Stealing Apollo’s chariot: the inner workings of solar panels

Electricity surrounds us. Whether in the grids of power cables blanketing every developed nation, or the currents coursing through the human bloodstream to keep the heart beating, electricity drives our lives. It has powered the great advances of the last century and the hunger for more — and a more efficient — means of generating electricity is always growing.

Generating the massive wattage that powers the globe is no small task, especially considering it requires ravenous consumption of resources like coal and gas. Natural resources are finite, however, and the processes of extracting and using them are often destructive. As technology advances and the global population swells, clean and renewable energy will become the holy grail. Avenues of research into renewable energy include methods such as cold fusion, but for the time being these are pipe dreams. There exists, however, a great and terrible source of energy which, if not limitless, will likely last billions of years. The source we speak of is the sun; the heart of the solar system and the most plentiful source of energy in our reach.

Ancient cultures often venerated the sun as a god, both for its blinding appearance and ability to make crops grow. While the worship of Aten and Helios may have died out, the sun continues to exert a primal influence on our planet, whether by nourishing the growth of entire ecosystems or killing them with drought. Now, with recent developments in technology, the sun may even provide us with unlimited power going forward.

The process of converting light to electricity is known as “photovoltaics.” The word photovoltaic derives from the Greek word “phos” (light) and the term volt, the unit of measurement for electromotive force. Photovoltaic cells are devices constructed to capture sunlight and convert it into usable electricity. Solar panels, the large surfaces that collect sunlight and convert it to electricity, made of many photovoltaic cells that carry out the process of generating an electrical charge from sunlight.

Semiconductors: doping without scandal

A solar cell is made of semiconductive material, such as silicon. Semiconductors fall between conductors and insulators in terms of their capacity for electricity to pass through them, hence the name. Silicon, though a relatively poor conductor on its own, touts a crystalline structure that makes it well suited for building semiconductors. Because the outer shell of a silicon atom is only half full of electrons, it will bind strongly with other atoms as it seeks to fill its shell.

To make silicon more conductive, it can be given “impurities” by combining it with other elements. This is a process called “doping,” and silicon doped with impurities allows for freer movement of electrons. With a silicon semiconductor, there are two parts, each doped with a different material. The first is doped with phosphorous, which possesses five atoms in its shell. When it bonds with the silicon, it leaves one atom left unbonded. Because this electron is only held in place by the nucleus, it takes less energy to knock it loose. This produces N-type (negative) silicon.

Silicon can also be doped with boron, which only has three electrons in its shell. This produces P-type (positive) silicon, which offers holes that free electrons can then fill.

When energy hits the silicon, it can knock the extra electrons in the N side free, and they will move to fill the holes in the P side. Afterward, the electrons from the N-type and P-type come together and form an electrical field. The solar cell becomes a diode, allowing electrons to move from P to N, but not the other way.

Sunlight hits the silicon, knocking loose the free electrons on the N side, which then move to fill the holes in the P side.

Of course, this process requires energy to hit the silicon cell. This is where sunlight comes in. Sunlight is made of photons, small particles of energy that can hit the solar cell and loosen the electrons on the N side. The free electron flow from N to P, creating an electric current as they pass.

Once the electrical field has been created, all that remains is to put it to use. A power inverter is often attached to the solar cell — or more commonly a cluster of cells referred to as a module — and will convert the electricity from direct current (DC) to alternating current (AC), rendering it ready to be transported to homes or businesses.

Inefficiencies and current research

Despite the (for all intents and purposes) limitless power of the sun, the technology to convert it into usable electricity is still rather inefficient. Not all of the sunlight’s energy is absorbed by a solar panel. In fact, most of it is lost. Generally speaking, the best solar cells will only convert 25 percent of the energy they receive into electricity. This is because sunlight, like all light, is comprised of a spectrum of several different wavelengths, each with their own level of intensity. Some wavelengths will be too weak to knock the electrons loose. Other wavelengths will be too strong for the silicon to make use of their full energy.

Furthermore, solar panels require very specific placement. The angle of the panels has to be just right to catch the maximum amount of sunlight, and as you might expect, the panels are only going to be useful in areas that get a lot of sunshine. Inclement weather can turn an array of panels into a very expensive and not altogether interesting art installation.

Research into more efficient solar panels is ongoing. Thin-film solar cells, manufactured from cadmium, are thinner than silicon cells and better at absorbing solar energy. They are also currently worse at converting that energy into electricity, though, their low cost and convenient size make them an attractive avenue for further research.

The other major development is “black silicon,” which sounds like a MacGuffin from a fantasy story, but is really quite innocuous despite the ominous name. Black silicon is simply silicon that has been treated to have a black surface. This is important because black objects absorb more light. A brief refresher on physics: visible light is divided into different wavelengths, each viewed as a range of color. We perceive objects as having a particular color because they reflect that particular wavelength while absorbing others. Black objects absorb all colors, reflecting none, hence they appear black.

Black silicon could be the future of clean energy, and would also make a great Joy Division album cover.

LP3

Black silicon has a lot of potential to make more absorbent solar cells, especially in areas where sunlight is sparse or where the sun typically hits at a low angle. The big drawback at the moment is that the process of creating black silicon gives it a higher surface area, leading to an increase in carrier recombination, an occurrence where a freed electron simply recombines with the silicon cell rather than traveling to join a different atom and producing an electric current.

Flaws aside, the research into black silicon is ongoing, and recently scientists in Finland have managed to reduce the instances of carrier recombination, thus increasing the energy conversion to 22.1 percent. It’s not quite as good as typical silicon, but a promising improvement nonetheless.

from Planet GS via John Jason Fallows on Inoreader http://ift.tt/1EpCROL
Will Nicol