an initial evaluation of this novel pump concept and potential applications and can be used to guide the design for future prototype of the AF hydraulic pump. [ABSTRACT FROM AUTHOR]

A digital displacement hydraulic motor uses actively controlled on/off valves instead of a valve plate for porting fluid. Displacement can then be varied by varying the timing of the on/off valves. To minimize throttling energy loss, the valve timing must also consider the precompression and decompression of the compressible fluid. In this paper, a rotary on/off valve, whose rotation controls the valve timing, is used. The desired motion of the rotary valve that encodes the ideal valve timing is a function of, and is periodic with respect to, the motor crank shaft angle. As the desired valve trajectory is not periodic in time unless the motor speed is constant, the conventional internal model based repetitive control is not applicable. A recently developed angle-domain repetitive control is used instead to control the rotary valve motion to achieve the desired valve timing. Experiments show that the proposed controller achieves good tracking performance for a broad range of varying motor speeds. The maximum error is reduced by six to ten times with the new control scheme over baseline fixed period repetitive or proportional-integral controllers. This in turn leads to efficiency gains for the digital hydraulic motor up to 45%. [ABSTRACT FROM AUTHOR]

McKibben actuators, comprised of an elastomeric tube wrapped in inextensible fibers, were developed in the 1950s and have been used primarily in pneumatic applications. Mc-Kibben actuators provide superior force and power density at a lower cost compared to many conventional actuators due to their simple design. This paper explores driving McKibben actuators with high-pressure hydraulics and reflects on some challenges associated with modeling the actuators at hydraulic pressures. Twelve actuators were designed, fabricated, and tested experimentally over a range of contraction ratios. Four modeling approaches were evaluated against experimental data including a new model that eliminates some of the limitations of existing models by predicting output force as a function of initial actuator geometry only and capturing the nonlinear variation in elastomer wall thickness as the actuator is strained. The experimental results suggest that the existing McKibben force models presented in this paper are not as accurate as the new model for tracking total axial force generation. The absolute value of the error from experimental values of each of the four models ranged from 9.1% for the new model developed in this paper to 9.9%, 10.0%, and 10.5% for the three other existing approaches, but the new model has unique advantages in modeling hydraulic McKibben actuators that require thicker tube walls. This study lays the ground work for future research in modeling soft robots comprised of compliant actuators powered hydraulically at high pressures. [ABSTRACT FROM AUTHOR]

Switch-mode hydraulic control is a compact and theoretically efficient alternative to throttling valve control or variable displacement pump control. However, a significant source of energy loss in switch-mode circuits is due to throttling during valve transitions. Hydraulic soft switching was previously proposed as a method of reducing the throttling energy loss, by absorbing, in a small variable volume chamber, the flow that would normally be throttled across the transitioning high-speed valve. An active locking mechanism was previously proposed that overcomes the main challenge with soft switching, which is a lock mechanism that releases quickly and with precise timing. This prior work demonstrated a reduction in energy losses by 66% compared to a control circuit. In this paper, a numerical model is developed for a switch-mode virtually variable displacement pump (VVDP) circuit, utilizing the proposed soft switch. The model is then used as a means of designing a proof of concept prototype to validate the model. The prototype design includes methods for controlling the soft switch spring preload, travel distance, piston displacement required to unlock the soft switch, valve command duty cycle, switching cycle period, and load pressure. Testing demonstrated that the soft switch circuit performed as expected in a baseline condition. The operating region for this prototype was found to be quite narrow. However, the model does a good job of predicting the displacement of the soft switch. [ABSTRACT FROM AUTHOR]

Switched inertance hydraulic systems (SIHS) use inductive, capacitive, and switching elements to boost or "buck" (reduce) a pressure from a source to a load in an ideally lossless manner. Real SIHS circuits suffer a variety of energy losses, with throttling of flow during transitions of the high-speed valve resulting in as much as 44% of overall losses. These throttling energy losses can be mitigated by applying the analog of zero-voltage-switching, a soft switching strategy, adopted from power electronics. In the soft switching circuit, the flow that would otherwise be throttled across the transitioning valve is stored in a capacitive element and bypassed through check valves in parallel with the switching valves. To evaluate the effectiveness of soft switching in a boost converter SIHS, a lumped parameter model was constructed. Simulation demonstrates that soft switching improves the efficiency of the modeled circuit by 42% at peak load power and extends the power delivery capabilities by 77%. [ABSTRACT FROM AUTHOR]

The bulk modulus of hydraulic fluid is dependent on the quantity of entrained gas in the fluid. In this paper, an effective fluid bulk modulus model that captures dynamic gas absorption during pressure transients is derived from the overall mass transfer theory. Optical measurement of a microgas bubble volume is used to determine the interfacial mass transport. Compared to traditional models, the proposed model is able to capture the 10% gap in the pressure profile between the first and second cycles, when simulating multiple compression cycles of an oil sample with 0.65% entrained gas by volume at 8MPa. [ABSTRACT FROM AUTHOR]

In order to improve the thermodynamic efficiency of an internal combustion engine (ICE), a Stephenson-III six-bar linkage is optimized to serve as a replacement for the traditional slider-crank. Novel techniques are presented for formulating the design variables in the kinematic optimization that guarantee satisfaction of the Grashof condition and of transmission angle requirements without the need for an explicit constraint function. Additionally, a nested generalization of the popular NSGA-II algorithm is presented that allows simultaneous optimization of the kinematic, dynamic, and thermodynamic properties of the mechanism. This approach successfully solves the complex six-objective optimization problem, with challenges for future refinement including improvement of the combustion simulation to attain better accuracy without prohibitive computational expense. [ABSTRACT FROM AUTHOR]

Interrupted-plate heat exchangers are used as regenerators for absorbing and releasing thermal energy such as in a Compressed Air Energy Storage (CAES) system in which the exchanger absorbs energy to cool the air being compressed or liberates energy to heat the air upon expansion. The exchanger consists of layers of thin plates in stacked arrays. In a given layer, the plates are parallel to one another and parallel to the exchanger axis. Each successive layer is rotated to have its plates be perpendicular to those of the layer below but still parallel to the exchanger axis. As flow passes from one layer to the next, new thermal boundary layers develop, beneficial to effective heat transfer. The interrupted-plate heat exchanger can also be seen as a porous medium. As such, it demonstrates strong anisotropic behavior when flow approaches the plates in a direction other than axially. Thus, pressure drops and heat transfer coefficients are dependent upon the attack angle. Mathematical models for anisotropic pressure drop and heat transfer behavior are proposed based on numerical calculations on a Representative Elementary Volume (REV), the unit cell model of the interrupted-plate medium. The anisotropic pressure drop is modeled by the traditionally used Darcy and inertial terms, with the addition of another term representing mixing effects. Heat transfer between the fluid and the plates is formulated in terms of Nusselt number vs. Reynolds number and approach angle of the mean flow. These models are used when solving, on the scale of the heat exchanger application, the volume-averaged Navier-Stokes equations that treat the exchanger region as a continuum. The analysis of the heat exchanger is used for design and optimization of the medium. [ABSTRACT FROM AUTHOR]