We investigate the thermodynamics of simple (non-interacting) transport
models beyond the scope of weak coupling. For a single fermionic or bosonic
level -- tunnel-coupled to two reservoirs -- exact expressions for the
stationary matter and energy current are derived from the solutions of the
Heisenberg equations of motion. The positivity of the steady-state entropy
production rate is demonstrated explicitly. Finally, for a configuration in
which particles are pumped upwards in chemical potential by a downward
temperature gradient, we demonstrate that the thermodynamic efficiency of this
process decreases when the coupling strength between system and reservoirs is
increased, as a direct consequence of the loss of a tight coupling between
energy and matter currents.Comment: 6 pages, 2 figures, to appear in EP
