  Motivated by recent experiments [1-3] we calculate the thermopower of
graphene incorporating the energy dependence of various transport scattering
times. We find that scattering by screened charged impurities gives a
reasonable explanation for the measured thermopower. The calculated thermopower
behaves as 1/\sqrt{n} at high densities, but saturates at low densities. We
also find that the thermopower scales with normalized temperature T/T_F and
does not depend on the impurity densities, but strongly depends on the fine
structure constant r_s and the location of the impurities. We discuss the
deviation from the Mott formula in graphene thermopower, and use an effective
medium theory to calculate thermopower at low carrier density regimes where
electron-hole puddles dominate.
