Science Interviews

Interview

Sun, 15th Jul 2007

We Can Drive, but How Would We Eat?

David MacKay, Cambridge University

Listen Now    Download as mp3 from the show Fuels of the Future

Chris -   Weíve heard a lot of options on todayís 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 itís 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 isnít that weíve got a fuel problem, if we want to get off fossil fuels, itís not so much the fuels as where are we going to get the energy from instead of fossil fuels.  Iíve 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.  Itís 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. Thatís if thereís no clouds, but itís cloudy two thirds of the time, so weíre down to 80 watts per square meter. Thatís the figure if youíre at the equator, and weíre 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, whatís the population density of Britain, and itís 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 youíre 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 weíre spending on transport at the moment.  You have got to bear in mind that we havenít 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 hadnít 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 weíve got President Bush saying weíre going to put across x amount of land to growing all these things to make biofuels, thereís not a sound ecological or scientific argument for doing that?

David -   Well, the population density of America is lower than ours, so Iíd certainly say that Britain canít 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 -   Theyíve still got to eat though.

David -   Yes, and I think thereís a big worry that in this rush to look green, people will actually end up doing something thatís 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 youíre establishing even more of a monoculture, youíve 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, thatís 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, weíll 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 ĎLibya, youíve got a nice low population density, lots of sunlight, please could we be nice to you and buy a bit of solar power from youí.

Chris -   I did a back of the envelope calculation and found that if you covered the entire Sahara desert in solar cells, assuming theyíre 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].  Thatís an equivalent of power to the gulf stream, thatís a lot of energy, thatís a million Gigawatt power stations.  Why arenít 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 arenít we doing that?  Well, why are we bombing Iraq?

Chris -   Donít go thereÖ  Well I suppose you could have said that for president Bush.  But to wrap up then, how sound is it to say to people Ďeverything makes a difference.  Turning off your TV when itís on standby, not leaving your phone charger plugged in when itís not charging anything, this will help to save the planetí.  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, itís using less than 1 watt, itís 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 UKís energy consumption, thatís how big a difference phone chargers will make.

Chris -   So weíre 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 youíre leaving a computer plugged in all the time, thatís using maybe 80W, if itís screen is switched on thatís 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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