Friday, 20 January 2017

Harnessing the sun's energy - Photosynthesis

This post is a part of the series An Acre of Sunshine.

Almost all life on earth is supported by solar energy that is originally harvested by plants and algae. Though there are some notable exceptions, such as the life found at deep sea vents, that isn't really applicable to a discussion of a farm in Quebec. In photosynthesis, plants and algae capture the energy of photons (particles of light), and use that energy to cause a chemical reaction, taking water, carbon dioxide and solar energy as inputs, which allows them to produce simple sugars and oxygen gas as outputs. The chemical process is as follows:

6CO2 + 6 H2O + photons → C6H12O6 + 6O2
More simply: carbon dioxide + water + sunlight  → sugar + oxygen

These sugars are the primary 'stuff' that plants are made out of. As was discussed here, sugars can be thought of as chemical 'springs', that are built and compressed by adding energy from the sun, and can be released at a later time to access the energy (motion) that they contain.

These same sugars are then the source of energy for nearly all other forms of life. The plants use the sugars themselves to support their ongoing needs, to grow and build their stems, leaves, roots and trunks, and to reproduce, making flowers, fruits and seeds. All animals and fungi are in turn supported by these plants, eating and recycling the energy that the plants have captured and stored away. As both the plants and their consumers go about their business, the energy stored in these sugars is converted many times into other forms, used to build other tissues, and is stored as many types of sugars and fats, and often actively used more actively in the form of proteins. Some of that energy is also converted into heat and motion, such as when a person heats up when they are out for a jog.

As with every type of energy capture and conversion, photosynthesis doesn't capture all of the energy coming in from sunlight, it actually converts only a small fraction of it into sugars. Without going into a long lesson about biochemistry, the takeaway is that the maximum possible efficiency of photosynthesis is around 11%, but that the typical efficiency of actual plants is more like 2%. If you would like to know more about those details, start here and here. This means that out of all the energy that is in the sunlight striking a plant, the plant only captures and uses two parts of each one hundred.

But it is even worse than that. This 2% figure is for plants that are growing in a good environment, with just the right soil, temperature, nutrients, rain, etc. At our hilly farm, we have conditions far from this optimal. The local soil is sandy and rocky, and doesn't have all the nutrients that are available in the good black soil found in local river bottoms. Of course soil can always be augmented with fertilizers and other nutrients, though this is not been done at our property. Fortunately rain is not an issue for us, as our area consistently has 10 or more days per month with at least some precipitation. For the sake of argument, lets say that these imperfect conditions reduce by half the total energy capture of our local plants.

Finally, it is cold and snowy for nearly half of the year. The only time that plants can capture the sun's energy is when they have leaves. We have at best 180 growing days, from early April to early October. Fortunately the summer is also when there is more sunlight, so only having leaves for half the year means that plants can take advantage of 70% of a year's sunlight (see monthly energy availability here). Putting all of these factors together, we can figure out how much energy an acre worth of plants can actually harvest in a year.

5,112,000 kWh/acre/year * .02 photosynthetic efficiency * .5 losses for imperfect conditions * .7 of annual solar energy captured = 35790 kWh/acre/year of energy harvested by plants at our land

How numbers fall, don't they? We are now down to a figure in the same ballpark as our annual energy needs per person. And remember, this is all the energy that plants harvest, not just the parts that we want (known in biology as gross primary production). That energy includes everything, the metabolism of the plant, the fighting off of disease, pests, and predators, and the growth of leaves, stems, flowers, seeds, and roots. If one wants to look at growth, that is called net primary production (see here and here for more information). No matter which part of the plants that people want to use, the useful amount is always going to be much less. Say we were growing corn (we'll get back to this in a later section on crops). In this case, we only really want the kernels of corn. The rest of the plant, the stem, the leaves, and the roots, are usually not harvested or used. What if we only wanted the walnuts from a walnut tree?

If we wish to be sustainable, these considerations set the limit for how much energy from plants could be harvested per year. As long as we eat plants and use wood products grown under the sun, these limits are going to constrain us. It is possible that with better plant breeding, irrigation, soil quality, fertilizers, pesticides, etc., that this figure could perhaps be doubled or even tripled, so that our plants could capture a much higher yield. Also keep in mind that these numbers are for a northerly clime - our solar resource, and our growing season, isn't as long or as bountiful as what might be found in other areas of the world. With more sunlight and a year-round growing season, fertile tropical areas could be much more productive than our farm in the north.

Now that we have an estimate for the amount of energy that plants can obtain from the sun, we can look at all of the different plants, and sometimes the animals that feed on them, that we could grow and harvest on our farm.

Estimate for photosynthetic production: 35790 kWh/acre/year

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