  We discuss a quantum thermal machine that generates power from a thermally
driven double quantum dot coupled to normal and superconducting reservoirs.
Energy exchange between the dots is mediated by electron-electron interactions.
We can distinguish three main mechanisms within the device operation modes. In
the Andreev tunneling regime, energy flows in the presence of coherent
superposition of zero- and two-particle states. Despite the intrinsic
electron-hole symmetry of Andreev processes, we find that the heat engine
efficiency increases with increasing coupling to the superconducting reservoir.
The second mechanism occurs in the regime of quasiparticle transport. Here we
obtain large efficiencies due to the presence of the superconducting gap and
the strong energy dependence of the electronic density of states around the gap
edges. Finally, in the third regime there exists a competition between Andreev
processes and quasiparticle tunneling. Altogether, our results emphasize the
importance of both pair tunneling and structured band spectrum for an accurate
characterization of the heat engine properties in normal-superconducting
coupled dot systems.
