Solar power is exactly what it sounds like: the derivation of electric power from the energy radiated by the Sun upon the Earth’s surface. The phenomenon responsible for the conversion of light energy to electric energy is the photovoltaic effect, the identical twin of the photoelectric effect.
The Photoelectric Effect
Despite rewriting the laws of gravity and thoroughly revolutionizing our understanding of mass, energy, space and time, it was the discovery of the photoelectric effect for which Einstein was awarded a Nobel Prize. However, while the discovery seems trifling on a macroscopic level, it was profound in a microscopic sense. It spawned what we now call modern physics, the central tenet of which is the dual nature of light and matter. The photoelectric effect is the proof of light’s dual nature.
In 1887, a curious Heinrich Hertz observed that a metal ejects electrons — signified by sparks — when it is illuminated by light. The phenomenon, which was rightly called the photoelectric effect, baffled 19th-century physicists trained in classical physics, according to which light behaves only as a wave, which was unable to convincingly explain why a tide of light should expel electrons from a metal.
However, in 1905, Einstein challenged a century of work and millennia of common sense to propose that light behaved both as a wave and a particle. According to Einstein, it is these particles that are responsible for the photoelectric effect. The particles, he postulated, are emitted as discrete packets of energy, which he called photons. When a metal is pelted with photons, the photons collide and push the electrons away, like a cue ball pushes away an 8-ball. Experimental results matched theoretical predictions, thereby proving his theory to be indubitably true.
However, a photon doesn’t necessarily eject an electron. When struck by photons of lower energy, the electrons aren’t ejected; they accelerate, but are still contained within the metal. This is called the photovoltaic effect, as it renders the metal conductive. While technically it is the photoelectric effect on which solar power is based, it is this minuscule difference for which we must make the distinction and attribute a photodevice’s ability to the photovoltaic effect.
The Photovoltaic Cell
A photovoltaic cell is simply two semiconductors glued to one another. A semiconductor, as the name suggests, is a material that conducts, but only partly. The conductivity of a semiconductor like silicon or germanium lies between the conductivity of a metal, like copper, and an insulator, like rubber. With conductivity neither immense nor negligible, semiconductors provide us the ability to control the current.
A semiconductor normally conducts at a high temperature, but we can also make it conduct by injecting it with impurities. This is called doping. A semiconductor is either doped with an atom that carries an extra electron, such as phosphorous, or with an atom that is deprived of an electron, such as boron. The vacancies left by migrated electrons are called holes and are considered to be positive charges. The former then becomes an n-type semiconductor, while the latter becomes a p-type semiconductor.
However, individually, the two semiconductors are incapable of conducting current. To create current, there must exist an electric field, and to create an electric field, there must exist a potential difference, which is created by gluing together the p-type and n-type semiconductors. Now, the electrons, when vigorously struck by the photons, don’t roll along a level surface. Instead, the gluing or the creation of a potential has rendered the “snooker table” sloped in the middle. The photons strike the electrons, which race down this slope to the other end, where they are seized by a conductor that is connected to, say, a bulb.
Of course, the current produced by a single cell is insufficient to power, for example, a toaster. Multiple cells are therefore connected in series to form grids or arrays. It is these grids, the shiny blackboards, or what are called panels, that we find soaking up sun on solar farms and rooftops. Each cell is adorned with an anti-reflective coating, which ensures that it absorbs as much light as possible. As a consequence of the unidirectional nature of the slope, the current created is DC. The panels’ output is therefore connected to an inverter that converts the DC to AC, which is what powers households and industries.
The Pros and Cons
Scientists know that energy, while it seems scant, is actually not. Our planet is replete with energy, but what we lack is the knowledge of how it must be harvested. For instance, two centuries ago, energy was solely obtained from burning coal. It still is, but back in the day it was pretty much the only source. This was a great source of anxiety, considering that the amount of coal concealed by Earth is finite.
Then, Einstein’s E=mc² revealed to us the astounding energy crammed into mere atoms. Still, while the energy — which we call nuclear — uranium produces is ample, the element is much scarcer than fossil fuels. Einstein’s equation is revolutionary because it purges us of the anxiety: a glass of water, according to the equation, stores about 1.5 Terra-Watthours (TWh) of potential fusion energy. To put that in perspective, the U.S. in 2010 consumed 4,000 TWh of energy. In other words, it would have taken just 2600 glasses of water to power the entire U.S. for a year. Assuming there are 2600 cities in the U.S., a glass of water could power each city. However, how can we unlock this energy? We have no idea.
Other seemingly inextinguishable sources are natural resources, such as the wind, water and of course, the Sun. The Sun has been Earth’s primary source of energy since the planet was born. It illuminates us with 122 PW worth of light energy, which is 10,000 times the average power consumed by all of humanity in 2005. Therefore, it would have been terribly foolish not to capture it in some way. We were oblivious to this potential until we became aware of the photoelectric effect. It is worth mentioning again that what is limited is not the energy, but our knowledge of the Universe. It is this knowledge that enables innovation.
What’s more, unlike fossil fuels, solar energy is not just copiously available, but it also doesn’t pollute the environment. It produces absolutely no greenhouse gases, which are responsible for escalating the planet’s temperature. Also, owing to the ridiculous scalability of semiconductors, solar cells can now be made thinner than a strand of human hair. They are both small and efficient! We find them glinting on calculators and watches, and today, entire cars are solar powered! However, solar energy is primarily used to power satellites and probes, since sunlight in space is more abundant and not absorbed in any way, as it is on Earth due to the atmosphere.
In fact, if the presence of the atmosphere weren’t enough, the clouds floating above also stifle the light. Solar power’s dependence on the weather is a major disadvantage…but not the only one. Yes, the panels don’t produce any greenhouse gases, but the machines employed in extracting the silicon and constructing and manufacturing the panels itself do. While the panels and the energy produced are admired for being pristine, what lurks behind those glaring panels is a medley of toxic chemicals. Also, let’s not forget that the product is immensely expensive. In this regard, it seems to me that we can never fool nature. We may evade the laws of nature, only to stumble into the merciless laws of economics. Renewable energy is surely the future, but it certainly is expensive.