Membrane Stirling Engine Isothermal Efficiency
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Solution Overview
Problem
Classic Stirling engines face limitations such as large temperature differences between heat exchangers and working gas, non-isothermal expansion and compression, and unavoidable dead volumes, which restrict their efficiency to a maximum of 50% of theoretical Carnot efficiency.
Innovation Solution
A diaphragm Stirling engine design featuring membrane envelopes with negligible flexural rigidity, hermetically closed at one end, and open to a regenerator box, where the working gas is enclosed in thin-walled membrane bags that immerse in hot or cold liquid, utilizing hydraulic pistons connected via an eccentric for phase-shifted movement, achieving isothermalization and minimizing dead volumes through pulsating membrane bags for enhanced heat transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If rigid cylinders and heat exchangers are used in classic Stirling engines, then structural strength and pressure resistance are improved, but dead volumes increase and heat exchange efficiency deteriorates
Solution Approach 1:
The patent replaces rigid cylinders and heat exchangers with flexible membrane envelopes that have negligible flexural rigidity. These thin-walled membranes allow direct contact between the working gas and heat transfer surfaces, eliminating dead volumes and improving heat exchange efficiency while maintaining pressure resistance through the hydraulic liquid environment.
2Area of stationary object
If finned heat exchangers are used to increase heat transfer surface, then heat exchange area is improved, but dead volumes and pressure losses increase
Solution Approach 1:
The membrane envelopes provide a large heat transfer surface area directly in contact with the working gas, eliminating the need for finned structures. The thin-walled membranes collapse to zero volume during operation, preventing the accumulation of dead volumes that would occur with rigid finned heat exchangers.
3Strength
If rigid displacement pistons are used, then mechanical strength is improved, but dead volumes between piston and cylinder walls increase
Solution Approach 1:
The flexible membrane bags replace rigid displacement pistons, allowing the working gas volume to change without creating dead spaces. The membranes can fully collapse to zero volume, eliminating the dead volumes that inevitably exist between rigid pistons and cylinder walls.
4Power
If polytropic expansion and compression are used instead of isothermal processes, then mechanical work output is improved, but thermal efficiency deteriorates
Solution Approach 1:
The membrane envelopes enable continuous isothermal expansion and compression by maintaining direct thermal contact with the heat transfer liquid throughout the entire cycle. The flexible membranes allow the gas to expand and compress while continuously exchanging heat, achieving true isothermal processes rather than polytropic cycles.
5Productivity
If large temperature differences between heat exchangers and working gas are used, then heat transfer rate is improved, but thermal efficiency deteriorates
Solution Approach 1:
The thin-walled membrane envelopes provide excellent thermal contact with the heat transfer liquid, enabling efficient heat transfer at small temperature differences. The membranes' high surface area to volume ratio and direct contact with the liquid allow rapid heat exchange without requiring large ΔT, thus maintaining both high heat transfer rate and high thermal efficiency.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design achieves significantly higher Carnot efficiency, up to 80% or more, allowing for efficient mechanical and thermal energy conversion at lower temperatures, and enables the use of medium-temperature heat sources like solar energy, industrial waste heat, or geothermal heat, with improved heat transfer and reduced material costs.
Implementation Method 1
very good heat transfer from hot or cold fluid through the thin membrane into the working gas
Implementation Method 2
hydraulic pistons (or similar technical means such as bellows, hydraulic cushions and the like), which can displace exactly the volume of liquid corresponding to half the maximum gas volume in the membrane bags
Implementation Method 3
an interposed heat regenerator, and working pistons for transferring the thermal pressure fluctuations of the Gas generated work outwards
Data Source
Figure 1~2
Figure 3~4
Figure 5~7
AI summary
The invention relates to a membrane Stirling engine, comprising a working gas, a hot part and a cold part, the working gas of the Stirling engine being located in membrane casings both in the hot part and in the cold part of the engine, said casings having two ends. The casings are hermetically closed on one end and open on the opposite end, the open end leading to the hot or cold chamber of a regenerator box in a tightly sealing manner.