Hydraulic heat engine

[Low-Q]

This idea is to use a looooong tube made of a non flexible material. This tube is painted black to absorb heat from the sun. Inside the tube there is water, and the end of the tube there is cylinder and a piston. It is water in the whole tube untill the piston, so there is no air inside.
When water is warmed up, it expands. From 277K to 373K the water expand its volume by 5%. That means if the tube is 10m long, the water will expand 50cm at 373K compared to 277K.
This is a force which is incredibly strong, because water is hardly compressable. The piston will face a force of about 50 atm. pressure if it is prevented to move inside the cylinder during the warmup of the water (It requires 50 atm. pressure to compress water 5%).
So how much energy can we expect to harness from a device like this?.

Say the surface of the cylinder is 100cm^2. The stroke is 50cm. The average pressure is 25 atm. The maximum energy output would be:
100cm^2 * 25kg * 9.81 * 0.5m = 12262 Joule. If we manage to heat and cool the water in 1 cycle pr. minute, we have a potential energy output of 735,7kWh.

We make a push-pull system, so we also can get the same force when the piston returns, so we get > 1.5MWh of energy.

What do you think?

Vidar

[fritznien]

This idea is to use a looooong tube made of a non flexible material. This tube is painted black to absorb heat from the sun. Inside the tube there is water, and the end of the tube there is cylinder and a piston. It is water in the whole tube untill the piston, so there is no air inside.
When water is warmed up, it expands. From 277K to 373K the water expand its volume by 5%. That means if the tube is 10m long, the water will expand 50cm at 373K compared to 277K.
This is a force which is incredibly strong, because water is hardly compressable. The piston will face a force of about 50 atm. pressure if it is prevented to move inside the cylinder during the warmup of the water (It requires 50 atm. pressure to compress water 5%).
So how much energy can we expect to harness from a device like this?.

Say the surface of the cylinder is 100cm^2. The stroke is 50cm. The average pressure is 25 atm. The maximum energy output would be:
100cm^2 * 25kg * 9.81 * 0.5m = 12262 Joule. If we manage to heat and cool the water in 1 cycle pr. minute, we have a potential energy output of 735,7kWh.

We make a push-pull system, so we also can get the same force when the piston returns, so we get > 1.5MWh of energy.

What do you think?

Vidar

how much energy to heat the water and tube?
100 sq cm X 10 meters=100,000cc of water
100 degrees k rise in temp X 100,000cc=10 million callories=41868000joules
one cycle per minute is 204 watts
12262/60=204
input power is 700 thousand watts
workable in pricipal but nobody is ever going to make one for the power.
fritznien

[Low-Q]

how much energy to heat the water and tube?
100 sq cm X 10 meters=100,000cc of water
100 degrees k rise in temp X 100,000cc=10 million callories=41868000joules
one cycle per minute is 204 watts
12262/60=204
input power is 700 thousand watts
workable in pricipal but nobody is ever going to make one for the power.
fritznien

That did not seem very promising That means that the conversion from heat to expansion is terribly inefficient.

Vidar

[Low-Q]

I did read more about compressibility of water. It happens to decrease its volume by 1.8% at 40 MPa, or approx 400 bar.

So the numbers changes a bit...

There is 100 cm^2 piston. The force at 5% compression of volume is 1.09MN. The average is 545kN. Distance is 0.5m, so the energy out will be 272.5kJ. That is still not more that 4.5kW per minute...

However, these figures are not correct, because the compressibility decrease with higher temperature and pressure. It is not easy to predict the energy out vs energy in.

Some litterature here: http://en.wikipedia.org/wiki/Bulk_modulus

[fritznien]

any way you cut it is going to have poor conversion.
solar pannels for sunlight and an old steam engine for plain heat will
do far far better.
fritznien