Solar panels are a serious investment for any homeowner. The price of a few panels for your roof may be expensive, but panels are becoming more and more affordable. The American Recovery and Reinvestment Act passed this past February will enable more people to purchase solar panels, by offering a 30% tax credit on the solar panels and also removing the previous cap, so you can actually recoup the entire 30%. It will take more than 6 years for the solar panels to pay for themselves, however they will make your life a lot more sustainable, and will certainly add value to your home. Here is how they work.
On a clear, sunny day, Earth’s surface is bombarded by about 1,000 watts of energy per square meter. This light could potentially strike the photovoltaic systems and produce enough energy to power our grids and our homes. Photovoltaic cells, which make up the solar panel, can convert sunlight into energy. The cells are made of semiconductors, such as silicon, which is most common. Once light hits the cell, some of it is absorbed by the semiconductor, transferring the energy from the light into the system. This energy knocks some electrons loose, allowing them to flow in the direction the solar system desires. This is done with one or more electric fields. The electrons that are now moving freely constitute a current, which with the addition of metal contacts at the top and bottom of the photovoltaic cell can be drawn off for other uses.
Silicon is not the greatest conductor in its purest form. A single atom of silicon will have an outer ring of electrons that is only half full (4), and as the Silicon atom seeks to complete the outer ring, it will meet up with other silicon atoms in order to share the electrons and become complete. This done on a large scale will produce what is called a crystalline structure, however, as stated previously, it is not the best conductor because it is pure, and the electrons have nowhere to go.
A solar cell needs impurities in order to facilitate the free flow of electrons. It might seem odd that impurities are what are necessary to enable the silicon, but it makes sense. Water follows the same principles as pure H20 will not be able to hold a current. It is the other chemicals and impurities that are found in water in most of its natural forms, as well as out of the tap, that enable the water to conduct electricity.
The element that adds most of the impurity to silicon is phosphorous. With its outer ring holding 5 electrons, the combination of silicon and phosphorous leaves one electron in the lurch, with the phosphorous electron being held in place by the proton in the phosphorous nucleus. Since this last electron is not tied up in a bond, it is much easier to knock loose, creating free carriers which are electrically charged. Without the phosphorous, there would be dramatically fewer free carriers, and it would take more of a jolting to get them free. Using phosphorous is called “doping”, and the resulting systems are called “N-Types” as they consist of a higher level of negative ions in the form of electrons.
Phosphorous is only one part of the equation. Another portion of the solar system will add boron as the impurity. Boron holds only 3 electrons on its outer ring, and as such creates a positive electric field (P-Type for positive). The extra electrons on the phosphorous side rush eagerly to the gaping holes of the boron side, causing the free carriers to flow in a specific direction. Without electric fields, solar systems would not work.
At a point, the positive side and the negative side will come into equilibrium, and it becomes harder and harder for electrons to move from the negative side, with all the extra electrons, to the positive side with the space for the electrons to rest. At equilibrium, there is an electric field holding the sides at bay. This field acts as a diode, and actually pushes electrons from the positive side to the negative side. The electrons can only move in one direction, as if the positive side is the top of a hill, and electrons are free to roll down, but not back up.
With electrons moving in one direction through an electric field that is acting like a diode, light from the sun is able to knock free some electron-hole pairs. A photon of light will be able to free one electron, and create a subsequent hole. If this happens close enough to the electric field, or if the electron and hole make it into the electric field’s range of influence, the field will cause them to swap sides, disrupting the electrical neutrality of the field. With the addition of an external path, the electrons will attempt to return to their proper side and unite with the holes that were sent there. The electron flow is the current, while the cell’s field causes a voltage. The combination of current and voltage gives us power.
The surface of silicon is very shiny, so an antireflective coating needs to be applied before it can be put in service. The coating can reduce reflection losses to less than 5%. After that, a glass cover plate is added to protect the cells from the elements. The entire photovoltaic model will, most likely, consist of 36 cells, with a positive and negative terminal on the back. After all this, the system can absorb at a maximum, 25% of the energy from the sun, with more realistic systems capturing 15% at most.
The reason this percentage is so low has to do with the electromagnetic spectrum. Visible light is a small part of the spectrum, and with different wavelengths come different energy levels. For crystalline silicon, the sun needs to add roughly 1.1 electron volts (eV) to break an electron off of its atom. This is called the band gap energy. Some types of light are under this threshold and float right through the apparatus as if it was not there. Other types have too much energy, and unless it is twice the 1.1 eV, the extra energy will simply be lost all together. The wavelength issue accounts for the loss of 70% of the radiation energy that hits the cell. The optimal band gap is 1.4 eV, and while it might seem like a good idea to lower the band gap to enable more light to create energy, it should be noted that the lower the band gap, the weaker the electric field and the more energy that becomes necessary to free the electrons in the first place.
Other losses come due to the fact that silicon is a semiconductor, and not the best material for creating a current. More energy is lost through the top of the system, as the glass needs to be transparent to let in the photons. Metal on the back conducts the electrons very well, however if this substance were on top the photons would not make it through. Some transparent conductors are used for this side, but not in every system.
Now that we have a better understanding of photovoltaic systems, it’s time to place them on top of the house. BUT, a very important factor in considering solar panels is the orientation and angle of inclination, as not all roofs are ideal candidates for a solar system. In the northern hemisphere, the panels should face directly south (orientation). The angle of inclination should match the area’s latitude for maximum energy year round. The module should never be shaded in any way by trees, chimneys or a neighbors home, as the blockage of even one out of thirty-six cells will cause energy production to be cut at least in half.
Be sure to consult meteorological data before you decide what size system you might need, as the amount of available sun in your region, and your desired energy input are key factors in the determination of whether a solar system is right for your home. Think about designing the system to react to the worst month of the year, so that you know you’ll be fine all year.
The system relies on batteries to store the electricity, which can add a lot of cost to the operation. However, many homeowners decide to link into the utility grid, which allows them to sell energy to the grid, and buy it back when they need it. Make sure to check with a solar expert when considering the system, as well as the utility company to make sure your system will produce identical energy, so that they will buy it.
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For more information on the potential payback from a photovoltaic system check out GREENandSAVE’s Return on Investment Table, which will be a valuable tool in any new Home Remodeling project.