Robots resembling humans are no longer purely science fiction, though we are far away from true stand in replacements of ourselves. Nevertheless, it reasonable to assume that over the course of this century they become a greater part of daily life. One important point we need to address is the energy supply for our lookalikes.
Most over our present day appliances run electricity and it seems logical to design humanoid robots or androids powered electricity. However, this raises the question of how to get electricity into an android. Given the mobile nature of this type of robot, we would not want to get them hooked up on power cables. So then the next logical option would be battery powered androids.
Unfortunately, electrical batteries have some serious drawbacks. First of all, they need to be recharges at certain intervals. This means that robots will need to be hooked at a power cable for some time once in a while. Yes, wireless charging is possible, but it is highly inefficient due to the electrical resistance of air, which also means that during charging the android need to stay close to the power source.
Also batteries, in particular those with a high energy capacity, require a good deal of scarce and hence expensive materials and with an increasing demand for android, the price of high performance batteries will also rise and hence humanoid robots will remain a expensive luxury.
But if robots can be made to resemble humans, can we not design robots to be powered like humans? How is the human body powered? By burning sugar. And sugar is relatively cheap as plants are build from long sugar chains, called cellulose and agricultural waste is an abundant supply of cellulose.
Bacteria could be used to convert biomass into either hydrogen or ethanol, which in turn to could be used in a fuel cell to produce electricity. The use of microbes to produce hydrogen is explained in the video below.
Of course, the down side of this approach is that robots that consume biomass, will need to dispose of the resulting waste. Fortunately, this will take less time than a full battery recharge.