TSGC Advanced Design Project:
Stirling Engine Demonstration

April 28, 1997
TCU Stirling Team Members
Tanya Hardy
David Meek
Nathan Moser
Majin Sierra
Winyu Vongstapanalert
Greg White
Aaron Williams

     The seven-member team at Texas Christian University has been working on the Stirling engine contribution to the Texas Space Grant Consortium's Advanced Design Project. This semester's activities consisted of studying the feasibility of using Stirling Engines as an alternative means of solar power collection.

     Everyday billions of Watts-hours of solar energy passes by our planet and into the rest of the galaxy. This energy could be harnessed to drive a Stirling motor; this "free" energy could then be transferred back to earth through microwaves. The mirror system can be one large mirror or a series of small individual mirrors focusing the sun's energy on the engine's receiver. The system needs a sun tracking system to rotate the solar concentrator about two axes in order to keep it pointed directly to the sun. In 1984, a terrestrial Stirling engine with a concentrating system was fabricated and implemented which demonstrates the awesome potential of the Stirling Engine.

     The system consisted of two main components:

  • Engine: United Stirling 4-95 MKII by McDonnell Douglas
  • Concentrator: Vanguard I by Advanco

     This system holds the world record of converting 29.4% of the collected solar energy into electricity producing 27.1 kW. The mass of the engine weighed 330 kg, and the concentrator was 10 meter in aperture diameter giving it a reflective surface of 91.4 m

     The basic design for the Stirling Engine was conceived and implemented in the early nineteenth century, years before the invention of the internal combustion engine. The two main types of Stirling engines are the kinematic and free-piston Stirling engines. Both types operate on the same thermodynamic cycle. For convenience, the free-piston set-up will be used to illustrate the cycle. The free-piston Stirling engine consists of a single cylinder, a displacer, a piston, and a heater. The displacer fits inside the cylinder loosely so that air can flow past it as it moves back and forth. The piston is sealed against the cylinder walls much like a piston in a car's engine. The piston is able to move up and down the shaft the displacer is connected to. The working gas contained in the engine is alternately heated and cooled, over and over again. When the gas is heated, its pressure rises, and it "pushes" against and moves the piston, thereby doing work. After the piston has been pushed as far as it will go, the gas is then cooled and its pressure drops, "sucking" the power piston back to its starting position at which point the heating cycle begins again. This cycle repeats very rapidly, sometimes as fast as fifty times a second. This is done by having one part of the engine at a constant, high temperature (the heater end, heated by the sun) and another part at a constant low temperature, and moving the gas back and forth between them.

     In order to better understand how a Stirling motor performs, we have purchased a model engine with a solar concentrator and also fabricated our own engine. The model Stirling engine, the Sun Runner, was purchased by our team from Solar Engines, a division of P.M. Research Inc. Attached to the motor is a parabolic mirror used to focus the sun's rays on the receiver. The heat is converted to rotary motion by the expansion and compression of the working fluid. We have fabricated our own Stirling engine with assistance from the machine shop at Texas Christian University and Nathan Widner, a local area machinist. This engine is based on previously tested and working motors. However, some modifications were made, such as material selection and the addition of a pressure release in the piston.

     In addition to studying and fabricating actual stirling engines, preliminary analysis has been performed regarding the benefits and challenges of using Stirling Engines as an alternative to solar cells for space power generation. Details of this analysis can be found in our final report.

TSGC Advanced Design Program


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