First, I’ll start by saying it was not the easiest thing to find information related to this subject that wasn’t from a solar energy information site, people trying to sell panels, etc. I didn’t want to use those, they seemed kind of biased, but the majority were those types of sites, so not all the questions are answered and not all the disadvantages and advantages are pointed out. With that being said, I split this up into two parts. The first part will talk about how solar panels work. Then the second will talk about the differences between the two types and the pros and cons.
Every hour the sun beams onto Earth more than enough energy to satisfy global energy needs for an entire year. Solar energy is the technology used to harness the sun’s energy and make it useable. Today, the technology produces less than one tenth of one percent of global energy demand.
There are two ways to get solar energy.
Concentrated Solar Thermal systems (CSP)are not the same as Photovoltaic panels; CSP systems concentrate radiation of the sun to heat a liquid substance which is then used to drive a heat engine and drive an electric generator. This indirect method generates alternating current (AC) which can be easily distributed on the power network.
Photovoltaic (PV) solar panels differ from solar thermal systems in that they do not use the sun’s heat to generate power. Instead, they use sunlight through the ‘photovoltaic effect’ to generate direct electric current (DC) in a direct electricity production process. When sunlight hits the cells, it knocks electrons loose from their atoms. A module is a group of cells connected electrically and packaged into a frame (more commonly known as a solar panel), which can then be grouped into larger solar arrays. As the electrons flow through the cell, they generate electricity.The DC is then converted to AC, usually with the use of inverters, in order to be distributed on the power network.
Photovoltaic cells are made of special materials called semiconductors such as silicon, which is currently used most commonly. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely.
Silicon has some special chemical properties, especially in its crystalline form. An atom of silicon has 14 electrons, arranged in three different shells. The first two shells — which hold two and eight electrons respectively — are completely full. The outer shell, however, is only half full with just four electrons. A silicon atom will always look for ways to fill up its last shell, and to do this, it will share electrons with four nearby atoms.That’s what forms the crystalline structure, and that structure turns out to be important to this type of PV cell.
The only problem is that pure crystalline silicon is a poor conductor of electricity because none of its electrons are free to move about, unlike the electrons in more optimum conductors like copper. To address this issue, the silicon in a solar cell has impurities — other atoms purposefully mixed in with the silicon atoms — which changes the way things work a bit. We usually think of impurities as something undesirable, but in this case, our cell wouldn’t work without them. Consider silicon with an atom of phosphorous here and there, maybe one for every million silicon atoms. Phosphorous has five electrons in its outer shell, not four. It still bonds with its silicon neighbor atoms, the phosphorous has an extra electron. It doesn’t form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place.
When energy is added to pure silicon, in the form of heat for example, it can cause a few electrons to break free of their bonds and leave their atoms. A hole is left behind in each case. These electrons, called free carriers, then wander randomly around the crystalline lattice looking for another hole to fall into and carrying an electrical current. However, there are so few of them in pure silicon, that they aren’t very useful.
But our impure silicon with phosphorous atoms mixed in is a different story. It takes a lot less energy to knock loose one of our “extra” phosphorous electrons because they aren’t tied up in a bond with any neighboring atoms. As a result, most of these electrons do break free, and we have a lot more free carriers than we would have in pure silicon. The process of adding impurities on purpose is called doping, and when doped with phosphorous, the resulting silicon is called N-type (“n” for negative) because of the prevalence of free electrons. N-type doped silicon is a much better conductor than pure silicon.
The other part of a typical solar cell is doped with the element boron, which has only three electrons in its outer shell instead of four, to become P-type silicon. Instead of having free electrons, P-type (“p” for positive) has free openings and carries the opposite (positive) charge.
Before now, our two separate pieces of silicon were electrically neutral; the interesting part begins when you put them together. That’s because without an electric field, the cell wouldn’t work; the field forms when the N-type and P-type silicon come into contact. Suddenly, the free electrons on the N side see all the openings on the P side, and there’s a mad rush to fill them. Do all the free electrons fill all the free holes? No. If they did, then the whole arrangement wouldn’t be very useful. However, right at the junction, they do mix and form something of a barrier, making it harder and harder for electrons on the N side to cross over to the P side. Eventually, equilibrium is reached, and we have an electric field separating the two sides.
This electric field acts as a diode, allowing (and even pushing) electrons to flow from the P side to the N side, but not the other way around. It’s like a hill — electrons can easily go down the hill (to the N side), but can’t climb it (to the P side).
When light, in the form of photons, hits our solar cell, its energy breaks apart electron-hole pairs. Each photon with enough energy will normally free exactly one electron, resulting in a free hole as well. If this happens close enough to the electric field, or if free electron and free hole happen to wander into its range of influence, the field will send the electron to the N side and the hole to the P side. This causes further disruption of electrical neutrality, and if we provide an external current path, electrons will flow through the path to the P side to unite with holes that the electric field sent there, doing work for us along the way. The electron flow provides the current, and the cell’s electric field causes a voltage. With both current and voltage, we have power, which is the product of the two.
There are a few more components left before we can really use our cell. Silicon happens to be a very shiny material, which can send photons bouncing away before they’ve done their job, so an antireflective coating is applied to reduce those losses. The final step is to install something that will protect the cell from the elements — often a glass cover plate. PV modules are generally made by connecting several individual cells together to achieve useful levels of voltage and current, and putting them in a sturdy frame complete with positive and negative terminals.