Wasting Sunlight

Feb 3, 2016

by Russ Conser

In my last post, I told the story of how grasses co-developed with grazing animals like cattle. This time, I want to give you a sense of the incredible amount of energy healthy grasslands actually capture, and the amount of energy that is wasted when sunshine falls on bare soil.

Let’s start with the bottom line – when sunlight falls on bare soil on US crop land alone, much of which is used to grow corn to feed cattle, the amount of energy wasted is equal to the total energy from coal, oil and natural gas that the US produces each year. In other words, we waste as much of today’s sunlight as we produce from yesterday’s sunlight. If you want to know more, read on…

This NASA database will tell you the actual average amount of solar energy (‘insolation’) falling anywhere on the planet by month throughout the year. As it’s designed to help you plan things like solar panels for your house, it accounts for clouds and other factors that obscure sunlight. At my house in Texas, for the last 22 years, I’ve averaged 4.42 kWh/m2/day. I checked our friend Allen William’s place in Mississippi and he gets 4.36 kWh/m2/day, and our friends Gabe & Paul Brown in North Dakota gets 3.84 kWh/m2/day.

So for the sake of making a simple calculation, let’s just say we all get around 4 kWh/m2/day – that’s 120 kWh/m2/month, or 1,460 kWh/m2/year. First, to give you a sense of how big that number is, my house uses an average of about 1700 kWh/month (we have heavy air conditioning, and way too many sep56b289374690c-figure_1.jpgTV/computer screens!). So that means if I could capture 100% of that sunlight and directly put it to use, it would take 14.2 m2 or 152 ft2 to power my house, or about the size of a small 11’x14’ bedroom.

But neither man-made nor natural technology is able to make use of 100% of that solar energy. If I capture that with a solar panel of 15% efficiency, it would take about 1000 ft2 or a large fraction of my roof.

Biology is less efficient, but much cheaper to cover large areas. As you might recall from your understanding of rainbows, sunlight is made up of light from different frequencies. To start with, plants can only make use of about 49% of that light. A good chunk of what remains of that light is used in the machinery to make the sugars from input CO2 and water. When all is said and done, C3 plants could, in theory, capture 4.6% of incoming sunlight, and C4 plants 6.0% as shown in Figure 1, In reality, the highest actual reported efficiencies for C3’s and C4’s are 2.4% and 3.7% respectively (Ref: Zhu, et. al.).

So let’s do some simple calculations to get a rough sense about how much energy that adds up to, and how much is wasted if we don’t catch it as well as we could.

Again to keep things simple, let’s say a good mixed grassland pasture captures 2% of the total sunlight falling on it or 1460 kWh/m2/year. In the lower 48 US States, there are 335 million acres of cropland and 775 million acres of grazing land (Ref: Nickerson et. al.). There are 4046.8 m2 in an acre, so that means if we made use of all the capturable sunlight on US agricultural we get some whopping large numbers…

• Cropland – 39,585,797,600,000 kWh or 39,586 TWh (“Tera” Watt-hours)
• Grazing Land - 91,579084,000,000 kWh or 91,579 TWh
• Total Crop & Grazing – 131,164,881,600,000 kWh or 131,165 TWh

The numbers are so big, it’s hard to grasp what they mean, so let’s compare to our current US energy production.

sep56b2897b26f89-figure_2.jpgThe US DOE tracks energy production in common but obscure units of “quadrillion BTU” but these can be easily converted to TWh’s, too (Table 1). Referring back to numbers above, that means that on combined crop and grazing land in the US, when working well (i.e. 131,165 TWh @2% of solar energy capture), our total farm system could capture 5 times the energy of all the energy produced in our economy. Or if we count coal, oil, natural gas and natural gas liquids (20,335 TWh) as sunlight from ancient yesterdays, we could capture 6.5 times the energy in plant biomass from today’s sunlight than all of the energy we dig up and reuse from yesterday’s sunlight!

OK, that’s an interesting comparison, but you’ll (rightly) say that those numbers aren’t really fair because you can’t power your kitchen lights with bails of hay. So let me tell you about what I see as the real tragedy – the very real effect of sunlight that’s wasted when it falls on bare soil. That’s real energy that really could have been put to good use.

The most common method of feeding cattle today involves growing corn and shipping it to a feedlot. For simplicity, let’s say these fields are fallow during half the year, and there’s plenty of bare soil between active plants during early growing periods such that over the course of the year, we only capture 50% of the sunlight that we could have captured on crop land. This means that on crop land alone, 19,793 TWh (19.8 ‘Peta’Wh) of energy that we could have used to grow a plant is wasted as heat just bouncing back into the air. This number compares almost exactly to the total annual energy production from all fossil fuel sources of 20,335 TWh (20.3 PWh). In other words, we waste as much energy by not capturing today’s sunlight on cropland as we produce from all of yesterday’s sunlight.

But wait, there’s more! The agricultural sector uses a total of roughly 1.6 Quadrillion BTU’s (469 TWh) including both direct (e.g. fuel) and indirect (e.g. fertilizer) uses. In other words, we waste maybe 1% of usable sunlight growing crops to feed cattle, a number that’s as large as the total US coal, oil and natural gas production system, and then spend another ~2% of all of that coal, oil and natural gas energy to plug the hole we dug with wasted sunlight!

But wait, there’s even more!! Crop lands alone are not the only guilty party. We have plenty of bare soil wasting sunlight on many or even most of our grassland pastures as well. Assuming the same under-performance of 50%, that would mean we’re wasting 45.8 PWh on grasslands – or more than twice the total US production of US coal, oil and natural gas combined!

Fortunately, there is a better way, and as grassland farmers and ranchers, we’re sitting with the right tools and right places to make a very big difference in the US energy equation. By managing cattle directly on the grass in a way that captures the energy from the sun on all of the soil area for as much of the year as we can, we put the potential waste back to use, and cut the need to fill those gaps with yesterday’s sunlight.

Hopefully that was a fun exercise in helping you understand the enormous magnitude of energy we waste when we fail to capture sunlight year-round. To learn more about things you can do to capture more sunlight on your farm, I highly recommend that you watch this video of Luke Linenbringer’s talk from the 2015 Grassfed Exchange conference in Michigan. And I hope to see y’all in Georgia where we can all learn even more cool tricks to capture more sunlight.

Russ Conser
Fulshear, TX

For more reading if interested:

1. Zhu, Xin-Guang, “What is the maximum efficiency with which photosynthesis can convert solar energy into biomass?” Current Opinion in Biotechnology (2008, 19:1-7).
2. Nickerson C., Ebel, R., Borchers, A., and Carraiazo, F., “Major Uses of Land in the United States, 2007” USDA Economic Research Service.
3. Beckman, J., Borchers, A, and Jones, C., “Agriculture’s Supply and Demand for Energy and Energy Products” USDA Economic Research Service (2013).

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