Bernoulli's principle states that an increase in the kinetic energy of a fluid occurs simultaneously with a decrease in pressure for isentropic processes. What then limits us from converting all of the gas's pressure into kinetic energy? Could not this kinetic energy then be used to perform useful work (e.g. move turbine)? This appears contrary to Carnot efficiency but I'm failing to see a very simple limit on what prevents us from being able to continually transform the pressure into work. Cannot a nozzle or similar device convert all this pressure into directed kinetic energy?
The one simple limit I can think of is shock transitions where the mach number goes from greater than 1 to below 1 (i.e. not an isentropic process). But since we're only going to higher and higher mach numbers by decreasing pressure and increasing velocity, we should be able to avoid this limit. Therefore, is it not possible to isentropically decrease pressure arbitrarily low at least until quantum mechanical effects become important?
Perhaps a better way to define my question is through a thermodynamic cycle:
One can define the enthalpies at each position of a thermodynamic cycle (i.e. $h_1, h_2, h_3, h_4$) and the overall thermal efficiency as follows:
$\eta_{th} = \frac {W_{out} - W_{in}}{Q_{in}} = \frac {(h_2 - h_3) - (h_1 - h_4)}{h_2 - h_1} = \frac {\eta_N h_2 - (h_1 - h_4)}{h_2 - h_1} $
where $\eta_N = \frac{h_2 - h_3}{h_2}$ is the enthalpy extraction ratio. We know that $\eta_{th}$ cannot exceed $1 - T_{cold}/T_{hot}$ by Carnot efficiency, but can we make any generalized statements about the relationship between $\eta_N$ and $\eta_{th}$? For example, must $\eta_N$ always be less than $\eta_{th}$? Must $\eta_N$ always be greater than $\eta_{th}$? Or can no general statement be made?