Present research indicates that diodes are a "free energy" device. Albeit, most diodes are useless in terms of producing usable power. Producing usable amounts of power would require billions of microscopic heavy doped Schottky diodes. Such a diode array chip, when mass produced, would be affordable. Manufacturing semiconductor chips is similar in nature to developing a photograph. Each layer of the chip is produce simultaneously by deposition. So there's no microscopic machine that creates each diode at a time. In the case of diode arrays, the design is minimized since there are a few layers-- just one type of component, a diode. This process is outlined at
Photolithography - Wikipedia
The entire process of fabricating semiconductor chips is outlined at
Semiconductor fabrication - Wikipedia
Creating a diode array chip consisting of a trillion microscopic diodes could be affordable, given mass production. One example, mentioned in this page details a proposed diode chip producing up to 169 watts/m^2. Such a diode device would direct the flow of ambient thermal energy to the appliance. The appliance would eventually convert most of such energy to heat, thereby heating the surrounding air. The heated air would flow to the cold device, thus forming a relatively closed loop. Eventually such a device would reach thermal equilibrium, in which case the temperature of the device would remain relatively constant. Energy would flow in a circular loop from the device, to the appliance, to the air, and back to the device.