Adaptive materials is the core concept. Star Trek already has defensive screens which snap on due to an automatic detection. Plus, use the body's own heat to provide both power and reducing EM output. The purpose to reduce signatures not full stealth.
There are already simple passive ways of doing that when it comes to blocking IR without needing extra electronics.
Adding Electronics in itself, even ThermoElectric Generators, creates a new sensor profile to target.
The more electronics & equipment you add, the more sensor target types the enemy can home in on.
They don't have to target your human biological profile, they can target electronics or any of the signals that your electronics will make.
That's the issue with the sensor / counter-sensor warfare.
The more gear you use, you create different target profiles for the enemy sensors to look for to track you down in the field.
It's really a game of complicated cat & mouse.
https://www.physlink.com/education/askexperts/ae420.cfm
And like I've told to Dex before, the amount of energy that the Human Body generates in Heat output
is at best 100 watts.
The average human (according to my Coke® can's Nutrition Facts label) consumes approximately 2000 Calories per day (it's actually usually somewhere between 2200 and 4000, but 2000 is a nice number). Using a simple conversion (1000 calories = 1 Calorie, 1 calorie = 4.1868 J), this amounts to 8.37 x 106 joules ingested per day.
Human energy equivalentThis means that the average person expends ~8.37 x 106 joules of energy per day, since most of us are in some sort of equilibrium with our surroundings. Assuming most of this energy leaves us in the form of heat, I calculate that on average we radiate ~350,000 J of energy per hour. Since Watt is just Joules per second, this is roughly equal to energy given off by a 100 Watt light bulb!
Typical ThermoElectric Generators operate at
5-8% efficiency.
Even if you applied the Carnot Theory Limit on Heat Engines, the maximum would be ~83% using the Seebeck Effect for a solid state TEG.
But applying all that heavy metal for a TEG on a person doesn't make much sense given the weight penalty.
Even if you solved the materials problem and found a way to use Flexible Fabric-Style polymers to hold the TEG, @ 83% of 100 watts, that's 83 watts of electricity.
You can only do so much with 83 watts of electricity.