Are Plants Energy Efficient?

by Gabriel Popkin and Neil Zimmerman
A version of this article was published in the Potomac Vegetable Farms newsletter, September 2009 edition (pdf), pp 2-3.

We hear a lot about energy these days, and most people believe that solar power is going to play a significant role in our future energy economy. But we already depend on solar energy, because the sun provides all the energy that goes into the food we eat. So we thought it might be interesting to ask how much of the sunlight that falls on plants actually becomes useful food energy.

As you know, each vegetable has its own nutritional profile. Some, like tomatoes, contain almost no calories, whereas others, such as corn, pack a considerable caloric punch. We thought it would be more interesting to look at a high-calorie vegetable, so we chose corn. We estimated that one corn plant occupies a square of ground roughly half a meter on a side, giving it a “footprint” (rootprint?) of .25 square meters. We also know that the earth is bathed in 340 Watts per square meter (W/m^2) of sunlight energy (averaged over the surface of the earth) all the time—almost 8,000 times more than the total global power demand, if only we knew how to harness it. You know how bright a 100-Watt light bulb is, so imagine 3.4 of these illuminating every square meter of the planet—that’s a lot of light!

But how much of this sunlight energy actually hits our quarter-square meter corn plant over the course of its lifetime? It depends a bit on the season, so let’s say the corn was planted on May 15th and harvested 90 days later. At our latitude of about 39 degrees north of the equator, it would receive an average of around 14 hours of sunlight per day. But this sunlight is most direct only at high noon, so we multiply by a factor of 2/pi (for reasons you can look up in a calculus book if you want) to correct for the more slanted morning and afternoon light. We also multiply by the cosine of 20 degrees to correct (roughly) for the fact that the sun never quite reaches directly overhead, due to the fact that we are north of the Tropic of Cancer (the northernmost latitude that ever receives direct sunlight). That gives us an average power of around 50 Watts shining on our corn plant’s quarter-meter-square home—one medium light bulb, rather than 3.4 bright ones. We make one final, very rough, approximation—we multiply by ½ to account for the fact that the corn plant only occupies its full .25 square meter allotment when it is fully grown (it takes up much less space when it is young), and also to take into account that the plant will receive less sunlight on cloudy and rainy days.

Even given all that, our plant takes in a lot of energy—roughly 27,000 Calories, in fact. But we know we don’t get 27,000 Calories from an ear of corn, or we couldn’t scarf down two or three at an afternoon barbecue. In fact, an ear of sweet corn only has about 90 Calories, so somehow a factor of 300 is lost between the sunlight energy that lands on the plant and the food energy we get out of it. Where does it all go?

More than half of it is never absorbed by the plant at all, because plants only absorb certain types of light. The sun emits light waves with a vast range of wavelengths, from long-wave infrared to those short-wave UV rays we’re all afraid of. But chlorophyll absorbs mostly red and blue light, which represents only about a third of total solar radiation at sea level. This means the chlorophyll in a corn plant can only absorb 9,000 or so Calories of sunlight in its lifetime, but still only about 1% of that becomes sugar that we can eat. What about the rest?

One place that a lot of this energy goes is the rest of the corn plant. Corn can be anywhere from five to twelve feet high or so, and the vast majority of the non-water mass of the plant comes from carbon dioxide in the air that was converted into sugars and starches through photosynthesis. It turns out that about half the energy of a corn plant is in the stalk and leaves, which are indigestible to us, although scientists are working on developing enzymes that can digest corn stalks, in order to produce biofuels.

Another consideration is that light is not the only requirement for photosynthesis. Even on the brightest day, if the temperature is too cold or if the local carbon dioxide concentration near a corn field is too low, photosynthesis will not proceed as fast as it would otherwise, and some light will be “wasted.” Physicist Freeman Dyson notes that “A field of corn growing in full sunlight in the middle of the day uses up all the carbon dioxide within a meter of the ground in about five minutes. If the air were not constantly stirred by convection currents and winds, the corn would stop growing.”

A third reason that most sunlight energy doesn’t end up as food is that the plant itself consumes energy to stay alive. Even though plants don’t move around the way animals do, they do transport minerals and fluids up through their roots, stalks and leaves; they build large molecules out of smaller buildings blocks; and they have immune systems that fight disease. All of these things take energy, and all of that energy comes from the sun.

So plants, it turns out, are pretty darn inefficient. If we had a worker who was one-third of 1% efficient, we would fire him or her on the spot. But before we fire our plants, we might want to ask, how efficient are plants relative to other systems that convert energy from one form to another? As we noted at the beginning of this article, we hear a lot about solar power and how it’s going to solve all our energy problems. The good news is that solar panels are a lot more efficient than plants at converting sunlight to energy we can use—the typical panels you can put on your roof today might be around 20% efficient, and the fancy ones NASA uses to power its satellites can reach efficiencies of over 40%. The bad news (beside the fact that we can’t eat solar panels) is that these panels are made of expensive materials like tellurium—one of the rarest elements on earth—and there is not enough of these materials to power the whole world. Scientists are working on developing new solar cells that can be made out of common and cheap elements like silicon. Plants solved this problem long ago, and build their molecules out of cheap elements like carbon, nitrogen, oxygen, and metals found in abundance. Chlorophyll, for example, has at its center a single atom of magnesium, which is the eighth most abundant element in the earth’s crust. So we may wish to learn a few tricks of the trade from our old-fashioned, inefficient plant brethren.

Other energy-conversion systems we could look at are cars, which convert about 20% of the energy in gasoline into motion; the electrical grid, which converts about 13% of the energy in coal or uranium into electrical energy at your outlet (another fraction of which is wasted by whatever device you power with it); and cows, which convert about 4 to 5% of the energy they consume into food we can consume, at least according to one source looking at feedlot cattle. With grass-fed cattle we may not care so much about this number, because we can’t get a single useful calorie directly from grass, and cows do us quite a service by turning it into something both nutritious and delicious. Just another great reason to eat grass-fed!

Finally, we could ask how efficient our own bodies are. We probably don’t care about how much food energy is stored in the human body, but we might care how much of the food energy we consume is available for useful work. That number seems to be within a few percentage points of 14%, averaged over the course of a day, meaning that, if we have a physically demanding job, we use about one in every seven calories we eat to do work, and the other six to maintain body temperature and other metabolic activities—staying alive, in other words.

Another way to look at it is that, if we were to fuel ourselves entirely from corn, we would only use 1 out of every 2,100 calories of sunlight that falls on a growing plant to do work. That’s bad enough, but if we eat a hamburger from a feedlot cow, it’s even worse—we only use 1 out of every 42,000 calories of sunlight that originally fell on the corn plants the cow ate! You can think about that next time you’re out there trying to set things up just right so those plants can do their wondrous photosynthetic thing.