Suspended-Load Backpack

[Todg]

Originally posted by Anthony:
Talking about wH/kg is kind of weird; one of the problems with radioisotope batteries is that you can't turn them off. The initial power output of tritium is slightly over 1W/g, and it will

OK, here's the question that was missed. Is the 1W/g the power of the beta radiation emitted, or the electrical power that is produced via the betavoltaic process? It's clear that the two aren't the same.

And what's weird, given the hazards of tritium, it's strange to see that you can find it from everything from emergency exits signs to key change fobs.

Do a search on ebay for 'tritium'. I wonder how much potential power is being wasted?

 

[Anthony]

The amount of tritium required to make an object glow in low light is well under 1W.

Efficiency of betavoltaics appears to be in the 20-30% range.

 

[Todg]

What's the source of the 1W/g number, BTW? I'm trying to reconcile that with the 10^7 watt-hours per kilo that I keep finding.

Not trying to be difficult. I just can't find a source for the output power of tritium radiation. Posting this to the TML as well.

 

[Anthony]

Atoms per gram: 2e23 (avogadro's number/atomic mass of tritium)
Half-life: 12.32 years = 3.89e8 seconds
1/e time: 3.89e8/ln(2) = 5.6e8 seconds
Decay rate = 2e23/5.6e8 = 3.6e14 decays/second/gram = 9,700 Ci/g (one Curie = 3.7e10 decays/second)
Energy per decay = 18.6 kEv = 3e-15 joules
Power = 3e-15 * 3.6e14 = 1.08W/g
Total energy = 2e23*3e-15 = 6e8J/g = 1.7e5 wH/g = 1.7e8 wH/kg. Presumably the 10^7 wH/kg includes efficiency losses and the weight of non-tritium components of the cell.

 

[SanDragon]

And why is this not in the betavoltaic thread?

 

[Todg]

Thanks, Anthony.

Filed away in the black hole of quality.

Looks like output is actually much lower. 18.6keV is actually the maximum. Average energy of the beta is about 5.7keV.

 

[Anthony]

Ah. Right, energy is lost to the neutrino as well (the neutrino is neither useful for power production, nor relevant as a radiation hazard).

 

[Todg]

Unfortunate that it works out this way. For a brief amount of time i thought I had an alternate power source for robots.

Dang!

 

[Anthony]

Radioisotope power sources (including tritium and Pu-238) are viable power sources for robots; not a huge power density, but a few hundred watts/kg is doable, and a lot of robots don't need a large constant power supply.

They just contain somewhat hazardous radioactive materials, so for most civilian purposes just having them plug into a wall outlet is both cheaper and safer.

 

[Todg]

OK. Mostly I was intersted in alternate power sources for robots in use by adventurers/military - some sort of alternative to fuel cells for areas that don't have a handy supply of hydrogen, or electrical outlets.

 

[Straybow]

And where would they be without a handy supply of hydrogen (the most available substance in the universe) but they remembered their T-powered batteries?