jfreissmann
Sink outlet temperature limited to T_crit
The sink outlet temperature is limited to T_crit. E.g., for propane, only 96°C can be set as the outlet temperature for the subcritical cycle. However, due to superheat, higher sink outlet temperatures are feasible and indeed quite common, especially for systems with high temperature glide at the sink. The sink outlet temperature should not be limited to T_crit. Rather, the simulation should show if a specified outlet temperature can or cannot be reached.
I concur. We were unsure how to set a sensible limit that makes sure users don't break the simulation and weren't sure how a more generous yet easily calculatable limit could look like. But I think you're right, that a graceful failure with clear "error message" is the preferable solution in order to avoid unnecessarily tight limits.
I have to back track here a bit, as I was confusing your issue with a similar hard limit set for transcritical cycles. The problem you are highlighting here stems from the fact, that TESPy's Condenser classes ttd_u (upper terminal temperature difference) does not equal the Pinch Point or the actual upper temp. difference in the counter flow heat exchanger (T_sup - T_sink_ff). The Condenser class is rather defined to set the difference between the heat sinks feed flow and the condensation temperature. Therefore, the critical temperature of a fluid in a subcritical porcess is the limiting value using this implementation.
A possible fix to this limitation is TESPy's new MovingBoundaryHeatExchanger which aims to detect phase changes and therefore allows to set the pinch point temp. difference. For now, it has some limitations as well and it'll take some time to replace the condenser with it, but I'll put in on the to-do-list.