  We derive expressions for the efficiency and figure of merit of two spin
caloritronic devices based on the spin Seebeck effect (SSE), i.e., the
generation of spin currents by a temperature gradient. The inverse spin Hall
effect is conventionally used to detect the SSE and offers advantages for large
area applications. We also propose a device that converts spin current into
electric one by means of a spin-valve detector, which scales favorably to small
sizes and approaches a figure of merit of 0.5 at room temperature.
