  We evaluate the Onsager matrix for a system under time-periodic driving by
considering all its Fourier components. By application of the second law, we
prove that all the fluxes converge to zero in the limit of zero dissipation.
Reversible efficiency can never be reached at finite power. The implication for
an Onsager matrix, describing reduced fluxes, is that its determinant has to
vanish. In the particular case of only two fluxes, the corresponding Onsager
matrix becomes symmetric.
