In this paper the design and operation of a prototype, high heat transfer, Stirling thermocompressor is described. The implementation of novel, deformable, large surface area, low dead volume, in-piston heat exchangers is described. Experimental data from the thermocompressor is compared with two models. One model is dynamic and takes mass flow restrictions and heat transfer limitations into account. The other model is quasi-static, derived from the ideal gas law, and assumes that mass flow is unrestricted and that the control volumes inside the thermocompressor are isothermal.
The models match experimental data, but there is some disagreement, possibly due to an air leak in the thermocompressor. The presence of this leak would describe why experimentally higher operational frequencies result in larger pressure swings while the model predicts the opposite (Figures 6.10 and 6.11). Future efforts will seek to remove this leak, increase the
1 1.1 1.2 1.3 1.4 1.5 1.6
0 0.5 1 1.5 2
Boost Pressure Ratio
Power (Watts)
Predicted Performance of Thermocompressor
h=10 W/(m2K), A=2,300 cm2, 2 Hz h=10 W/(m2K), A=2,300 cm2 10 Hz h=100 W/(m2K), A=61 cm2, 2 Hz h=100 W/(m2K), A= 61 cm2, 10 Hz
coefficient of heat transfer for the collapsible heat exchangers by preventing them from clumping, implement check valves, and refine the dynamic model.
It is demonstrated that higher rates of heat transfer are capable of producing larger pressure swings in the working fluid, and that the in-piston heat exchangers are capable of adding to this heat transfer capability.
Acknowledgement
This work was supported by the Center for Compact and Efficient Fluid Power, an NSF Engineering Research Center, grant EEC-0540834.
References
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7. Conclusion
Summary
The major contribution of this work is the unique design and control of three energy conversion devices. A prototype bridge vibration energy harvester, free-piston engine compressor, and Stirling thermocompressor were modeled, designed and constructed. Each device was fully instrumented and experimental data was produced to validate its dynamic model and evaluate its performance.
Although these projects differ in many important ways, this dissertation described how to cast widely different energy conversion devices into a common impedance matching framework.
The three energy conversion devices were presented and described in terms of this framework.
Each device emphasized different aspects of the three major conceptual components of this approach: the energetic source, the source impedance and the load impedance. By considering the relevant conceptual components for each device, insights were gained into the fundamental mechanisms needed to transfer energy across energetic domains.
The steps taken for the projects described in this document could be applied to a great number of energy conversion devices. The first step in applying this framework is to create a simplified dynamic model of the system so that a bond graph can be used to concisely show the transfer of energy between components. Although converting the system dynamics into bond graph form requires simplification of complex dynamics, this form is useful because the barriers to fast and efficient energy transfer can be quickly identified. In this form it is clear where energy crosses energetic domains, which is where major bottlenecks are likely to occur. In this form it also becomes clear that the project’s dynamics can be logically separated into common components. By converting the system to a Thevenin type electrical circuit, fundamental goals of the device become clear and decisions pertaining to active control or physical design can intelligently be made.
With some the energy conversion dynamics separated into an exogenous voltage source, this energetic source can be maximized without affecting the other dynamics. The components comprising the energy transporting source impedance can then be modified to properly mate the energy source to its intended destination. In some cases one or all three of the conceptual components are fixed and do not permit enhancement through design or control. In this instance it is important to identify these components’ dynamics so that its adjacent components can accommodate it.
Contribution
As summarized above, this work describes the design and control of energy conversion
devices. The key contributions for the five manuscripts are listed as follows:
Bridge Vibration Energy Harvester
Manuscript 1 (Chapter 2): From: Experimental Research Platform for Structural Health Monitoring, Babjak, B., Szilvasi, S., Pedchenko, A., Hofacker, M., Barth, E. J., Volgyesi, P., &
Ledeczi
Presentation of prototype bridge vibration energy harvester
o Compliant mechanisms utilized as low friction 1-DOF mechanism
o Linear motor used for power generation, active control, and as proof mass o Fully instrumented with linear potentiometer, encoder, and accelerometer, servo
amps, and data acquisition
Derived control using the maximum power transfer theorem for simplified harvester o Validated in simulation and experimentally
o Enables the collection of the maximum amount of available power across a broad spectrum of frequencies
o Cancels complex part of harvester’s impedance by eliminating inertial and stiffness elements
Manuscript 2 (Chapter 3): Multi-Domain Impedance Matching Applied to a Bridge Vibration Energy Harvester, Hofacker, M, Pedchenko, A, Barth, E. J.
Determined canonical control law for a revised bridge vibrational energy harvester prototype
o Verified analytically and in simulation
Found control law to approximate ideal behavior o Stable, causal controller
o Found through constrained optimization
o Shown in simulation to produce significantly more power than passive canonical alternative
Free-Piston Engine Compressor
Manuscript 3 (Chapter 4): Design and Validation of a Figure-Eight Free-Liquid-Piston Engine Compressor for Compact Robot Power, Barth, E. J., Hofacker, M, Kumar, N, and Manuscript 4 (Chapter 5): An Experimentally Validated Figure-Eight Free-Liquid-Piston Engine Compressor, Hofacker, M, Kumar, N, Barth, E. J.
Presentation of functional third generation prototype free-piston engine compressor o Standalone device with onboard data acquisition, control, and voltage regulation o Constructed custom printed circuit boards for signal conditioning and voltage
regulation