Abandoning mercury, again

I have been writing about using mercury as the working fluid in the bubble pump for two reasons:

  • Mercury does not wet glass, so achieving ideal flow of separate slugs of fluid and gas is feasible.
  • Mercury has density over 12, so the pressure produced by a given hydrostatic head is 12 times higher, and the volume required to contain the resulting compressed gas is 12 times smaller than if the fluid were water.

However, there is not much mercury available compared to the amount of water available. Dealing with high pressure is difficult and dangerous. Getting heat to flow through the high pressure container is difficult. Mercury liquid does not compare to a refrigerant in carrying heat. Using low pressure hyperbolas permits the use of thin walls surrounded by refrigerant to carry heat in or out of the gas. And the size reduction possible due to high pressure may not result in a cost reduction because of the necessary materials.

So, I now plan to use a cheap fluid like water, some tubing the fluid does not wet (in the case of water, something hydrophobic), some patterned method of making many hyperbolas at once in a flat sheet with parallel tubes, and refrigerant to carry heat between the heat source or sink and the tubes, evaporating from the hot surface, rising to the cold surface, condensing, and flowing back down to the hot surface.

The machine pumps air from a large low-pressure reservoir into a large high-pressure reservoir, at perhaps double the pressure, unless there is a need for producing air at a particular output pressure. It has a simple layout. Bubbles and fluid flow up through the expansion hyperbolas into a separation tank. The top port feeds the hot low-pressure gas countercurrent heat exchanger, and the bottom port feeds the hot low-pressure fluid countercurrent heat exchanger. The cold low-pressure gas output feeds the low-pressure gas reservoir and the cold low-pressure fluid output feeds the top of the compression hyperbola. Low pressure gas is periodically injecteded into the descending fluid at the top of an inverted U before the nearly horizontal entrance of the hyperbola. The hydrostatic pressure at that point is lower than the gas pressure in the low-pressure reservoir.

At the bottom of the compression hyperbola, the bubbles and fluid separate in the high-pressure separation tank. The cold high-pressure gas and cold high-pressure liquid enter the high-pressure sides of the two countercurrent heat exchangers. The hot gas output feeds the high-pressure reservoir, while the hot liquid feeds directly into the expansion hyperbola. High pressure gas from the reservoir is periodically injected into the rising fluid in the expansion hyperbola, at a point where the hydrostatic pressure is lower than the reservoir gas pressure.

The rate of fluid flow is controlled by adjusting the rate of gas injection into the expansion and compression hyperbolas. Work is produced when excess high-pressure gas is allowed to expand in a turbine or other mechanism.

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2 Responses to “Abandoning mercury, again”

  1. 0xff Says:

    A picture here would help.

    I am trying to envision this a the size of a refrigerator, probably too small.

    They need access to the heat from the sun and the cold of the (night, earth)?

  2. archimerged Says:

    The height of the hyperbolas times the fluid density dictates the maximum pressure increase possible. meters x kilograms / meter^3 = kilograms / meter^2. Or 1 atm = 33.9 feet H2O.
    You have to allow for the bubbles, which reduce the pressure because they don’t weigh much. If slugs of gas and fluid are equal size, that cuts the pressure in half. I suspect slugs of fluid are larger.

    Sources of heat and cold depend on what is available. In death valley, heat would be ambient during day (or enhanced by solar collector), and cold would from the nearby mountain top, by using a gravity feed heat pipe to carry heat up the mountain. The machine might even be able to operate at night using heat from underground, a gravity feed heat pipe carrying heat up out of the sand.

    If the machine runs one cycle per day, then it can use day and night temperature differences. Then you need large reservoirs. It’s up to the person who owns the site to figure out what is best for that site.

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