  We consider thermoelectric transport properties of the edge states of a two
dimensional topological insulator in a double quantum point contact geometry
coupled to two thermally biased reservoirs. Both spin-preserving and
spin-flipping tunneling processes between opposite edges are analyzed in the
presence of electron-electron interactions. We demonstrate that the
simultaneous presence of spin-flipping processes and interactions gives rise to
a finite longitudinal spin current. Moreover, its sign and amplitude can be
tuned by means of gate voltages with the possibility to generate a pure spin
current, with a vanishing charge current.
