Pression engine

[Gabriele]

Hello. Can somebody help me doing some computation?
System is simple. We have a tank of water deep 20m and wide 10 meters.We have a vertical piston on the top-right from 0 to 10m,and an orizzontal one at 20 m of deep. We compress the water from the bottom by an half of the volume doubling his density. We bring the bottom piston from 20m to 10m so the density of water doubles.We let expand the upper piston to the right releasing energy needed to copress the liquid,plus the energy involved considering now the density from 0 to 10m to the right became 2g/cm3 from1g/cm3 (that is 0.25g/cm3x10mx10m) NOW,we move down the lower piston from 10 to 20m and from right to the left the upper right piston... it results to me the energy spend is in an assolute value 200 and a energy gained is 175. This is what i consider... the weight of water i need to rise from 20 to 10 to compress so -20kgx10. the energy in more i gain from expanding the upper right piston that is 0.25 x 10m x10m(should be 1 but on the right of the wall we have a spring that compensate natural pression on the wall of 10m of water) and then i gain also 10kg to 20kg from the fall of the lower piston so 15x10m... -200+25+150 = -25... why? Thanks....(here i post an approximation of what i would do...)

[Gabriele]

It can be done with gas too... damn... what have i lost by the way (excluding the brain )?

[LibreEnergia]

Water is not compressible, so the system would not work as you presume.

If you were to use a gas instead then it should be self evident there is no energy to be gained by cycling the gas over any range of pressures.  First law of thermodynamics applies...

[Gabriele]

From wiki: The compressibility of water is a function of pressure and temperature. At 0 °C, at the limit of zero pressure, the compressibility is 5.1×10−10 Pa−1.[31] At the zero-pressure limit, the compressibility reaches a minimum of 4.4×10−10 Pa−1 around 45 °C before increasing again with increasing temperature. As the pressure is increased, the compressibility decreases, being 3.9×10−10 Pa−1 at 0 °C and 100 MPa.

The bulk modulus of water is 2.2 GPa.[32] The low compressibility of non-gases, and of water in particular, leads to their often being assumed as incompressible. The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.[32]

http://en.wikipedia.org/wiki/Properties_of_water#Compressibility

If i can comprime water by 2%,the behavior don't change...i simulated it... (bycalcolus)...perhaps i miss something...

For a gas is the same concept insted. (each centimeter of fall,pression rise by the amount of the weight of a cm of gas!)

[LibreEnergia]

From wiki: The compressibility of water is a function of pressure and temperature. At 0 °C, at the limit of zero pressure, the compressibility is 5.1×10−10 Pa−1.[31] At the zero-pressure limit, the compressibility reaches a minimum of 4.4×10−10 Pa−1 around 45 °C before increasing again with increasing temperature. As the pressure is increased, the compressibility decreases, being 3.9×10−10 Pa−1 at 0 °C and 100 MPa.

The bulk modulus of water is 2.2 GPa.[32] The low compressibility of non-gases, and of water in particular, leads to their often being assumed as incompressible. The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.[32]

http://en.wikipedia.org/wiki/Properties_of_water#Compressibility

If i can comprime water by 2%,the behavior don't change...i simulated it... (bycalcolus)...perhaps i miss something...

For a gas is the same concept insted. (each centimeter of fall,pression rise by the amount of the weight of a cm of gas!)

As you correctly point out water is somewhat compressible at extreme pressures.

However, you were talking about 'doubling' the density of the water. You would need enormous forces to do that and its is simply not practical.

In any case compressing liquid or a gas results in no net energy gain around a cycle so why bother.