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This paper presents the design and dynamic modeling of a second generation prototype combined Stirling engine pump. The Stirling pump is intended to fill the technological gap of a compact high energy density power supply for untethered fluid power applications in the 50W to 500W range. Specifically, this prototype is intended as a compact and quiet, untethered, hydraulic power supply for an ankle foot orthosis testbed associated with the Center for Compact and Efficient Fluid Power. The energy source for the unit is flexible and can include propane, butane, methane, natural gas, or other high energy density hydrocarbon source of heat. The target output pressure of 7 MPa (1000 psig) is obtained from a pumping stage that is driven by a sealed engine stage that utilizes high pressure helium as the working fluid. The separate pumping stage utilizes the differential pressure swing inside the engine section to pump hydraulic fluid to the desired output pressure. This paper presents the system dynamic model of the Stirling pump, and includes (1) heat transfer from the heat source to the working fluid in the hot space of the engine, (2) heat transfer from the working fluid in the cold space of the engine to the heat sink, (3) energetically derived pressure dynamics in the hot and cold spaces, (4) mass flow around the displacer piston in between the hot and cold sides, (5) work output to the pump driving section, (6) pumping piston inertial dynamics, (7) flow losses through the pump's check valves, and (8) hydraulic power output. This dynamic model allows components of the Stirling pump to be sized. The paper includes results from the dynamic model.

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This paper presents the design and dynamic model of a novel “controlled Stirling power unit” with an independently controlled displacer piston. Breaking the coupling traditionally seen in Stirling devices between the power piston and the displacer piston, realized either kinematically or dynamically, allows an additional control degree of freedom that can be used to shape the thermodynamic cycle independent of the load. The device presented combines such a controlled Stirling engine (called a pressurizer) with a power extraction unit. The dynamic models of three different power extraction units are presented. The dynamic model builds on a previous experimentally validated first-principles model of a Stirling pressurizer. The model is a lumped parameter compressible fluid power dynamic model that captures the pressure dynamics of the high pressure helium working fluid as it is affected in time by volume, mass and heat flux changes. The dynamic model of a pressurizer combined with a linear electric generator is used to study different displacer motion profiles with regard to the shape of the thermodynamic cycle, and the effect on the power output and efficiency of the device.