Isothermal bubble pump heat engine

A bubble pump comprises a tube filled with alternating slugs of liquid and gas. The tube must be narrow enough so that surface tension keeps the liquid slugs completely isolated from one another. Preferably, the liquid should not wet the walls of the tube.

The isothermal bubble pump heat engine is an amazing device. It comprises a hot heat source, cold heat sink, many identical cleverly shaped loops of sealed tube containing alternating slugs of a heavy liquid and the working gas threaded through the heat source, heat exchanger, and heat sink, and finally, a means for extracting energy from the motion of the liquid. For example, magnetic particles suspended in the liquid would interact with an electromagnetic field, transferring energy to the field as the expanding gas does work on the slugs of liquid. An applied electromagnetic field is also required to start the flow any time the engine is stopped, and if the engine is operated in reverse, it will pump heat from the cold sink to the hot source.

The slugs of fluid and gas move as rapidly as the heat flow and energy extraction permits, and the amount of energy extracted is expected to be close to the maximum possible given the temperature difference between the heat source and sink, allowing for losses to friction, parasitic heat flow, temperature drops between the sources and the gas, etc.

The bubble pump loops take the shape of the heat engine's PV diagram (with pressure increasing downward):  the height of the tubing in the ambient gravitational field is proportional to the pressure of the slugs of gas at a given location, and the volume available for a given slug is greater as the tubing is more horizontal.   

It is necessary that the slugs of liquid be larger than the slugs of gas, or else the relationship between height and pressure will not be uniform, but this can be allowed for in the shape of the loops.

The isothermal expansion and compression segments form hyperbolas.  The expansion segment will be at higher pressure.  The heating and cooling segments must be inside a countercurrent heat exchanger.  For isobaric heating and cooling, the segments are horizontal but the gas volume does not necessarily vary as the horizontal position.  Vertical segments would not generally cause constant volume heating and cooling, but as expected the pressure would not be constant.

Does this really produce isothermal expansion and compression?  Not precisely.   A more complicated mechanism with reservoirs and a means for feeding alternating slugs of gas and liquid into the isothermal tubes would operate efficiently under a wider variety of conditions, but with some control over the heat flow, this machine will operate efficiently, and it has an appealing simplicity.
Now to build one…


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