27 July 2007

Energy from the sun

At The Oil Drum Robert Rapier argues biomass energy has a very limited role to play as compared to solar photovoltaics. (Of photovoltaics isn't as exciting if you're a large-scale farmer looking for a subsidy.)

The fundamental problem here is that photosynthesis is not very efficient. Consider the rapeseed oil yield above. Gilgamesh made a table that is basically the solar capture/conversion to oil from various crops. The gist is that only a few hundredths of a percent of the incoming solar energy gets converted into liquid fuels. Of course some did get converted into other biomass, which could be otherwise used for energy, but generally we get a very low capture of the sun's energy for use as liquid fuels. (This exercise can still be proven by assuming the theoretical limit for photosynthesis. One must just make more assumptions and it is not as easy to follow for a general audience).

Consider instead direct solar capture. Let's not even consider the record 40+% efficiency that Spectrolab announced last year. Let's not consider any of the more exotic technologies that are pushing the envelope on direct solar capture efficiency. BP's run of the mill silicon solar cells operate with an efficiency of 15%. That's about 250 times better than the solar to rapeseed oil route. Or, to put it a different way, you can produce the same amount of energy with direct solar capture in a 13 ft. by 13 ft. area that you can by photosynthesis in 1 acre of rapeseed. And odds are that you have a roof with an area that size, which could be used to capture energy without the need to use arable land.

Rapier has reached the same conclusion I've preached for years: We need to develop the technology we need to shift to electric power for transportation.

Of course the disadvantages are 1). The costs for solar are still relatively high; and 2). We have a liquid fuel infrastructure. But in the long run, I don't see that we have any chance of maintaining that infrastructure. If we are to embark on a Manhattan Project to get off of our petroleum dependence, we should direct our efforts toward an eventual electric transportation infrastructure.

Biomass energy will eat up too much land and agriculture will generate too much pollution with a poor ratio of Energy Return On Energy Invested (EROEI). Synthetically created photovoltaic materials will convert sunlight to energy far more efficiently than plants can manage. This fact might one day enable nanobots to outcompete DNA-based life. But for the foreseeable future the use of photovoltaics instead of biomass energy will protect nature.
Airplanes probably aren't going escape their use of liquid fuels when cars shift to electric power. So the relative cost of air transportation will rise vis a vis ground transportation. But overall the cost of transportation will fall once we can power vehicles off of electricity.

5 comments:

Norma said...

Stopping by for a looksee. Yes, you meet your title, but you've done a bit more than swerve. Still, well-written and thoughtful, at least the four or five I've looked through, and that's unusual for a left sided blogger.

pygalgia said...

Well thought out, but it isn't an "either/or" situation. In the short term, cellulosic ethanol can help reduce our petroleum consumption, and algae is a very good source stock. Photovoltaics are a great way to produce electricity, much better than our current coal plants for example. It will be a long term project to change from liquid fuel vehicles to electric vehicles, and it's well worth doing. In short, we need to tackle our energy system from multiple angles, with both short term and long term strategies.
"How do you eat an elephant?"
"One bite at a time"

Karlo said...

Thanks for the left-handed compliment Norma!

BadTux said...

Indeed, you've pointed out the principle flaw with biomass. However, we will continue to need hydrocarbons for the foreseeable future because we will still need lubricants regardless of what we do with the cars themselves. Given that, even if growing corn to turn into lubricating oil (a known/proven process -- Red Line is the primary vendor of such synthetics here in the USA) is inefficient and requires more energy than you get out, it will still need to be done. It just won't be the solution to our energy requirements.

For that matter, even the higher energy density of solar panels is not going to do the job. We need something even denser than that to meet our baseline needs. Which means, regardless of what some greenies whine about them, nuclear power plants. People whine about safety and point at Chernobyl, but Chernobyl was an early 1940's weapons reactor design that was never designed for safety or indeed even originally designed to generate power, it was designed to produce plutonium for bombs, Modern designs like pebble-bed reactors are so safe that if the cooling system fails, they self-throttle -- they can't melt down because when they heat up the neutrons become too active and escape the pebble bed, thereby no longer contributing to the fissioning of uranium. Heavy water plants have the same attribute -- if you dump the water, no more fission (in the case of heavy water plants it's because the water is the moderator that slows the neutrons down enough so they don't escape and instead smash into uranium to cause reactions).

But you point out the energy density issue to some of the rabid lefties and they whine that we'll just have to "change our lifestyle." Ain't happening. Modern skyscrapers are uninhabitable without air conditioning, as are places like Phoenix and Los Angeles (inland area) in the summer. When I lived in Phoenix, it was 90F at 5am in the morning in August. In addition, the modern transportation infrastructure requires significant energy to transport the requirements of civilization around, we're long past the point at which everything you need to survive can be made within 100 miles of your home, we'd have to kill off 5 billion of the people on the planet to get to that point and go back to a hunter-gatherer lifestyle hunting small animals with stone-tipped spears for that to happen. Of course, the most rabid of the lefties think that'd be a GOOD thing, but I sorta like civilization myself, even if it's only a thin veneer over the inner monkey of the jumped-up apes with delusions of grandeur who call themselves "humans".

Anyhow, 'nuff of that. Time to get some work done...

- Badtux the Energy Penguin

Karlo said...

I must confess that I have my more luddite moments of longing for the days of spearing fish in the local river but I'm sure you're right.