This post was inspired by this video about the “CO2 train”. How much energy, money and space does it take to offset a household’s CO2 footprint?

First, let’s start with some assumptions about chemistry and physics. I’m going to assume that to fully “offset” CO2, the molecule needs to be split up into free carbon and oxygen. Not turned into dry ice that must then be kept cold, or under pressure. Not partially transformed into something barely useful, like gaseous methane. But the molecule actually removed from existence, ready to be re-created.

These are back-of-the-envelope calculations. Don’t base your PhD thesis on them. It is possible that creating a liquid like ethanol requires less energy, so consider this discussion an upper bound. Furthermore, I will assume that all CO2-equivalents (CO2e) are actual CO2 gas molecules.

From Carbon Dioxide, the molar mass is 44 g/mol, which translates to 22.7 mol/kg. From Carbon Dioxide (data page), the bond energy of CO2 is 804 kJ/mol. That is an average per C-O bond, and there are two bonds, so 1.61 MJ/mol. With 1.61 MJ/mol / 44 g/mol = 36.5 MJ/kg.

For comparison, burning 1 kg of ethanol releases 29 MJ, or 20% less. This is why I said above that converting the CO2 to ethanol instead of atoms may be cheaper. You need to get some water in there too, though.

A US household causes around 50 T/year of CO2e. Thus requiring 36.5 MJ/kg * 50,000 kg/year = 1.83 TJ/year to break it down. Normalizing that power value to base units, that is 1.83 TJ/year / 365.25 / 24 / 3600 = 58 kW. Quite a lot!

An average US household uses around 10,600 kWh/year of electricity, or 1.21 kW. So the CO2 bond energy footprint is 48 times as large as the electricity used.

Another way of looking at it is the energy released from burning 1 kg of diesel, 44 MJ, suggesting a US household is causing CO2e equivalent to burning 42 tonnes of diesel per year. That’s 115 kg per day.

At 0.14 USD/kWh, or 0.039 µUSD/J, this would cost $71,400 per year. With a median US household income of $71k per year, this does not seem sustainable!

Solar Panels

What if we installed solar panels? After all, the sun is the only truly sustainable energy source (as in: not using the energy will not change the fact that it is burning fuel.)

From the video, a 1.5x1m solar panel produces 1.44 kWh/24h = 60 W on average, or 40 W/m². Thus, we are looking at 1 450 m² of solar panels, or 38x38 m². That’s for one household; the size of a large garden in a Swedish town. Of just solar panels.

A solar panel costs around 250 USD/m², suggesting this would cost 363,000 USD to buy, excluding cables, etc. With an expected life time of 25 years, that becomes 14,500 USD/year, or 20% of the median US household income.

Pandora’s Box

There are many stories that can be summed up as a Pandora’s Box. Using oil to add leverage to our output is a new one we can add.

It’s easy to get hooked on something that suddenly makes you many, many, times stronger than you were before. Not to mention that no politician wants to kill their nation’s GDP unless forced to. Perhaps it would have been better not to open this box.

Conclusion

Any talk about carbon offsetting, carbon capture or other shennaningans are distractions. The only way we’re ever going to get our CO2e under control is by using less energy. No economy will crash itself for the greater good of the planet, so the only thing we should focus on is being able to do the same we do now with less energy. That means those in power should (continue to) set limits on energy usage, and keep lowering those quotas. Force the innovation we need.

Perhaps you don’t need to fly to the other side of the world to snap a picture of yourself in front of a monument millions of others have already taken a photo of. Photos that are available a mouse-click away, for a fraction of the energy.