Monday, April 13, 2009

Ways to Save the Planet 3

We've reached the end of the series with two more ideas for fighting global warming.

First up, solar panels on satellites. The idea being that there is twice as much light up there as down here, and, if concentrated by fresnel lenses, many times more. Then the energy would be transmitted back to earth by microwaves, SimCity style.

This episode left many obvious questions unasked and unanswered:

1. How effficient is this microwave transmission of energy? Get twice as much energy from your solar panel, and lose half on the way down, and you haven't gained anything by being in space.

2. How many solar panels could we have on land for the price of putting one in orbit? My guess would be a number in three figures. Maybe less if you have to buy the land, but this world - even much of the West - is hardly short of deserts.

3. Launching objects into space is highly energy intensive, as is any subsequent maintenance. So what is the embedded carbon cost of one of these satellites? Compare this to a similar device, on land, albeit generating half the power, during daylight. It can't be good. How long will it have to last?

Of course you don't get answers to "how long will it last" while the whole project is at a mere proof of concept stage. But it seems to me very hard for these numbers to add up.

Finally we had a design for an active scrubber of CO2 from the atmosphere. Air blown into the device at one end would flow against a caustic soda solution, reacting with it, removing the carbon dioxide.

True to form, much was made of building the prototype - welding the blades on to the fan, setting the thing up in a football stadium, and staying up all night to see if the numbers - on the stadium scoreboard - indicated success, which they did. Success was defined as extracting more CO2 than is generated by the energy required to run the thing. A modest goal and clearly not the limit of ambition for efficiency.

What was less clear was how it could possibly be more efficient to extract CO2 from the air, where it is sparse, rather than from the exhausts of power stations where it is abundant. My understanding of CCS technology, such as it exists, is that it substantially diminishes the fuel efficiency of the plant, because the CCS process takes a lot of energy to run. Extracting CO2 from a lower concentration in air, the energy cost is surely even bigger.

The other issue ignored by the programme was where we get all this caustic soda (NaOH) from. All the machine is doing is stirring the reagents in this reaction:

CO2 + NaOH -> NaHCO3

Now - we are not told - the caustic soda can be reclaimed from the sodium bicarbonate by reacting with lime (CaO).

NaHCO3 + CaO -> NaOH + CaCO3 

What's that we've got now - CaCO3 - calcium carbonate (calcite/chalk). OK , the lime can be retrieved from this for reuse and the CO2 extracted for storage, by heating to 825 degrees C. 

 CaCO3 + heat -> CaO +  CO2 

Whew. But hang on there's a lot of processing here. A lot of energy, surely, required to heat the calcium carbonate, to compete the cycle. This sort of chemical cycle necessarily requires an energy input, even though the outputs are identical to the inputs. We are just using chemistry to move CO2 from A to B. And this, presumably, is why CCS has a high energy cost.

Finally, there is the issue of whether and how CO2  can be safely stored. This being the Discovery channel, the idea they tested was fabricating torpedos out of raw solid CO2 - dry ice - and dropping them into the ocean where they would embed themselves in the sediment of the ocean floor and stay there for a good long time. In theory. So we have a delivery mechanism that is cool and probably works, but nobody checked whether the CO2 actually stays put. There wasn't even a picture of the torpedo hitting the ocean floor. It wouldn't be difficult to drop a number of these torpedos, and then to dive down and dig one up every month and see how much it has shrunk. Actually it would be difficult and expensive, but it is the critical question. 

And even if it works, this is another energy intensive step in the process. It is the whole process that has to cover its whole footprint, preferably many times over, not just one step or another.


So where does this leave us? From the whole series I quite liked the wave powered pumps - although they may end up doing more for fish stocks than for global warming. But eating more fish may free up land for biofuels or renewables, so that could work. And I liked the cloud albedo management idea. The rest seemed just too weak. They stack up very poorly against existing renewable technologies, transport options, nuclear power, best energy efficiency practise and so on.

Maybe some of these ideas will come good. Further research should be encouraged, but banking on any would be madness. Our policies today should be based on technology that already exists.

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