Friday, February 3, 2012
Reality Check on Joule
Companies entering commercialization tend to make some pretty bombastic claims. Among them is a company called Joule Unlimited, which was recently reported by Biofuels Digest to claim a yield of up to 25,000 gallons of ethanol per acre from the sugar produced by their product, comparing it to a yield of roughly 800 gallons per acre planted in sugarcane.
Numbers like that don't tend to inspire confidence in me. Over thirty times the yield? Time for a reality check.
There are really two ways we could go about this. The simplest one is to simply compare the amount of energy in the ethanol Joule claims to yield and compare it to the amount of insolation, or solar input. That's right, it's the chemical engineer's old friend, the energy balance.
For context, there are a couple of things that we should know. Sugarcane gathers energy Since Joule's system uses cyanobacteria, we can look at typical photosynthetic efficiencies among plants and microorganisms - for high efficiency plants like sugarcane, 1%; for high efficiency blue-green algae, 2-3%. Finally, we need to take a look at what solar resources are available. This map comes originally from NREL and describes the input to a tracking solar panel (it follows the sun), so it should serve as a good proxy. Let's take a good insolation value in the US to be 6 kWh/m^2/day.
For a 25,000 gallons of ethanol per acre per year yield, the energy content is 2.22e6 MJ. On the other side of the equation, total solar input is 3.2e7 MJ/acre over the course of a year. If we divide the two to get a simple efficiency, we get about 7%.
In reality, Joule's system is not designed to produce ethanol but sugar, which involves even greater energy input. However, we can already see that the claim that they have is more than twice what can be considered the best of what nature has produced, unless they really have discovered something that radically defies all previous biotechnology. If we assess this claim on the basis of sugar, then we're likely to discover even more ridiculous stuff, since the native sugar contains more energy than ethanol.
In an anaerobic fermentation, two ethanol molecules are produced from each sugar molecule. Returning again to what 25,000 gallons of ethanol implies, it's 1.62e6 moles, which implies an origin in about 8.1e5 moles of sugar. Wikipedia tells me the combustion enthalpy is -2805 kJ/mol, so the total production of Joule's claimed system is 2.27e6 MJ. This boosts the efficiency up to about 7.1%, not much more, but still twice as good as the best nature produces.
Joule has failed the reality check. That was an ideal case, where we assumed no photosynthetic metabolism to help the bacteria in cellular processes or reproduce; the likely metabolic requirements to produce the sugar means the claimed efficiency of the cyanobacteria Joule uses must be even higher. Their photosynthesis levels are, roughly speaking, over two times higher than I'd even believe advanced biotech is able to produce on the basis of sugar yields alone; if you include cell growth and other uses of the sugar, I'd estimate four or five.
This analysis does not say that Joule will not have a great product. This is also not to say that Joule isn't run by smart people. Note that even reducing their reported yield by a full order of magnitude is still about three times better than sugarcane, the one first generation biofuel that is truly, in my opinion, going to be with us to the end. But these incredibly bombastic claims are emblematic of a lot of biofuels companies pretty much playing very fast and loose with science, and hoping nobody will notice.