STIRLING ENGINES

In 1817 the Scottish minister Robert Stirling invented a "hot-gas motor".
Stirling engines theoretically achieve the highest possible energy conversion efficiency of all heat engines. They can use any kind of energy and used as a generator, motor, heat-pump or cooling system they produce extremely low emissions. Nevertheless the Stirling engines have still not achieved commercial breakthrough as a mass product.

Working Principles

Stirling engines as thermodynamic machines are subject to the law of Carnot, ideally achieving the efficiency (n):

n(th)=(T1-T2)/T1

T1 is the higher temperature in °K
T2 is the lower temperature in °K

With increasing temperature differences the theoretically achievable thermodynamic efficiency increases toward 100%. The development of Stirling engines over the last decades has focused on constructing high-efficiency machines through high temperature differences. The result were Stirling engines filled with helium/hydrogen and sealed, working with temperatures of up to 800°C, with a pressure of 50 to 190 bar.

Schematic Presentation

TWO PISTON TYPE STIRLING ENGINE

DISPLACER TYPE STIRLING ENGINE



Source: Stirling engine homepage

Model of a Stirling Engine

Source: Stirling engine homepage

Advantages

• They run quietly, with less vibration and they have an even torque.
• There are no valves, exhaust or spark plugs.
• Energy supply is external to the motor and generates less emission.
• A wide range of energy can be used with Stirling engines (fossil energy, solar, biomass).

Problems

• The heat exchanger of the engine does not work very efficiently due to high frequencies.
• The extremely high temperatures lead to high head loss.
• The high velocity of the working gas in the cylinder leads to high friction losses.
• The exchange of gas between the hot and cold zones of the cylinder is incomplete.
• At high pressure, leaking occurs frequently as do problems with the material, especially when conventional crank engines are involved.

Additional Uses

• They work well in applications like power generation on pleasure boats because they are very silent and sufficient cooling water is readily available.
• They would also work very well in airplanes where the air gets colder as the plane climbs to altitude. The planes would be very silent and also have low vibration levels.

Because of the high temperature, pressure and frequency for high-performance Stirling engines the costs are very high. The present trend is to build Stirling engines involving temperatures in excess of 1,000°C at the hot end and toward the employment of new materials (ceramic, special metals) and improved manufacturing technologies.

Sources:BOMIN SOLAR, SIGNON SAN DIEGO, FUTURE DRIVE, WORLD WATCH.

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