  We study port-Hamiltonian systems with two external ports, and the strategies
and limitations for conversion of energy from one port into the other. It turns
out that, apart from the cyclo-passivity of port-Hamiltonian systems, this is
related to the internal connection structure of port-Hamiltonian systems. A
source of motivation for energy conversion is provided by thermodynamics, in
particular the Carnot theory of conversion of thermal into mechanical energy.
This is extended to general port-Hamiltonian systems which are satisfying
structural conditions on their topology; thus generalizing the Carnot-Clausius
theory of heat engines. In particular, the operation of Carnot cycles is
extended, which is illustrated by the example of a precursor to the Stirling
engine, as well as an electromagnetic actuator. Furthermore, alternative energy
conversion control schemes such as energy-routers are discussed.
