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By Robert Rapier on Jul 23, 2007 with no responses

The Future is Solar

I have done a lot of research lately into various alternative diesel technologies as I was working on my renewable diesel chapter. One thing that became very clear to me is that the world will not be able to displace more than a fraction of our petroleum usage with biofuels. I already knew that this was the case with ethanol, but now I believe that is true of all liquid fuels. Consider this sneak preview (still in draft form) from the book:

There are approximately 4 billion arable acres in the world. There are many different feed stocks from which to make renewable diesel, but most biodiesel is made from rapeseed oil. Rapeseed is an oilseed crop that is widespread, with relatively high oil production.

Consider how much petroleum could be displaced if all 4 billion acres of arable land were planted in rapeseed, or an energy crop with an oil productivity similar to rapeseed. The average rapeseed oil yield per year is 127 gallons/acre. On 4 billion acres, this works out to be 33 million barrels per day of rapeseed oil. The energy content of rapeseed oil is about 10% less than that of petroleum diesel, so the petroleum equivalent yield from planting all of the world’s arable land in one of the more popular biofuel options is just under 30 million barrels per day. This is just over a third of the world’s present usage of petroleum, 85 million barrels per day. Yet this is the gross yield. Because it takes energy to grow, harvest, and process biomass into fuel, the net yield will be lower, and in some cases may even be negative (i.e., more energy put into the process than is contained in the final product).

The fundamental problem here is that photosynthesis is not very efficient. Consider the rapeseed oil yield above. A reader at The Oil Drum made a table that is basically the solar capture/conversion to oil from various crops. I tried to recreate the table, but it was taking far too much time (Blogger has a terrible quirk about tables), so here is a link.

Basically, 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 when an acre of rapeseed/canola is planted, we get about 0.06% conversion of the sun’s energy into oil. (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).

Consider now 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.

Of course the disadvantages are 1). The costs for solar are still relatively high; 2). We have a liquid fuel infrastructure; 3). Storage is still a problem. But in the long run, I don’t see that we have any chance of maintaining that infrastructure. The future is solar.