Solar Panels for RVsThe operating environment on the roofs of RV's is unique and requires a different approach than would be taken in other situations or applications. Since we are mainly concerned with what is appropriate for use on RVs, this discussion will not cover other uses.
There are presently three commercially available types of solar cells based on silicon: Amorphous (thin film), Poly-
First, let's briefly cover how they are made and how they work so that you will have a better understanding of what I am going to present to you later about voltage ratings and what affects output.
How they are made.
For the poly and mono-
The silicon is then blended with a small amount of boron, melted down and used to grow crystalline ingots. The ingots are then sliced into very thin wafers and impregnated on one side with phosphorous. This gives them an electrical charge, one side has a positive (+) charge and the other side has a negative (-
The wafers are then screen printed with a grid of silver paste. These wafers with grids are then called "Cells". A group of these cells (sandwiched between glass on top and a weather proof back layer) is then called a laminate. When the laminate is framed it is called a module (or panel).
The amorphous panels are made by depositing a silicon rich gas (silene) on a substrate which is then etched and scribed into cells and modules. This is about as brief as I can get and still give you an idea of how they are made. In actuality, the process is a lot more exacting and complex.
How they work.
The cells have a positive and a negative side. In the middle of the cell where these two sides meet (the P-
Regardless of the size of the cell, it has a potential voltage of about 0.5 volts (0.475 to 0.625 volts depending on the manufacturer). So roughly one volt is produced by having two cells in series (2 x 0.5 = 1). In order to develop enough voltage to charge a battery, you need 36 cells in series. According to our model based on 0.5 volts output per cell, a 36 cell panel would produce about 18 volts (36 x 0.5 = 18).
In actuality, you'll find 36 cell panels on the market that produce any where from 16.5 volts to 19.5 volts depending on the type of cell and who makes it. This may seem to be overkill on voltage considering that you are only trying to charge a 12 volt battery! However, there are other factors that cause a voltage drop in these panels, but more on that later.
The size of the cell has everything to do with how many charging amps are produced (even if the voltage isn't affected by cell size). The bigger the cell, the more sunlight it can receive, and therefore, the more amperage it will produce. It really is that simple.
All our solar panels are high grade and operate above 17.7 volts! This extra voltage really produces a big boost to the charging amperage when used in combination with our 2512iX and the New TriStar maximum power point tracking charge controllers. Check out our DIY Solar Ware House System Cores which incorporate these products.
How they are rated.
Panels are rated in Watts of output. This wattage rating is derived by multiplying the panels peak power voltage times its peak power amperage (Watts = Volts x Amps). These ratings are based on standard test conditions (STC) of 1000 watts/square meter of light input, a cell temperature (not air temperature!) of 25 Degrees C (77 Degrees F), and an air mass of 1.5 (slightly above sea level with the Sun at a slight angle to the panels).These standard test conditions are rarely found in "real world" operating conditions. For example:
1000 watts/square meter of sunlight would only be reached around solar noon, with the panel squarely facing the sun, just after a rain shower has washed all the dust out of the air. "Real world" input is usually around 750 to 800 watts/square meter on a bright day (when you factor in dust and air pollution and consider that the panels are laid flat on the roof and are therefore not square to the sun).
When you consider that solar cells are dark blue to almost black, they soak up sunshine and get quite hot so they are operating at temperatures considerably higher than 25' C (77' F). This increased cell temperature translates into a voltage drop and therefore less output.
The air mass changes as you move from sea level to mountaintop. The atmosphere is thicker at sea level so more sunlight is interrupted by dust and pollution and less gets transformed into solar electricity. Conversely, the same panel operating from a mountaintop will see more intense sunlight and will produce more power.
This is not to imply that panel manufacturers are purposely trying to deceive you. It is because "real world" operating conditions are so variable that they had to come up with some standard test conditions so that all panel ratings are derived after being subjected to the same conditions as every other panel. Therefore, you have a basis for comparison between manufacturers. Just realize that on average you will probably only see about 80% of the rated output of solar panels in "real world" operating conditions. See our discussion of Standard Test Conditions (STC) versus Normal Operating Cell Temperatures (NOCT) for further understanding.
How reliable are they?
There is very little that can go wrong with a solar panel short of physical damage. In fact, all panels pass Jet Propulsion Labs Block V tests, which are: withstanding 125 m.p.h. wind loading, surviving one inch hail at terminal velocity (52 m.p.h.), and thermal cycling at temperatures beyond what you will find here on Earth (short of tossing them into molten lava !!).
There are no moving parts to wear out and they don't consume any fuel. As long as there is enough light to cast a shadow on the ground, they will produce electricity. They are so reliable that Crystalline panels come with 25 year warranties and the Amorphous panels now come with 5 to 25 year warranties.
The Crystalline panels have been around long enough to have earned their 25 year warranties. They are fully expected to last longer than 35 years. The Amorphous panels are relative new comers and the earlier versions had some troubles with power degradation over time and delamination. The newer versions claim to have overcome the worst problems and have 25 year warranties. However, they have not actually been around long enough to prove that they will survive those 25 years.
Amorphous panels may prove to be the panels of the future eventually, but right now they haven't "earned their wings" so to speak. Considering that they cost essentially the same per watt as crystalline panels and may be dead in 10 years, I firmly believe the best investment for my money would be the crystalline panels.
How efficient are they?
Since roof space on an RV is at a premium, efficiency is worth considering. Efficiency in this situation is defined as: How much of the available energy in sunlight is transformed into usable D.C. electricity? In other words: How many of those 1000 watts/square meter coming from the sun will be available to you?
Amorphous panels are about 6 to 8% efficient. So you would expect to have about 60 to 80 watts/square meter. Amorphous panels are the least expensive per watt but require twice the roof area to equal the power of the crystalline panels.
Screen Printed Poly-
Screen Printed Mono-
Our InnoTech ECO220 Back Contact Mono-
As far as their efficiencies at the extremes of the operating environment (low light or high cell temperatures), they each perform a little differently.
In high cell temperature conditions ALL panels experience a voltage drop. Crystalline panels will perform well as long as they have at least 36 cells and an operating voltage of about 17 volts or higher. All our solar panels are custom made for us and operate above 17.7 volts!. With high voltage like that, you never have to worry about voltage drop due to high temperatures.
What affects output?
There are several factors that affect the output of solar panels. The most important ones are listed below:
Light Intensity. The brighter the sunlight the more power the panels will produce. This is effectively a straight line correlation. So, if there is 1000 watts/square meter of sunlight ,you will see the full rated output of the panel. If there is 500 watts/square meter, you will see half the rated power of the panel.
Shading. The shade caused by tree branches, TV antennas, satellite receivers or air conditioners will dramatically affect the output of solar panels. So, place the panels where they won't be in the shade and keep them clean! This is most important during the 3 hours either side of solar noon (9AM to 3PM Standard time or 10AM to 4PM Daylight Savings time) as that is when you receive about 95% of the days charging power. Morning and evening sun is at a lower angle and it is sometimes hard to avoid some shading from objects that are permanently attached to your roof. But those hours are less powerful and don't contribute that much to the days charging amperage anyway.
Cell Temperature. The cells of solar panels are dark in color and operate at much higher temperatures than what the air temperature is. The hotter these cells are, the more of a voltage drop is experienced. This is why it is important to use panels that have operating voltages well above 17 volts. It takes 14.1 to 14.4 volts to fully charge most batteries (some want 14.6 to 14.8 volts) and a 17 volt panel may drop about two volts when they get hot. This means that they only have about 15 volts to charge with. When you add in the other voltage drops caused by wire size, connections and the controller, you may not be able to push the batteries up to the finish voltage they require. So, pay attention to the operating voltage of solar panels when you are considering buying them!
Angle of Sunlight. If the panels are not squarely pointed at the sun, you will lose some of that sunlight to reflection off of the surface of the panel. For the most part, the panels are mounted flat on the roof, so, you can expect to lose some power to reflected light. This is most pronounced in the winter when the sun is at a lower path across the sky. There are tilt mount options that will allow you to tip the panels into the sun and, therefore, recapture those rays. Just remember to lay them back down before you take off down the highway!
Improper Wiring. Poorly made electrical connections create resistance and result in less power going to your batteries. Make 'em tight and keep 'em clean! Also make sure to use the proper gauge wire to carry the amount of power you are producing. Using too small of wire will result in voltage drop (remember the discussion on voltage drop above?)