High Power Stirling Engine Design

This dissertation provided a strong basis for the design of a high power Stirling engine that could be a potential candidate for commercial utilization in the proposed solar-thermal — electric technology. The goal is to design a Stirling engine with 2 to 3 kW output power. It is desired to keep the operating frequency below audible range. In addition, it is desired to keep the flow friction dissipation below 25 W, as that is the main loading for the displacer piston, and hence, for its actuator. The flow friction losses are strongly dependent on the flow speed, which is partly dictated by the frequency of operation. Therefore, 10 Hz appears to be an appropriate choice of operating frequency.

Output power of the engine is, to the first order, proportional to the displacer swept volume. To keep the cost low, it is proposed to use flexures to provide the bearing function for the displacer mass-spring subsystem rather than a linear motion ball bearing. Flexures are simple and very easy to manufacture. However, it appears to be a challenge to design a flexure for large piston excursions. Hence, the displacer piston excursion and diameter are chosen to be 2 cm and 13 cm, respectively. Based on these dimensions, the mean pressure of the working fluid needs to be about 75 bars to produce 2.5 kW of mechanical work. Table 6.1 tabulates the dimensions of a possible design and corresponding calculated non­ideal effects of the engine components. This design is projected to achieve thermal efficiency of 16% that is about 65% of the Carnot efficiency at 27°C and 130°C temperatures. As expected from the foregoing theoretical models and experimental efforts, the dissipative

High Power Stirling Engine Design

Figure 6.1: Energy balance diagram for the high-power Stirling engine. Compare to Fig­ure 3.3.

Effects of the engine are a small fraction of its output power (2.2% in this design) at higher pressures. Figure 6.1 depicts the power balance diagram of the design indicating the loss contributions of different components.

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