The world “renewable” has quickly become a synonym of words like “clean”, “environmentally friendly” and “sustainable” in our language. Even the companies and governments have used it with commercial, informative and even propagandist goals. The truth is that “renewable” stuff is part of contemporary life since some years ago, especially when energy is involved.

Strictly speaking, there is no sense in the words “renewable energy” since it is not possible to renew something that cannot be exhausted or get old. Thus, what can be labeled as renewable is the source of that particular energy. However, the expression is accepted due to, I guess, brevity and because it describes well enough the idea behind it, which is that the source produces energy again and again.

The most renowned types of renewable sources are solar and wind energy, mostly because of their accelerated development these years, its simplicity and a bit of some added social glamor. However, we forget that hydro (potential energy from falls of water) is the most important renewable source worldwide and by several orders of magnitude in comparison with any other renewable source. Geothermal and biomass are some other examples of developed renewable sources of energy. But just exactly why are they renewable? What feature makes them renewable? Why oil and uranium are absent from this list?

Renewability has little to do “cleanness”. Products of biomass could be even more polluting than a well-refined oil and, in between, they release a higher amount of greenhouse gases. Renewability has little to do with being “environmentally friendly”. Big hydroelectric plants have a huge impact on the environment, completely changing the face of several acres of terrain, the river itself and more often than not, displacing entire communities out of their land. Furthermore, renewability has almost nothing to do with the source of energy itself. It is not only a matter of what the source is made of: radiation or some sticky black stuff from underground.

A source of energy is renewable when the rate of utilization is less than the rate of production. This definition includes the nature of source but also how fast it is being exploited. Oil could be renewable if we use it every million years, which is around its rate of production. I know, a stupid idea, but just because we are reaching seven thousand millions of people. If we were a million people, “renewable oil” would be a reality.

On the other hand, hydro is renewable. The time that takes to replenish the bodies of water is in the order of days. Thus, it is renewable because the energy usage rate is less than rain precipitation. However, this situation can change because climate is globally changing and in some regions, those days may become weeks and, just like that, you have a shortage and a non-renewable source of energy which is what happened in Panama some months ago.

It doesn’t hurt to take a look at the time scales at which sources produce energy. Solar radiation is produced faster than a second, wind comes and goes in hours, biomass may take a year and uranium is simply not renewed. But this alone, doesn’t tell us if they are renewable or not. Rate of consumption is the most important part in the picture.

For example, let’s go to our “most renewable” source: the sun. The rate at which we receive solar radiation on Earth is 174,000 TW, some ten thousand times what we require (17 TW). These 17 TW increases by 2% per year. A simple financial expression show us that, depending exclusively on solar energy, in just 466 years our consumption rate will match the 174,000 TW. We have some 500 years to keep naming renewable to the solar energy.

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