  We present a formalism to study the heat transport and the power developed by
the local driving fields on a quantum system coupled to macroscopic reservoirs.
We show that, quite generally, two important mechanisms can take place: (i)
directed heat transport between reservoirs induced by the ac potentials and
(ii) at slow driving, two oscillating out of phase forces perform work against
each other, while the energy dissipated into the reservoirs is negligible
