# Working Fluids

Generally the working fluids in a power generating system is either water (steam) or air. This I imagine is because they are widely available fluids. The specific heats of these ranging from some 1 - 2 kJ/kg-K.

For the sake of argument, let's assume that hydrogen or xenon are equally available and comparably priced to water and air. Let us also say that the explosive nature of hydrogen not a matter of concern.

Specific Heat Hydrogen = 14 kJ/kg-K
Specific Heat Xenon = 0.16 kJ/kg-K

It would seem to me that a hydrogen system has the potential to generate the power of 7 steam plants at the same temperature due to the larger heat capacity.

The xenon seems like it would have a higher efficiency due to being able to reach higher temperatures faster.

Is this line of thinking correct? It would seem to me in binary power cycles the working fluids with the lower specific heat would be a better choice.

Wouldn't this be worse - it takes 7x as much energy to heat the hydrogen?

Anyway, in a steam plant you are normally boiling the working fluid to get an increase in volume and so pressure - something with a low latent heat vapourisation would be good

russ_watters
Mentor
Generally the working fluids in a power generating system is either water (steam) or air. This I imagine is because they are widely available fluids. The specific heats of these ranging from some 1 - 2 kJ/kg-K.

For the sake of argument, let's assume that hydrogen or xenon are equally available and comparably priced to water and air. Let us also say that the explosive nature of hydrogen not a matter of concern.

Specific Heat Hydrogen = 14 kJ/kg-K
Specific Heat Xenon = 0.16 kJ/kg-K

It would seem to me that a hydrogen system has the potential to generate the power of 7 steam plants at the same temperature due to the larger heat capacity.
No. Two problems with your logic:

1. I'm not sure if your last sentence is meant to imply higher efficiency, but the working fluid is just a carrier for energy. In order to get 7x the power generation, you need 7x the input energy.
2. More directly, it isn't specific (sensible) heat that is what makes it a good working fluid, but rather the phase change from liquid to gas that allows it to absorb the most heat, while pumping occurs in the liquid phase for least loss. So you can't use hydrogen or xenon since they don't condense to liquid at near room temperature without ridiculously high pressures.

Ok so the phase change is generating high pressures. Through heating a gas a pressure increase can be achieved but, you suggest that it isn't nearly as dramatic as a phase change.

This brings up something else that I have had on my mind in the past; is there a way to calculate the pressure generated by converting the water into steam? Also, can I calculate the pressure increase due to a chemical reaction say, gasoline burning in a cylinder?

As far as the efficiency, I was trying to make the connection between specific heats and Carnot efficiency (if there is one). I guess you could leave the fluid in the heater longer to achieve the same temperatures. So it would appear that the relation between them is a matter of time and energy "stored."

phase changes occur in equilibrium at constant pressure and temperature

there is no connection between carnot efficiency and the working substance