For all stories about heat engines running between smaller temperature differences, the smaller the temperature difference, the less excited you should be. For example, I have a device that reaches the Carnot efficiency across temperature gaps of 0K! HINT: It's a rock. Or a paper clip. Or a wet tissue. Or...
Here's a detailed look at the thermal control system for the ISS, for the curious: https://www.nasa.gov/pdf/473486main_iss_atcs_overview.pdf
I know I'm on HN, as opposed to one of the political sites I read, when I run across that sort of remark.
“RTGs use thermoelectric generators to convert heat from the radioactive material into electricity. Thermoelectric modules, though very reliable and long-lasting, are very inefficient; efficiencies above 10% have never been achieved and most RTGs have efficiencies between 3–7%”
Also interesting, the price of RTGs:
"Plutonium 238, which is used to power Radioactive ThermoGenerators such as the ones that power the Voyagers, Galileo, and Cassini probes has not been produced in the US for 25 years, but NASA and DOE have budgeted $50 million to restart production of about 2 Kg per year for 5 years…an average cost of about $5 million per Kg. That is expensive stuff!"
One SNAP-27 unit, the type used in Apollo missions, providing only around 70 W, had around 4 Kg, which means its Pu price alone would be at least around 20 million, if it would be produced now.
One can sort of "feel" this result by imagining the voltage pushing a cloud of electrons around a circuit, producing shaft work in an electric motor, and/or heat if there is any electrical resistance.
> The [thermoelectric] conversion efficiency by Eq. 1 is the product of the Carnot efficiency (ΔT/Th) and a reduction factor as a function of the material’s figure of merit Z = S^2 ρ^−1^κ−1, where S, ρ, and κ are the Seebeck coefficient, electrical resistivity, and thermal conductivity, respectively.
Heart Pacemaker require around 10uWatt and this new power source seems to provide 12uWatt for 1 cm2. If temperature difference could be maintained, then could provide a low weight, low power source that do not require changing battery.
On a more practical note, I wonder how feasible a thermoelectric powered earpiece would be...
In combined cycle power plants they use the exhaust gasses of a turbine engine to power a secondary steam power plant. This obviously would work in a car if it weren't for the huge complexity and cost.
This technology is probably more applicable to situations where you have a source of low quality heat and you want to extract a tiny bit of power.
Bolting a heat engine to the internal combustion means that the excess heat from the engine block and the exhaust gases can do some work before convecting, diffusing, or radiating away. The usual problem is not the additional weight of the heat engine part, but the enormous radiator you would need to maintain a proper cold well. This would likely be a ribbed (finned) aluminum plate covering the entire underside of the car, with scoops and fans to ensure sufficient airflow across it.
The combustion engine part could then be redesigned to produce higher temperatures, as the heat engine portion can be actively driven if necessary to cool the engine block--or to warm it, as might be needed for diesel startup.
There's no need to have "excess" heat in an engine.
In a fixed volume (the cylinder, on the time-scale of ignition), pressure is proportional to temperature.
Higher temperature in the cylinder bleeds more heat into the engine block, but also produces more force on the pistons.
Nitrous oxide systems do this, at risk of overheating the engine. If you were to actively drive a Stirling integrated into that engine, it would actively cool the engine, forcing its heat into the cold well. You would overheat your oversized radiator, instead of your engine.
If up front cost and/or weight/packaging were the issue you'd see them employed at least occasionally in marine/rail/off highway/stationary industrial applications where physical trade-offs aren't as big of an issue and the up front cost is can be more easily amortized over the long service life of the equipment it's tacked on to.
This was known as a turbo compound engine: https://en.wikipedia.org/wiki/Turbo-compound_engine
These improved fuel efficiency by 15-35%. It's actually the reason my mind jumped to recovering energy from exhaust gasses. Doing this directly from heat would be elegant if it could ever be made cost effective.
> high-power density of 12 microwatts per 1cm2
Still I agree it sounds misleading.
> shortened the silicon nanowires to 0.25nm
That's a very short wire. Isn't that about the diameter of a silicon atom? I thought the smallest wires we could make on silicon were about 20nm _wide_.
for an intuitive sense, it would take 125 hours to charge a standard phone with 1 sqM device
So the potential for the application you cite is whole watts of power, enough to recharge a phone or a lantern overnight. (though, at current silicon fabrication costs, the price would be prohibitive)
For 2W you need ~20m² (~200sqft). That is the size of two rooms. Probably using the surface of your whole roof you can get 4W or 5W.
5°C Delta T generation is pretty impressive though. There are some commercial systems that can operate with 60°C temperature Delta (organic rankine cycle) and recover about 12% of that energy.
By bringing the temperature differential down, we could use a fairly weak heat source to charge up a device.
Would we be able to use a tiny amount of shielded radioactive material to provide a constant heat source and use that heat for the temperature differential?
A radioactive device that gives off that much heat wouldn't make sense for something like your TV remote control. However, maybe it could help power your off-the-grid house.
Does a radioactive material exist that could give off a few Celsius degrees of heat for years?
Would it be feasible to have homes use that material (properly shielded) without being a threat to national security and safety?
No - the "hotness" is pretty much intrinsically unsafe, people can break into the shielding, and if you limit it to easily containable alpha emission it has a short life.
> without being a threat to national security and safety?
Spaceships are essentially thermos bottles (metal tube surrounded by vacuum).
So lots of surface with large temperature differences.
Probably the problem of this application is that it's much better to have very good insulation in the walls than to try to capture the escaping heat and try to use it for something useful as heating.
Already possible using 1970s mechanical tech. Or solar. Im sure there are uses, but dont go with wristwatches for your explanations.
Or 1980s: https://en.wikipedia.org/wiki/Automatic_quartz