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• 09 Oct 07
  

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• 03 Oct 07
  

  
  Here's a story that's doing the rounds on the blogs -Scientists Invent 30 Year Continuous Power Laptop Battery.The story is written in peculiarly convoluted prose, but appears to be saying that a breakthrough in nuclear energy technology means that we'l

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• 02 Oct 07
  

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  • Here's a story that's doing the rounds on the blogs - Scientists Invent 30 Year Continuous Power Laptop Battery.
     
     The story is written in peculiarly convoluted prose, but appears to be saying that a breakthrough in nuclear energy technology means that we'll all be running around with magic batteries in mobile and laptop, in "two to three years". They'll be perfectly safe, will save the world, and will probably last longer than you will.
     
     Sadly, no. As with the best techno-rubbish, there is a story in there, but you'll be pootling around the skies in jetpacks before you're powering your Dell from neutron decay.
     
     That story is betavoltaics. This is a way of generating electricity much as solar cells generate power from photons, only by using high energy electrons generated from the beta decay of certain radio-isotopes. If you pick your isotope well -- the examples given use tritium, a radioactive form of hydrogen - the other decay products are inert, and in theory you can generate useful amounts of power for a reasonable length of time. As with every radioactive system, it has a half-life depending on the isotope; tritium's half-life is around twelve years, so every decade or so your battery will halve in power - but that won't change, no matter how little or much power you take out in the duration.
     
     Beats Duracell, right?
     
     Again, sadly, no. There are a few small problems.
     
     One is that the sort of atomic structures that generate power when bombarded with high energy electrons are the sort that tend to fall apart when bombarded with high energy electrons. While solar cells have the same problem, it's to a much lesser extent. There's a lot of research into making materials that don't suffer so much, but it remains a serious issue.
     
     Secondly, while it's true that a tritium-powered battery will eventually turn into an inert, safe lump of nothing much, and while it's also true that a modest amount of shielding will keep the radioactivity within the battery the while, there's the small problem that if you break the battery during its life the nasties come out.
     
     Thirdly, they don't have a great conversion efficiency. Around 25 percent is the best you can get - which is pretty good, but leaves 75 percent sloshing around as heat. That means a 25 watt battery will get plenty warm.
     
     Lastly, they're not very good batteries. Even the latest devices, which are very clever in the way they saturate a porous structure with the gas and thus usefully capture quite a large number of the energetic electrons, have an energy density of the order of twenty five watts per kilo. Lithium ion batteries, the sort you have in your laptop, manage 1.8 kilowatts per kilo. That's 72 times more bang per gram. Do you fancy carrying a battery 72 times heavier than the one you have at the moment, especially if it's hotter than a sixty watt light bulb?
     
     Which is not to say that betavoltaics aren't useful. There are some interesting ideas for using small betavoltaic cells to constantly trickle charge more conventional batteries, which helps circumvent some of the capacity issues.
     
     And there are plenty of places where it's difficult or dangerous to have to change or recharge the power source, and many of those only need small amounts of juice.
     
     Those places do not currently include your lap. You may possibly be relieved to hear this.

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