"Energy can be transformed (changed from one form to another), but it can neither be created nor destroyed".
The modern version of the steam engine has been in vogue from 1780 when James Watt invented conversion of reciprocating motion to rotary motion. As the key trigger for the industrial revolution steam is and remains the medium for bulk energy transformation.
Irrespective of the energy source, be it coal, bio-fuels, geothermal, solar thermal or nuclear the heat energy transformation takes place first to steam and then to rotary motion. Steam as a transformation medium is also used for other purposes like domestic heating and other industrial processes.
Let us see what properties of water and steam makes it the most suitable intermediary for energy transformation.
Water is the most abundantly available substance on earth. Even though bulk of this is saline water, the fresh water sources itself is huge enough to make it easily available in almost all parts of the globe.
Handling is easy and natural. No special precautions are required. It is not toxic or harmful to humans and vegetation. There is no problem of spills or cleanup (if free from toxic or polluting chemicals). It is not a potential pollution hazard.
Change of State.
One of the things that make water or steam the ideal thermodynamic medium is the easy of change of state. The boiling temperature and the condensing temperatures are within ranges achievable by using normally available materials and designs.
To get the most out of the steam energy transformation steam should be made to condense at temperature near ambient. This means the pressure at which it condenses will be at vacuum. The properties of water allow condensing at ambient temperatures to achieve very high vacuums even in tropical climates. Ambient cooling media is sea water, water from large water bodies or ambient air (using a Cooling Tower) which is abundantly available.
The volume of unit mass of steam as it exhausts in the turbine is almost 25000 times of water. In the boiler where the water converts to high pressure and temperature steam the volume increases almost 25 times. The steam then expands almost 1000 times in the turbine. The kinetic energy released during this expansion is what rotates the turbine. The much smaller volume of water makes it easy for handling and pumping and recycling it back to the boiler. This smaller volume also requires lesser heat transfer surfaces.
This high volumetric change from one state to another is one of the main properties that make the water- steam cycle the best option.
Water has the highest specific heat (the amount of heat per unit mass for unit temperature increase) of all the commonly available substances. This is almost four times more than that of Air. Even superheated steam has a higher Specific Heat than air. Even though Hydrogen has a much higher Specific Heat more than three times that of water, handling, availability and the explosive nature works against the use of hydrogen as an energy transfer medium except in special cases.
These are the main factors that make water – steam cycle still the best medium for energy transformation.