By Marcos Chiquetto
The Sun is the main source of energy for us here on our small planet Earth. Nowadays, ever greater efforts are being made to take advantage of solar energy by means of photovoltaic cells, which transform the energy from the Sun into electrical energy.
But what about the other types of energy we use? Where do they come from?
Well, leaving aside nuclear energy, volcanoes and hot springs as well as movements produced by the gravitational force exerted by other heavenly bodies, such as the tides, all the other forms of energy available to us come from the Sun.
— Hang on. When I fill up the tank of my automobile with fuel, I’m not using solar energy. Likewise, when I eat a hamburger, the energy doesn’t come from the Sun.
Or does it?
Energy from the Sun reaches the Earth in the form of electromagnetic waves. A large portion of these waves are detected by our retinas, and we call this light. Upon reaching the Earth, the light from the Sun is absorbed by plants, which store its energy in chemical bonds by means of photosynthesis. It is the energy from photosynthesis that enables plants to exist, to generate the tissues they are made of, to grow and to reproduce, in a process that has been taking place for some 500 million years.
So, when you burn wood or charcoal, you are transforming the solar energy that was stored in the plant into radiant energy and producing heat and light associated with combustion.
The emergence of plants hundreds of millions of years ago also led to the emergence of animals, from microscopic creatures to fishes, birds, reptiles and, lastly, mammals.
Animals can be herbivores, which feed on plants. In this case, the energy they use to live and reproduce comes from plants: it is solar energy stored in plants by photosynthesis. Carnivores, in contrast, ingest energy that was previously solar energy, was then stored in plants and, finally, accumulated in the tissues of other animals.
Ever since living creatures first appeared on Earth, these have continually died and decomposed. Some of the plants that decomposed millions of years ago turned into coal, which is one of the main sources of our energy. In addition, the decomposition of plants and animal remains at the bottom of lakes and seas at high pressures and temperatures produced oil in a process that also lasted hundreds of millions of years. So, the energy in coal and oil comes from dead plants and animals, which fed directly or indirectly on solar energy.
— And when I turn on the light in my house and the energy comes from a hydroelectric power station? Are you going to tell me that is also solar energy?
Yes. When it is absorbed by the water in seas, rivers and lakes, the Sun’s energy increases the energy level of molecules in the water, which evaporates. Because it is less dense than atmospheric air, water vapor rises, cools down and ends up condensing once again into droplets to form clouds. When the water in clouds is compacted into drops, rain is formed and the water in the rain returns in rivers to the sea. We then stop the natural water flow with dams and make the water go down through tubes where it moves turbines connected to generators, producing electrical energy. The energy produced in a hydroelectric power plant therefore used to be solar energy, which ended up being converted into electrical energy by us after it was absorbed by water molecules and transferred to the movement of rivers.
So, all the energy we have is solar energy?
No. Not all of it. Nuclear energy doesn’t come from the Sun. It comes from the formation of atoms of chemical elements in a process that has been taking place continuously since the beginning of the universe, which according to current theories occurred billions of years ago.
Nor does the energy in volcanoes and hot springs come from the Sun; instead, it comes from the thermal energy generated by the compression of the material that formed our planet. Another form of energy that doesn’t come from the Sun is the energy in tides, which are a result of the gravitational influence of the Moon and the Sun on our planet. (In this case, it is not the energy generated by the Sun but merely the gravitational force produced by the Sun’s mass.)
Finally, there is the wind energy. Winds, like tides, have a strong influence of gravitational forces, but they are essentially produced by the Sun, for winds are primarily the result of differences in temperature at the Earth’s surface.
To sum up:
- When you generate electrical energy with a photovoltaic cell, you are using solar energy that is reaching our planet at that very moment.
- When you eat food of plant origin, you are using solar energy that reached our planet a few weeks or months ago and was converted by photosynthesis and stored in the plants’ tissues.
- When you eat beef, you are using solar energy that reached our planet a few years ago, was converted by photosynthesis and stored in blades of grass, leaves of alfalfa or other fodder, being then accumulated in the tissues of the cow that ate the fodder.
- When you burn gasoline in your car, you are using solar energy that reached our planet hundreds of millions of years ago, was converted by photosynthesis, was stored in the tissues of plants and animals and ended up producing chemical bonds in the hydrocarbons that form the oil. A fraction of these hydrocarbons was distilled, transformed into gasoline and put in the tank of your automobile.
- When you spend energy from a hydroelectric power plant, you are using solar energy that reached the Earth some time ago and produced rain, which formed rivers, whose water powered the turbines in a power plant.
However, when your home is supplied with energy from a nuclear power plant, you are not using solar energy, but energy stored in the nuclei of atoms, which have been formed continuously for billions of years, in a process that started long before the appearance of the Earth.