Solar Energy Harvesting

What about solar energy?

What about it?

Life on Earth depends on extracting energy from the solar radiation that reaches the planet, or eating stuff that does it. Professor Sun is the main source of energy required for us to live. Professor Sun generates energy by transforming Hydrogen into Helium by means of nuclear fusion, a process that excretes energy in the form of light that in turn reaches our planet after 8 minutes.

How much of this energy is being used by life on the planet? The question is a complex one, but I decided to do some back of the envelope calculations to the order of magnitude of the light that could be used in principle, is used in practice, and would be needed for Human consumption.

Energy of the Light that Reaches the Atmosphere (1LRA) per year: 10^25 Jules/year ~ 1LRA/y.

This is the total energy that gets to the planet every year, most of it useless to life.

Energy of the light that could be absorbed by photosynthetic organisms: 10^24 J/y ~ 1LRA/month.

This is the total energy that could in principle be used by living organisms. This calculation accounts for light of frequencies that cannot be used by photosynthetic complexes, and also for light falling on areas on the planet where it couldn’t be used anyway. For example, a lot of the energy that hits the surface of the oceans is reflected. The one transmitted gets scattered and some of it is lost.

In order to get a sense of the scale of these processes, we should compare it to the energy consumption of the human race.

Energy consumption of humans: 10^21 J/y ~ 1LRA/minute.

We humans consume about 1minute of the total solar energy that reaches the planet in a year. This includes not only energy spent on machines, but also the actual food we eat. Remember, those plants that we eat, or that our cows eat, harvest energy from the sun. Plants store their unused energy in the form of carbohydrates.
The energy (think: calories) of all the living things that can harvest energy from the sun gives us of an upper bound of much energy could be extracted from processing them.

Energy stored in the total photosynthetic biomass: 10^21J

This is about the total energy of all photosynthetic organisms. If you kill each bacteria, algae, plant in the world and magically extracted all of its energy in a perfectly efficient manner, it would barely provide the human race with energy for one year.  And of course, there would no more plants to replant!
Proposals that suggest extracting energy from the stored carbohydrates, such as ethanol-from-corn, are even worse; only a small fraction of the total biomass of the planet is carbohydrates. In other words, we cannot plant enough to ever extract enough energy to fulfill our current energy needs.

Comparing this number to the amount of fossil fuel highlights how small it is. Of course, how much fossil fuel there is in the planet is not known, much less how much of it can we actually reach.  Do not take these numbers too seriously, but think about them to have an idea of the order of magnitude of how much energy there is in photosynthetic organisms represent.

Guesstimated Fossil Fuel Reserves: 10^22J
Guesstimated Total Fossil Fuel in Earth, including the unreachable fuel: 10^23J

The fossil fuel reserves are 10 times more than the total current photosynthetic biomass! This is very suggestive: any source of energy that uses biological systems to directly extract energy from the sun, such as harvesting algae, will not be a major factor in any long term energy solution for the human race.
However, look at that very first number. There is a lot of energy falling into the planet. Harvesting this energy directly, by means of photovoltaic solar panels, is a reasonable strategy.

Author: minustwofish

I am a quantum physicist.

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