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By Robert Rapier on Jul 9, 2009 with no responses

Technical Feasibility is the Easy Part

A couple of people have now written to ask for comments on the story from Green Car Congress about the Polish CO2 to methanol scheme. Here is the story:

Report: Polish Power Plant and University to Cooperate on CO2 to Methanol Trial

Here is the bit I immediately focused on:

Nazimek says his “artificial photosynthesis” process is based on the photocatalytic conversion of water and carbon dioxide under deep ultraviolet light. Synthesis of 1 kmole (32 kg) of CH3OH from CO2 and H2O requires 586MJ of energy, according to Nazimek’s calculations. (Methanol has a HHV of 22.7 MJ/kg, or 726 MJ/kmole).

So the implication there is that you are getting more energy in the form of methanol than you put into the system (input of 586 MJ for an output of 726 MJ), for a positive net energy. However, like the Steorn system, this interpretation would unfortunately violate the laws of thermodynamics. Perhaps something has been lost in the translation. Otherwise, either all of the energy into the system is not being measured, measurements are being done inconsistently, or there is some other error.

Here is one problem. Methanol’s high heating value (HHV) is quoted above. However, when considering energy that you can practically get out of a system one should not use HHV. Why? Because that presumes that you have condensed the water from the combustion products and taken everything back down to room temperature (25 C). That doesn’t happen in practice. Just feel the exhaust coming out of your auto.

So the comparison of energy input into the system to HHV for the output can be misleading. If you consistently use HHV for input and outputs, then you should get a consistent answer for the net energy, but if you mix lower and higher heating values you could easily conclude that you are creating energy when in fact you are simply subtracting apples from oranges.

Having said that, I think artificial photosynthesis has great potential for energy production. I have often speculated on this. Natural photosynthetic efficiency is very low, but it does result in captured solar energy in plants all over the world. Plants do take CO2 in and convert to biomass. The trick is that they do take in more BTUs in the form of solar energy (and maybe also energy in the form of fertilizer) than are found in the the biomass they produce.

So I am in no way trying to diminish the work. This sort of work needs to be done. I just want to inject a dash of reality into the energy balances. It’s like I tell people all the time – you can in fact run a car off of water. You can turn combustion products like CO2 and water back into fuels of all sorts. The catch in both of these cases is that you must always input more energy into the system than you can get back. That’s how the laws of nature unfortunately work.

So while technical feasibility can often be easily demonstrated, there are many more hurdles that must be jumped before you would operate a scheme like this in practice. For instance, what is the source of energy? If you are using sunlight, then it may be perfectly acceptable to input 100 BTUs of sunlight and get back 10 BTUs of liquid fuel. But it wouldn’t be a good idea to input similar quality fuels and get back fewer BTUs.

A second consideration is energy required to purify the final product. The story above indicates that the product is in water at a 15% concentration. This is quite similar to the concentrations of ethanol that corn ethanol producers make and then have to purify. The water has to be removed, and it takes energy to do that. So even if I had a perfect conversion of 1 BTU of energy input to 1 BTU of energy out, the net energy will fall as I input energy to purify the final product. (A 3rd major consideration is the capital costs, which keeps many fine ideas in the lab).

So in conclusion, technical feasibility of so many of these schemes is not in question. (Of course as was the case with Steorn or (possibly) with Cello, sometimes technical feasibility itself is the problem). But beyond technical feasibility are all sorts of considerations that can render a seemingly wondrous invention into something that never escapes the lab. If you hone in on the mass and energy balances of the system (a chemical engineer’s bread and butter), you can often see why a promising experiment in the lab won’t work in practice.