The present invention provides a TEG device comprising a first unit comprising a thermopile unit and a second unit comprising a cold plate and/or a radiator. One of the first and second units is adapted for being embedded or implanted into a body, while the other of the first and second units is adapted for being placed at the outer surface of the body, i.e. in a fluid. The first and second units are adapted for being thermally connected to each other through the surface between the body and the fluid when in use.