Photosynthesis vs. Solar Cells?

[Dr. Anomaly]

This thought was sparked by mention in another thread of some progress towards "artificial photosynthesis", and I didn't want to derail that thread, so I thought I'd start a new one instead.

 

One of the ongoing problems with solar cells is that they just don't put out much power. And that started me thinking about photosynthesis, which also gathers solar power, albeit into a chemical energy source (sugars).

 

Now, leaving aside for the moment the difficulties with getting at, in a usable (probably electrical) form, the energies stored in the sugars by photosynthesis... how does photosynthesis stack up vs. current solar cell technology in efficiency of energy capture?

 

That is, for a given surface area that is solar cells, vs. an equal surface area that is photosynthesizing, which captures more of the incident energy?

 

(I realize there are all kinds of variables in this, not the least of which is the exact make-up of the solar cells in question, and the particular species of photosynthesizing plant. I'm hoping for some kind of general comparison.)

 

If this kind of information exists, I have no clue where to go looking for it. Does anyone else have any ideas?

[Cancer]

Re: Photosynthesis vs. Solar Cells?

 

I know that when used in detector mode, silicon has a remarkably high quantum efficiency. Astronomical CCD detectors used at their peak sensitivity wavelength, about 600 nm, have QEs up around 60%. But that's a mode where energy is expended to make sure as many photons as possible are captured, with the chip cooled to liquid nitrogen temperatures, etc. My memory is that when used to extract power, the efficiency is much lower, but I couldn't tell you more than that off the top of my head.

[McCoy]

Re: Photosynthesis vs. Solar Cells?

 

Photosynthesis is not an efficent process. There is a very slow enzyme involved.

[Wyrm Ouroboros]

Re: Photosynthesis vs. Solar Cells?

 

Nothing that directly converts solar energy to stored energy of any type is 'efficient'. The reason behind this isn't because of the conversion efficiency, it's because direct solar power isn't all 'that much' power in the first place. Want more power? Get your solar panel closer in to the sun, where the area covered gets hit by a greater quantity of that wonderful solar energy.

 

The reason why oil products are so 'efficient' is because they have considerably more time worth of 'stored' solar energy per whatever your measuring unit happens to be. Millions of plants absorbing and storing millions of days worth of sunlight, compressed, 'purified', stored ... refined by man. Even biodiesels take hundreds of plants and a summer's worth of solar energy, refine it, and finally pump it through your engine.

 

The trick isn't to increase efficiency, it's to increase the quantity of solar days per unit of measurement. Imagine a solar cell that managed, in one day's worth of exposure, to absorb two days, five days, fifty days' worth of energy ...

[Cancer]

Re: Photosynthesis vs. Solar Cells?

 

Well, you'll never do better than about 1360 Watts per square meter here at Earth from direct solar. But capturing it at a closer distance gives you the problem of transmission or storage. I am very, very wary of any sort of space-to-Earth-surface energy transmission scheme ... the energy densities needed to make it worth it are stupendous, and will quite literally cook anything in the beam. If the downlink beam should lose lock on target ...

 

The energy density in fossil fuel is high because of that time concentration you mention ... but if you had to pay for that concentration instead of consuming it opportunistically, that would be a grossly inefficient process.

[Cancer]

Re: Photosynthesis vs. Solar Cells?

 

Here's a phenomenological way to attack that OP question.

 

Suppose that agriculture is about as efficient as we are likely to get in conversion of solar energy to usable form. This is, of course, assuming the answer.

 

Picking sugar cane as relevant (because sugar has very little nutritive value besides energy), I take (from here) a rough guess of agricultural sugar cane yield of 3 tons per acre per year. (Hawaii looks to be higher than that in the table, but it notes above that in the text that mainland sugar uses an annual harvest while Hawaii is usually alternate years ... that pulls Hawaii down to within a factor of 2 or so of that 3 tons number.)

 

Wikipedia lists the food energy of sugar at about 1600 kJ per 100g. So that 3 tons per acre per year turns into about to be about 4.8e+7 kJ per acre per year; divide by 4050 (roughly, m^2 per acre) to get roughly 12000 kJ per year per square meter.

 

The solar constant is 1360 W/m^2, 1 year is pi times 10^7 seconds, but the Sun is available only half that time, and those numbers are for US fields which are at 30 degrees latitude so factor in cosine 30 degrees = 0.866, get 1.85 * 10^10 J/year or 1.85 * 10^7 kJ/year from an acre.

 

Divide the 12000 kJ/yr by 1.85e+7 kJ/yr and get 0.00065, or 0.065 per cent efficiency.

 

The numbers I've seen for photovoltaics are above 1%. The highest I've seen is about 35%, but I don't know if those are just lab figures or the real efficiencies of production-grade cells.

 

Now, this is admittedly a rigged estimate, because it omits the biomass of the non-sugar part of the cane (sugar makes about 10% of the mass of the cane plant as harvested), that you might recapture in some way. If you could do that well, then that could give you back something like a factor of 10, maybe. Now you're within howling distance of a percent, and you can now scratch your head about whether you want to factor in somehow the benefit of the CO2 --> O2 conversion that is a by-product of photosynthesis. But, there's also the assumption that you can use the full energy content in the sugar at 100% efficiency, which is probably bogus.

 

Does that help?

[McCoy]

Re: Photosynthesis vs. Solar Cells?

 

One of the reasons photosynthesis is inefficent is that it uses just a fraction of the available energy. Get a prism and pass sunlight through it, or look at a rainbow. Remember there are frequencies above and below what you see. Now of all those photons falling like a hard rain, photosynthesis uses the red, and a bit of the blue.

 

I believe, in theory at least, solar cells can be made that can capture most (more than half anyway) of the sunlight reaching the surface.

[Dr. Anomaly]

Re: Photosynthesis vs. Solar Cells?

 

Yes, Cancer, that does help; thanks.

 

Crud. I was hoping this might be one of the cases where Nature had an answer that we could imitate / fine-tune / whatever to do a better job than stuff we created from scratch ourselves; looks that that isn't the case.

 

Ah, well... back to the ol' electronic brain...

[Dr. Anomaly]

Re: Photosynthesis vs. Solar Cells?

 

Oh, and McCoy? I know that photosynthesis uses only part of the available spectrum; my thought was that if the process were efficient enough to be worth looking at, we might be able to do some tinkering (genetic engineering) to produce a plant that used a wider slice of the spectrum, grabbing more energy.

[McCoy]

Re: Photosynthesis vs. Solar Cells?

 

Oh' date=' and McCoy? I know that photosynthesis uses only part of the available spectrum; my thought was that if the process were efficient enough to be worth looking at, we might be able to do some tinkering (genetic engineering) to produce a plant that used a wider slice of the spectrum, grabbing more energy. [/quote']

Maybe. Might be worthwile looking into as a side project, but I wouldn't spend a lot of time or funding on it.

 

Kinda think if it could be done, it would have evolved by now. I could be wrong.

[Zeropoint]

Re: Photosynthesis vs. Solar Cells?

 

There's one big, BIG advantage to photosynthesis: plants = solar-powered, self-repairing, self-replicating molecular assemblers.

 

A plant is composed almost entirely of air and water. To be sure, a plant draws nutrients form the soil, and those nutrients are vital to the plant's growth, but the vast majority of the mass of a plant is carbon, hydrogen, and oxygen atoms taken from the air and water around it. Obviously, carbohydrates and hydrocarbons are going to be the easiest things for a plant to make, but that covers a lot of ground. Genetically engineered crops could, in theory, provide a source of various useful substances that could be harvested with relatively low labor, and more importantly, a low technology infrastructure.

 

If a crop of plants could provide a useful fuel with little manpower or sophisticated technology, that might prove to be more important in some situations--such as a newly colonized planet--than pure efficiency. Liquid fuel can be distributed and used with lower technology than electricity, too.

 

Also consider that plants could make things other than fuels, such as plastics. Or drugs. They could also collect minerals from the soil and concentrate them for easy harvesting, although I suspect that would only be useful for elements and compounds that are very valuable and used in small amounts.

 

Anyway, you get the idea. A solar cell is much more efficient at turning sunlight into usable energy, but plants can do a lot of other things with the energy.

[Cancer]

Re: Photosynthesis vs. Solar Cells?

 

There are multiple substances which perform photosynthesis, capturing different portions of the spectrum. Compare the absorbance curves for chlorophyll a & b as seen in the "Chlorophyll" entry of Wikipedia. I have the impression that there are other photosynthetic pigments also, but I can't find hard examples in my quick search now. Anyway, a suite of different pigments each with different absorption peaks is functionally equivalent to a chlorophyll with a broader wavelength range of absorption.

 

EDIT: Some carotenes are photosynthetic too, but I haven't found absorbance curves for them yet. And I'm pretty sure that there are some alternate metabolic pathways that harness light ofr energy without participating in the "regular" oxygen-rich chemistry, too.

[Markdoc]

Re: Photosynthesis vs. Solar Cells?

 

There are multiple substances which perform photosynthesis' date=' capturing different portions of the spectrum. [/quote']

 

I think Rhodopsins are what you want But in truth, that doesn't help much. The problem is that biological energy conversion isn't very efficient - and because as already noted, the energy density at Earth's surface is low.

 

One potential way around this problem is to concentrate the energy available via solar. And that's already being done.

http://news.bbc.co.uk/2/hi/science/nature/6616651.stm#graphic

 

I actually went to see the power station listed here, the year before it went commercial. Right now, it's only producing 11 Megawatts, but the plan is to up that figure to 300 MW in the next 5 years - enough for a medium-sized city. And these systems are relatively cheap and simple to build and operate, even compared to conventional fossil fuel-fired plants. Much more cost-efficient than individual solar cells

 

cheers, Mark

[McCoy]

Re: Photosynthesis vs. Solar Cells?

 

There are multiple substances which perform photosynthesis, capturing different portions of the spectrum. Compare the absorbance curves for chlorophyll a & b as seen in the "Chlorophyll" entry of Wikipedia. I have the impression that there are other photosynthetic pigments also, but I can't find hard examples in my quick search now. Anyway, a suite of different pigments each with different absorption peaks is functionally equivalent to a chlorophyll with a broader wavelength range of absorption.

 

EDIT: Some carotenes are photosynthetic too, but I haven't found absorbance curves for them yet. And I'm pretty sure that there are some alternate metabolic pathways that harness light ofr energy without participating in the "regular" oxygen-rich chemistry, too.

Yet AFAIK no plant has both red and green chlorophyll. Wonder why?

 

Chloroplasts, like mitochondria, have their own DNA and may have at one time been free living organisms that became symbiotes. Maybe even if red and green chlorophyll cannot coexist in one chloroplast, red chloroplast could be transplanted into a plant to see if they could coexist alongside green ones.

 

I was wrong, I would give funding for this if I could.

[Cancer]

Re: Photosynthesis vs. Solar Cells?

 

There are plants with both chlorophylls and carotenes.... trees/shrubs which have the dark red leaves in summer are one case, and the multicolored leaves of the coleus plant are another example.