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February 13, 2007

Corn’s Day in the Sun

It’s highest priority for national policy to have alternatives to (your choice): a) importing as much oil as we do from the places we get it; b) putting more greenhouse gasses in the atmosphere; c) both of the above.  Everybody agrees.  Corn to ethanol is well-represented in congressional lobbies as just such an alternative and is getting a lot of money.  Silicon Valley VCs invest in it; President Bush wants to increase funding for it; tax breaks make ethanol-laced fuel more competitive than it would otherwise be.

But is it a good way to achieve whichever noble goal you chose?  Some people say more energy goes into making a gallon of ethanol from corn than you get out of it.  I posted pointers to pretty good evidence that corny ethanol does save oil and reduce greenhouse emissions.

The questions really are how much of the energy from the sun which falls on an acre of land do we harvest when we make and use ethanol and is there some better way to harvest and use that energy.  Turns out that very little of the energy ends up in our gas tanks and that direct use of sunlight to produce electricity results in capturing two orders of magnitude more energy per acre than farming.  But there are a couple of gotchas before we switch gears and tell Archer Daniels Midland and the rest of the ethanol crowd that the gravy train has stopped.

The numbers: in the sunbelt regions of the US, every acre receives sunlight with a daily energy equivalent of about 24 megawatt-hours of electricity.  Photovoltaic (PV) cells at the current commercial state of the technology convert only 15% of that – 3.6 megawatt hours.  Concentrated Solar Power (CSP), which consists of a lot of mirrors focusing the sunlight on something that boils, is about twice as efficient so yields 7.2 megawatt-hours/acre/day.  PV technology is advancing rapidly, partly because it shares so much engineering with chip design and LEDs. In the lab, PVs have efficiencies over 40%.  Let’s just stick with 7.2 megawatts for now and convert it to 26,000 megajoules (MJ) daily or something over 9 million MJ annually.  This number should be cut in half because you can’t use every inch of ground for collecting; you need access and structure etc. etc. so call it 4 million.

Using the most optimistic study of corny ethanol I’ve seen, the GROSS annual yield per acre is just 73,500 MJ annually – not even two percent of the 4 million MJ we get with CSP.  And this is gross energy production – actually enough other energy goes into the process of planting crops, fertilizing them, irrigating them, and harvesting them, and then fermenting them to get the ethanol so that the net yield is less than half of this.

So, if you owned an acre of farmland (or desert land), would you plant it in corn or in solar arrays?  Why are we doing ethanol rather than straight solar? Turns out there are some good reasons besides the fact that the first presidential primary is held in Iowa.

First reason for NOT immediately turning a field from corn to solar is upfront capital costs.  It costs approximately $1.25 per annual gallon of production capacity to build an ethanol plant so the capacity for the 915 gallons of ethanol we can expect to get from our acre needs a capital outlay of about $1150.  Of course there was also a cost for making the land into a corn field; there’s capital equipment needed to plan, tend, and harvest the corn; and there was a capital cost to build the roads that get the corn to the still – but this is all sunk capital.  Comparably little new capital needs to be spent for corny ethanol

Comparable capital costs for solar are much higher.  Covering an acre (at 50% coverage) with PV panels would cost about $750,000 at today’s prices just for the collectors.  The capital cost for an acres worth of CSP with today’s technology and prices is close to $2 million (this a shaky calculation I did based on data here).  Moreover, new capital would have to be spent on transmission lines to take the power from where it is being generated to where it is going to be used or at least to the nearest high-voltage line.

Net-net: you get one hundred times the net energy by using CSP on a field rather than planting corn.  You don’t have to pay for planting, weeding, irrigating, and harvesting.  But you have to spend at least 1800 times as much capital. That’s one reason that solar plants aren’t growing higher than an elephant’s eye in Kansas.

The second reason that corny ethanol is attractive despite the low energy yield per acre is that Ethanol displaces gasoline and reduces our demand for imported oil.  Solar electricity reduces the demand for coal, which we have plenty of, and natural gas which, when imported, comes mainly from Canada. So, if reducing dependence on Mideast oil is the main factor to be considered, ethanol helps more in the short-term than solar power.

Next posts on this subject give some circumstances under which it might well be a good idea to front the huge capital required for solar electricity rather than continuing to build ethanol plants and some ideas on where this capital might come from.

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