Wednesday, 9 April 2008

Fuel cells and decentralising energy

My one of my undergrad dissertations (I had two: one on placement year, one as a three-month project before my finals, this is the latter) was on the subject of hydrogen storage, which got me thinking about energy issues. We're currently highly dependent on hydrocarbons as fuels for electricity generation, cooking/heating, and vehicle fuels, as well as to a smaller degree in chemical feedstocks for everything from drugs to polythene bags. These resources are finite, and the oil's largely locked up in a part of the world we're obsessed with destabilising, so it'd be good to have alternatives. The energy isn't going to be anywhere near as plentiful as energy from fossil fuels, at least at first (due not so much to scientific limitations as political heel-dragging), so there's the problem of belt-tightening and energy efficiency for a while.

Our energy generation is very centralised. It's easier to build one big nuclear (or coal, or gas) power plant than several small ones, after all, and electricity is fairly easy to distribute from such a central point. The losses involved in distributing electricity to the end users aren't huge. With the race towards electric vehicles, we're going to be drawing on the grid to fuel our cars and trucks too, so it's important to think about how we're making and using our energy. One of the biggest drawbacks with centralised power generation, as it turns out, is efficiency. Electricity generation systems based on heating up water (such as nuclear, gas and coal power plants) waste maybe two thirds of the heat they generate. You can claim back some of that energy, but due to pesky thermodynamics you can't get it all. The excess heat can't really be transported to where it'd be useful (our windswept, chilly lab for a start) so it's dumped straight into the environment, which kind of sucks for the climate locally and globally.

Earlier in the year we had a presentation from Prof. John Irvine of St. Andrews University. He's working on fuel cells, and in particular solid oxide fuel cells. A fuel cell, in general, is a device which uses up fuel to generate electricity. There's a lot of interest from industry. What's really striking is the sort of product Rolls Royce are aiming at - it's a fuel cell which runs off our existing natural gas supply, but it generates electricity at the same sort of or better efficiency than a gas-burning plant, and it's the size of a domestic gas boiler. Here's the fun part: any inefficiency in generating the electricity just heats that person's house! Although it's obviously not sustainable, it gives us a "stop-gap" that allows much more efficient use of our existing fossil fuel reserves. It's a nice idea, and it highlights a possible benefit of shifting electricity generation out to people's homes.

In the long term, energy generation may decentralise, or it may stay central. Many of the big green options (solar, wind, wave power) can be decentralised, and it's been suggested that we'll all set up our own little solar farms and whathaveyou. Again, this makes it easy to catch waste heat and use it more productively, and the losses incurred in transporting the electricity are reduced. Maybe neighbourhoods will gang together. Other systems, like geothermal power or nuclear fusion, are clean but still need to be centralised, and still have the dumping-lots-of-hot-water problem. It'll be interesting to see how everything's mixed up in 50 years time.

As far as vehicles go, in the short term everything's based on existing rechargable battery technology. This means that it's largely dependent upon how the electricity is made, and the concerns above. In the long term it looks like we may wind up with a "hydrogen economy". Current batteries are pretty crappy in terms of energy density - you can fit a lot of energy into a particular volume (your fuel tank) but they're very heavy compared to petroleum, and the batteries have to be disposed of, recycled, whatever after a couple of years. Instead of charging batteries, the theory goes, we can use the electricity to make hydrogen gas from water. Hydrogen gas has a great energy per unit weight (it's the lightest gas, and it's got a literally explosive amount of energy crammed into it) but it takes up a lot of space compared to petroleum (something we're working on). You get the energy back out by running it through a fuel cell, getting your water and electricity back, which can then power your vehicle.

There are a lot of things to think about when it comes to the efficiency of a hydrogen powered vehicle. As well as the efficiency of the electricity generation, you've got to worry about the efficiency of making the hydrogen using that electricity (there are some interesting ideas on that front, although of course nature got there first) and then you've got to store the hydrogen, and transport it! When you've got hydrogen-fuelled vehicles moving around hydrogen fuel, there's a lot of room for inefficiency in the system as a whole. Many of ideas for storing hydrogen in smaller volumes look like they'd need large-scale chemical industry work, so it's not clear how we could decentralise this and thereby cut back the shipping costs (in terms of efficiency). We may be stuck with a situation like we've got with oil at the moment, moving tankerloads of mysterious pellets around the country to wherever they're needed. This is a big argument in favour of sticking with old-fashioned rechargable batteries, if we can make them lighter, longer-lived and easier to recycle.

In a lot of ways, thinking about energy efficiency is like thinking about the Drake Equation. You've got a lot of subtle variables in there, like the energy involved in making your energy-efficent gizmos and recycling them at the end of their life. These could counterbalance the benefits you get from your gizmo in the first place! So it's a complex problem, and a lot of good debate and argument's come out of it.

Sorry to my regular readers (are there any?) for the delay in updating. I've been a bit preoccupied. I'll do a theory update about intermolecular bonding (once atoms stick together to make molecules, how molecules stick to eachother to make materials) on Sunday with any luck, and get back to a regular update schedule soon.

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