Betavoltaics

I ran across a very interesting post today about betavoltaics, a method of generating electrical power similar to photovoltaics but based on beta particles (electrons) rather than photons. Despite what the gormless author of the linked article says, this is nuclear power based on radioisotope that is a beta-emitter. Usually this is tritium (which decays to produce the beta radiation, it is not produced by it as author states).

One of the interesting things about these batteries is that with tritium's 12ish year half-life you can have a battery that produces power for decades without recharging. Non-recoverable tritium is produced naturally by the action of cosmic rays on the atmosphere but for industrial purposes it is produced in nuclear reactors. While production is common enough to be a factor in routine nuclear processes, only an estimated 225kg have been produced in the United States since 1955.

Tritium is pretty cool stuff and is used in such things as gun sights and glowing keychains ("Trasers" or "Glowrings"). Alas, the United States and Canada have recently decided that keychains are a frivolous use of nuclear technology and so the totally harmless and very cool keychains that I'd very much like to have are banned in the US. Items like the keychains and gun sights contain only a very very small quantity of tritium, but also have a fairly steep price tag (about 7 to 10 bucks).

The power output of tritium is pretty low, about 24 watts per kilogram. A couple of years ago the price of tritium in Canada was $30,000 per gram. If you wanted to power a 10W device for 8 hours each day for 10 years you'd need a battery that could produce 80 watt-hours per day after one half-life. A kilogram of tritium produces 576 watt-hours per day, so you'd need 138 grams to produce 80 watt-hours per day. Double that to get the quantity at the start of the half-life, 277g. After it was all done you'd be looking at $10 million for the battery.

To compare, a D cell battery is about 23 watt-hours (depending on how fast you drain it). You'd need about 4 batteries a day to produce 80 watt-hours, or about 14,600 for 10 years. At 2 bucks each that's $30,000.

If you used a rechargable NiMH D cell that is good for 200 cycles, costs $9 and is good for 18 watt-hours you'd need 91 of them at a cost of $821 for all 10 years. Add a solar panel to charge them; you'll need a 15W panel at a cost of $60, for a total of about $880 or $0.24 per day. The tritium battery would cost about 10,000 times as much, but you have the advantage that you could stay in a cave for 10 years! With the 91 rechargables you could only stay in the cave for about 2 weeks before needing to charge up again (which would take about 2 weeks unless you got a bigger solar panel).