Solar Module Voltages
Until recently silicon was typically made in 5″ ingots. The ingot would be sliced with a diamond saw and 72 5″ cells would fit in a metal frame, producing a photovoltaic panel called 24 volt nominal. Now silicon is usually produced in 6″cells and 60 cells now fit in a regular sized frame; these 60 celled PV panels are called 18 volts nominal. Most panels are currently made with 6″ cells.
A 12 volt panel, for example, doesn’t put out 12 volts but it produces enough voltage to charge a 12 volt battery. It produces around 18 volts and has an open circuit voltage, without a load, of 21 volts.
An 18 volt panel puts out around 24 volts and its open circuit voltage is around 36.
A 24 volt panel works at around 32 volts and its open circuit voltage is around 45 volts.
So you can see that the voltage of a panel can be confusing. With an 18 volt panel, you can put more of the panels in series without getting too high a voltage for a charge controller or an inverter, and at the same time you get more amps — and it is the amps that do the work.
Silicon modules can be made in either a mono or polycrystaline configuration. A silicon solar panel that is mono crystalline uses a more pure form of silicon and this can produce the most power. Unfortunately, It is more difficult to manufacture a mono crystalline panel and hence they often are more expensive. Poly crystalline panels are easier to make and the advances in technology with poly crystalline makes them approach the efficiencies that used to be only achievable in mono crystalline panels. Mono crystalline photovoltaic panels also have the ability to perform better in high heat areas although some poly crystalline panels can also perform well in high heat. In the spec sheet for a panel is a temperature coefficient. This tells you how much electricity you will lose for a temperature rise in degrees. Panels are normally rated at 25 degrees C.
When an ingot of silicon is cut, the middle slices produce less power than the end slices, which is a major problem for solar panel manufacturers. The low output cells used to be mixed up with the high output ones, but these low output cells bring down the voltage of the whole the string of cells to the lowest producing cells. Recent manufacturing techniques separate low output cells from those with a higher output and a good company can make the same panel with different wattage outputs. The tolerance for a panel with segregated cells would be usually 0 +3. This means that you may find a panel in your order that is three percent higher than the rated output but the panels wouldn’t go below the rated output. This is why the zero tolerance is so important.
Another group of panels are called thin film modules. These were created when silicon supplies were scarce but now thin film solar panels have a hard time competing against the new technologies that allow pure silicon to be made out of an inferior grade mined silicon.
Typically a thin-film PV panel can be CIS orCdTe. The CIS panels are Copper Indium Selenide and the CdTe are the Cadmium Telluride panels. CIS panels only have a 15 year warranty and they don’t have as good efficiency as a regular silicon solar panel. In addition, Cadminum Telluride panels have to be recycled as hazardous waste and for that reason we have never been fond of them. They also suffer from a reduced efficiency and a reduced life expectancy.
The PTC rating of a panels is called the Pacific Test Condition. It takes into account that the panel would be producing less in the morning and evening while the maxium output would be at miday when the sun is most perpendicular to the panel. The PTC rating is a more accurate determination of how much power a solar panel would produce. The STC or standard test conditions is the output of a panel when the sun is most pependicular to the panel. This is the rating that is used when companies tell you how may watts their panels put out.
There are many solar manufacturers that buy cells from large companies and then these companies put the cells in a frame, put glass on the front with some sort of waterproof backing and you have another company manufacturing a solar panel. The trick is to find a manufacturer that is going to be around in 25 years. The issue is not to doubt the panels’ performance in 25 years — solar panels have been around for decades and the old panels often test out to their original specs and most PV solar companies warranty their panels’ power output to be 80% in 25 years. The issue is more one of being able to purchase replacement panels in case of breakage, or to expand an existing solar system. However, you may need a crystal ball to see which companies will still be around in 25 years.