Stirlingmotor mit stapelförmigem Aufbau

Stirling engine with a stack-shaped structure

November 9, 2023

Principle and previous development

In 1980, Prof. Ivo Kolin first realized a heat engine based on the Stirling process, which used warm water as a heat source and thus worked with a temperature difference of less than 50° Celsius: a volume filled with air (working medium) that was closed off from the surroundings Shape of a flat cuboid or cylinder is bordered on two opposite sides by a cold and warm plate. The temperature difference of these plates corresponds to the working temperature difference of the machine. A displacement plate with a suitable thermal mass is intermittently moved back and forth between these surfaces and periodically brings the enclosed air into contact with the warm and cold heat exchanger plates. The heating and cooling results in a pressure fluctuation in the working gas, which acts on a coupled working piston and is thus converted into mechanical power. This machine produced a mechanical output of around 1 watt with a working volume of around 1.5 liters. The further development to even smaller drive temperature differences was carried out by James R. Senft, who rediscovered the old principle of pneumatic displacement control (according to O. Ringbom) and thus realized machines that can be driven by temperature differences of approx. 1° Celsius. However, the performance of these machines is only in the milliwatt range with a machine volume of approx. 0.1 liters.

Requirement

The requirement for a new motor is the ability to exploit temperature differences, such as those that occur as fluctuations in the outside air temperature in the day-night cycle, and at the same time achieve a mechanical output of approx. 10-100 watts.
At an assumed working temperature of

T_max = 20° C = 293 ° K

and

T_min = 10° C = 283 ° K

results in a maximum efficiency of

η <= 1 – T_min / T_max = 0,034

Under ideal conditions, a heat output of at least 30 watts must flow through the machine per watt of mechanical power generated. In real machines this value will certainly be a factor of 2 to 3 larger. In order to transfer this amount of heat, a correspondingly large dimensioning of the heat exchanger surfaces is essential and is the central design task.

New patented design of a low temperature Stirling engine

In order to provide the entire required heat exchanger performance, the entire working volume is divided into individual cells, each of which is bordered by two heat exchanger surfaces. The whole machine consists of a stack of alternating hot and cold heat exchanger plates. The heat is supplied or removed via two separate water circuits through the heat exchangers designed as hollow chamber panels. By stacking the exchanger plates, the entire required exchanger area can be accommodated in a compact machine. In between are the actual air-precipitated working volumes in which the displacement plates move back and forth discontinuously (due to cyclical pressure fluctuations - ring bomb coupling) and so the air periodically flows towards the warm or cold exchanger plates. This results in periodic heating or cooling of the working gas.
There are two separate displacement stacks. These stacks mechanically connect the displacement plates, which must move synchronously. The two stacks of plates are mounted separately on the top and bottom plates in order to achieve their opposite movement.
The individual cells are pneumatically coupled and perform work on a common working piston. According to the original Kolin idea, this is designed as a membrane with a working connecting rod and coupled to a flywheel on which mechanical work can be carried out. A patent was granted for this idea. The corresponding publication patent DE 195 34 379 A1 can be downloaded as a PDF document.