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Thermocouple is a material (junction between two different metals) which generates potential difference in terms of millivolts, isn't it?

Why don't we create a matrix of thermocouples (many thermocouples which are connected in series and parallel) to generate voltage in terms of volts or even kilovolts, and can supply current in terms of amperes?

Today's scientist are able to put billions of transistors inside a tiny chip. Why don't they put billions of thermocouples on a one meter-square plate? Isn't that feasible; if so, why? If it is feasible, just why don't they do it?

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For a thermocouple to work there has to be a temperature difference. You have to heat one side or cool the other side, otherwise it will not produce any power.

You can buy what you want, that is , there are Peltier elements for sale. A Peltier element is just the same thermocouple but it is usually used in reverse (connect power to thermocouple and one side of it cools down while the other heats up) for cooling.

The problem with them is that the efficiency is very low in power generation mode. Part of it is because the temperature difference is too low (power plants heat the steam to more than 100 degrees for a reason - higher temperature = higher efficiency). Also, you need good electrical conductors for a thermocouple, which means that they will most likely conduct heat pretty good, so part of the heat goes trough the thermocouple. So, for high temperatures they are worse than a steam turbine, while you cannot get much power from low temperatures (where you cannot use a steam turbine) anyway.

In any case, thermocouples are used for power generation in Radioisotope Thermoelectric Generators (RTGs).

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