  We study a thermoelectric ratchet consisting of an array of disordered
nanowires arranged in parallel on top of an insulating substrate, and contacted
asymmetrically to two electrodes. Transport is investigated in the Mott hopping
regime, when localized electrons can propagate through the nanowires via
thermally assisted hops. When the electronic temperature in the nanowires is
different from the phononic one in the substrate, we show that a finite
electrical current is generated even in the absence of driving forces between
the electrodes. We discuss the device performance both as an energy harvester,
when an excess heat from the substrate is converted into useful power, and as a
refrigerator, when an external power is supplied to cool down the substrate.
