Addictive

Unfortunately, the most that our simple cell could absorb is around 25 percent, and more likely is 15% or less. Visible light is only part of the electromagnetic spectrum. Electromagnetic radiation is not monochromatic; it is made up of a range of different wavelengths, and therefore energy levels.

Since the light that hits our cell has photons of a range of energies, it turns out that some of them won't have enough energy to form an electron-hole pair. They'll simply pass through the cell as if it were transparent. Still other photons have too much energy. Only a certain amount of energy, measured in electron volts and defined by our cell material, is required to free an electron. We call this the band gap energy of a material. If a photon has more energy than the required amount, then the extra energy is lost.

Unfortunately, our band gap also determines the strength (voltage) of our electric field, and if it's too low, then what we make up in extra current, we lose by having a small voltage.

We have other losses as well. Our electrons have to flow from one side of the cell to the other through an external circuit. We can cover the bottom with a metal, allowing for good conduction, but if we completely cover the top, then photons can't get through the opaque conductor and we lose all of our current. If we put our contacts only at the sides of our cell, then the electrons have to travel an extremely long distance to reach the contacts. Its internal resistance is fairly high, and high resistance means high losses. To minimize these losses, our cell is covered by a metallic contact grid that shortens the distance that electrons have to travel while covering only a small part of the cell surface. Even so, some photons are blocked by the grid, which can't be too small or its own resistance will be too high.

Silicon happens to be a very shiny material, which means that it is very reflective. Photons that are reflected can't be used by the cell. For that reason, an antireflective coating is applied to the top of the cell to reduce reflection losses to less than 5 percent.

One way efficiency has been improved is to use two or more layers of different materials with different band gaps. The higher band gap material is on the surface, absorbing high-energy photons while allowing lower-energy photons to be absorbed by the lower band gap material beneath. This technique can result in much higher efficiencies. Such cells, called multi-junction cells, can have more than one electric field.

The structure of a solar pannels, with its layers.

There are a couple of problems that we'll have to solve. First, what do we do when the sun isn't shining? No one would accept only having electricity during the day, and then only on clear days, if they have a choice. We need energy-storage - batteries. Unfortunately, batteries add a lot of cost and maintenance to the PV system. One way around the problem is to connect your house to the utility grid, buying power when you need it and selling to them when you produce more than you need. This way, the utility acts as a practically infinite storage system. The utility has to agree, of course, and in most cases will buy power from you at a much lower price than their own selling price.

Some people use sophisticated panel mounts called "trackers" that follow the path of the sun during the day. These automatic systems can increase output 50% in the summer and 20% in the winter, but this only increases the difference in output between the seasons. They are also expensive.

You can adjust your panels' position manually to get the best tilt angle for each season. Take your latitude and add 15° for the winter, and subtract 15° for the summer. At the spring and autumn, the best angle is equal to your latitude. If you leave your panel in a fixed position, you can decide to leave it at the best angle for the winter to help even out seasonal performance. It is advisable to have at least a 15° tilt to avoid rain accumulating on your panels.

The other problem is that the electricity generated by your PV modules is direct current, while the electricity supplied to you is alternating current. You will need an inverter, a device that converts DC to AC. Some PV modules, called AC modules, actually have an inverter already built into each module, eliminating the need for a large, central inverter, and simplifying wiring issues.

Normal panels cost between $300 and $700. The 'trackers' panels cost between $1000 and $2700. In the school, we would be using the normal panels. Since we need an average of 120 kilowatts of electricity to run a school per day (a small school, with 20 computers and 7 classrooms - 12 hours on per day), and each panel produces an average of 4 kilowatts per day, we would be looking at 30 panels in the school. That means that it would cost between $9000 and $21000 to have solar panels at school.