  A consequence of the Second Law of thermodynamics is that no thermodynamic
system with a single heat source at constant temperature can convert heat into
mechanical work in a recurrent manner. First we note that this is equivalent to
cyclo-passivity at the mechanical port of the thermodynamic system, while the
temperature at the thermal port of the system is kept constant. This leads to
the general system-theoretic question which systems with two power ports have
similar behavior: when is a system cyclo-passive at one of its ports, while the
output variable at the other port (such as the temperature in the thermodynamic
case) is kept constant? This property is called `one-port cyclo-passivity', and
entails, whenever it holds, a fundamental limitation to energy transfer from
one port (where the output is kept constant) to the other port. Sufficient
conditions for one-port cyclo-passivity are derived for systems formulated in
general port-Hamiltonian form. This is illustrated by a variety of examples
from different {(multi-)}physical domains; from coupled inductors and capacitor
microphones to synchronous machines.
