Monday, June 2, 2008

The last thing we should talk about

Wallace Broecker has been promoting the idea that `artificial trees are the way to solve global warming'. Pushed for details, he says that `brilliant physicist Klaus Lackner has invented a method to capture CO2 from thin air, and it doesn't require very much energy'. Broecker imagines that the world will carry on burning fossil fuels at much the same rate as it does now, and 60 million CO2-scrubbers (each the size of an up-ended shipping container) will vacuum up the CO2.

I think it's a very good idea to discuss capturing CO2 from thin air, but I feel there is a problem with the way this carbon scrubbing technology is being discussed. The problem is energy: how much energy does Lackner's CO2-capture method require. `Not very much'? Come on, we need numbers, not adjectives.

Here are some of the numbers required for a coherent conversation about carbon capture. Grabbing CO2 from thin air and concentrating it into liquid CO2 requires energy. The laws of physics say that the energy required must be at least 0.24 kWh per kg of CO2. What does Lackner's process require? In June 2007 Lackner told me that his lab was achieving 1.3 kWh per kg. Let's imagine that further improvements could get the energy cost down to 0.7 kWh per kg of CO2.

Now, let's assume that we wish to neutralize a typical European's CO2 output of 11 tonnes per year, which is 30 kg per day per person. The energy required, assuming an exchange rate of 0.7 kWh per kg of CO2, is 21 kWh per person per day. For comparison, British electricity consumption is roughly 17 kWh per person per day.

So as a ballpark figure, the Broecker/Lackner plan requires an amount of energy equal to current electricity production.

When I call carbon capture from thin air `the last thing we should talk about', I don't mean that we shouldn't talk about it. I definitely think we should talk about it, in detail, to help drive us towards more radical action now, to reduce the need to create these mega-vacuum-cleaners.

P.S. What about trees?. Trees are carbon capturing systems; they suck CO2 out of thin air, and they don't violate any laws of physics. They capture carbon using energy obtained from sunlight. The fossil fuels that we burn were originally created by this process. So, the suggestion is, how about trying to do the opposite of fossil fuel burning? How about creating wood and burying it in a hole in the ground, while, next door, humanity continues digging up fossil wood and setting fire to it?
From the minutes of the Select Committee on Science and Technology, the best plants in Europe capture carbon at a rate of roughly 10 tonnes of dry wood per hectare per year. Or in equivalent CO2 terms, that's about 15 tonnes of CO2 captured per hectare per year.
So the area of forest per person required to fix a European output of 11 tonnes of CO2 per year is 7500 square metres per person. (And then you'd have to find somewhere to permanently store 7.5 tons of wood per year!) Taking Britain as an example European country, this required area, 7500 square metres per person, is twice the area of Britain.


Jurgen Van Gael said...

How about trees? How do they compare to carbon scrubbing technology?

Robin Smith said...
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Robin Smith said...

I ran some numbers on this topic over a year ago. I wanted to see if I could win the Virgin Earth Prize (US$25M) which means I would have to find a way to sink 1 gigatonne of CO2 (I think it was CO2 not C) per year for 10 years.

The method I chose was to use Biomass with CCS as this is good way to both generate electricity and sink CO2 obviously. (given the tech is available which it isnt)

The result was that I could win theoretically. In practicable terms though it was a non starter. It would have required harvesting biomass the area of Greater London every year to sustain the carbon sink.

Peter Harper said...

The trouble with biomass is that photosynthesis is relatively inefficient. David's own stats on energy and power densities show some other renewables to be better. Trouble is they are usually intermittent. This is a problem for real-time energy systems, but might not matter so much if they were used as the energy input for chemical air-capture systems, located wherever conditions were exceptionally favourable. Deserts come to mind. The nice thing about air-capture is that unlike 'geoengineering' systems it simply restores the status quo ante (in principle at least) and could even permit a certain degree of 'row-back' from a situation of overshoot. I think the idea deserves more consideration, at least as an emergency measure to allow the new energy infrastructure to be built up.

Unknown said...


You're getting a reasonably fair write-up at El Reg:

Maybe you could create a new entry to discuss some of the points in it or your book generally. For example at home my family has reduced our total primary energy use to net 4kWh/day electricity and ~20kWh/day gas heating/DHW. And we got there by following through on the don't-leave-it-switched-on thought! See for slightly obsessional details if you're short of better things to do...



Anonymous said...
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FutureNerd said...

It's a little confusing that you use average European CO2 production and average British electricity consumption. True, you are using rates per person per day, and one wouldn't expect those figures to vary too much between Britain and the rest of Europe.

Also, I think it would be more striking (but still fair) if you count the CO2 that would be produced by average current electricity-generation methods, then find the real break-even point.

For instance, if generating those 21kWh per day only produces another 5.5 tonnes per year of CO2, then you need another 10.5 kWh per day to clean that up, which generates another 2.75 tonnes/yr, which requires another 5.25 kWh/day, etc.

21 + 21/2 + 21/4 ... = 42

Of course if generating 21kWh/day produces more than 11 tonnes CO2/yr, then the series diverges and running the scrubbers makes things worse the more of them you run.

LegendMaster said...

have you done any work on ocean seeding to promote phytoplankton growth, which might potentially sink out and take the CO2 with it?

Jon Turney said...

Just read parts one and two of the book - a very useful compilation of data and arguments. I've been struggling with the climate/energy chapter for a book on the future and looking for an unbiased approach using "guerilla physics", I know realise. Glad to have found it. It helped firm up a number of conclusions I was heading towards, though I'm still kind of hoping Craig Venter can save the world, even if you've convinced me that Wally Broecker probably won't!

Unknown said...

Rather than liquifying CO2, why not react it with water, producing hydrocarbons, and oxygen. The artificial oil thus produced can be used as as a means of energy distribution, when electricity is impractical.

I've seen back of the envelope estimates that oil produced this way would cost a few dozen dollars per barrel. Such oil is more useful than liquified CO2, both as fuel, and as a feedstock for the chemical industry, but I'm not sure what the costs would be.

magnificolm said...

Great talk at Oxford tonight. I am inspired. So ...

What about trees?

First, there's an argument that only biological systems are capable of storing carbon at the scale the world needs.

Second, generalise "trees" to all organic production from which the carbon is capable of being stored. Opens other thinking, e.g. organic waste; bioengineering; algal blooms and the food chains above them (New White Cliffs of Thamesmead?); etc.

More importantly, look very hard at better forms of storage. Our present problem is not only the result of fossil fuel burning, but also land use and the consequent release of soil carbon. So why not put it back?

Charcoal, or more generally biochar, is a soil improver with a very long history - search "terra preta" for example. I believe that:

- its production, in closed retort technology rather than traditional burning, can cleanly release some usable energy in gases, over and above the energy content of the char (which in this scenario would not be used);

- it certainly ameliorates depleted and acidic soils, and may improve others to a worthwhile degree depending on environmental factors;

- it (greatly?) increases a soil's water and nutrient holding capacity;

- it creates a welcoming environment for soil microfauna, including artificially introduced "good guys" such as microrrhizal fungi, adding further to the soil's carbon content.

The list may be longer, but a triple whammy of carbon removal, soil recovery and desert reversal seems enough to be targeting calculations at. There's an interesting comparison between the use of biomass for energy and its use for black carbon sequestration.

One proviso: subsequent soil management must be of the mulch-and-no-till school, if you want soil carbon to stay there. Present technology for this is not new, but perhaps could be bettered.

I believe that Edinburgh and East Anglia unis take a serious interest.

Robin Smith said...

What if the idea of climate change, is caused by wrong thinking. That is, despite the science being correct (a big IF still), the cause of a high carbon energy system has been overlooked by thought. Deeper still, that wrong thinking is now obscured by the political agenda which now forbids further scrutiny of climate change given its systemic adoption by the collective world view? And then further still, the wrong thought will result in even worse consequences even if the world was made carbon free. Is anyone in the world, especial the academies of science and learning, considering this. I hear you. the first authentic scientist to stand up and attempt it will have a very short career.