  Employing universal relations for the Onsager coefficients in the linear
regime at the symmetric point of the single impurity Anderson model, we
calculate the conditions under which the quantum scattering phase shift should
satisfy to produce the asymptotic Carnot's limit for the thermoelectric
efficiency. We show that a single quantum dot connected by metallic leads at
the Kondo regime cannot achieve the conditions that cause the best
thermoelectric efficiency. We study a system of serial double quantum dots
without inter-dot correlations. We show that maintaining one dot in the
electron-hole symmetric point makes it possible to obtain conditions for the
quantum phase shift linked to charge fluctuations in the other quantum dot that
satisfy the conditions associated with enhancing the thermoelectric efficiency.
We also discuss the presence of bound states in the continuum (BICs) and
quasi-BICs associated with the quantum scattering interference process that
improves thermoelectric efficiency. We identify two types of quasi-BICs that
occur at low and high temperatures: The first is associated with single Fano
resonances, and the last is with several Fano processes. We also discussed
possible temperature values and conditions that could be linked with the
experimental realization of our results.
