  We analyze an electronic nanoscale thermal machine driven by time-dependent
environment: besides bias and gate voltage variations, we consider also the
less prevailing time modulation of the couplings between leads and dot. We
provide energy and heat current expressions in such situations, as well as
expressions for the power exchanged between the dot+leads system and its
outside. Calculations are made in the Keldysh nonequilibrium Green's function
framework. We apply these results to design a cyclic refrigerator,
circumventing the ambiguity of defining energy flows between subsystems in the
case of strong coupling. For fast lead-dot coupling modulation, we observe
transient currents which cannot be ascribed to charge tunneling.
