The Liquid Piston...                          ...or, return to Home Page

... Thermodynamic Dreaming.

We were working on a micro-hydro turbine in Ireland - 6Kw, handy enough... But all its ills came from the control circuitry, the electrics. A water turbine needs to run under a constant load - cut this load and the rotor will over-speed, sometimes to destruction.

It all seemed a bit complicated given the task in hand - a few lights, yes, but to heat the bath-water, to keep the freezers running...

Heat-Pumps are essentially compressors. It occurred to us that the pressure of the water coming down from the mountain could be converted directly to pressure in a refrigerant medium.

With one stone, two birds - cold at one end, hot at the other.

Traditionally compressors have rigid pistons in rigid cylinders. This requires a sliding seal between the two - friction, wear, bother...

  Why not, we thought, use the surface of the water as the Piston - the seal would be 100% gas-tight, there could be no friction, and especially, no wear.

    The loose-fitting, floating 'pistons' prevent heat transfer and reduce evaporation and splashing. They equally provide the motive power, at the right moment, to inverse the input-output vane on the feed pipe.

    So far so good - if one disposed of a head of water of 300 metres, which would produce directly the 30 Bar needed to construct a heat-pump with a friendly refrigerant medium, such as Carbon Dioxide. No Freons please...

     Such a head of water is hard to find, and the penstock would have to be chunky at the bottom of the hill.

    Opposite - a perspective view shows the "Splash Traps" which prevent water from the main cylinders reaching the Intensifier cylinder. Valves are provided for draining them, and also to regulate the static pressure in the primary system.

    30 Bar heads being hard to come by, clearly some means is needed to raise the air-pressure to a useful level.



 The Pneumatic Intensifier -

Here  -  (is a Page with Flow Diagrams and 2D explanations...)  

     - Consists of four concentric stainless-steel cylinders, highly polished. A double-ended  piston slides freely, as may be seen below:

    The air-pressure from the Liquid Pistons moves this free-piston to and fro, acting on the large surfaces.

     The central, much smaller cylinders are formed either side of a fixed central seal. These interior cylinders have a smaller surface area. But they enjoy the same force as is acting on the larger air-drive cylinders.


Pressure is (let me get this right...) inversely proportional to the product of the force and the surface area acted upon, so much higher pressure is produced. The hydraulic intensifier, using this principle, has been known for two Centuries.

    The innermost cylinder is divided along its length by a partition - one side serves to feed the refrigerant medium, as a low-pressure gas, and the other to carry the compressed, heated medium to the condenser, where it will liquefy, give up the latent heat of evaporation gathered in the cold end of the system. Concentric sheet-steel valves govern this flow, and its non-return.

    Perspective view of the Intensifier: The red in the secondary circuit represents the compressed refrigerant gas, the blue the low-pressure gas. In contrast to the Liquid Piston, the intensifier has 5 toroidal sliding seals. We don't think there's any way to reduce this number - the concentric design has no connecting rods, so there is no need for 'stuffing box' seals. The primary circuit could be filled with Nitrogen, for example...

    Liquid piston with part of the control-gear - Click to enlarge - Full-screen 1024 x 768.