Rankine cycle

[skittlesmonkey]

I don't understand why you would ever run the Rankine cycle with a working fluid that has a high heat of vaporization. The fluid is heated, evaporates, runs across a turbine, condenses on the cold sink, and is pumped back into the boiler. why would one use something like water or a refrigerant to run this cycle when it is thermally much cheaper to generate a unit of gas of lets say heptane, which has the same boiling point as water but a lower heat of vaporization. Just as i fill a steam engines boiler with heptane ( pretty much the same boiling point as water) it runs faster and does more work per unit energy compared to water. So why would one use working fluids like water or refrigerants ( when there are alternatives with lower heats of vaporization and the same boiling point)? I keep being told a high heat of vaporization (per mole that is) is desirable because it makes for a low flow rate, but don't u want a high flow rate and put more flow across your turbine which is the point of the whole thing. When mole of liquid is evaporated it makes 22.4 liters of gas with very little variation from this law no mater if high or low heat of vaporization so why use such a working fluid?