  For thermoelectric power generation in a multi-terminal geometry, strong
numerical evidence for a universal bound as a function of the magnetic-field
induced asymmetry of the non-diagonal Onsager coefficients is presented. This
bound implies, inter alia, that the power vanishes at least linearly when the
maximal efficiency is approached. In particular, this result rules out that
Carnot efficiency can be reached at finite power, which an analysis based on
the second law only, would, in principle, allow.
