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Check out the fundamental limits of turning Mechanical Energy to Heat energy.
It is the mechanical equivalent of heat.
As far as I know, there is no such thing as "mechanical heat energy"; there is mechanical energy, and there is heat energy.
The mechanical equivalent of heat is 4.2 Joules per calorie, so 4.2 Joules can heat 1 gram of water by 1 degree C.
It will still be heat energy, but it can be converted to mechanical energy