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Heat is the graveyard of energy. Everything that uses energy, or is energy, is actually just energy on it's way to the graveyard.

The energy of the universe is a pool of water a top a cliff. Water running off this cliff is used to do stuff (work), and the pool at the bottom is heat.

The "heat death of the universe" is referring to this water fall running dry, and all the energy being in this useless pool of "heat".

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Do thermophotovoltaic cells operate on different kind of heat?

Is it impossible to convert heat into other forms of energy without "consuming" materials like in the case of steam, geothermal or even the ones that need a cold body to utilize thermoelectric effect.

TPVs don't rely solely on the temperature of an object being high, they instead rely on two objects on either side having different temperatures. As heat moves[1] from one side to the other some of the energy from that movement is turned in to electricity.

[1]: Technically the movement itself is heat, the objects don't contain heat, rather they contain internal energy, but the two get mixed up more often than not.

That movement is effectively “consuming” the differential.

What thermal energy sources actually exploit is temperature difference, not heat. And in the end that difference averages out.

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