Wednesday, 18 January 2017

Insolation (aka Sunshine)

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

If we are going to live within the energy budget that we could generate renewably, would we be able to support our energy needs from what we can harvest at our farm? As was mentioned in the introduction, the basic realization that I had was that for most, if not all, sustainable uses of our property, that sunlight was the energy source from which all our other activities would spring. In terms of sustainability, this is as close as it comes to getting 'free' energy - sunlight is going to be shining down on the earth's surface regardless of what we do and will continue to do so for a few billion years.

The amount of solar energy that arrives at any particular spot on the earth's surface is quite variable. This is largely due to the combination of the location of the sun in the sky as it moves throughout the day as well as the weather. For the orientation of the earth to the sun, the closer the sun is to straight up in the sky, the more energy reaches the surface. Annually, the maximum amount of solar energy is generally for those places where the sun is most directly straight up in the sky for the greatest part of the year. This is mediated by clouds, which are the biggest way that weather has an impact on solar energy. When it is cloudy, much of the sun's energy is intercepted, with only a relatively small amount reaching the surface in a form that can be captured for other uses.

The equator is where, not counting weather differences (cloudiness), the most light energy comes down each year. The Ottawa region, where our farm is, is halfway between the equator and the north pole, at 45 degrees latitude. There is an excellent sun visualization tool from the University of Nebraska, from which I've made a couple of schematics below.

Summer solstice (June 21) sun path in Ottawa
Here in Ottawa, the sun never is straight up in the sky. The yellow line with the sun on it shows where in the sky the sun passes across the sky through the day. As can be seen in this first image, the summer sun in Ottawa reaches fairly close to straight up in the sky at noon on the summer solstice, 68 degrees up off of the horizon (90 degrees would be straight up). And in the winter, the sun never gets very high in the sky, as you can see in the image below. On the shortest day, December 21, the sun reaches only 22 degrees above the horizon.

Winter Solstice (Dec. 21) sun path in Ottawa

When one actually compares total annual solar energy to other areas around the world, Ottawa doesn't fare as bad as one would think. Ottawa receives about 2/3 as much solar energy as Cairo, Egypt, which receives as much solar energy as any large city on earth. How does this happen, you ask? Ottawa, and any other locations far from the equator, have something very interesting going on. They have very long days at the same time as the sun shines from highest in the sky; we call this time 'summer'. This also means that Ottawa gets most of its solar energy in those few summer months.

Because of the variability in the energy available from sunlight, we will describe the amount of sunlight over the course of a complete year. In Ottawa, the total insolation (solar energy) over the course of a year is: 1263.4 kWh/m2/ year. This is energy per square meter per year. A square meter is a bit smaller than a typical door inside a home. Though the annual total is important, notice in the chart below that the amount of solar energy at our location varies tremendously, with July getting over 4 times the energy from sunshine as January. As discussed above, this is due to sun angle, day length, and prevailing weather.

(chart data courtesy of

To discuss land use, one seldom speaks in units land as small as one square meter. A more typical unit that is discussed in land use, property ownership and agriculture is the acre (especially in the US). One acre is roughly the size of a football field, minus the endzones. This is a decent sized chunk of space, but very manageable when discussing forests and wheat fields. Here is a picture of one of our fields with lines added to show exactly how big one acre is.

Converting out from our square meter, this single acre, as well as every other acre of property in the area, receives:

1263.4 kWh/ m2 / year * 4046.86 m2 / acre = 5,112,000 kWh/acre/year

Over five million kilowatt hours per acre per year, that is an enormous amount of energy! This is especially true considering that we calculated that my family uses 80,000 kWh/year in our personal lives, and that the total annual energy use per capita in the US is only about 90,000 kWh/year. By this math, it looks like one acre should be able to support the energy needs of over 50 people on each and every acre. It almost seems like there is no problem to be solved, that there is so much energy that we can be as inefficient as we want. If only it were so simple. The problem is, as was discussed here, that we have to have some way to capture, convert, and make that energy useful for fulfilling our needs, and we have to account for all the inefficiencies of converting energy from one form into the next.

What do we have at our disposal that can capture that solar energy so that we can put it to good use? Though there are some technological options that we will discuss later on, we will start with the solution that life on earth figured out billions of years ago, photosynthesis.

Estimate for solar energy reaching the ground: 5,112,000 kWh/acre/year

Previous page: So how much energy does a person really use?
Next page: Harnessing the sun's energy - Photosynthesis

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