Filling up the laptop
Originally published 2003 in Atomic: Maximum Power Computing Last modified 03-Dec-2011.
You know what's wrong with the world today?
Batteries.
OK, OK, maybe batteries aren't the world's biggest problem. But they're a significant one.
Computing technology has improved out of sight in the last few decades, but battery technology hasn't advanced nearly as quickly.
For low power devices, that's not a big deal - modern batteries can deliver days of mobile phone standby time, and modern portable audio devices can run for a respectable number of hours from one or two AA cells. But there are plenty of relatively high power devices that could benefit considerably from a better energy source.
There's nothing wrong with the concept of the battery - an electrochemical reaction that produces a potential difference between two poles of a cell, multiple cells stacked together making up a battery - but the darn things just don't have enough energy density. Energy density, which is commonly measured in watt-hours per kilogram (Wh/kg), tells you how much juice you can get out of a given power source. A one kilogram, 100Wh/kg battery will give you a hundred watt-hours; it could run a 100 watt bulb for an hour, assuming it didn't have trouble delivering its power that fast.
Lead acid batteries are cheap and last well, but they'll give you only about 35Wh/kg, shading up to 50Wh/kg for expensive versions. Nickel metal hydride batteries - the most common type for cheap portable devices, these days - manage about 70Wh/kg. Lithium ion batteries are common enough these days too, and manage 120 or so. Lithium polymer's still pretty exotic; it manages 150-odd Wh/kg.
Alkaline batteries can manage about 140Wh/kg, but they're no good for high-drain applications, and you can't recharge them. Rechargeable alkalines can take a few tens of charges to some large fraction of their original capacity provided you haven't run them flat, but they're still no good for high-drain use.
Petrol, for comparison, manages something in the order of thirteen thousand watt-hours per kilogram. Petrol engines are miserably inefficient, but even an implausibly good 90% efficient electric drive system, in a car carrying around five hundred kilograms of lithium polymer batteries, will only give you three quarters of the motive energy that you'd get from a 50 litre petrol tank feeding a cruddy 20% efficient four stroke engine.
One day, we'll have batteries that pile up free electrons in quantum pockets in a trifistrium matrix, or something, and deliver the energy density of a plutonium bomb.
In the meantime, if we want to get away from burning fossil fuels in horribly inefficient mobile engines and powering our portable gadgets from low-energy-density batteries, fuel cells look like a good idea.
Fuel cells electrochemically convert fuel into electricity, without bothering with the awkward combustion process that dooms regular engines to inefficiency.
The fuel that most currently workable fuel cells run on is hydrogen, which is a bit of a pain to store and transport. A device called a "reformer" can be used to convert methanol and water (much easier to store) into carbon dioxide and hydrogen, but reformer-based fuel cells aren't very efficient, and small models for portable electronic devices are less efficient still.
"Direct methanol" fuel cells (DMFCs), on the other hand, run from un-reformed methanol. DMFCs seem most likely to end up in your laptop or mobile phone or super-loud boom box that can annoy a whole beach for a whole day without a break.
Eventually.
A DMFC with currently-unremarkable 35% efficiency ought to give you 170-odd watt-hours from only 100ml of methanol. That's not incredible, compared with current laptop batteries, but it's good. And - the big plus - you get another 170 watt-hours every time you slap in another 100ml fuel cartridge.
The companies developing small DMFCs envisage methanol cartridges selling like batteries, except rather cheaper on a watt-hours-per-dollar basis.
If tweakers and skinflints use syringes to refill their cartridges and, regularly, poison the catalyst layer inside their fuel cells, I will so not be surprised.
All of the practical DMFCs so far require about a 3% solution of methanol in water to run. Fortunately, this doesn't mean that 100ml of methanol has to be hiding in 3.3 litres of mainly-water "fuel"; these things make water in the process of running, so they just need a small water reserve to get started, recycle most of it, and emit a little bit of water vapour.
One major DMFC catch is that when you do the watt-hours-per-kilogram or watt-hours-per-cubic-centimetre comparison between methanol and the battery technology of your choice (in Wh/kg terms, 35% efficient methanol-to-electricity conversion beats lithium polymer by a factor of about 14), you're not taking into account the size and weight of the fuel cell itself. The fuel is light and small; the fuel cell is not. Pretty much all of the small DMFCs so far are more than double the size of the gadget they power, and they're often obvious prototypes that're about as stylish as a breeze block.
Another major DMFC problem is that small DMFCs don't have anything like the current capacity needed to run things like laptops, yet. They've got the energy, but they deliver it in a trickle, not a flood. High current models are, thus far, much too big to be practical. But that'll change. NTT DoCoMo is promising fuel cell powered mobile phones by 2005, and Toshiba's demonstrated a DMFC the size of a portable printer that can run a pretty low powered (20 watt) laptop.
And then there are the fringe options, like microbial fuel cells that run on sugar water, enzyme-catalysed ethanol "batteries" that don't need expensive metallic catalyst material, and even microengines coupled to tiny generators.
If you're really hard core, of course, you're going to hang out for Mr Fusion, so you can run an aluminium smelter on three banana peels a day.
But a pocket full of wood alcohol cartridges will, I think, beat the heck out of batteries, for the rest of us.
