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Comment by eloff | Hacker News Reader

So a typical nuclear reactor in a car is a terrible and unworkable idea. But what about an EV powered by nuclear batteries?

Could that be economical or safe? You'd have pretty much unlimited range.

http://www.bristol.ac.uk/news/2016/november/diamond-power.ht...

My gut feeling is it can't compete with lithium ion batteries in terms of rate of discharge or economics. But if money was no object, I think it may be possible.

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"Nuclear batteries" aren't, strictly speaking, batteries. They're RTGs, and they have very low (and constant) power output relative to other energy sources.

It would take days to fully charge an EV from a typical RTG and there's no way an RTG could put out enough power to even keep an EV rolling on a flat highway at highway speeds. They are good for remote, unmanned stations but not much else.

An RTG powered electric car may not be able to drive cross country at highway speed, but most cars spend the vast majority of their time parked. Having a RTG constantly top off the battery of an electric car would probably work quite well, assuming you worked out the many, many issues surrounding the RTG itself first.

A "large" RTG puts out a few hundred watts of electrical. A very efficient electric car might consume 25 kWh/100 miles, so about 250 watt-hours per mile. If your car sits parked 6 days per week, it might work.

Also: RTGs are typically 3-7% efficient, so your trunk will put out thousands of watts of waste heat continuously. Hope you live in a cold climate.

So if you're charging at a rate of 2 miles per hour you can drive 48 miles a day on average, which is a fair bit more than most cars do. Even a 1 mile per hour charge rate would be enough for some people, although at that point you'll probably supplement with solar panels or something.

One advantage over solar charging is that this won't be affected by the weather and works all night long. The excess waste heat is a problem, but it is way down the list of problems with running a RTG.

That still sounds incredibly optimistic. The most modern RTG:

https://en.wikipedia.org/wiki/Multi-mission_radioisotope_the...

Puts out 125W electrical, 2KW thermal, and costs $109M each (!)

Most cars could support 600w of solar panels. Even in bad weather you'd still come out ahead.

The price is almost meaningless. We don't have enough refined plutonium available to build more than three MMRTGs right now.

You wouldn't use a plutonium based RTG in civilian use for obvious reasons.

> Also: RTGs are typically 3-7% efficient, so your trunk will put out thousands of watts of waste heat continuously. Hope you live in a cold climate.

Ouch. It might be possible to harness some of that with e.g. Thermal to electric circuits to improve the efficiency.

The RTG is a thermoelectric generator -- it's already doing all it can with that thermal energy.

RTGs also avoid having moving parts or working fluids. More efficient energy recovery systems are available if you're willing to accept some maintenance items.

I don't see how you can have it both ways. Either there isn't much waste heat, or there is lots of it and you can potentially harness that still.

Let me make an analogy: Your petrol car has a heat engine. It converts about 20% of the heat energy released by combustion into mechanical energy. The rest goes out the tailpipe. Yet harnessing that remaining energy is not so simple because the temperature gradient is (relatively) small.

The thermoelectric effect also requires large temperature gradients, and efficiency is proportional to the gradient.

I think the car is a bad example because the temperature gradient is high and it should be possible to harness some of that waste heat. We just don't try. With hybrids we could definitely put that electricity to use, so I'm surprised no attempt is made to harness it. Especially for long haul trucking it could be worth it, but they're aren't diesel electric hybrid trucks that I'm aware of. Or at least they're not common.

I do understand your point though.

We do try! Some power plants and ships pipe turbine exhaust through a boiler and steam engine to increase fuel efficiency:

https://en.wikipedia.org/wiki/Combined_cycle_power_plant

Might be tough to fit all that under the hood though.

I was talking about in cars specifically. It would seem to me that adding some kind of TEG around the engine and part of the exhaust system might be worth it on a hybrid vehicle. https://en.wikipedia.org/wiki/Thermoelectric_generator

I guess it would depend on if you can make it cheaply enough.

Sounds like folks are working on it:

https://en.wikipedia.org/wiki/Automotive_thermoelectric_gene...

The numbers being floated about seem to be 4-5% fuel efficiency gain. Significant, but as you say it depends on the price.

A thermoelectric generator is to heat what a hydroelectric plant is to water. Neither one of them consumes its respective resource; they just capture some energy as it naturally flows from one space to another. Seeing water at the bottom of a dam, or heat on the exterior of the RTG, doesn't mean that any energy has been wasted; it just means it's reached its resting point.

Yeah, that makes sense. If the heat at the bottom of the dam, to mix your metaphors, has no further to fall, that is the temperature gradient compared with the external environment is low, then you're right, you can't really do anything useful with it in a practical way.

The work required for moving the RTG up a slight hill might just negate the energy you gain from it. An RTG is not really meant to move itself very fast. Consider the Mars rovers, they move extremely slow but benefit from the stable, long term power supply.

Any fission material in a consumer vehicle is unacceptable due to drastic consequences of a crash.

That's not how nuclear batteries work.

The one I linked is radioactive carbon 14 formed into diamonds. That'd be fine in a crash.

It also has power density on the order of 1 µW/cm^3 (= 1 W/m^3), which is incredibly low. An entire truckload of those batteries wouldn't even be able to power the truck's headlights.

That's a good point.

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