Future Work

Without a doubt, building a high-power engine and assembling a complete solar — thermal-electric system is the most important task in pursuing the proposed technology. The following paragraphs suggest some areas of research that have the potential to help improve the engine design in many respects and to provide more practical designs and low-cost components for the system.

A flexure is an appropriate and low-cost replacement for the magnetic spring within the displacer subsystem. A flexure is very stiff in the radial direction and provides the required stiffness in axial direction to set the operating frequency. It obviates the linear motion ball bearing and the shaft, eliminating the sliding friction, enthalpy and conduction losses that are an inseparable part of the current single-phase engine prototype. Therefore a research and design effort in this area will be rewarding for the next generation engine. However, careful consideration is necessary for a reliable design which guarantees 25 to 30 years of continuous operation.

In the single-phase engine prototype, the displacer piston is part of an electromagnetic system and is actuated by flowing alternating current through the windings. The magnetic circuit is an air-core system and, hence, is not very efficient. Other actuation systems could replace the current mechanism to make it simpler and more efficient. For instance, piezoelectric actuation might be an appropriate candidate for the task. Hence, further studies in this direction could lead to outstanding solutions.

This dissertation outlined a low-temperature Stirling engine which, converts low- quality thermal power into mechanical work and then into electricity. Obviously, solar — thermal-electric power generation is not the only application for such an engine. Waste heat recovery from industrial plants or even geothermal resources are examples of other areas for which a low-cost low-temperature Stirling engine could find its niche applications in generating electricity from a source of energy that otherwise would be wasted. This, by itself, could be a great research work in future.

Table 6.1: Engine design parameters.

Working fluid



Th: 130 °C Tk: 27 °C


10 Hz


75 bars

Indicated powers

Adiabatic model Input: 15 kW Output: 2.4 kW


Open area: 250 cm2 Length: 1 cm

Hydraulic diameter: 0.1 mm Flow friction loss: 4.32 W Temperature drop: 0.5 ◦C


Open area: 250 cm2 Length: 1 cm

Hydraulic diameter: 0.1 mm Flow friction loss: 4.32 W Temperature drop: 0.5 ◦C


Open area: 600 cm2 Length: 1 cm

Hydraulic diameter: 70 fim Flow friction loss: 11.72 W Effectiveness: 98%

Displacer piston

Diameter: 12.7 cm Stroke: 4 cm

Power piston

Diameter: 15 cm Stroke: 4 cm

Gas spring hysteresis loss

33 W

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