Sugar Cane vs. Solar Panels
As I was reading the crazy ideas about the future in The Singularity is Near, I had a crazy idea of my own: what if humans could be solar powered? Right now, humans use plants to capture the sunlight's energy and convert it into carbohydrates which we can digest and use to power our bodies. What if we could cut out the middle man? What if we could wear a solar paneled hat or body suit that would capture the sunlight and turn it into the glucose that the body needs for energy? And if it were possible, how large of a solar paneled hat would you need?
And this lead me a second question: which collects more usable energy, an acre of sugar cane or an acre of solar panels?
So I ran the numbers. I chose sugar cane because it produces the largest number of calories per acre of any agricultural product. From sucrose.com we learn that you can harvest 10 tons of sugar per hectare = 10,000 kg/10,000m^2 = 1 kg /m^2. This means a square meter of sugar can yields a kilogram of sugar a year. As there are 4 calories (or kcal) per gram this gives you 4,000 kcal/m^2.
At this site we learn that a square meter of 11% efficient solar panels collects around 550Wh a day in Austin Texas. So for a whole year that gives you 550 * 365 = 200 kWh/yr/m^2.
Then as everyone knows (or knows how to use Google Calculator knows) 1 kcal = 1.16222222 watt hour. So our 4,000 kcals of sugar = 4.65 kWh. That means the 11% efficient solar panel generates 200/4.65 = 50 times more usable energy a year than the sugar cane does. And as you can now get 22% efficient solar panels that would go up to 100 times.
Solar panels are therefore 2 magnitudes of order better at creating usable energy per m^2 than sugar cane.
Aside: This is just the usable energy. Plant leaves themselves are closer to 3-6% efficient in capturing sunlight (vs. 10-20% for solar panels) as this site explains. Only light within the wavelength range of 400 to 700 nm (photosynthetically active radiation, PAR) can be utilized by plants, effectively allowing only 45 % of total solar energy to be utilized for photosynthesis. Furthermore, fixation of one CO2 molecule during photosynthesis, necessitates a quantum requirement of ten (or more), which results in a maximum utilization of only 25% of the PAR absorbed by the photosynthetic system. On the basis of these limitations, the theoretical maximum efficiency of solar energy conversion is approximately 11%. In practice, however, the magnitude of photosynthetic efficiency observed in the field, is further decreased by factors such as poor absorption of sunlight due to its reflection, respiration requirements of photosynthesis and the need for optimal solar radiation levels. The net result being an overall photosynthetic efficiency of between 3 and 6% of total solar radiation.
If only there was a way to convert electricity from the panels directly into sugar or glucose. Then we could free up a lot of crop land (99%) and produce the sugar directly from solar cells. You would definitely lose some energy in the conversion, but you could lose 98% of the energy and still come out with twice as much sugar. I tried to find out if there was a way you do this, but I couldn't find anything. I checked out artificial photosynthesis but not much there. If anyone knows of a way or any research in this direction, leave a comment.
Ideally, you would want a little black box that could convert the electricity to glucose, which you could then add to water and use with an IV drip to get it in the blood. Then you could go for days without needing any food. This could be cool if you are in the military, or an astronaut or a hiker. Don't bring food, just your solar panels and your glucose maker and you are good to go.
How many square meters of solar panels would a person need if they could live directly off of it? A human male needs around 2,700 kcal a day. Our 22% efficient solar panels in Austin give us 1100Wh or 1275 kcal/m^2 a day, so we would need a little over 2 square meters (assuming no energy losses in conversion). A little big for a hat, but hey everything is bigger in Texas.
RSS Feed
21 comments:
Very Interesting sums, thanks. Do you mind if I publish them on my site (http://www.yourgreendream.com/articles_inspirational_maths.php) and give you some credits back? Let me know at admin@yourgreendream.com.
Thanks, either way.
I just posed the same question to myself (Aug '07)before discovering your blog, analyzed it from a slightly different perspective and came up with a similar answer.
After 30 years of experimentation, Brazil produces about 870 gallons of ethanol per acre per year at 76,100 BTU's per gallon, or about 66 million BTU's per acre per year.
Wikipedia suggests that a PV collector can generate 1kWh per square meter per day, the equivalent of 3,400 BTU's. 4046.85 (SqM per acre) x 365 (days per year) gives you 5 trillion BTU's per acre per year, about 75 times the amount of energy produced by sugar cane/ethanol.
A couple of caveats: You probably can't install 1 acre of solar collectors on 1 acre of land. And if you efficiently process the bagasse, the sugar cane residue left over from ethanol production, you can produce as many BTU's as the ethanol itself.
An interesting aside: corn yields 1/2 the ethanol per acre as sugar cane, and it requires 3 BTU's of energy to produce and transport every 4 BTU's of ethanol.
Bill Mantis
Bill,
Glad to see that our numbers more are similar, given that we took different approaches to the problem.
Interesting points on Brazil and corn ethanol. I have done a little bit of research on these topics before in my Ethanol and Corn and Brazil and Ethanol posts. You might want to check those out and see if my numbers also jive with yours on those.
you did not compare energy lost planting, harvesting, transporting, refining sugar to energy lost in your fictitious glucose maker.
anon,
You are right that I did not include planting, harvesting, transporting, or refining sugar in my numbers. Neither did I include manufacturing of solar panels or distribution of electricity for the other side of the comparison.
How much energy is consumed manufacturing the PV pannels.. I have heard that the amount of overall energy needed to create a PV pannel is more than what it will produce in its liftme.. Is that a myth?
Hi David,
That is a myth. According to this research, it takes about 2 years for a solar panel to produce as much energy as it took to produce. Solar panels generally last for 25 years or more, so panels produce much more energy that it took to produce them over their lifetimes.
Consider this: If a solar panel required more energy to produce then it could ever produce, add to this the cost of materials, labor, transportation, and installation. The manufacturer could never make any money.
If the manufacturer had to pay for lets say 10,000 kwh and that's how much the PV would ever produce, they would have to sell it for at least as much as they made it for in order to break even, and then there are the other costs I already mentioned. Why wouldn't the buyer of the PV just buy the same 10,000 kwh? At least then he wouldn't have to pay for the materials, labor, transportation, installation, and extra for the PV manufacturer to make profit.
It is a 100% myth that a PV requires as much as it will ever make to produce. Just silliness.
In response to:
"If only there was a way to convert electricity from the panels directly into sugar or glucose. Then we could free up a lot of crop land (99%) and produce the sugar directly from solar cells. You would definitely lose some energy in the conversion, but you could lose 98% of the energy and still come out with twice as much sugar. I tried to find out if there was a way you do this, but I couldn't find anything. I checked out artificial photosynthesis but not much there. If anyone knows of a way or any research in this direction, leave a comment."
I followed a class on the subject last year, and I can tell you that artificial photosynthesis is a very recent development, and not much progress has been made yet. The idea has been around for longer, but people have only now begun to understand the photosynthesis process thoroughly enough.
Most efforts focus on trying to accomplish the most elementary aspect of photosynthesis, and that is the breaking up of H2O into it's constituent parts and harvesting hydrogen from it. Research has focused on finding a decent catalyst for the reaction, both for breaking the hydrogen from the oxygen (hard) as well as for efficiently turning 2 H+ into H2 (easier but also hard).
Doing something more complex as turning Co2 and H20 into glucose however..is a bit farther off for now.
Thanks Ronald.
what about hemp?
Another angle on this is the fact that photosynthesis and PV cells use different components of the spectrum. Ideally, you'd find a way to fracture the light so that plants would get only the sunlight they needed and so be less likely to overheat while the solar panels would get only the fraction of the spectrum that they needed as well (hard UV).
Plants would actually be more efficient with the hard UV light removed, since it would prevent them from overheating.
If such a process could be done efficiently (PV cells that reflected light in the 400-700 nm range onto plants that used the incident energy) it would make sun drenched desert areas like Arizona a lot more farmable.
Also, if the PV's electricity could be used to efficiently add CO2 to a pool of algae you could increase the efficiency of carbon fixation.
But personally I think the best route is to just go with iron fertilization of the earth's oceans and use that as feedstock.
Ryan,
Interesting idea. I like it.
I have wondered if it would be possible to genetically engineer plants that could utilize a larger spectrum of light. The fact that plants are green shows their inefficiency as they are reflecting off the green light rather than absorbing it.
I also am a fan of iron fertilization for greater production of life on Earth.
Awesome article!
I read that scientists are against it because of some (possibly uncontrolable) after effects from iron fertilization. We should indeed solarfy the deserts (and with molten salt heat reservoirs)!
The Sahara forest project says to simply evaporate ocean water in greenhouses, use fans and you could grow anything. I came here (after others) searching sugarcane to Kwh and realized that it would be too expensive to build greenhouses in the deserts for such being that ethanol yield is only a few thousand dollars per acre. So, it would be really cool to have solar farms that reflect the portion of light needed by crops so as to avoid the costs of greenhouses. Since concentrated solar power (by use of mirrors) is just over 100 times more efficient than sugarcane to ethanol, one may ask "why do we even need to try to grow crops in the deserts?". I think we need to for sequestering carbon. People complain about converting 10,000 square miles of deserts into mirrors for CSP but don't complain every time they drive. So, I'm curious as to how much cropland it takes to sequester all of the world's fossil fuel use...
you'd have a great tan! (someone had to make a stupid comment!)
The ideas expressed by Fat Knowledge and commentators are very interesting and it is indeed good to see that people are actually thinking about this important issue. However, there seems to be an over-emphasis on the economics. I guess this is because we have been nurtured on the belief that everything depends on the bottom line. As such, whenever there is a problem we run for the calculator and start adding the dollars.
May I be allowed to take a different approach. Forget the dollars for now because all of the costs will be market driven and in many cases do not reflect "true"value.
FACT 1. Mother nature is very systematic in her approach and many of the issues we face can be solved through the understanding of the basic science that is involved. I DID NOT SAY SIMPLE SCIENCE. Plants use CO2 to produce oxygen and energy (sugars)while animals use oxygen and energy to produce CO2. It seems to me then that there is a natural symbiosis here.
FACT 2. There is more CO2 being produce than can be utilized by plant.
QUESTIONS: Why? Where? How? By who or what?
FACT 3. Animals also produce another green house gas, Methane.
FACT 4. Man rears millions of animals on farms to provide food (energy)
FACT 5. Methane is an excellent source of energy.
QUESTION: Why is there not a greater effort to harness this methane to get more energy for heat, light, transportation etc.?
Is it not interesting that the greatest wealth produced in the 20th century was through petroleum which is the major source of the imbalance between CO2 produced and CO2 utilized?
Are the petroleum companies involved in the discussions on climate change?
Why do we want to trade in Carbon (CO2) and not Oxygen?
Just some idle thoughts.........
Leroy McClean.
Problem with this analysis is that you have no comprehension of cost.
1m^2 of sugar cane costs $1.00 to produce
1 m^2 of solar panels cost $2000 to produce
Do you want to pay $2000 times as much for your electricity?
it would be really cool to have solar farms that reflect the portion of light needed by crops so as to avoid the costs of greenhouses
lol the same idea occurred to me, but inhead of a hat, an skin replacement! :D
Luckily I'm not alone! xD. Yay! :P
Maybe a "better" approach would be to reduce the amount of biological energy needed. Amputate your arms and legs and replace them with electrical appendages that could be directly driven with PV panels. Now the head and torso would need a lot less energy, significantly reducing crop sizes needed to sustain an individual.
I know this is an old thread, but... I agree with the last post.
Post a Comment
Note: Only a member of this blog may post a comment.