Monday, December 29, 2008

Understanding a solar cell - The solar panel building block

I am build a few homemade solar panels, and I am working on writing up an instructable and a video to show how easy these are to make. Before I post that information however, I wanted to introduce the basic building block of a solar panel and describe how to look for them, how they work, and their energy output. Once this is understood, building a solar panel becomes an easier task.

So what is a solar panel and what are solar cells? A solar panel is basically just a box that holds a bunch of individual solar cells that are connected together. A solar cell is the component that actually converts the sunlight into electricty. It takes a lot of these individual solar cells working together (in a solar panel) to produce enough electricty to be useful. When you look up solar panels, you will see that they come in all types of various wattages - ranging from 15watts, 30 watts, 60 watts, 240 watts, and higher. The number of solar cells in the panel, and the way that they are wired is what determines the solar panels overall power production.

To explain what I mean about how solar cells ultimately determine the overall power of a solar panel, I will discuss the power rating of an individual solar cell, solar cells and overall wattage, wiring solar cells together, and how to connect solar cells together in your wiring schema.

Solar panels are rated on three measures of power: Watts, volts, and amps. The first thing you need to know about solar cells is that typically all of them creates a little more than .5volts of DC electricty - no matter what physcial dimension they are. The only thing that changes with the size of a solar cell is their current, or AMPS. So if we take our 3"x6" solar cell example and look at its electricity output - it is more than likely rated at .5volts with 3 amps. If we were to break that solar cell into half, we would get two solar cells that rate at .5 volts and 1.5 amps each.

When determining the overall wattage of a solar panel made up of combined solar cells, we use this simple equation:

Watts = Volts x Amps.

When building a solar cell, you should always know the final wattage, volts, and amps that you want the solar panel to produce, and you should always use the same size and same type of solar cells. Larger cells produce more current, or AMPS, and smaller cells produce less current. Larger cells will produce more power, but will make your solar panel heavier, and smaller cells will keep your solar panel lighter, but it will not produce as much power as larger cells. You also do not want to mix different sized cells together because the current or AMPS of your solar panel will be limited by the smallest cell in the group - wasting the power of the larger cells. SO ONLY USE SOLAR CELLS OF THE SAME SIZE AND SIMILAR AMPS.

So now that we have an idea how individual solar cells are rated and how their power production is rated - how do we know how many solar cells to use to make a final solar panel with a pre-determined amount of wattage? We do this by using the equation for wattage:

Watts = Volts x Amps

So lets say that we want to create a solar panel that puts out 60 Watts. We know that our 3"x6" solar cells put out .5 volts and have 3.3 amps. So we take 60 watts divided by 3.3 to get our overall solar panel voltage. 60watts/3.3amps = 18volts.

Now that we have our overall solar panel voltage, we can find out just how many solar cells we need to get 18volts out of the solar panel. To do this we take:

Solar Panel Voltage / single solar cell voltage = Number of cells needed per panel

So in our example, we found out that a 60 watt solar panel will have 18 volts. So now we take 18volts/.5volts = 32 solar cells needed. So we know now that it will take us 32 .5volt solar cells to create a single 60 watt, 18 volt solar panel. This means that we will need to wire these solar cells together in a series to achieve the voltage increase we need.

There are two ways that you can wire your solar cells together in your solar panels

1) Series wiring (positive to negative and negative to positive)
2) Parallel wiring (positive to positive and negative to negative)

Wiring solar cells in series will increase the voltage of the solar cell, but will not increase the amps. Wiring solar cells in parallel will increase the amperage, but will not increase the voltage.

Using our 3"x6" solar cell with .5volts and 3.3amps I will explain how these two different methods of wiring solar cells together drastically changes the power output of your solar cell.

Series Wiring
If we take two 3"x6" solar cells by connecting the positive terminal of one cell to the negative terminal of another cell, and the negative terminal of the same cell to the postive terminal of the other cell, then we will have series wired the two together creating an increase in voltage to 1 volt with the rated 3.3 amps not changin. If we were to series wire six of the solar cells together, we would get 3volts (.5x6) at 3.3 amps, and so on.

Parallel Wiring
Parallel wiring refers to connecting solar cells to increase amps, but not volts. If you have two .5 volt solar cells rated at 3.3 amp hours, for example, by connecting the positive terminal of one solar cell to the positive terminal of the other, and the same with the negative terminal, then we will have parallel wired the two together. In this case, we now have a .5 volt solar cell and the rated 3.3 amps increases to 6.6 amp hours.

A single solar cell will not yield a lot of useful electricity, so you need to connect them together into a solar array - inside the solar panel. Connecting solar cells together is relatively easy, but it is a slow, gentle process that should be done carefully and with a lot of attention. Each solar cell has postive leads on the bottom of the cell, and negative leads at the top of the cell. There are two ways to connect solar cells together. The first way is to solder the tabs (or wires) that are already connected to the solar cells to the tabs or another solar cell, and the second way is to buy a metal solar cell ribbon and solder a strip of it to the back of each cell you want to connect (soldering to each of the six rectanglar points on the cell)

It is important to note that not all solar cells are sold with the tabs already connected, but that all solar cells have connection points on their back-side that look like a metal rectangle (6 total) which are used for soldering the ribbon into place. I recommend that if you can find solar cells with the tabbing already connected - that you purhcase these. They are easier to solder, the will cost you less in material, and they are faster to install with less steps and less soldering. Using the solder ribbon to connect solar cells will more than double the time it takes you to create a solar panel, and leaves more room for errors and connectivity issues.

When soldering solar cells together, I recommend the use of a flat-tip 30 watt soldering iro and silver bearing solder. You can find both of these at Radio Shack.

Be sure to solder the connections in series or parallel, based on your application.


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James Curtis said...

Accurate and informative explanation.

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Maria said...

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Welcome to the Harness Solar Power Project Blog. I created this blog to help teach people about solar power, solar panels, solar electricity, and how easy solar power is to use. I set out to show you how you can start using solar power in your everyday lives. Take a look through the posts to start learning about solar power projects you can easily do around your home to start harnessing the sun's ability to create electricity.