Lesson 4: Photovoltaics

What is a photovoltaic system?

A photovoltaic system uses energy from the sun to make electricity. However, the idea of using solar power for electricity is not new. The first solar power experiments on the “photovoltaic effect” were done in 1839 by a French scientist named Alexandre-Edmond Becquerel. The photovoltaic effect is the generation of an electric current using light energy. Becquerel put two brass plates into a conductive liquid and shined a light on the plates, which resulted in an electric current.

In 1880, Charles Fritts noticed that the element selenium was sensitive to light and created the first selenium solar cell. In the 1950s, scientists at Bell Laboratories found that the element silicon was also light sensitive and and developed a silicon based cell, which is similar to the solar panels used today. This type of solar cell has been installed on satellites and spacecraft. As of the year 2005, solar panels power one million homes worldwide.

How do solar panels work?

One solar cell is made up of two layers of silicon crystals, where the top layer, called the n-layer, is infused with phosphorus and the bottom layer, called the p-layer, is infused with boron. The silicon, phosphorus, and boron have a very convenient relationship based on the number of electrons orbiting each atom. Silicon has four valence electrons, which means four electrons occupy the silicon atom’s outermost “shell.” [See Chemistry Basics] Phosphorus has five valence electrons and boron has three. The silicon atoms neatly form crystals, where the 4 electrons bind to the 4 electrons of other silicon atoms in a lattice formation. The n-layer’s lattice has phosphorus atoms filling the space between the silicon atoms and the p-layer’s lattice has boron atoms filling the space between the silicon atoms.

When sunlight strikes the surface of the solar panel, electrons are dislodged from the phosphorus-infused silicon lattice on the top, where there are more electrons, and are captured by the boron-infused silicon lattice on the bottom, where there are spaces for missing electrons. This flow of electrons is what creates the electric current that can be used to do work. In other words, the difference in charge of the two layers creates an electrical field. [See Electricity Basics] For a great visual representation of this concept and to get a more detailed explanation of the chemistry behind solar panels, check out this link from University of Pennsylvania.

Solar panels are actually made up of multiple solar cells. Cells are wired together into modules, modules are wired together into strings, and strings are wired together into arrays.

What kinds of photovoltaic systems are there?

There are two major types of PV systems: 1) independent systems which are considered “off the grid” and 2) interconnected or “grid-tied” systems. Independent systems supply electricity directly from the sun, and there is no connection to the regular power company. Grid-tied systems DO work together with the regular power company in a two-way type of communication. Sometimes they supply energy to the power grid and other times they draw power from the power grid. Some grid-tied systems involve a battery back up while others do not.

Why would you want a particular type of system? Independent systems introduce more long term savings in electricity costs than grid-tied systems and also allows the user to be completely independent of the utility grid. This is effective for remote areas where it is difficult to connect to the grid. However, grid-tied systems have a lower start-up cost. Grid-tied systems without batteries cost less than grid-tied systems with batteries.

There are also solar thermal systems that use water as a medium to transfer energy. Two main types of systems exist – drainback solar thermal systems and closed loop pressurized solar thermal systems. Drainback systems directly heat potable water by passing it through solar collectors while pressurized systems use a “solar fluid” frequently made of distilled water and anti-freeze that is pressurized and circulated through the collectors. The solar fluid then transfers it’s heat to the potable water via a device called a heat exchanger.

What are the advantages and disadvantages of solar systems?

Advantages

As discussed above, PV systems allow the user to be independent of the utility company. They provide an opportunity to reduce CO2 emissions because solar power is a “clean” energy source. Since solar arrays depend only on the virtually unlimited light energy from the sun, they are considered to be renewable. PV systems are also very reliable. They can last 25 to 50 years with fairly minimal maintenance because they have almost no moving parts.

Disadvantages

One of the biggest disadvantages of solar energy is the high start up cost. It also requires fossil fuels to manufacture solar panels, not to mention the mining of the requisite raw materials. While we have not yet found a way to circumvent the challenges in obtaining the raw materials, companies like Solar City have figured out clever ways of making solar energy affordable for the average homeowner. They absorb the initial start up cost of equipment and installation, and then sell the solar energy directly to the consumer at a lower rate than the utility company. Weather is also a factor in how much energy solar systems can produce, so power output can vary based on the amount of sunlight available. As such, energy storage for low light and night time use is also a major hurdle because batteries can be expensive and have limited reliability and lifetime.

References

  1. http://www.scientificamerican.com/article/how-does-solar-power-work/
  2. John Balfour, Michael Shaw, and Nicole Bremer Nash. The Art and Science of Photovoltaics: Review Guide for the NABCEP Entry-Level Exam
  3. https://fling.seas.upenn.edu/~electric/dynamic/?page_id=244
Advertisements