Farming the Sun in Iowa: Corn versus Photovoltaics
By Richard Stovall
I was having lunch with some of my students and one of them handed me the local newspaper, the Des Moines Sunday Register, and on the front page was a special report entitled “Threats to Iowa’s Bio-energy Industry”. In the report were a series of articles including the following: Fossil Fuel Attacks on Mandates, Put Renewable Energy at Risk. It provides an analysis of the various forms of renewables used in Iowa (ethanol, biodiesel, and wind) and the threats they are facing from the fossil fuel industry to their existing subsidies. In this multi-page special report what was particularly notable to me was the complete absence of the word “solar” or “photovoltaic”.
We have attempted to correct this lack of solar awareness through a series of seminars we’ve been delivering around the state, including one entitled “Understanding Iowa’s Solar Potential and Viability”. As part of the seminar we attempt to dispel many myths about solar PV in Iowa, the most significant of which is that there is not enough solar resource (sunshine) to make it worthwhile. We know that in Iowa on average there are 4.8 peak sun hours per day. This means that there are 4.8 kilowatt-hours per square meter per day of available “fuel” from the sun. Most Iowan’s are surprised to know that is equal to the available solar resource in Houston, Texas.
We were in Iowa for most of 2012 and watched the corn growing cycle from tilling, planting, and growing through to its burnout from drought. In a year with enough rainfall, that corn would be making great use of the 4.8 peak sun hours available. But knowing that 60% of all the corn grown in Iowa is for ethanol production and that there were a record 14.6 million acres planted in Iowa in 2012, I can’t help but try to make some comparisons about the effectiveness of farming the sun by capturing solar energy through growing corn for ethanol versus farming the sun by capturing solar energy through photovoltaic systems.
Since most corn ethanol is used as a gasoline additive, I thought is would be fun to make a comparison between the how much land it would take to grow the corn to provide the ethanol fuel to go from New York to Los Angeles versus how far you could travel in an electric vehicle if you used a photovoltaic system of the same land area to generate energy.
We know that it requires 1.5 gallons of ethanol to equate to 1 gallon of gasoline, so a Corolla that gets 30 MPG will get only 20 MPG on ethanol. If the distance in miles from New York to Los Angeles is 2774 miles, then it will take 138.7 gallons of ethanol. We know that one acre is good for about 7110 pounds of corn which can be distilled into 328 gallons of ethanol. This means that it will take about .42 acres to get the required ethanol.
Now lets compare that to a photovoltaic system. Assuming the use of mono or poly-crystalline silicon, based on the kilowatts per acre of some large installed systems we can install about 160KW per acre (assuming ~6 acres per megawatt). On those same .42 acres we can install a 67KW array, which in Des Moines, would produce about 86,560 kilowatt-hours per year.
Now we just need to know how many miles you can go in an electric car per kwh. As it happens a gentleman was traveling across the country in his Tesla and stopped at our KOA campground in Newton, Iowa. On his blog roadsteronroute66 he notes that he is getting 4.5 miles per kilowatt-hour.
At 86,560 kilowatt-hours per year, at 4.5 miles per kilowatt-hour one could travel 389,520 miles. That is 140 times the distance one could travel with ethanol.
Taking it one step further. That corn crop must be rotated each year and I’m told the soil must rest for 2 years between plantings. Therefore, it would take three times as much land area for the corn if this is to be a sustainable process. This means that for a given area in Iowa, the sun’s energy harvested through photovoltaics to power an electric car will get your 420 times the distance that the same solar energy harvested through corn and ethanol production for use in a combustion vehicle will.