How does solar energy work?

How does sunlight shining on a solar panel generate electricity? You may have seen solar panels along the highway and thought you understood how they work. Simply, you may state that when the shun shines on the panel, electricity is generated.

^[i.n.]You would be correct, but this answer is much more complicated than it first appears, as the act of sunlight shining on a panel does not create electricity out of nothing. To understand how a solar panel works, there are certain laws of nature that must be made clear. One of the fundamental theories of the universe is the Conservation of Energy.

The Conservation of Energy

The theory of the Conservation of Mass states that energy cannot be created or destroyed, only transformed. Therefore, in the case of solar panels, the electricity (energy) generated by the panels is not magically created from nothing when the sun shines. The electricity generated is thus a transformation of the only other energy source available, light. An easier way to think of this concept is to imagine baking a cake.

All of the ingredients required to make a cake; the eggs, flour, milk etc. can be thought of as a type of energy source.

They all exist initially and once they are mixed together to form the batter, none of the initial ingredients have been created or destroyed. The eggs, milk and flour are all still in the bowl, just mixed together. Nothing has been lost. The oven facilitates the transformation of the batter into the final product of a cake, or the second form of energy.The solar panels are simply a method of facilitating this transformation of energy, from light to electricity, much like the oven transforms the mixture of ingredients into the final form of a cake.^[i.n.]

So how do solar panels facilitate this energy transformation?

A solar panel is made up of many individual photovoltaic cells.

A photovoltaic cell has a design similar to a common battery, containing both positive and negative charges, that when aligned properly can release electricity. However, a photovoltaic cell relies on the use of a thin wafer made of a semiconductor, usually silicon. The semiconductor wafer is specially treated to form an electric field, with a positive charge on one side, and a negative charge on the other. These individual cells are grouped together in groups of 12 to form a module, similar to a 12 V battery. These modules are then combined to form a single solar panel commonly seen on rooftops. A semiconductor is a material that has the conductivity, or the ability to carry an electrical current, between that of a good conductor, such as metal, and that of an insulator, such as rubber. Semiconductors are used as they can be altered and controlled quite easily to obtain the necessary level of electrical resistance required to perform electrical operations. Common metals heat up too quickly and conduct electricity too well, making it hard to use them in modern electronics. Just as we were told as children not to stand near metal during a lightning storm, utilizing a metal conductor in a solar panel is just as dangerous. The solar panels would overheat as there would be no resistance to the electrical current, and the panels would likely catch fire.^[i.n.]

So how do photovoltaic cells work?

Sunlight is made up of photons, atomic sized particles that radiate from the sun. When sunlight shines on the solar panels, the individual photons strike the silicon and transfer their energy to the electrons of the silicon, knocking them loose from the silicon atoms. A photon can be thought of as the white (cue) ball in the game of pool, and the electrons as the remaining stripes and solids. Just after racking the balls (electrons), they are "stuck" in that position and will not move unless a force acts upon them.

When the cue ball (photon) smacks into the other balls during the break, the energy is transferred to the other balls and they are knocked free of their initial position. These loose electrons are now free of the silicon, but need to be directed to form an electrical current. This is achieved by the photovoltaic cell by the use of an electric field.

What is an electric field?

An electric field is achieved by creating an imbalance within the photovoltaic cell. This is done by taking two slightly different types of silicon and placing them near each other. The electrons on the two types of silicon are arranged differently, giving one side a slightly positive charge, and the other a slightly negative charge. The two work together to create a path for the loose electrons to move along, forming an orderly electric current. As electrons are negatively charged, they will be pushed away from the negative side and towards the positive side. Just as water will always flow downhill due to the effects of gravity, an electron will always move away from negatively charged objects and towards a positive charge. This scientific principle will always hold true, unlike the laws of nature when depicted in cartoons.

The electric field directs this current to a node, which transfers it out of the individual photovoltaic cells and converts it to an alternating current (AC) form of electricity, which is able to be used on the grid.

How much energy can be generated by solar panels?

Solar panels have a notoriously low energy efficiency. Solar panels have an average efficiency of about 22%, meaning that only 22% of the electrons knocked loosed by the photons are captured in the form of an electrical current. The remaining energy is usually lost in the form of heat. The effectiveness of a solar panel is thus determined by how well the electric field created by the semiconductor can direct the electrical current to the node which transfers the electricity out of the photovoltaic cells. Going back to our pool example: A good pool player may hit a few of the balls from the break into the pockets, but most will remain on the table. The balls (electrons) that fall into the pockets represent the 22% efficiency of the panels, as a majority of the balls still remain on the table. New technology is attempting to improve the efficiency of solar panels by changing the surface of the silicon semiconductor. Thus more of the electrons would be captured by the electric field and transferred out of the cell as electricity. The improved technology would be in effect, cutting channels in the pool table that guide the balls to the pockets, increasing the likelihood they would go in.

What have we learned?

Solar energy is quite easy to understand once these basic concepts have been explained. Sunlight, which is made up of photons, strike the photovoltaic cells contained within the solar panels and knock electrons loose from the silicon semiconductor. These electrons are then directed by the electric field created by the PV cell into an electric current that can be transferred out of the cell and onto the power grid. The efficiency of the panels at directing these loose electrons into a current have a direct impact at the cost of energy generation and improving the efficiency of the panels remains the most important research in the solar technology field.^[i.n.]

Florida Solar Energy Center Solar Energy Basics http://www.fsec.ucf.edu/en/consumer/solar_electricity/basics/index.htm Web. 20 August 2016

National Aeronautics and Space Administration http://science.nasa.gov/science-news/science-at-nasa/2002/solarcells/ Web. 18 August 2016

The Physics Classroom Electric Field and the Movement of Charge http://www.physicsclassroom.com/class/circuits/Lesson-1/Electric-Field-and-the-Movement-of-Charge Web. 21 August 2016

NW Wind and Solar How do solar systems generate electricity? http://www.nwwindandsolar.com/solar-power-in-seattle-and-the-northwest/how-do-solar-systems-produce-energy/

Peer review

Dan wrote a Science Blog composition based on the question: "How do solar panels generate electricity?". He starts with an outline on the history and technical aspects of solar panels which is followed up by the actual answer.

The rhetorical situation here is clear, a question is being answered. I would imagine an audience of scientifically literate, curious people. This genre is appropriate for this situation, as answering a question or explaining some scientific principle is the reason for this genre's existence. As this genre is meant to make complex scientific problems more understandable, Dan's word choice and sentence structure are more or less sound. There are some syntactical problems such as the last sentence in the answer just ending. An aspect of the genre that Dan does not take up is the visual aid. An example of where a visual aid could come in handy could be to better illustrate the photoelectric effect. There is an analogy but there is potential for more. As I discussed previously (the last three sentences), using the rhetorical devices of visual aids and analogies would assist in making this piece more successful. As I will discuss later on, conforming more towards the schema would also help the piece by making it more digestible.

The schema outlined by group 1 is not particularly well conformed to in this composition. For instance, the first element in the answer, a "specific_topic" element, raises a schema error. As far as I can tell this is because a "specific_topic" element must be preceded by a "technical_principle" element. Also a problem with this composition is the code left over from the template file. All of this code should be removed and replaced with one's own. Lastly, the "sources" element/section of the blog was left entirely untouched.

In terms of rhetorical situation and response, Dan's piece reflects research done by group one quite well. However in terms of schema conformity, it could use some work. My main piece of advice is to get rid of all the boilerplate XML that comes with the template. Simply doing this will, in my opinion, enhance readability. A corollary to this advice is to clean up schema errors. Finally, adding such devices as visual aids and analogies would help in conforming to the genre.

Instructor Note

Nice work here--there are a few moments where I had questions (see comments above), but on the whole I think you pulled off the genre very well and with an appropriate topic and audience in mind.