Along with a growing demand for PV systems comes pressure for lower-cost and higher-efficiency equipment, safer operation, and faster installations. Manufacturers of PV equipment—inverters, modules, racks, and other balance-of-system components—respond with innovative devices, drawing upon feedback from installers and designers.
Changes in PV modules happen in two areas. First, manufacturers make incremental improvements that result in efficiency—and power—gains. Second, major design changes happen, like increases in the size or number of cells in a module, different frames or mounting methods, and different materials used for the front and back protection.
The PV industry experiences a seemingly inevitable march toward more powerful, larger modules: In 2000, 75-watt modules were commonplace; in 2009, 200 W modules were plentiful; in 2012, 250 W modules were becoming the norm. Today, there are more than 1,000 different models of 300 W or greater modules on the market and manufacturers continue to expand the selection in this size range.
Larger modules mean more power installed more quickly—15 years ago, a 3,000-watt array would have meant racking and wiring 40 modules. Today, that can be accomplished with 10 modules. Fewer wired connections mean savings for both the system owner and the manufacturer. For framed modules, the ratio of aluminum frame material to the module footprint decreases with wattage increases, which lowers costs for manufacturers and reduces mount costs.
Most higher-wattage modules, in the range of 300 to 445 W, are designed with 72 (or 96) cells in series, rather than 60 cells, which was the standard for many years. Six-inch-diameter cells have replaced 5-inch cells—a 44% increase in surface area, and more area means more current per module.
Because all those cells are wired in series, higher voltage is produced as well. The open-circuit voltage on 72-cell modules can exceed 45 volts (60 V on 96-cell modules). Higher-voltage modules can limit design options for arrays connected to step-down charge controllers or string inverters. Higher voltages equate to bigger “building blocks” and means fewer choices for PV string sizing. Depending on the equipment’s voltage window, it may be impossible to specify the right number of 72- or 96-cell modules that won’t exceed the equipment’s high-voltage limit on the coldest days, nor drop below its low-voltage window on hot days. This is especially true for climates that experience wide temperature swings.
Higher-wattage modules can sometimes limit the overall array size. For example, if a string inverter has a voltage input window of 300 to 600 VDC, in many locations when temperatures are hot at least 10 of the 72-cell (300-watt) modules need to be in series to keep the inverter humming. This translates to a (minimum) 3,000 W array—with only one string. Furthermore, with average 72-cell module specifications, if the expected low temperature is 14ºF (-10ºC) or less, then the maximum number of modules in series would be 11. This results in a very narrow range of system size options: 3.0, 3.3, 6.0, 6.6 kW, etc.
One solution is to use an inverter with a wider maximum power point tracking (MPPT) window (such as the PowerOne Aurora PVI models, which have a window of 200 to 530 VDC). This could allow as few as six or seven modules in series. AC modules or microinverters (see below) are another option for high-wattage modules.
Higher-wattage modules also have a larger footprint. For example, while 72-cell modules are generally about the same width (about 39 inches) as 60-cell modules in the 180 W to 260 W range (six cells wide), they are 12 cells high instead of 10—about 12 inches taller and 8 pounds heavier. This increases the load on the rack attachment points, with the same amount of rail holding more module weight. Also, more caution is needed when moving larger modules—typically it requires that two people carry each module.