What is the technology here? How does heating hot water turn into USB charging current?
I have a lantern / radio combo that works this way.
FYI, $100 is about what you can get a made in China gasoline generator from Harbor Freight or ebay or Amazon. Cheap models will produce enormously more power - usually about a kilowatt - but yes, they are not very fuel efficient and they are extremely noisy.
Peltier devices are kinda nifty. They are made out of semiconductors so they have no moving parts. Apply electricity to a peltier and the semiconductors act like a bunch of itty bitty heat pumps, so one side gets hot and the other side gets cold. Reverse the polarity of the electricity, and the hot side becomes the cold side and the cold side becomes the hot side.
They also work in reverse. If you apply a heat differential across the device (like a candle on one side and cooler water on the other side) and they generate electricity.
They’ve been around since the 1970s. They were originally ungodly expensive back then, and were used in space satellites.
As heat pumps, they have a lot of disadvantages. They are fairly inefficient, so if you use them as a cooler, they generate their own heat, which you have to get rid of or else the device will overheat. So when they are used as CPU coolers or small portable picnic coolers, there’s always a fan or a really big heat sink on the hot side to draw the waste heat away from the device. They also can only generate a certain amount of heat differential, so if you want to heat or cool more than that, you need to stack them.
When you use them as a generator, again, they aren’t all that efficient. On a larger scale, other technologies will generate more electricity for the amount of heat pumped into the system.
Their advantages are that they are small, lightweight, and have no moving parts which makes them pretty darn reliable. And these days, they are fairly inexpensive.
They are often called Thermoelectric Generators (TEG) or Thermoelectric Coolers (TEC), depending on how they are used.
You’re being generous here - they are very inefficient. 
So much so that they are only used in niche applications.
My favorite application for Peltier devices is to generate electrical power on those probes that are sent out to explore the solar system. (Link.)
Somehow my thoughts turned to a ideas of a Stirling engine built from polished brass and steel, driving a generator. Just seems much more the part. For a small desk mounted one, the efficiency is probably no better sadly.
yep, the Voyager probes have them, as Radioisotope Thermoelectric Generators (RTGs.) They use the decay heat from slugs of plutonium to heat the “hot side.” I think they’re still used even in newer spacecraft.
I remember a while back when there was a teenage girl all over the news for building aPeltier junction flashlight. (Somewhere I have one of those tiny Peltier juction switchable heater/coolers that can hold maybe 2 or 3 cans of soft drink. Bought it on impulse around 15 years ago because it was very cheap, but have never even used it because it had only a cigarette lighter plug and not a home AC plug.)
These also spam up the science press such as breathless articles “MIT finds a way to harness the waste heat from a car”.
Yes, you can grab a couple percent of the waste heat, but that hardly justifies adding the extra mass and complexity of the junctions to a car. And you’re never going to be able to get more than a few percent since the dT is low and the solid state junctions themselves are terribly inefficient compared to gas expansion engines.
I think the reason is the general public is under the impression that reclaiming all of the waste heat is feasible. It isn’t, but that requires understanding entropy balance in a process.
Curiosity has one. Mars 2020 rover will get one too.
I remember that the Dept of Energy restarted production of [sup]238[/sup]Pu to supply these RTGs. Apparently production will cover two more spacecraft after Mars 2020.
As I heard it described, humans have now landed a nuclear-powered robot with a rock-vaporizing laser on the surface of another planet. We have become the villains of a 1950s bad science fiction flick.
I keep seeing sales on Peltiers on Amazon for very, very little. Tempted to get one to put on a cpu just to see what’s what. But the inefficiency means that there’s a lot more heat generated which means the fan would blow faster. And I’m on a keeping things quiet kick now.
To cool quietly requires other, non-cheap, options.