The astrophysics of solar panels: how do self-consumption and surplus energy work? | Cosmic Void | The USA Print

In the month of January, the solar panels that I installed four months ago at home produced more energy than I consumed (I repeat, in January!). After compensating what I have taken from the network, the electricity company has even kept, for free!, part of the energy that I have produced.

According to Plato, “knowledge without justice should be called cunning instead of wisdom”, so here are some astrophysical considerations (I leave materials engineering or environmental and sustainability issues to experts), to define knowledge about what it is or it would be fair, about what cunning implies and what wisdom really would be in this matter of solar panels.

I start strong: nuclear energy (and gravity) is the basis of life on Earth. Everything we know on the face of our planet gets its vital energy, its power source if we make a play on words between engineering and biology, directly or through intermediaries, from the great nuclear fusion power plant that is the Sun. The Sun reaches Earth between 1,361 and 1,362 watts per square meter (physicists are lazy and write W/m²), which implies that the Sun is tremendously stable (pardon me for the series Blackout, which does not tell a big lie either, the thing can happen). It is also inferred that if we had a one square meter solar panel (the ones that are usually 2-3 square meters) we could simultaneously provide power to 136 10 W LED bulbs (a lot of light for a house!), or 5 refrigerators powerful, or 9 televisions, or almost 2 washing machines, or half an induction cooker, or a more normal combination of all those devices. The Sun gives a lot of energy!

The thing is, all of the above calculation is wrong for various reasons. We leave aside the efficiency of solar panels (that is, how much energy that reaches them from the Sun they are capable of converting into electricity and how much is “lost”) and we get into more astrophysical concepts. We have already given one: those 1,361 W/m² is what is known as the solar constant (we should add “terrestrial” to it, Uranus has a solar constant 1,000 times smaller). The 1,361 W/m², which is a power per unit area, what is known as a flow (if it is already difficult to understand the difference between energy and power, here is another physical concept), is what is received from the Sun outside of our atmosphere. But we are under the atmosphere, and it is not completely transparent.

Nuclear power (and gravity) is the basis of life on Earth. Everything we know on the face of our planet is fed by the great nuclear fusion power plant that is the Sun.

In fact, the atmosphere is opaque, thank goodness, for the most energetic rays of the Sun, the gamma or ultraviolet rays. It’s not completely transparent to blue rays, which explains why the sky is blue (we’ve never talked about that in this section, but there are so many articles on the subject!). Our atmosphere is quite transparent, but not completely for the light to which we are more accustomed and which gives us the colors of everyday life, all those of the rainbow. Then it becomes opaque again in the mid-infrared, where the rays that reach us from the cosmos serve to heat and excite water molecules and nothing reaches the Earth’s surface (which explains why we sent the James Webb telescope into space).

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Solar panels in Lavapiés, Madrid.
Solar panels in Lavapiés, Madrid.

Finally, it becomes transparent again in radio waves. The atmosphere is very much his own and he lets through what he wants. Something that has sculpted our existence is not trivial. So the solar panels that we can put on our homes or work buildings receive much less energy than the solar constant implies. If the Sun were high in the sky, at the zenith it is said, the atmosphere would typically eat up 20-60% of the solar constant. The exact value varies in minutes or hours, it depends on whether we are talking about blue or red photons, and what is in the atmosphere at that moment. If there is dust, the blue photons can disappear almost completely, if there are clouds a lot fewer photons arrive (but not 0! Because there is sunlight on cloudy days it is a good topic for another article).

The atmosphere has sculpted our existence, it is not trivial: opaque to the most energetic rays such as gamma or ultraviolet rays. It is not transparent to blue rays, which explains the sky

But the Sun is not normally at the zenith, that only happens once a day in intertropical latitudes, not in Spain. In peninsular and Balearic Spain, forgive me my Canarian friends, the highest point the Sun reaches, right on the summer solstice, is at a height of almost 75 degrees, the height of the stars is measured in angles from the horizon, with the zenith at 90 degrees. At the winter solstice, however, the Sun only rises to a height of about 25 degrees.

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And why is the height of the Sun important? Imagine yourself in a swimming pool. They want to get to the bottom. If they dive vertically they will arrive earlier, they cover less distance. But if they dive obliquely they will have to go further to reach the bottom and it is easier to run out of air. Well, the same thing happens to photons from the Sun. If the Sun is at the zenith, it is said that they have to “cross an atmosphere” to reach the surface. If the Sun is at a height of 30º, the photons pass through 2 whole atmospheres, and if it is at 45º, like 1.4 atmospheres. Total, the lower the Sun, the less photons reach the surface, and also the problem is not linear, the number of photons that the atmosphere eats grows exponentially as the Sun goes down in height. This effect is exactly what we see in the insolation curves of solar panels, for those of you who already have them or have seen those bell-style graphs of energy produced.

Electricity production and consumption at user level.
Electricity production and consumption at user level.Pablo G. Perez Gonzalez

As the Sun gets higher and higher as we get closer to the summer solstice, the maximum energy production will be higher and higher. December 21 is when the Sun reaches the lowest maximum height of the year (in the northern hemisphere), 25º in Madrid. Not that day, it was cloudy in Madrid, but on the 26th, my panels produced 3 kW at peak, at noon; astronomical noon, defined according to that maximum height of the Sun, neither clock noon nor the heretic noon that many use to call lunchtime. It is the lowest production, photons from the Sun must pass through the equivalent of 2.4 atmospheres. On February 17 we are going for 4 kW at the peak, the Sun reached 37º of altitude, equivalent to 1.7 atmospheres. By the time we get to the summer solstice, the photons will only travel 1.04 atmospheres and we should reach 7 kW at the peak. But that, if the atmospheric conditions are the same, and they won’t be, there will perhaps be more dust from the Sahara, more ozone, whose levels rise in summer, less pollution particles from heating…

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Once the knowledge has been treated, where does the justice and cunning that we spoke about at the beginning remain? Justice: why the compensation of surpluses, that is, the energy produced that is not used and injected into the general network, is paid for months? Winter is not the same as summer, it should be apportioned in a year, the astrophysics of the problem works like this. Wasn’t there any astrophysicist reviewing the law that governs self-consumption?; or someone with common sense, the issue of when it is more sunny is not so unknown either. And cunning: the answer to the previous question involves her.

I am afraid that there are many interests in the subject, from a new way of producing energy that is ending models that have lasted decades (remember the “sun tax”), to this new business opportunity with which many are making a lot of money. And they will want to earn more. There are already distinguished minds that remind me of the millionaire of Contact (Robert Zemeckis, 1997) or that of the Weyland-Yutani corporation, speaking of carpet the Sahara of solar panels.

Corollary: I prefer a wise way of producing energy than applying our knowledge unfairly and cunningly, we will have to be vigilant. So far this edition of cosmic voidpublic service version with useful information for day to day.

Cosmic Void is a section in which our knowledge about the universe is presented in a qualitative and quantitative way. It is intended to explain the importance of understanding the cosmos not only from a scientific point of view but also from a philosophical, social and economic point of view. The name “cosmic vacuum” refers to the fact that the universe is and is, for the most part, empty, with less than one atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillions of atoms per meter cubic, which invites us to reflect on our existence and the presence of life in the universe. The section is made up of Pablo G. Perez Gonzalezresearcher at the Center for Astrobiology; Eva Villaver, researcher at the Center for Astrobiology; and Patricia Sanchez Blazquezfull professor at the Complutense University of Madrid (UCM).

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