  We study thermoelectric and viscous contributions to the ratchet effect, i.e.
radiation-induced generation of the direct electric current, $J_{\rm rat},$ in
asymmetric dual-grating gate structure without inversion center. Previously
[E.M\"onch et al, Phys. Rev. B {\bf 105}, 045404 (2022)], it was demonstrated
that frequency dependence of the $J_{\rm rat}$ is essentially different within
hydrodynamic (HD) and drift-diffusion (DD) regimes of the electron transport: $
J_{\rm rat}^{\rm HD} \propto 1/\omega^6 $ and $ J_{\rm rat}^{\rm DD} \propto
1/\omega^2 $ for $\omega \to \infty.$ Here we analyze previously neglected
thermoelectric contribution and find that it yields high-frequency asymptotic
$1/\omega^2$ even in the HD regime and can change sign of the response. Account
of the finite viscosity of the electron liquid yields contribution which scales
at high frequency as $1/\omega^4.$ We also find plasmonic resonances in the
$J_{\rm rat},$ and demonstrate that asymmetry of the structure allows for
excitation of the so-called directional travelling plasmons.
