Solar Hydrogen at Night
Ever since I can remember, scientists have been trying to figure out photosynthesis–and do it ourselves. In elementary school, back mid-twentieth century, we were shown a film (the kind you feed through a projector, and hope it doesn’t break) where a little cartoon creature labeled “photosynthesis” says, “And I’m NOT going to tell you!” Outside immunology, bacterial and plant photosynthesis is the most complex topic in my textbook–the one that took three editions to get straight (almost).
Yet the fundamental aim of photosynthesis is surprisingly simple. You absorb light, split water, and store the energy in chemical bonds. Plants mainly store it in sugar, amino acids, vitamins–messy molecules useful to them. But some bacteria, for unaccountable reasons, store much of their energy in hydrogen–and get rid of it.
Why do the bacteria give up hydrogen, losing some of their energy for sugar, amino acids, etc.? New-age evolutionists would suggest that the hydrogen help out their fellow microbes somehow, down in the muck of the bogs where these purple bacteria live, and that overall their fitness is enhanced–well, read the book for details. Or check out Carrie Harwood’s research.
Tom Meyer and colleagues try to imitate plants and bacteria. They’ve made a chromophore (did you find the structure on the internet? I don’t see it, surprise.) that specifically splits water and stores almost all the energy as H2 (supposedly; I don’t see the details published). The idea of such a chromophore is to imitate what chlorophylls have done for at least three billion years. Chlorophylls are amazingly beautiful and intricate molecules that absorb a photon, store it in a chain of conjugated double bonds, and apply it to a precise reaction such as splitting water. And releasing hydrogen fuel–the kind we’ve been saying could drive your next car, and maybe store at night as formate. In fact the next target of Meyer’s group is “to reduce carbon dioxide, a greenhouse gas, to a carbon-based fuel such as formate.”
Could we actually use solar to get rid of some of our excess CO2?
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Ultraphyte 
A key question in my mind is the efficiency of this process. Solar cells can generate electrical energy directly at ~ 25% efficiency, and new techniques could extend that to 50-70% of all light energy impinging on the cell. Electrolysis of water is about 50% efficient. So overall, we can get 6-17% efficiency for hydrogen production. Overall plants are around 2% efficient at best, although I am not sure about algal conversion efficiencies, but green plants don’t make hydrogen directly.
So if we use bacteria, they need to be put in containers, fed some macro and micro nutrients, and have sunlight at some level piped in for the system to produce hydrogen. Conversely, solar energy can be cheaply concentrated and directed at systems to generate power and hydrogen, whose production can be varied between the two forms under complete control. If we eventually manage to build solar power satellites for near 24 hour power generation, they are going to be more robust and convenient than bottled bugs.
The obvious value of bugs is that they are self replicating and the equipment for hydrogen production simple, somewhat like biogas generators. The hydrogen generated could be most cheaply made into useful energy by burning. This suggests to me that tropical communities using the gas for cooking and transport might be a good market for bacterial H2 production. In the industrialized world, especially in cities where land area per person is limited, I’m not seeing an obvious case where H2 produced by bacteria makes more sense than using other processes. Even in the home, electrolysis makes more sense than bacterial production.
We’ve seen that algal biofuel production, while better and more efficient than corn ethanol, is not making much headway despite the early enthusiasm (hype?). I think the reasons are fairly clear and they would apply to bacterial H2 production as well. So I see this as an interesting, but niche, technology.
Alex, not sure where your numbers come from, but depending on how you calculate, plant photosynthesis is 90% efficient. There’s no reason why solar electrolysis can’t be 90% efficient if you have an optimized catalyst. The researchers don’t say what they’ve got, of course. Meyer is not using bacteria directly; he develops molecules using principles based on study of biological chlorophylls.
I agree that space solar is the ultimate way to go. But we will always need various forms in which to collect, store, and transport energy.
Plant photosynthesis only uses a small fraction of the available spectrum and hence energy in sunlight. Solar cells absorb a wider spectrum, but still not complete coverage. Your 90% is probably based on the spectrum that could be absorbed by chlorophyll and accessory pigments.. The 2% is low because it isn’t just conversion efficiency, but efficiency of fixation. Much of the fixed carbon is used for maintenance. Finally, seasonality impacts when plants can harvest sunlight. My data on electrolysis was based on some references I found a few years back. I think the exact values were around 46% efficient. I know that the Univ of Hawaii was working on some techniques to improve that, I think using ultrasonics (some variant of sonoluminescence?)
When thinking about technologies, it is important to compare them to existing ones and for economic viability. A rule of thumb is that a technology will replace an earlier one if the price and cost combination is 1/10 existing systems. Niches work because of local conditions, e.g. solar panels in remote areas (and satellites) reducing refueling difficulties. Bio ethanol in the US is only sustainable by fiat. Algal biofuels are not economic, although they may ultimately be if integrated with CO2 emitters and carbon taxes raise fossil fuel costs.
You focus on your area of expertise to highlight interesting possibilities. I tend to put on the green eyeshades and look at such ideas through the lens of economics to see if the idea might pass muster as a working technological solution. Unfortunately the majority of the universe of interesting ideas will fail for economic, and sometimes non-economic, reasons.
That’s interesting about the 1/10 limit for technology replacement. Examples?
Were Ford’s first automobiles 1/10 the cost of horses and trains?
Wikipedia link for photosynthesis. Note peak efficiency for chlorophyll A for the peak wavelength is 25%. That must include intensity, as plants max out on fixation and so cannot use the full intensity of the absorbed spectrum.
The 1/10 rule was something that was a given at B-School in my technology lectures. I see it repeated in various books and essays, so it is a meme that may or may not be correct. It has some real world consequences – c.f. “The Innovator’s Dilemma”.
Cars had a clear performance advantage in power production over horses. Cost of fuel was very low compared to feeding and caring for horses. That is why horses, especially used for pulling delivery carts and coaches rapidly declined. Even early cars went faster than horses, could sustain speeds indefinitely, and needed only a quite fuel refill to continue. No stabling costs, horse changes, feed costs needed. No dead animals in the street. Today internal combustion engines are orders of magnitude more powerful than horses (my Prius is 134 horsepower and can travel at over 100 mph. A horse and buggy – 1 HP and maybe 15 mph). I can leave my car unattended and maintenance free, while a horse must be fed every day, working or not. A horse costs ~$3000 to buy and several $1000’s per annum to maintain. My Prius cost $23000, will last perhaps 10 years, and needs a few hundred dollars maintenance per year. Price and performance.
I think that plant-based energy as described here is more difficult to nail down in terms of ‘efficiency’ because the chain of reactions is so long and tangled. Very unlike machines which are typically designed to use the shortest and, seemingly, the most ‘efficient’ path. If you could add up all of the energy transfers along each path, they’d both have to add up to the same amount … according to the most basic law of physics, i.e., conservation of energy. So what you’re really evaluating is the energy distribution channel effectiveness/length … how fast can I get from here to there.
Normally I would say that plant efficiency is the amount of carbon fixed vs. the impinging sunlight. Photosynthesis maxes out fairly quickly, so as sunlight intensifies, efficiencies decline. Then you have to look at the plant life cycle and other resources needed for growth. I don’t think you can reasonably add in the energy due to respiration as this isn’t useful for exploitation, it is just a cost of production. Bottom line, plants are fairly inefficient converters of solar energy to useful energy. In the case of Joan’s article, we really care about the efficiency of hydrogen production that will be fed into formate synthesis for storage. It will be interesting to see what the efficiencies are in vertical farms, where nutrients can be optimized and the available sunlight distributed to a larger leaf area than you get in flat fields. Multiple cropping should also help. However, having said that, I think technology solutions will prove more efficient and economic except for a few niche markets.
Agree – however, given the diversity of ‘plant life’ I wonder whether there are any plants (possibly algae) that are extremely simple and therefore have a very short chain of reactions/energy usage which could be adopted/adapted to use as energy farms.
Algal production is higher than terrestrial plants, although I’m not entirely clear why. Single cell algae can also be easily genetically manipulated to increase production. George Church talks about his work in this area in “Regenesis”. Photosynthetic bacteria might well be similarly amenable to this approach. Spectral absorption in bacterial pigments are broadly similar to green plants, but stronger on the red end of the spectrum. A useful overview ref is: How Photosynthetic Bacteria Harvest Solar Energy
Alex, your ideas are good but there are more recent references.
Here’s an idea for an “artificial leaf” made of purple bacteria:
http://onlinelibrary.wiley.com/doi/10.1002/btpr.406/abstract
And a provocative argument for artificial photosynthesis and the “hydrogen economy”:
http://www.nature.com/nphoton/journal/v6/n8/abs/nphoton.2012.175.html
Certainly there are some interesting lab experiments to increase efficiency. Unfortunately I can’t tell how they compare to non-bio systems. Somewhere I read about targeting a theoretical maximum of 30% efficiency. For the sake of argument, suppose biological systems can match the performance of existing, off the shelf, solar and electrolysis systems. At that point we will then need to look at economic and market factors. Costs, convenience, ease of production etc. There is no question that I can buy a solar cell today and make H2 by electrolysis. It is child’s play. The system will work under a variety of conditions. Large industries are looking at new materials to further reduce the cost of solar panels. Mirrors can be used to increase their energy output. The question is, why would I prefer to buy a biological device to do the same task? On an industrial scale, is algal or bacterial replication a sufficient cost offset over cheap solar panels? I don’t know the answer, because we don’t have any experience with biological H2 production system. maybe we will have a better idea in 10 years? I’m open to using the best, most sustainable system available. The real issue today is what can be done to replace fossil fuel with renewables asap to bend that CO2 concentration curve.
“The question is, why would I prefer to buy a biological device to do the same task?” — because mono-culture/energy gets you into trouble.
I can think of many everyday things found in probably every home in the Western world that are substitutes for each other, including food sources. I’m not a fan of buying every gizmo that comes out, but do feel that some overlap/substitutability as well as built-in multipurpose usage is worthwhile/desirable. At present, my chief beef with solar and wind is that each technology has been designed to perform in only one way. What a waste!
How would you envsion an artificial leaf performing in either a different or multiple ways? Maybe burning the H2 as a gas for gas appliances or the car, and providing electricity via a fuel cell, or conceivably by direct electron capture (like a PV cell)? Wind power can be applied as mechanical energy, usually for pumping, and electricity.
I agree with diversity, primarily to avoid monopolies. But diversity also incurs integration issues (e.g. inverters for solar PV to match the mains supply). Then there are other factors – off grid heat and power is much less convenient, and usually more expensive. For example, propane deliveries are much more expensive than piped natural gas. Gasoline electric generators are far less demand elastic than the utility mains.