Friday, 31 March 2017

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.

Previous Page: Food from the land - Annual crops
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