Energy from the land - Photovoltaic solar panels

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

While all of the posts that I've written so far have focused on the energy that we can harvest through the plants and animals that we grow on the land, these are not the only way to make the sun's energy useful to us. There have long been ways of harvesting some of the sun's energy as heat, and it has become now become feasible, and even economical, to convert the sun's energy directly into electricity, and humanity uses a lot of electricity to make our technologically intense world go round. Photovoltaic (PV) panels are not the only way of generating electricity from the sun, but have become a very practical way to provide power at both a small and large scale. All of the most complicated work of assembly is done in a factory, and once wired into place, the panels need little to no maintenance for their lifetime of several decades. 

 Solar panel arrays at the author's home

I won't bother going into the details of the history of photovoltaics, or their chemistry for that matter, but I do think that it is important to compare and contrast PV with photosynthesis at a higher level. Plants evolved photosynthesis over a very long timescale, figuring out through trial and error how to capture some small fraction of the energy pouring down in sunlight and passing it along through a quite long series of chemical reactions until it reaches a form that can be used to grow and maintain a plant. As was discussed here, this process has an efficiency of about 2%, and that is only when conditions are just right. When it comes to photovoltaics, scientists and engineers were inspired by photosynthesis, but free to explore the possibilities afforded by any materials available, not just those organic molecules that make up plants. Metals, glass, inorganic compounds of all kind were fair game as they tried to figure out how to harness sunlight. It has also turned out to be the case that it is easier to generate electric current than it is to build up sugars, fats, or other chemical energy storage. Put together this means that the PV panels widely available today can turn about 15% of the sun's energy into electricity, and can work on any day of the year; they don't take the winter off the way that our local plants do. These panels can create a steady stream of electrical energy any time they are exposed to sunny skies, and even cloudy skies to a lesser extent.

Estimate #1. From first principles.

We only really need one estimate here, as the numbers are really quite straightforward. First is the question of how efficiently PV panels can convert solar energy into electricity. At the moment, the typical commercially available panels are roughly 15% efficient, though more expensive ones approach 20%. Some laboratories are pushing to 30% or beyond with new architectures and chemistries. For the sake of argument, we shall stay with that 15% number.

 Les Mées Solar Farm, Photo by Jean-Paul Pelissier/Reuters

The second aspect to consider is how much of the ground is actually being covered with the panels. Native ecosystems often have leaves spread over every inch of ground, whether it be a forest canopy or a field of waving grasses. While one could simply spread out solar panels flat on the ground covering every inch, this isn't an efficient use of resources. Instead panels are tilted so that they are as close as possible to perpendicular with rays of sunlight streaming down. And because one doesn't want the panels to shade each other out, it is necessary to space them out on the land. In larger installations, this spacing also makes for easy access between the rows of panels for doing any needed maintenance. Solar farms often actually cover only about 25% of the surface area where they are found. With these two figures we can do the same calculations for annual harvest that we have done for other land uses:

5,112,641 kWh/acre/year of sunlight * 15% efficiency * 25% packing factor = 191724 kWh/acre/year

For those of you keeping score, this is tremendously more energy than anything that can be harvested from plants. This is 10-15 time the energy that one could get from our most productive plant of corn, and 50 times the energy that can be harvested from cutting timber. The two arrays seen at the top of the page at my house are capable of producing about 10,000 kWh/year, roughly the same as what 3 acres of forest can do. Electricity can't be easily turned into food or furniture, but for anything that electricity can do, this makes photovoltaics a very easy winner.

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Food from the land - Annual crops

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

Going along with the main theme of this series, the following post gives some quick estimates about the energy yields that different crops can produce, starting with a more in-depth discussion of corn.

Estimate #1 for Corn - From first principles

Cereal crops are plants that are grown for their starchy seeds, including corn, wheat, barley, oats, and others. When it comes right down to it, these are some of the most important plants for feeding the world. Corn makes a good example, and is one of the most productive crops per acre, period. The following estimates are for field corn, which is quite different than the sweet corn that you have eaten at dinner. It is much richer in oils, yields much more energy per acre, and is primarily used for animal feed, ethanol fuel production, as well as thousands of other uses in processed foods and chemical products.

In looking at photosynthesis, we found that our farm has about 36000 kWh/acre/year of basic photosynthetic energy production. Out of that amount, a plant needs to grow, metabolize, fight off predators, as well as to create that portion of the plant that is useful to us. In this case, what we actually want is the kernels of corn on each ear. Research on this subject shows that approximately 50% of a mature corn plant's energy is found in the kernels on the ears of corn, while the other 50% is in the stalk, leaves, and root system. This is actually a tremendous proportion of the energy of a corn plant that is found in the kernels. It is pretty incredible that these plants are able to funnel fully half of their energy into their seeds and that such a surprisingly small proportion is needed to grow the rest of the plant.

The other thing to account for is what proportion of the energy that a plant captures is put towards growth, and what proportion to maintain the health of the plant as it lives day to day, known as respiration. One source estimates respiration on a global scale at 20%, so as I didn't quickly find an actual figure for corn, we shall use that number. With this calculation, we get:

35788 kWh/acre/year * .5 (proportion of stored energy in seeds) * .8 (losses for respiration) = 14,315 kWh/acre/year of harvested corn kernels.

Estimate #2 for Corn - Real world yields
As I was not able to easily find Quebec data, I will instead use Ontario estimates of corn production to make an estimate. These recent data state that corn yields are typically around 150 to 170 bushels per acre per year of field corn (a bushel of corn is 56 pounds). As our farmland is of a much lower quality than the average farmed acre in Ontario, it could produce perhaps only about 2/3 of the average production. This means that one of our acres could produce:

150 bushels/acre/year * 2/3 (reduction for poor quality land) * 56 pounds/bushel * 1550 Calories/pound * (1 kWh/860 Calories) = 10093 kWh/acre/year


Other crops
In my last post, I showed a graph that included Calorie yields for many staple crops, and those are easy to convert to our usual unit of kWh. I also found plenty of sources (e.g., here and here) that listed the productivity of crops of all kinds, which often end up being much lower total energy because of how few calories many vegetables have (having high water content, high fiber, low fat). I've put a few of those estimates in the following table.




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Food from the land - Growing domesticated crops

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

Domesticated crop plants are quite peculiar. As was discussed in my hunting and gathering post, wild plants don't tend to produce very much human food. The selection pressures that are in place on wild plants are for their own survival and reproduction, and while they often have edible seeds, fruits or roots, how good a food they are for people was a non-factor in their survival. The adoption of agriculture changed plant selection drastically as it became people doing the hard work of ensuring the survival and reproduction of their crops, while selection pressures were refocused on making bigger, better, and more nutritious edible parts that are easier to harvest. And when you look at today's crop plants, they look downright bizarre compared to their wild counterparts. All of the parts that we like to eat and use are comically large when compared to those of their wild brethren. A typical corncob is close to a foot long and weighs over a pound, whereas for corn's wild ancestor teosinte, you could hardly call the tiny seed pods a cob (see picture below). Modern corn is amazingly good at providing food for people, but would not fare well for long without people to plant and tend to it.  And of course this sort of breeding change for size is only one of a multitude of ways in which people have changed both plants and their growing environment.

Image courtesy of http://evolution.berkeley.edu/evolibrary/news/070201_corn

While plant harvest often focuses on seeds and fruits, it can also be based on many other parts of a plant. Cabbage provides a wonderful example of how a single wild plant can be bred for many different foods, and wild cabbage is the progenitor of a dozen different vegetables today. Broccoli and cauliflower are flower clusters, kohlrabi is a part of a stem, while cabbage, brussel sprouts, kale and others are all modified leaves.  Really any part of a plant that grows in such a way as to have edible sugars, fats and proteins is viable as human food. And then there are the crops for non-food purposes like fiber or oil.

 Farming and yields

Whether organic or not, mechanized or not, genetically modified or heritage breed, the goal of farming is generally to have the highest possible yield per acre. This generally means creating a relatively simple ecosystem that provides the crop plants as close as possible to 'perfect' growing conditions. Important considerations include:
-Maintaining good nutrient levels, often with fertilization of some sort.
-Maintaining proper amounts of moisture, sometimes with irrigation.
-Reducing competition between desired plants and other plant species. While there are many ways to achieve this, the most common are some form of weeding or herbicides.
-Reducing predation on the crop plants from insects, birds and mammals.
-Reducing the detrimental effects of microorganisms, be they bacterial, viral, or fungal.

The vast majority of farming today in the western world uses a very technology heavy approach, with large tractors and implements, and heavy loads of fertilizers and pesticides. Traditional small-scale farming, and such modern reinventions of it as Permaculture, have a very difficult time competing economically with these conventional broadscale farming practices. These traditional techniques generally require large amounts of human labor, and don't benefit from the economies of scale that can be gained when farming 500 acres instead of just a few. And these modern farming techniques are only increasing their yields. See below for a graph of the yield trends for a number of major staple crops.

Graph courtesy of Math Encounters Blog

Our farm and its crops

Our own farm and those around it were first developed in the last decades of the nineteenth century by Irish immigrant farmers. The Moran family founded our farm, and the neighbors had names such as Flynn, Egan, and Brennan. They arrived with, or soon after, the wave of loggers coming up the Gatineau River. In those early days the first step was to open up the forest to create fields, which required cutting down any trees remaining after the loggers passed through, followed by digging out all of the stumps in order to make it possible to till the soil. They were probably only able to open one or two acres per year, and on our property they converted a total of 18 acres of some of the less hilly terrain on our property over to fields.

The early days of our farm mixed subsistence and market farming, growing a little bit of everything, plant and animal, to provide for the needs of the family. Any excess could then be sold on to the logging camps or down to the Ottawa area. At this time, the farmers grew a wide variety of crops, from garden vegetables to row crops like wheat. Since they were growing most or all of their own food, it was absolutely necessary to maintain variety so as to have a relatively balanced diet throughout the year.

An abandoned wheat thresher on the author's property

As with small family farms all over North America, this model began to make less and less sense as the twentieth century progressed. With mechanization and additives like pesticides and fertilizers, small-scale farms just couldn't compete. This was especially true in an area like ours, with hilly and relatively infertile soil that didn't have as high of yields and was much less conducive to industrialized farming techniques. The farms in our local area slowly consolidated so that many fewer farmers each farmed much more land, and shifted to one of the only models that remained economically viable, beef cattle farming. So while our farm isn't likely to go back to annual crops anytime soon, no discussion of land use would be complete without them.


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Food from the land - Raising beef cattle

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

Every time I pass in or out of our property I have to open and close two cattle gates as our small one lane road passes through a neighbor's pasture. If our little road saw any more traffic than it does, the road would need to be fenced out of the grassy areas beside them, but for now we often have to slow down and honk to get the cattle to shuffle off to the side of the road. You really get to know the cows when you have to shoo them out of the way on a regular basis.

In the area immediately surrounding our property most of the agriculture consists of raising beef cattle, as well as some draft horses. The soil is too rocky and the hills too steep to allow our area to be economically competitive in growing row crops, but the sloping fields grow grass just fine. The fields on our own farm have been used primarily to support cattle for about the last 50 years. Before that there was a wider variety of agriculture, but these others were mostly abandoned as cattle became the mainstay of the local farms.

For these operations, the farmers are in the grass business every bit as much as the cattle business. Grasses only grow from the spring through the fall, but since cows need to eat during the winter also, farmers must harvest enough grasses to provide for the snowy months. During the summer, technically May to November, cattle are brought to the grasses, to feed on pasture. This reduces the work of the farmer tremendously, as the cows harvest their own food. The farmer does have to fence off the paddocks, ensure water supplies, and move cattle between the fields, but this is less intensive than preparing for the winter months. To provide for the winter food needs, the farmer needs to maintain other grass fields for hay, cutting and baling the growth and setting it aside to be doled out as needed to keep the animals well-fed. The same fields can be used for both haying and pasture, but can only primarily provide for one of these in a given year.

It doesn't seem like that much would be required to grow grass, just to cut down any trees, and then let the cattle come through to eat as they would. But in actuality good pasture is a crop like any other, just that it is a perennial crop that only needs to be replanted every 20 or 30 years, rather than each spring. The usual way of establishing pastures is quite similar to planting row crops. One plows up a field to prepare the soil and kill off competing plants, and then the seeds of a variety of grasses and forbs are planted, with names like alfalfa, orchard grass, Timothy, fescue, and clover. These fields often are helped by the addition of trace nutrients as well as fertilizers. Once the fields grow in, they can be maintained for many years. The degradation of pastures and hayfields can be from nutrient depletion or changes in the species composition of the grasses present. When cows are allowed a lot of space, they work through and eat only the choicest morsels, leaving all of the less desirable plants standing. Over time, these undesirable plants can come to dominate the entire fields, to such an extent that the fields must be plowed and replanted.

Once the growing of the grasses is accounted for, one has to look at the business of actually raising beef cattle. Every year there is a seasonal ebb and flow that takes advantage of the natural cycles of the region. During the winter, the herds are almost completely made up of pregnant females, with just a few bulls that are only there to sire the next generation. In the spring all of the cute little calves are born, drinking only milk for their first weeks of life, transitioning over the summer months to the adult diet of grasses. As soon as the fields are showing good signs of growth, the herd is released out to pasture for the summer. The calves put on an amazing amount of growth throughout their first year of life. In the late fall, at around the same time as the pastures go dormant for the winter, the vast majority of the calves are sold off, most often to a feedlot where the calves will continue to put on weight for up to another year before becoming someone's dinner. The calves that are kept by the local farmers are generally the best females, which become the next generation of mothers. These cows, known as heifers, can be bred when they are as young as 15 months.

The cycle actually begins again during the early summer, as cows have a gestation period of about 280 days. This means that in order for a cow to have one calf each spring, it needs to be bred the prior summer, when the prior calf is only 2-3 months old. This also means that all cows should be pregnant in the fall when the calves are sold off, and very often those cows that didn't conceive are sold at the same time. It turns out that roughly 15% of cows don't get pregnant in a given year, which can be due to age, illness, or just random chance in whether the bull did his job. Cows may continue to breed for 10 years or more before age catches up with them.

Putting all of this together, one needs to grow enough pasture and cut enough hay to maintain a mother cow for the entire year to produce an 8 month old calf for sale. Those calves sold and destined to become beef will be fed mostly grain, including a lot of corn, for the rest of their lives. We won't include this feedlot part of cattle production in our calculations here, though I'll try to return to it at a later time in another post. So how much cow does an acre support?


Estimate #1. First principles
As discussed in 'Energy capture, conversion, and storage, a good first rule of thumb is to assume that each time a new level of an ecosystem consumes energy, that only 10% of that energy goes into the next level. We already made an estimate of the total amount of energy captured by photosynthesis, which in this case would be by the grasses. This energy is then used for metabolism, growth, reproduction, etc., of the plants, and only roughly 10% would of that energy would be available to the cows in the form of leaves and stems. Then the cows of course have their own metabolism and growth to deal with, meaning that only about 10% of the energy that the cows consume will end up in the form of cow flesh, which is what the farmer is most interested in.

35788 kWh/acre/year of photosynthesis * .1 for leaves and stems eaten by cows * .1 for efficiency of cows in turning food into weight gain = 358 kWh/acre/year of cow produced


Estimate #2 Going on available data for actual production
We can also look at typical agricultural yields, and see how much food a pasture typically produces, as well as the data on how efficient cows actually are in their growth and reproduction. I wasn't able to easily track down data for western Quebec, but did find what should be roughly comparable data, from the Manitoba Forage Council. This data shows that pasture produces from 2000 to 4000 pounds of forage per year, depending on plant species, fertilization, and water availability. Lets call it 3000 pounds of dry matter, as it is called, for the sake of calculation. Hay contains roughly 800 Calories per pound, so...

3000 lbs/acre/year * 800 Calories/pound * 1 kWh/860 Calories = 2790 kWh/acre/year of grasses

Further, a cow (pregnant and/or milk producing) requires roughly 30 pounds of food a day, or 10950 pounds through a year. In that same year, the calf will grow from an embryo up to roughly 500 pounds by the time of sale in late fall. The calf primarily drinks milk for the first couple of months, transitioning to the adult diet of grazing over the summer. All told, that calf will consume perhaps 1500 pounds of forage on top of the mother's intake over the summer and fall. Put together, it then takes...

10950 lbs forage (for cow) + 1500 lbs forage (for calf) * 800 Calories/lb * 1 kWh/860 Calories = 11580 kWh to maintain a cow for a year and to produce a 500 pound calf.

Finally, how much energy is harvested out of this system in a year? It is of course all of the calf, but it also ends up being the mother cow, around 15% of the time. As mentioned above, the cows generally aren't kept another year if they do not get pregnant over the summer. These cows average about 1200 pounds. When butchered, about 50% of a cow is meat, distributed over lean and fatty cuts. Some rough estimates suggest that this meat averages around 800 Calories per pound. It was difficult to find data on the embodied energy in the rest of the cow, including entrails, bones, skin, etc., so I will assume that these other parts have the same energy density as the meat. Put together, this means that...

(500 lbs (calf) + 1200 lbs (cow)*.15 (harvest rate of female cows)) * 800 Calories/pound * 1 kWh/860 Calories = 633 kWh of energy per year from raising a cow/calf pair.

The last step is to level out this amount of energy from a cow/calf pair back to a single acre:

2790 kWh/acre/year * 1 cow calf pair/ 11950 kWh * 633 kWh/ 1 cow calf pair = 148 kWh of energy in the form of cow harvested from one acre in a year.


Estimate #3 Actual production from our farm
Finally, we can make an estimate of the productivity of our farm from the actual production that we have observed over the last few years. We have 18 acres of pasture, and have used these fields both as pasture and hayfields over the last five years. In the first couple of years after the purchase of our property, we had one of the farmer neighbors put cow/calf pairs out to pasture on our farm. Since then, another local farmer has cut hay off of the same fields.

When we had cattle on our property, this was a herd of 20 cow/calf pairs which were rotated between our property and another nearby, such that the herd was on our property half of the time. This effectively makes 10 cow/calf pairs for the six month growing season. The calculations from estimate #2 can be adapted, as we know that each cow/calf pair needs 11580 kWh per year:

11580 kWh of forage /cow calf pair * 20 pairs/18 acres * 1/2 of the year * 1/2 of the time = 3217 kWh/acre of forage produced in a year.

In the years since we switched to haying, there has been quite a bit of variability in the weather, including both one very wet as well as one very dry summer. The summer of 2016 was a more average year, and in this year the property produced 50 large round bales of hay from a cut in mid-summer. In many climates farmers can get multiple cuttings of hay from a single field in a year, but with the relatively poor soils and shorter growing season, most of the local fields are cut only once. This does mean that the late summer and early fall growth aren't available for cattle unless the cattle are allowed to graze through later in the season.  Below is the calculation for the amount of energy found in those bales, with weight and Calorie estimates for the hay drawn from here and here.

50 bales/18 acres * 1000 lbs/bale * 800 Calories/pound * 1 kWh/860 Calories = 2580 kWh/acre/year of grasses

These two estimates, that our fields produce between 3217 and 2580 kWh, are very much in line with Estimate # 2 so going forward we will go with that the final figure estimated there, of 148 kWh/acre/year of cow being harvested per year from grassy fields.


Visualizing this growth
As discussed above, before getting to cattle one has to have grasses. Below is a picture of a big round haybale, weighing around 1000 pounds. Each acre of our fields can produce about 3 of these per year. Averaged out over the entire year, each acre is growing 7 pounds of grass per day, a big handful.

Round haybale with author and son for scale

In the picture below, the calf is approaching that 500 pound size, typical for when they are sold off in the fall. The mother is still over twice that weight, around 1200 pounds, and stands around 5' tall. To support that mother and calf for the year, it requires about 4 acres of hayfield and pasture.

Growing other animal species, or for other products
The above discussion was all about cow and calf cattle farming, the mostly small scale operations feeding their animals on pastured grass. I didn't address the 'finishing' process for beef cattle, where the calves live in a more constrained environment eating more grains as they put on additional weight and size. Nor did I discuss growing other animals for food, or such products as the milk or eggs that can be obtained from those animals. In terms of the amount of energy that can be converted from sunlight to the end agricultural product, growing beef cattle is one of the least efficient. The adult cows must be maintained for many years, and they usually have only a single calf per year. Further, cows are somewhat less efficient than some other types of animals at converting feed into weight gain. And just look at other examples like chickens or pigs. Each of these produces many more young in any given year, as well as those animals reaching market size much more quickly. The takeaway is that these other types of animal husbandry can produce significantly higher yields per acre. The upside of beef cattle is that they take much less effort on the part of the farmer, they can be raised on land of marginal quality, and there is high demand and therefore a good price for beef. Needless to say, I may try to make a more quantitative comparison in a future post.


Estimate for total cow production: 148 kWh/acre/year

Previous Page: Fossil Fuel Footnote
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Fossil Fuel Footnote

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

The process of biomass growth and harvest that I've described in other posts for other land uses is of course very similar to how fossil fuels were formed, but occurring over thousands to millions of years. Today's oil, natural gas, and coal were originally plant matter that did not immediately decay and became buried. All this organic matter was then subjected to time, heat, and pressure in the earth's crust, and slowly transformed in these conditions to become the fuels that we use today. Of course, only a tiny portion of the energy that was in the original plants actually is still accessible in fossil fuel deposits today, but this energy is exceptionally concentrated and has powered the world for over a century.

I found one calculation of the amount of plant matter needed to create fossil fuels, here. These researchers found that it took 200,000 pounds of original plant matter to create 1 gallon of today's gasoline. If we were to tuck in some of the numbers from my discussion of firewood, we get the following:

(89000 kg biomass needed per gallon of gasoline) * (2.2 pounds/kilo) * (1 cord of maple firewood/4600 lbs) * 7034 kWh/cord = 299,000 kWh of wood to make 1 gallon gasoline (37 kWh).

This energy conversion, from ancient plants to today's oil, preserves only 1 part per 10,000 of the energy found in the original plant growth. To put this into the terms of other posts in this series, this means that a year's growth for an acre of primeval forest led to the formation of about 1/100 of a gallon of gasoline, .4 kWh/acre/year. In a typical car, this would take you less than half a mile down the road.


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