Why Only Water

Because water has a great ability to expand in volume when changing phase from liquid into steam, something like 1600X, if memory serves. Plus, it's cheap, abundant, noncorrosive and environmentally sound.

It's not, back in 50's and 60's a lot of research was done in the use of liquid sodium in the heat exchange cycle in nuke plants. At least two commercial units using this technology were constrcuted.

I'm not sure why sodium, in particular, was chosen for nukes (likely, corrosion and radiation characteristics, maybe high boiling point and relatively low melting point) - but it's job is not to evaporate, but to transfer heat from the reactor to the steam generator.

So its purpose is very different from that of water, whose job is to expand to propel the turbines.

Chris

Quite.

Water is used in the boilers for Thermonuclear power stations for a number of reasons:

1) First, it should be pointed out that all power stations (except for Hydro Stations) use some form of heat, whether it be from coal, natural gas, nuclear or other heat generating methods, to turn water into steam. The steam is then used to power turbine generators to generate electricity.

Having said that, the reason why water is used is that it is cheap, readily available in large quantities and it`s thermo-dynamic properties are very well understood.

Hope this helps

Back in the 1930's General Electric built and tested a Mercury boiler/turbine in Schenectady NY and ran it on test for about a year. It was a small machine that worked pretty good but public concern for the hazard's of a mercury leak (at these temperatures and pressures) did not warrant the energy savings.

As a boy 10 years old, I saw this turbine and the same day, had a ride on the New York Central "Empire State Limited" train from Rochester to Schenectady.

You seem to be getting at some thing. Is it the different boil/flash points of other liguids that you could suggest instead of water, using less heat to have the same effect. Not every where water is cheap, abundant, recycable and for every one to use. I wonder what this question is leading to. Good luck.

In addition to the excellent comments above, also water has a large latent heat: upon change of phase between liquid and vapour a huge change in energy is involved. It makes its application as an energy carrying facility between heat and power more attractive than using any other substance.

I may be wrong, I thought the latent heat that heat required on phase changes is a loss. As we require the steam to generate heat, what is the advantage of the loss of energy in changing the phase from liquid to vapour.

When water changes to steam pressure is created. That pressure can be converted to mechanical energy to turn turbines, etc. That's the purpose of a power station - to convert the energy stored in a burnable fuel to mechanical energy that can turn machines that make electricity. Obviously, there's a net energy loss involved.

Actually, when water is converted to steam, pressure is only generated if the volume available for the steam to occupy is limited.

The real work of the steam process in power generation is done in raising the temperature of the steam well above its condensation temperature.

Just as the boiling of water causes a volume to be occupied by the generated steam, as the steam temperature is raised it expands (if it can) to fill still more volume.

The usable energy transfer from the coal / oil / gas / nuke heat source is in the superheating of the steam; the energy in that superheated steam is used to spin the turbines (which are in the 'expansion path' for the steam). The energy used to boil water is wasted energy. Think of the cooling towers you see at power stations. Their job is to cool the (now low energy steam, and therefore low pressure) - possibly to the point of a phase change to water, prior to it being returned to the heat exchanger.

In summary - if the only way you use your energy is to turn water into steam, the only way you can get that energy back is to return the steam to water - which kills a turbine. So the working energy transfer is all carried in the superheating of the steam, and not in the boiling of water.

Chris

No doubt there are subtleties that I missed from my study of the engine on "The African Queen", but the essentials are the same,

heat energy -> mechanical energy -> electrical energy

I think you are right. The latent heat of conversion to change water to steam is a major loss of energy. This energy is added at the boiler and absorbed in the condenser following the turbine. The useful energy comes from the temperature/pressure drop through the turbine. Not the latent energy.

I have been under the impression that the heavier materials will impart more energy upon impact with the turbine blades. That is why the GE Mercury Turbine was successful.

In nuclear reactor training, I was told that water (or heavy water as a moderator) is well suited for cooling in the primary containment vessel. In the secondary turbine circuit (following the heat exchangers) other materials might be considered.

Note: I am talking about BWR (boiling water reactors) such as the Westinghouse designs which is the only one that I have had experience with.

Snakers

the latent heat that heat required on phase changes is a loss.

http://en.wikipedia.org/wiki/Sensible_heat and http://en.wikipedia.org/wiki/Latent_heat explain more. Latent heat is not a loss during the boiling step. Losses occur when energy leaves a system in a form otherwise than that required, like the radiator on a vehicle engine, as a simple example, or an un-lagged pipe in a power station.

In a typical condensing double-expansion compound steam engine, for illustration, the cycle is:

1. cold water is heated to boiling using sensible heat from the heat source, raising its temperature
2. boiling water is converted to saturated steam at constant temperature and pressure using the heat source; this is latent heat
3. steam is dried by raising its temperature with the heat source, above the boiling temperature at constant pressure, a process known as super-heating that uses sensible heat
4. superheated steam is expanded through the high pressure cylinder, producing mechanical work, during which its temperature and pressure are reduced while the fluid remains in the vapour phase (ideally)
5. this same steam is expanded through the low pressure cylinder, producing mechanical work, during which its temperature and pressure are reduced still further
6. whatever is left is exhausted to a condenser, which in effect produces a partial vacuum on the mechanism, and more work is extracted from it. The latent heat reappears here as a loss to the cooling water passing through the condenser, though the extra shaft work one obtains as a result of so doing outweighs the loss incurred.
7. The condensate is pumped back into the heating circuit, and
8. round it goes again to 1.

Not that it's done these days, one could couple the output of such a machine to a generator to produce that new-fangled electrickery. More usually the above cycle can be found in engines in old ships, though it serves well enough to illustrate the principles.

A turbine is an advance on the above mechanism that uses fewer moving parts than reciprocating engines. Turbines are less tolerant of water droplets, which is why several stages of re-heat may be found in the cycle of a modern power station, to keep the temperature of the steam up and improve its dryness.

Total thermal efficiency of the above? 45% on a good day with a compound reciprocating engine, perhaps? 55% with a turbine with multiple re-heat? - these figures are near enough to illustrate.

Remembering that some of the combustion heat goes up the flue as the hot gases nitrogen, water vapour and carbon dioxide from the combustion process, and the reality is that a multi-stage turbine with re-heat is a surprisingly effective mechanism for converting the heat stored in fuel to power under the circumstances, which is why they are used in power stations. Now, try to achieve that on any fluid other than water and one will have trouble getting the efficiency to that figure, and the plant required will be wondrously more complex, simply because of the ease of interchanging a simple, everyday substance like water between the liquid and the gaseous state at convenient temperatures and pressures. Consider a power station running on ammonia instead of water? Hmmmmmmmm... It could be done, though why would one want to (rhetorical question)? Which sort-of-explains why it isn't done.

http://www.simetric.co.uk/si_steam.htm illustrates the difference in energy between water as a liquid, the column h_f, and water as a vapour, h_g. The column h_fg is the value of the latent heat at constant temperature, and indicates the amount of energy that is either needed for, or obtained from, a change of phase.

http://en.wikipedia.org/wiki/Thermodynamic_cycles explains more, as does 'Stirling Stan', and much better than can be done in this reply.

Phew! Time for a lie down!

sodium was (still is?) used in experimental nuclear plants, instead of water, because with it you can work above critical point (at 600ºC and at right pressure liquid turns instantly to superheated vapour ). Russians had at least one experimental unit, where liquid sodium was used at 900ºC.

http://www.insc.anl.gov/neisb/neisb4/NEISB_3.2.A2.html

Upon reading it, note that water is still used in that example and the steam turbine is still present.

Bhankii said it all.

Any different liquid you have in mind?

Air it if you dare!

Steam-Engines [Railways] Technology was well known before the Thermal-Power generation; including waters thermal & Non-corrusive, non-toxic, Environ-Friendly, abundant in quality, readily availabillity & all the factors make it the top-chioce-candidate.

I would like to refer the abundant availabillity of Distilled-water for industry & other at much lower cost as it was & is the bye-product of steam-engines /prime-movers charging stations