April 24, 2007
Solar Energy Creates Chemical Energy
The ability to use solar power to generate liquid carbon-based fuels has the potential to generate energy for transportation much more efficiently than biomass, without biomass's water limitation, and with a much smaller land footprint. With all that in mind, some UCSD researchers have used solar power to partially reduce carbon dioxide.
Chemists have shown that it is possible to use solar energy, paired with the right catalyst, to convert carbon dioxide into a raw material for making a wide range of products, including plastics and gasoline.
Researchers at the University of California, San Diego (UCSD), recently demonstrated that light absorbed and converted into electricity by a silicon electrode can help drive a reaction that converts carbon dioxide into carbon monoxide and oxygen. Carbon monoxide is a valuable commodity chemical that is widely used to make plastics and other products, says Clifford Kubiak, professor of chemistry at UCSD. It is also a key ingredient in a process for making synthetic fuels, including syngas (a mixture largely of carbon monoxide and hydrogen), methanol, and gasoline.
A method to attach hydrogens to the carbon, totally displacing the oxygen, would produce hydrocarbons. Hook them up into long enough chains and the hydrocarbons become liquid at room temperature. Then you can put it in a gas tank and cruise.
A cheap method to use of solar power to create chemical feedstocks and liquid fuels would solve one of solar's biggest problems: the sun does not always shine. We could use nuclear power as baseload electric power. Then use solar power to create liquid fuels. That could entirely break our dependence on fossil fuels.
Given a sufficiently advanced and cheap enough battery technology we could use solar or nuclear power to charge batteries for transportation. But batteries are not the only potential way to use solar power for transportation. Solar power could run chemical processes to produce liquid fuels too.
If we could produce all our liquid hydrocarbon fuels from carbon dioxide extracted from the atmosphere then burning of liquid hydrocarbons would no longer increase atmospheric carbon dioxide. Instead we'd have an artificial carbon cycle in parallel with the natural carbon cycle.
This is not so impressive because they are still using a photovoltaic process to make the electricity that drives the catalytic reaction. Photovoltaic processes are inherently ineficient (10-15% at best). REAL work (understanding of chemistry) needs to be done in order to develop the photochemistry (which is FAR more efficient than photovoltaics) in order to eliminate the need for photovoltaics.
But generation of electricity brings one big advantage: The electrons can be dynamically allocated between driving reduction of carbon versus versus local uses of electricity versus distribution into electric networks.
In other words, excess electricity could be allocated to creation of liquid hydrocarbons. The creation of liquid hydrocarbons allows easy storage of energy. See this slide show by Nathan Lewis at CalTech: The most reasonable use of hydrogen is to reduce carbon to make hydrocarbons.
If generated electricity works for reducing carbon, why not build nukes to serve peak demand and use the excess electricity during lighter periods of use for hydrocarbon production?
Yes, but if you need electricity, is solar photovotaics the most efficient source of this electricity? Is the solar voltaics an integral part of the process? Or can you use electricity from any source (nuclear, etc.)?
Sure, nuclear power would work as well. Right now nuclear power is much cheaper than photovoltaics. So if some scientists and engineers developed a really cheap way to use electricity to reduce carbon from the atmosphere then nuclear power would be a more sensible source of electricity to use to reduce carbon.
But photovoltaics will drop in price. Nathan Lewis at Cal Tech thinks he can find a way to make nanotubes in cheap Titanium Oxide paint line up in a way that generates lots of photovoltaic power. Saw him interviewed about this. Then went and found Nate Lewis' interesting energy slide show which I commend to your attention. Lots of interesting facts. He looks at things from a terawatt perspective.
"the potential to generate energy...without biomass's water limitation"
I imagine that biomass uses water very inefficiently relative to this process, but wouldn't you still need some to provide the hydrogen?
"photovoltaics will drop in price."
Nanosolar seems to promise this in 2007 - I haven't heard anything since they started building their plant in California, but it seems to be real.
I've seen reports that photosynthesis is 99% efficient in theory, and that it's 3% efficient in plants. What's the source of the discrepancy?
The hydrogen needed to bind to carbon to make hydrocarbons chemically would be orders of magnitude smaller in quantity than the water that evaporates from plants. Very little of plant water hydrogen goes to fixation with carbon in photosynthesis.
I'd love to see an analysis of the amount of electricity, and capital expenditure needed for the synthesis of liquid hydrocarbons. CO2 is free (or better, they'll pay you to take it), water (in these quantities) is very cheap, so the cost is in the electricity and the capital expenditure for plant & equipment.
It appears to be feasible, from what I've seen elsewhere, so it appears to be just a question of conversion efficiencies and capex. How cheap does electricity need to be to compete with $70 oil? Or, conversely, how expensive does oil need to be to make $.10/kwh electricity a cheaper substitute?
Kurt, I'd have to say I disagree that photovoltaics are inherently inefficient; 10 - 15% of something as abundant as sunlight is still an abundance. Whilst increasing the efficiency of conversion of sunlight to electricity or chemical fuel like hydrogen or hydrocarbons is worthwhile, increasing the efficiency in mass producing solar technology is more worthwhile.
Cost is the biggest impediment to mass use of Solar, not it's conversion efficiency - we aren't short of sun exposed area to use. There are innovations that can and will improve that efficiency but there are other clever innovations in the process of being commercialised that will make solar cheaper - such as Nanosolar's continuous roll printing of CIGS PV using nanoparticle inks, or Origin Energy's Sliver cells that etch-cut silicon into hair thin slivers making the expensive silicon go much further, or Solar Heat and Power's low cost variant on the thermal trough collector using troughs shallow enough to use flat glass mirrors bent into shape (much cheaper) combined with Fresnel covers under the collector pipes to maximise the heat - SH&P also plan to have thermal storage to make it's solar farms put out power 24 hrs a day. There are other innovations in the pipeline, so there is no one clear winner - a mixed blessing since there's a lot of room still for better and cheaper, yet having one clear and obviously superior technology would attract the massive investment needed to win in the energy market place.
"Cost is the biggest impediment to mass use of Solar, not it's conversion efficiency"
Reasonable assertion up to a point, but the conversion efficiency can't be TOO low or it does become the show stopper.
Low conversion efficiency is the main reason why biofuels are not considered a viable replacement for fossil fuels. (takes too much land to produce too little biofuel)