We Can Drive, but How Would We Eat?

15 July 2007

Interview with 

David MacKay, Cambridge University


Chris -   We've heard a lot of options on today's programme about things that you can do to live a cleaner, greener, meaner life.  Is there a scientific argument for adopting these approaches?

David -   Well it's definitely exciting to have these opportunities for things like hydrogen and other fuels derived from biomass, but an important thing to think about is the actual energy.  Where is the energy coming from? It isn't that we've got a fuel problem, if we want to get off fossil fuels, it's not so much the fuels as where are we going to get the energy from instead of fossil fuels.  I've done a little calculation like your
lightening strike calculation just adding up the numbers for Britain, could we live on our own biofuels for example?  So we start with sunlight, which is a thousand watts per square meter at midday.  It's not midday all the time so we lose out at night time and evening and morning, so it comes down to 250 watts per square meter on average. That's if there's no clouds, but it's cloudy two thirds of the time, so we're down to 80 watts per square meter. That's the figure if you're at the equator, and we're quite far north, so it comes down to about 50 watts per square meter on average is the power of sunlight.  And then the best plants for making carbohydrates out of are 1% efficient, so that gets us down to half a watt per square meter. Now you just need to know, what's the population density of Britain, and it's 4000 square meters per person.  If I get to be dictator of Britain and I say lets have 75% of Britain devoted to growing biofuels, how much energy do we get from that?  Well the raw carbohydrate that you're getting out in the form of plant material is 36 kilo Watt hours per day, and in Britain 36kWh per day is pretty much the amount of energy we're spending on transport at the moment.  You have got to bear in mind that we haven't processed that plant material into fuel.

Chris -   So in other words if we put across all of the space we have available, we could just about be energy self sufficient using plants, but what about food?  Would we have to buy it all in from France?

David -   Exactly.  Earlier on in the show you were saying we could like on organic food, but that organic food needs that land to grow on as well.  So we really have a crunch, and bear in mind that we hadn't actually produced the biofuels from that plant material, and that requires energy too.  Many of these biofuel processes lose a lot of energy along the way, you have to put in extra energy.

Chris -   So when we've got President Bush saying we're going to put across x amount of land to growing all these things to make biofuels, there's not a sound ecological or scientific argument for doing that?

David -   Well, the population density of America is lower than ours, so I'd certainly say that Britain can't live on biofuels for transport in the way that we currently live.  In America, their population density is maybe 5 times lower than ours.

Chris -   They've still got to eat though.

David -   Yes, and I think there's a big worry that in this rush to look green, people will actually end up doing something that's very bad for the poorer people of the world, who would like to use the land for food rather than for our biofuel.

Chris -   Also, is there a risk that if you suddenly switch all this land away from agriculture you may actually make the environment worse?  Because you're establishing even more of a monoculture, you've got lots of one particular type of crop growing for you to make lots of oil which we can turn into biodiesel, and that could have knock on effects for the environment.

David -   Yes, that's right and another environmental effect is the water requirements if we did take over lots more land and start growing lots more crops on it, we'll end up with a world water shortage as well.

Chris -   So what is the answer, do we travel less?  What do we do to get around the problem?

David -   I think we need to be looking at lifestyle changes to be able to live self sufficiently on our own renewables.  Alternatively, we need to be really nice to other countries and say 'Libya, you've got a nice low population density, lots of sunlight, please could we be nice to you and buy a bit of solar power from you'.

Chris -   I did a back of the envelope calculation and found that if you covered the entire Sahara desert in solar cells, assuming they're about 30% efficient and the sun shines for 12 hours in a day, we could generate about 10to the power 15 Watts [1,000,000,000,000,000 Watts].  That's an equivalent of power to the gulf stream, that's a lot of energy, that's a million Gigawatt power stations.  Why aren't we doing this?  This surely should be the answer.

David -   I completely agree, I think solar in the desert is one of the options for humanity.  You could certainly power Europe and North Africa, all at a European standard of living, using just a small fraction of the Sahara desert   Why aren't we doing that?  Well, why are we bombing Iraq?

Chris -   Don't go there...  Well I suppose you could have said that for president Bush.  But to wrap up then, how sound is it to say to people 'everything makes a difference.  Turning off your TV when it's on standby, not leaving your phone charger plugged in when it's not charging anything, this will help to save the planet'.  Will it?

David -   Some of these things definitely do make a difference; but the phone charger thing is crazy.  If you take a typical Nokia charger, it's using less than 1 watt, it's a really tiny trickle.  Probably about 1% of 1% of your energy consumption is going into the phone charger.

Chris -   But the argument is that there are so many people with so many of these things plugged in that en mass, if you add them all up it makes a huge difference.

David -   If you add it all up, you get 1% of 1% of the UK's energy consumption, that's how big a difference phone chargers will make.

Chris -   So we're being penny-wise, pound foolish with that argument.

David -   I think so.  But there are other things on standby which really do make a difference; if you're leaving a computer plugged in all the time, that's using maybe 80W, if it's screen is switched on that's another 100W maybe, a laser printer sitting doing nothing at all is 17W, and all of these things really do add up.  If everyone were careful in switching off those sorts of devices, we could be saving maybe 10-20% of our electricity consumption.


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