Alternate Energy 1 - Solar Power

Welcome to the first in a series that looks at various alternative energies, their scientific basis, and their potential for future application.

Solar Power

Technically speaking, solar energy has under its umbrella (...ella...ella) wind and wave power as well, since these two phenomena are indirect products of the sun's rays. Solar power in the sense that is usually meant refers to photovolataics, or photocells, used in assemblies or arrays to use the sun's incoming light to convert photons to electricity.

There is another form of solar energy, passive solar energy, which is a fascinating branch of science and architecture involving utilizing exposure, materials, and other solar properties to better control the heating and cooling of houses, or offset HVAC costs.

Solar power relies on the photoelectric effect. This effect occurs when light strikes a metal surface. It was discovered that if an anode was placed a small distance from a metal plate and light shone on the plate, electricity would flow through the system. Further investigation revealed that light causes some materials to emit electrons. When an incident photon adds its energy to an electron orbiting the molecules, the energy, when high enough, causes the electron to jump up, or escape from the atom. These free electrons can form a current when external load is applied.

Semiconductors

Solar cells are made using semiconductors. Semiconductors are a special breed of material which can be explained if a closer look is taken at what makes an insulator or a conductor. A conductor is usually a metal wherein the outer electron shell is half full. This allows the electrons to move to a SSolar Powerolar Powerhigher level more freely, and requires less energy to move them. An insulator, on the other hand, has a full valence shell and moving electrons to form a current is difficult because of the large amount of external energy needed to excite the electrons all the way to the next shell. The shells in question here can be around an atom, or in compounds, around whole molecules.

*Sometimes the properties of the individual valence shells can shift or redistribute when combined in a molecule – bonding must be taken into account. If you're interested, research band gaps and molecular bonding.

A semiconductor is a material with a full valence shell, but a very small energy gap between the valence shell and the next shell. This means that a small amount of energy is required to start a current. They also have extremely low resistivity, but one of their mot important characteristics is that their conductivity can be controlled. For example, as the heat in the semiconductor is increased, the electron energy increases and they can more easily jump to the next energy level and therefore make a current possible. (Many interesting applications of semiconductors are made in space, where heating is an issue).

The properties of a semiconductor are also able to be closely tuned by a process called doping, or adding a very small amount of impurities to it. Since a semiconductor consists of atoms with full valence shells, adding a few atoms with full-plus-one shells or full-minus-one shells adds or subtracts a few extra electrons. This allows precise control of how much current can flow. For example, adding arsenic to silicon allows one extra electron per molecule of arsenic. When current is induced, the extra electron can add to the current. When doping introduces extra electrons, it is called an n-type (or negative type) semiconductor. When an atom is added that subtracts an atom, it is called a p-type (or positive) semiconductor. (The positive is because as the charge migrates through the metal, the missing electrons are always being filled by others and so current appears to move with positive charges).

First-generation solar cells use silicon doped with various materials, involving a solid crystal of silicon. While peaking at a functional efficiency of about 24% and a potential efficiency of about 33% (units of energy out over units of energy in), they are relatively expensive to manufacture and require high-temperature processing, further adding to their overall inefficiency. After all, if they take more energy to make than they will ever produce, it doesn't matter how efficiently they do their job of processing solar energy – in the closed system of the planet, the net energy used will not be worth it.

Second-generation photovoltaic cells are composed of, as opposed to silicon, two different semiconducting crystals plated on top of each other. Semiconductors like this are called p-n heterojunctions, because the two semiconductors usually comprise a p-type and n-type.

The heterojunctions have some interesting properties. For one, the two are usually materials with staggered band gaps. This means that is is energetically economic for charge to flow from one to another when the electrons are excited by the photoelectric effect.

Second, where they interface, the inherent extra charge on either side causes some electrons to jump from the p to the n-type and build up a voltage.

The outcome of this is that when photons strike the metal, electrons are constantly being shifted to higher energy levels, feeding the supply of charge that can move and increasing the potential difference across the band gaps.

Compared to a semiconductor like silicon, the heterojunctions have one major advantage; they are much more cost effective to produce. They do not require high temperatures to make but can be plated in thin layers. They also do not require large crystals but can be plated onto glass or clay, reducing the amount of material required. So far, though, their peak efficiency is about 19-20%.

Solar power's greatest application is in off-grid areas and is only slowly gaining presence on grid. The great appeal of solar power is its inexhaustibility - the sun isn't going to stop shining when we use up reserves.

However, its limitations are that it isn't particularly power-ful - the efficiency leaves something to be desired, and it's only ever sunny half the time, if you're lucky. (Issues with storage of naturally-derived power generation will be covered in subsequent posts). Also, the costs will need to come down and manufacturing made more efficient for it to be a truly competitive alternative to coal and gas.