

Green leaves basking in sunlight, doing lots of juicy photosynthesis

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In photovoltaics, efficiency has a clear definition we can all agree on. Namely, it is the amount of electrical power one can get from a solar cell divided by the power in the solar radiation that shines on it. Sunlight liberates electrons which in turn can provide power proportional to their total number times the “pressure” which backs them. If you are getting a 20% efficiency from your solar panel, you are doing pretty good. The problem with comparing photovoltaics to photosynthesis though, is that while the former generates electrical power, the later generates chemical products. To convert photovoltaic power into something comparable to what photosythesis does, analysts typically take an additional step and require that the electrical power in turn be used to split water and generate hydrogen, which reduces the efficiency by at least half.

Photosystem II, diagram

A “photocurrent conversion efficiency” of 95% is often quoted for photosystem II, but this actually has little to do with overall photosynthetic efficiency. For most plants and bacteria, less than half of the incoming photons are of the proper wavelength to be be exploited by photosynthetic machinery. After accounting for various other inefficiencies, including maintenance costs and lost photons which never make it to reaction centers, real efficiencies are actually closer to 5% — but 5% of what?

The challenge ahead for Rensselaer researchers is to understand all the intermediate steps that occur when water is split during photosynthesis. To do this the group will use a combination of high speed spectrographic imaging with femtosecond light sources, and petascale computer simulation. They note that it takes four photons to split one molecule of water. The hydrogen that is liberated from water is not stored or combusted in plants, but rather used to power a rotary engine that generates ATP. The ATP is in turn used to synthesize sugars which are the real deliverables of photosynthesis. Different sets of enzymes cooperate at all levels in these conversions which makes it difficult to arrive at the 5% efficiency spec solely by counting hydrogens and sugars.

Photovoltaic solar cellsThe problem with plants is that, while they work pretty well at medium levels of solar radiation, their output quickly saturates with increasing levels of light. Photovoltaics, on the other hand, continue to scale their output more or less linearly as things heat up. If plants tried to operate across the full dynamic range of solar input at peak capacity, they would probably burn themselves out rather quickly. Plants are clearly not optimized for the generation of electrical power, or for hydrogen, but rather for a complex combination variables that might loosely be interpreted (at least for those farmed) as biomass.

Recreating photosynthesis in some kind of a machine may soon be possible, but how useful such a machine would be remains to be seen. The production of hydrogen would be one useful application, but methods of harvesting it efficiently have yet to be developed. To compete with photovoltaics, research on many fronts will be needed, but a better understanding of the basic operation of photosystem II is perhaps the most fundamental need.


Research paper: DOI: 10.1039/C2EE21210B – “The structure and activation of substrate water molecules in the S2 state of photosystem II studied by hyperfine sublevel correlation spectroscopy”

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  • Protagonist66

    Partially offtipic: They are all jabbing about global warming and they haven’t even tried to make artificial photosynthesis… All they’re doing is making more taxes to compensate CO2 emissions… What if they would have used all that effort and time to create working photosynthesis not based on energy but cleaning the atmosphere. We have fission power and maybe soon even fusion power to make all the power we need.

    • Bob Dobbs

      “They are all jabbing about global warming and they haven’t even tried to make artificial photosynthesis”

      Sure they have. There are carbon scrubbing “fake trees” that take up CO2. They’re expensive and not really used much (for that reason), but they do exist.

    • jhewitt123

      Saving the planet is huge business, not to mention a free pass toward many unspeakable horrors.

    • Eldi Kaiser

      Yeah, it’s not like natural photosynthesis grows on trees. :)

    • Yoduh99

      How does it make any sense in your head that it would be smarter or easier to try and invent brand new artificial photosynthesis technology to counteract the unnecessary levels of human CO2 production, when we can simply cut down on CO2 production right now with technology that already exists? and thinking that nuclear power, fission or fusion, is going to all of a sudden replace fossil fuels anytime within the next few decades (if that) is a pipe dream my friend.

      • Protagonist66

        Nuclear power would easily overcome the need for power and cutting down CO2 production doesn’t solve anything, just giving few years more time. As I see it, Developing something similar to a fully working artificial photosynthesis to reduce CO2 levels would be best option for the long run. And as someone stated there is some sort of CO2 “removers”. Only I was searching few months ago about this and only problem I found about this is that it wouldn’t produce power as it removes CO2 (so such station would cost monthly) so it won’t interest big corporations or even states it’s easier to tax people cause it makes money. But we all know that paying for sins to wash away doesn’t actually do anything else than rise our status. That goes for both the global well being and environmental purity

  • jack

    Like water, we have plenty of energy. Its an issue of quality and pollution.

  • Angel Ham

    The day someone succeeds at replicating photosynthesis will also be the day everyone will be forced to pay a licensing fee for having a garden..