Designing and building a simple solar array is a good father-son project or even a good science fair project for any young scientist. The array that we will build in this article will produce 12 volts with a maximum load current of 3 Amperes.
Knowing how something works is just as important to the DIY electrician as the knowledge of how to build it is. Knowing how something works is important because if something goes awry during the construction phase and you have to troubleshoot your project you will need to know how it was suppose to work before you can effectively troubleshoot it. Back in the days when I was getting my start in electronics there were two types of people working in the field, technicians and parts changers. The parts changers didn’t really have a good understanding of the theory involved; they didn’t understand how a particular circuit operated, so when something went wrong they just haphazardly changed parts until they stumbled across the one that solved the problem. They eventually solved the problem but it was a very time consuming method and a very expensive method for their customers because they ended up paying for many parts that they didn’t need. The technician, on the other hand, understood the theory of how a circuit operated, knew how to effectively test it and isolate the problem quickly. The technician didn’t replace good parts and that kept their customer’s cost to a minimum. For the DIY electrician, that could mean the difference between spending $70 for a Clothe Drier motor when all they need to replace was a $10 centrifugal switch,
Before we get into building our solar array let’s spend a minute or two talking about the history of the solar array and the theory of how solar cells work. I’ll spare you as much of the physics and math as I can but an overview is important. Photovoltaic’s, as a science, has a long history. Photovoltaic’s is the science of converting light energy into electrical energy. As you probably recall from your physics classes, the First Law of Thermodynamics, the Law of the Conservation of Energy, states that energy cannot be created or destroyed but it can be converted from one form to another.
In 1838, 19 years old physicist, Edmund Becquerel, observed and wrote about the phenomenon. In 1873, 35 years after this young scientist first observed the phenomenon; Willoughby Smith discovered the Photovoltaic properties of the element Selenium while working on undersea cable for the Telegraph Construction Company located in Great Britain. This observation led to the eventual invention of the Selenium solar cell in 1883 by Charles Fritz. The Fritz Selenium cell was the first solar cell to produce usable electrical energy but with a conversion efficiency of 1 to 2 percent, no commercial applications were forthcoming. In the late 1920’s, copper oxide, another metallic semiconductor was also known for its Photovoltaic properties. The first real use of the Selenium and the Copper Oxide cells were in light measuring instrument like the photometers used by the photographers of the day. There conversion efficiency of approximately 1 percent limited their usefulness. No real advances were made in Photovoltaic’s until 1954 when Bell Laboratory scientists, G.L. Pearson, Daryl Chapin, and Calvin Fuller, discovered that Silicon when doped with certain impurities produced minute electric currents. The first modern photo cell emerged from Bell Laboratories with an efficiency of approximately 6 percent. Today solar cell efficiency has reached 37 percent with the use of light concentration technology.
Modern solar cells are a three layered device based on solid physics. In essence they are a P-N Junction Photodiode with a large photosensitive area. The first of the three layers is composed of an N-Type semiconductor material which simply means that it’s a semiconductor material is rich in electrons. The second layer is the P-N Junction, called the “absorber layer.” The third layer is composed of a P-Type semiconductor material which is electron “poor.” The P-Type Semiconductor material is rich in “Holes” which means that current flows across the junction from the N-Layer to the P-Layer. The light enters the cell through a metal grid, the positive electrode. The negative electrode is a solid metal plate covering the back of the cell.
Designing the array
The two solar cell parameters that we are going to be concerned with as we select cells for our project will be their Voc (Maximum Open Circuit Voltage) at zero output current and its Isc (Short Circuit Current) at zero Output Voltage). A single 2” square solar cell produces a Voc of 0.5 Volts at 0.5 Amperes. To achieve our goal of 12 Volts and 3 Amps we will need to connect our cells in a series-parallel configuration. Connecting cells in series increases the voltage while the output current of the series circuit remains the same as that of a single cell. Since one cell produces 0.5 Volts, we need to connect 24 cells in series. The number of cells needed equals the required output voltage divided by the voltage of a single cell or #=Vout/0.5. Paralleling batteries increases the current available while keeping the voltage the same. To achieve our goal of 3 Amperes we need to parallel 6 solar cells. To achieve 12 Volts at 3 Amperes we need to parallel 6 series strings. To construct a series string, connect the solar cells negative to positive. To parallel the series circuits connect all the positive terminals of the last cell in each string together and connect all the negative terminals of the first cell in each series string together.
Mounting the cells
For your 12 Volt, 3 Ampere array you will need to mount a total of 144 solar cells. Since each cell measures 2 inches to a side your completed array will fit on a 24” X 24” sheet of ¼” plywood. The physical dimensions can vary from one manufacturer to another so you may have to adjust the size of the mounting board. Radio Shack is a good source for the solar cells. These cells sell for approximately $5 each so your array will cost you about $720 to build. Solar power isn’t inexpensive but once the array is built it is relatively cost free power.
