[As requested, here are answers to "where the heat comes from, why this is a superior manner in which to harness said energy"]
Heat to operate this device comes from any pair of hot and cold temperature reservoirs. For example, it could operate between the condenser of a steam power plant and the
cooling towers, absorbing heat from the condenser and delivering 90% of it to the cooling towers and 10% of it as work to a generator. Or it could operate in places where Stirling engines are used now, with solar collectors or using landfill gas. Or it could operate in a place like Death Valley, taking heat from the ambient and delivering it via a heat pipeline up a nearby mountain. A heat pipeline is just a large gravity-feed heat pipe, say a 60 inch diameter pipe carrying refrigerant vapor up the mountain and a smaller pipe carrying condensed liquid refrigerant back down.
It should be superior to existing Stirling engines because it doesn't have solid moving parts (so is more efficient) and is inherently larger scale. Also it ought to do a better job of keeping the working gas at constant temperature.
How does the heat energy get converted to work? See the paragraph about molecules of gas. Gas molecules collide with slugs of liquid accelerating them and slowing (cooling) the gas molecules. The slow molecules collide with hot tubing walls and get accelerated, raising the temperature, while the atoms in the walls vibrate less, reducing the temperature.
[revised description, of a slightly different machine, and I will be revising it further.]
Injecting bubbles into moving fluid
May 7th, 2006 by archimerged
The isothermal bubble pump is very simple, but not so flexible. There is no control over the spacing or size of bubbles once they are created when the machine is manufactured. So I propose a machine made of two hyperbolic segments of bubble pump with injectors at the inputs and collectors at the exits. The injectors are under automatic control. One injector feeds cold low-pressure gas bubbles into the downward moving fluid stream at the nearly horizontal top of the cold compression hyperbola. The other feeds hot high-pressure gas bubbles into the upward moving fluid stream in the nearly vertical bottom of the hot expansion hyperbola.
Alternating slugs of fluid and gas flow through the two hyperbolas, but elsewhere in the machine, flows of fluid and gas are separate. Heat is exchanged between high and low pressure fluid using a fluid-only countercurrent heat exchanger. Since there is no gas involved, a U tube easily brings the hot low-pressure fluid down to the level of the high-pressure fluid and returns the cold low-pressure fluid to the original level. Incidentally, inside the heat exchanger the fluids have equal hydrostatic pressure; importantly, the pipes are in direct thermal contact.
Separately, heat is exchanged between high and low pressure gasses in a gas-only countercurrent heat exchanger which can be placed anywhere in any orientation. Typically, it would comprise a series of insulated low pressure tanks of descending temperature, filled with coils of high-pressure pipes flowing in the opposite direction so cold gas enters the coldest low pressure tank, absorbs some heat, flows to the next warmer low pressure tank, absorbs more heat, etc. The volume of these tanks is unimportant so long as very little heat is lost and there is very little net heat flow in or out either end. The amount of heat gained by the high pressure gas should be essentially equal to the heat lost by the low pressure gas. The more tanks provided, the longer the gas stays in the tanks, and the larger the volume (so the relative surface area is lower) the better this goal is achieved. A single heat exchanger can serve many pairs of bubble pump hyperbolas.
This machine does not require a jump-start because the bubble injectors and collectors are inherently irreversible, and it cannot function as a heat pump unless the temperatures of the heat reservoirs are reversed. The liquid always flows down the compressor and up the expander, heat always flows from heat source to heat sink, and the motor always runs forward, but if the heat source is colder than the heat sink, forcing the motor to turn forward will pump heat from the cold source to the hot sink. Forcing the motor to turn backward will just pump fluid around the circuit with no gas flow.
The bubble collectors are simply vertical tubes leading to gas reservoirs. These tubes have such large diameters that the bubbles detatch from the walls of the tube and float freely (and irreversibly) upward. The gas pressures are always high enough to prevent fluid flow upward, so fluid never fills the gas reservoirs or flows into the gas-only tubes above the reservoirs. Hot fluid leaving the nearly horizontal upper end of the hot expansion hyperbola takes a sharp bend downward to the hot input port of the fluid countercurrent heat exchanger, but the bubbles escape upward into a short vertical pipe leading up to a gas reservoir. Cold fluid and bubbles arriving at the bottom of the cold compression hyperbola flow around a sharp bend from nearly vertical to horizontal. After a short horizontal segment, the bubbles escape up into the high pressure gas reservoir while the fluid flows to the cold input port of the fluid countercurrent heat exchanger.
Hot gas from the hot low-pressure reservoir and cold gas from the cold high-pressure reservoir flow slowly through the gas countercurrent heat exchanger and into the cold low-pressure reservoir and hot high-pressure reservoir. From there, the hot high-pressure gas feeds the bubble injector at the nearly vertial bottom of the hot expansion hyperbola, and the cold low-pressure gas feeds the bubble injector at the nearly horizontal top of the cold compression hyperbola.
A positive displacement pump somewhere in the fluid-only circuit converts fluid flow to rotary motion or rotary motion to fluid flow, always in the forward direction (the pump cannot run backward). A motor-generator converts electricity to or from shaft motion.
Thinking of gas at the level of moving molecules, one can imagine how and why the expansion hyperbola provides motive force to the liquid. At the bottom where the gas bubbles are injected, the hydrostatic pressure is so high that the bubbles are just big enough to touch the walls of the tube, and they are not expanding. But the fluid is moving upward in the nearly vertical hyperbolic tube, and the hydrostatic pressure drops as the amount and weight of the fluid above decreases. So the gas expands, doing work on the slug of liquid behind and ahead. Molecules of gas collide with the liquid, accelerating the liquid and decelerating the gas molecules, so the temperature and pressure drop. Molecules of gas also collide with the hot walls of the tube, gaining energy so the temperature and pressure of the gas rise. The faster heat flows into the gas, the more work it can do on the liquid and the faster the liquid moves. Nearer the top of the hyperbola, the slope of the tube decreases, so that a larger increase in volume is permitted without reducing the pressure so much. The goal is isothermal expansion: the pressure-volume product is constant and all of the energy removed from the hot walls of the tube is used to accelerate the moving fluid while the gas stays at constant temperature.
Thus, if no energy were being extracted from the fluid motion, the fluid would continue to accelerate. This is generally true of heat engines they speed up as long as heat (microscopic disordered kinetic energy) is supplied faster than energy is removed, storing the excess as macroscopic ordered kinetic energy. Here, the moving fluid plays the role of a flywheel.
[original description follows]:
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
No doubt someone wants a diagram. I think in words, usually. So a word picture:
Isothermal compression starts in a tube which is almost horizontal and slopes down to the right. The slope increases as the depth increases. The slugs of gas are at maximum volume for their temperature, which is cold because the isothermal compression tube is in thermal contact with the cold heat sink. The gas and liquid is moving at a reasonably constant velocity to the right and down. Farther down, the slugs of gas decrease in volume, because the liquid behind is being accelerated by gravity while the liquid ahead is being decelerated by the higher pressure gas ahead of it. The gas temperature increases and heat flows into the cold heat sink. If the temperature tends to continue rising (because the engine is running too fast for the cold heat sink to carry away heat) then the pressure will increase more and tend to slow down the engine. The hyperbolic shape of the curve will result in compression occuring as fast as possible without increasing the temperature.
Isobaric (constant pressure) heating occurs in a horizontal segment of tube at the maximum depth and maximum pressure of the engine. Ideally, the heat is obtained via countercurrent heat exchange with the isobaric cooling segment, but that segment operates at minimum pressure, at the top of the machine. Gravity feed heat pipes will not accomplish the job: capillary action heat pipes are needed. Countercurrent heat exchange is important because the heat capacity of the liquid may be substantial, and that heat should not be coming from the heat source or ending up in the heat sink, but should stay in the hot portion of the liquid.
After the gas and liquid are heated to the hot temperature, the isothermal expansion step begins. The liquid and gas slugs move suddenly upward and to the left, at a steep slope which decreases smoothly to a nearly horizontal path before connecting to the isobaric cooling unit. This phase produces the force which keeps the liquid and gas moving against friction and the electromagnetic load applied by the external field to the magnetic particles in the liquid, and of course, against the resistance of the compression step. As the weight of the liquid above the gas decreases, the applied pressure decreases and the gas expands. It does work on the slug of liquid ahead of it and behind it. The energy for this work comes from the heat absorbed in keeping the gas at constant temperature.
Does the isothermal bubble pump need a check valve in the circuit? With one, it shouldn't need to get a push start, but then we couldn't say the machine has no valves. Inertia serves a similar function to the check valve. When expanding gas warms back to the isothermal temperature, it applies additional force to the slug in front and the slug behind, decelerating the slug behind and accelerating the slug ahead.