Can a biological system be used to power an android?
"it needs to eat and drink like a normal person" // "The technological level is space-age"
Yes, this can be done
And it doesn't need to be 'space age tech' either.
I can see ways of doing this with existing technology.
In fact by now I think it's probably already been done in a crude way with existing technology : Let me introduce you to Slugbot [LinkOne LinkTwo LinkThree LinkFour LinkFive].
A simple internal fermentation (with the assistance of yeast of course) & distillery system can convert food into ethanol which can then be burned to produce electricity through a small electric generator.
Solid residue can be compacted to squeeze out any excess water (no doubt heat assisted by being placed near the ethanol burning system) & the resulting briquettes themselves burned as fuel to drive a small internal steam turbine for more electricity if desired, though that may not leave you with very convincing 'stools' (little briquettes or pellets of compacted ash) if that's important to you.
Or other bacteria such as those in septic tanks might be used to decompose the food into something with a more authentic human faeces appearance texture & smell while producing methane as a fuel for your internal generators in the same way (if at a micro scale) as Energy from sewage systems.
Excess water can be extracted from the system treated with other bacteria to provide the requisite colour & smell & excreted as 'urine'.
Of course if you're covering your 'android' with real flesh & skin as part of it's disguise you won't need too much treatment of any food taken in to make the faeces & urine look authentic, because you then have to include organs for digesting food & pumping blood (lungs too) anyway to keep the skin alive.
Those of course are for a hard state robot, if you're simply seating a computer in a biological body you can simply use muscle power to generate electricity, with tiny hydro generators in larger blood vessels perhaps or simply lacing bits of the same Piezoelectric material used in some flashing shoes through the body in appropriate places. This small device produced 52 microwatts.
A minimum (if any) handwavium is required for what you want & with (though you don't need it) 'space age tech' thrown in to explain more efficient bio engineered bacteria & minituristion of the systems required you really have nothing to worry about with your idea.
But all that aside your main concern appears to be that it doesn't need to plug itself in thereby spoiling the illusion it's a person & potentially giving itself away & if that's your major concern I can think of at least three non-biological supplementary power sources that could be used.
- A flexible subdermal sheet of solar power cells beneath the skin.
Cup your hands over your eyes with the fingers tightly squeezed together now turn your face to the sun, see what I mean, & that's your whole hand with a reasonable thickness of flesh not just a little thin skin, yes the efficiency of power generated will be considerably reduced by a layer of skin but it's not going to be non-existent.
Crystal radio style harvesting of radio waves for electricity, it's what we're beginning to use as a power source for a lot of small wireless devices these days & though not a lot it's still a bit, used to trickle charge batteries while 'sleeping' perhaps.
Nuclear batteries as used in pacemakers.
A desktop PC uses around 0.05 kWh (50,000 milliwatts) & this new prototype battery produces 3,300 milliwatt-hours per gram so a 16 gram one of these should more than suffice if you only want to power an android 'brain' in a human body & as Nickel-63 has a half-life of 100 years it should last more than long enough for your purpose. To power more than just the 'brain' you need a bigger one of course.
Personally I'd probably go for a mix-an-match of elements of several or even all of those ideas.
Sidebar : If you go for one of the human body / android brain options perhaps only replace the higher functions & leave the brainstem in to handle all the autonomous & reflex actions (breathing, heartbeat & flinching if cut or from anything really hot or cold etc).
It may even be plausible to only remove the higher reasoning centres of the brain & retain any learned skill-sets like walking, swimming or playing the piano from your body's 'donor' but I would suggest you ask another question for that if you want to consider it.