Construction and testing of a low temperature differential Stirling engine for power generation 2
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
This thesis presents the design and construction of a low temperature differential (LTD) Stirling engine for electric power generation. The target energy sources were geothermal, industrial waste heat or solar heated water. These sources would supply a source temperature of around 90 °C. Assuming that the sink is kept at around 20 °C, the engine was designed based on a temperature difference of approximately 70 °C. The initial design and basic structure of the engine was completed in a previous project utilising first order design methods. The goal was to develop a low cost prototype engine capable of producing up to 500W electrical output power. A novel gamma type engine was proposed utilising a rotary reciprocating displacer and industrial steam piping to form a low cost pressurised chamber. This project concentrated on advancing the design and construction towards completion with particular emphasis on the electrical control, measurement/instrumentation components, and gas flow through the regenerator. At the completion of this project the displacer piston actuation system has been redesigned. In order to achieve the displacer’s specified 2 ㎐ actuation, both the displacer’s structure and the actuation system were altered. The displacer’s aluminium shell and foam centre were removed and replaced with a pine superstructure coated in depron foam, reducing the moment of inertia from 0.4488 ㎏ ∙ ㎡ to 0.0984 ㎏ ∙ ㎡. A secondary motor was added to the actuation system to increase the actuation power. The gearing ratio was also altered from 10:1 to 2:1 to increase the peak displacer speed. The regenerator was designed and built to suit the unusual wedge shape requirements of the original design. A ribbed structure was conceived to allow fluid flow to be manipulated within separate sections, producing an even pressure drop over varying regenerator lengths. Simulations were run to optimise both the number of sections and the mass of wire wool to be placed in each segment. The final regenerator design has axial ribs placed at radii of 93, 134, 192, 276 and 392mm, creating four sections. These sections are filled with 0.68, 0.97, 1.40 and 1.90kg of #0 mild steel wire wool. As Stirling engines are not self-starting the generator was required to be run as a motor when starting the Stirling engine. To achieve bidirectional flow of current within the starter motor/generator control system, a field oriented control (FOC) inverter from Texas Instruments was purchased and set up to run the 1kW, 3 phase, permanent magnet generator in both motor and generation modes. This will allow the Stirling engine to be brought up to speed with the generator operating as a motor and then switch to generation mode when the motoring current falls below a set limit. Both pressure and temperature measurement systems were developed, constructed and tested in order to collect information about the performance of the engine under operation. Three pressure transducer circuits were designed and constructed with measurement ranges of 10 ㎪, ±0.99 ㎪ and ±6.66 ㎪. These circuits were integrated with a PiocLog1012 analog to digital converter and PicoLog recording software. Eight K-type thermocouples were used for temperature recording. These were sampled with a Pico Technology TC-08 temperature thermocouple data logger which in turn was connected, via USB, to a computer running PicoLog Recorder software. Thus far all component testing has been carried out with test rigs that model the relevant parts of the engine. The displacer actuation system and phase angle control of the displacer and power piston has been tested. Temperature and pressure measurement systems have been independently tested. Motor/generator speed control and switching has been simulated and tested. Unfortunately completion of the engine assembly was not achieved within the scope of this project and therefore fully integrated testing of all components was not carried out. Once mechanical assembly is completed fully integrated testing of displacer actuation, piston position, generator speed control and measurement systems can be achieved.