Slippery_Pete,
you stated several time the idea
Less heat transferred to the fluid = more useable energy available
I may be misunderstanding the point you intend to make, but I think that the statement is either wrong or more or less correct but very misleading.
Your sentence suggests that we add energy to the fluid and that this energy is split between useful work and heat. And so, the less heat the more work.
That's misleading.
Actually, the sequence of events is:
- we take a gas that has an internal energy U1
- optionally, we increase its energy to U2
(this is the compression phase. Mandatory for turbines. For steam machines, just assume U2=U1)
- we pour in some heat Q and get the internal energy U3 = U2 + Q
- we extract a percentage of the internal energy U3 into work through an expansion of the gas (usually adiabatic expansion).
- the resulting energy U4 = U3 - W
The percentage of extracted energy depends on the pressure ratio.
Actually it depends on the temperature ratio T4/T3 and is 1 - T4/T3 .
For adiabatic expansions, it can be shown that 1 - T4/T3 = 1 - (p4/p3)^((γ-1)/γ)
You can see that, for instance if the extraction efficiency is 40%, we extract 40% of U3 which is 40% of U2 plus 40% of Q. The more heat is added to the fluid, the more work is extracted.
This picture is, of course, a simplification. I looked at the efficiency of just one phase of the cycle rather than looking at the efficiency of the whole cycle. However it shows that:
- heat is the object of the energy extraction efficiency and is not an adjustement variable of the efficiency
- theoritical efficiency is just linked to the temperature ratio during expansion phases or to the pressure ratio which can be considered as a surrogate for temperature ratio
Note that the 40% U2 extracted is not magic energy ; it is only if the gas has been compressed initially from U1 to U2 with U1 being 60% of U2 that the final pressure ratio allows a 40% reduction. So the 40% extracted of U2 is just some invested energy that is reclaimed.
If there is no initial compression (steam engine) the increase of pressure obtained by adding Q in a fixed volume results in a pressure increase that allows a very poor extraction efficiency, much less than Q/U1.
Note also that the final state U4 is equivalent to the initial state U1 except that we extracted only a part of Q. So grossly, U4 = U1 + 60%.Q
And the final temperature of the gas is not a variable that can be acted upon ; it is dictated by be heat that could not be extracted.
Luc