The A-Z of PV Module specs – explained
In this article we will investigate PV module Spec Sheets or known also as “cut sheets”. Since there are literally thousands of modules available on the market it is necessary to know how to use that information most efficiently. Let’s start with the most obvious features of the cut sheet, the mechanical aspects of the module.
These will be given in millimetres and inches, which also determines the surface area – it gives installers an idea how the modules should be fitted on the roof, ground – or pole – mount installations. After the layout is determined, the space between modules has to accommodate for mid clamps and end clamps, usually one inch apart for each clamp. The area of the module is crucial to determine weight load or to calculate wind forces acting on the modules and structure they are attached to. The thickness of the module will determine type of clamps the installer is going to use. Quite often mid clamps differ only with the bolt length, whereas end clamps are shorter or longer depending on module thickness. Weight is also listed in most specifications, since there is always a limit it can be added to the roof structure. Point to remember: many permitting authorities will accept PV modules to be mounted on pitched roofs without professionally engineered design, so long as there is only one layer of existing roofing material present.
There are different types of solar cells: mono-crystalline, poly-crystalline or thin film. There are variable numbers of cells per module from 36 to 108 but most common are 60 and 72 cell modules. Most PV modules cells operate t near 0.5V and quite often they are connected in one string in series, yielding 36V per module so called Vmpp at maximum power output. However, other connections like 2 strings of 36 cells will yield around Vmpp = 18V per module. the surface area of the cell will determine current output, Impp.
Most if not all solar modules have a plastic back sheet that seals the cells against environment. Its material is usually white but some modules come with black back sheet to match the customer’s aesthetic needs. It has to be handled with care since it is fragile underbelly of the module. To protect crystalline cells and simultaneously provide transparent surface modules use low-iron, high-transparency tempered glass with an anti-reflection surface treatment. Low iron glass has high clarity, and tempered glass shatters into small fragments, instead of sharp shards, if broken. Modules are strenuously tested for weight loading and impact resistance, and the front glazing of a module is extremely durable. Thin-film modules may use a polymer film (plastic) as the front sheet, which is designed for arrays in high-impact environments.
The module lead’s connector type is important. Often called “quick-connects,” many new products are on the market. The old standard—Multi-Contact MC4 has been joined by Tyco, Radox, Amphenol, and others. The 2011 NEC mandates that these connectors be touch-safe and, for circuits greater than 30 volts, require a tool for opening. Most of these connectors are not cross-compatible, so mixing modules will require properly mating connectors. Factory-installed module leads will be listed in the spec sheet with wire size, insulation type, and length of the leads (positive and negative leads are not always the same length). Wire diameter generally ranges from 14 AWG to 10 AWG; or they may be listed in square millimetres (mm2).
Shading a small part of a PV module can have a disproportionally large effect on its output. Additionally, when a module is partially or completely shaded, the current flowing through the module can reverse direction and create hot spots, which can lead to deterioration of the cell, the internal connections, and the module back sheet. A bypass diode stops the reverse flow of current and also directs electrical flow around the shaded section of the module. Nearly all modules come with factory-installed bypass diodes, with the exception of some thin-film modules. A typical 72-cell module with all the cells in series will have three bypass diodes, each bridging a series of 24 cells that can be bypassed if any or all of those cells are shaded.
Standard test conditions (STC) are the conditions under which a manufacturer tests modules: 1,000 W per m2 irradiance, 25°C (77°F) cell temperature, and 1.5 air mass index. Real-world operating cell temperature is often 20 to 40ºC above the ambient temperature. STC (bright sun and a relatively low cell temperature) are not typical for field operation of modules, but they do provide a consistent standardized reference to compare modules. An I-V curve (current-voltage) curve is generated at STC for every cell and module manufactured. The I-V curve contains five significant data points (Pmax, Vmp, Voc, Imp, and Isc; discussed below), which are used for system design, troubleshooting, and module comparisons. I-V curves can also be diagrammed for any operating temperature and irradiance level, but the points listed on a module specification sheet and those printed on the back of the module are at STC unless otherwise stated. Peak Power (Pmax or Pmp) The specified maximum wattage of a module, the maximum power point (Pmax), sits at the “knee” of the I-V curve, and represents the product of the maximum power voltage (Vmp) and the maximum power current (Imp). This wattage is produced only under a very specific set of operating conditions, and real environmental conditions (changing irradiance and cell temperature) will alter a module’s Pmax.
At STC and tested under load, voltage at max power (Vmp) is the highest operating voltage a module will produce. Vmp, adjusted for highest operating cell temperature, is used to calculate the minimum number of modules in series.
Open-circuit voltage (Voc) occurs when the module is not connected to a load. No current can flow in an open circuit and, as a result, Voc occurs at the point on the I-V curve where current is zero, and voltage is at its highest (Note: the module produces no power under open-circuit conditions.) Voc is used to calculate the maximum number of modules in a series string. Because voltage rises as the temperature drops, calculations are performed for the coldest expected operating conditions. This ensures that NEC parameters and equipment voltage limitations are not exceeded.
At STC, and tested under load, the maximum power current (Imp) is the highest amperage a module can produce. Imp is used in voltage drop calculations when determining wire gauge for PV circuits. This is a design consideration rather than an NEC ampacity calculation, for minimizing voltage drop and maximizing array output.
Short-circuit current (Isc) is the maximum amperage that the module can produce. There is no voltage when a module is short circuited, and thus no power. Isc is the measurement used to size conductors and over current protection, with safety factors as required by the NEC.
Frequently, nominal operating cell temperature (NOCT) specifications are also listed on a manufacturer’s sheet. These are measurements calculated at different conditions than STC, using a lower sunlight intensity (800 W per m2); an ambient (not cell) temperature of 20ºC; and a wind speed of 1 meter per second; with the module tilted at 45°. The NOCT value itself is the cell temperature—given in degrees Celsius—reached under these conditions, Compared to the STC 25ºC cell temperature, the NOCT value will always be higher, usually by about 20ºC. NOCT values are used to mathematically calculate other test condition data points without resorting to further laboratory tests. NOCT conditions tend to more closely resemble the field conditions PV arrays generally operate in, and so give a perspective on “realworld” module operation.
Power tolerance is the range within which a module manufacturer is stating the module can deviate from its STC-rated Pmax, and thus what the manufacturer warranty covers. Common values are +/-5%, -0%/+5, and up to +/-10%. A 200-watt module with a +/-5% power tolerance could produce a measured output of 190 to 210W. Finding modules with a -0% power tolerance can ensure the best value per dollar spent, and keep arrays operating at closer to predicted output. Module Efficiency & Cell Efficiency Efficiency is the measure of electrical power output divided by solar input. At STC, power in is equal to 1,000 W per m2 and power out is the rated Pmax point. Assuming a module sized at exactly 1 square meter, and rated at 150 W Pmax, module efficiency would be 150 W per m2 ÷ 1,000 W per m2, which equals 15%. The typical crystalline efficiency range spans 12% to 15%, but there are high-efficiency modules over 19%, and amorphous silicon modules on the low end with efficiencies around 6% or 7%. Cell efficiencies will be slightly higher than module efficiencies because there is usually a small amount of empty space between cells. When deciding what module to purchase, if W per square meter (known as power density) is the driving factor, then a module with high efficiency should be chosen. But in many instances, there is plenty of room for an array and price per watt will be given higher priority than module efficiency. Temperature Coefficients Modules are directly affected by both irradiance and temperature, and because of environmental fluctuations, also experience power output fluctuations. When exposed to full sun, the cells will reach temperatures above the STC temperature of 25°C. And sometimes cell temperatures are lower than 25°C, such as on cold winter days.
Temperature coefficients are used to mathematically determine the power, current, or voltage a module will produce at various temperatures deviating from the STC values. The temperature coefficient of open-circuit voltage is used to figure out the PV array’s maximum system voltage at a site’s lowest expected temperatures. The temperature coefficient of power can be used along with pyranometer measured irradiance to calculate the power an array should be producing, which can be compared to actual output to verify proper performance.
A limited warranty for module power output based on the minimum peak power rating (STC rating minus power tolerance percentage) means that the manufacturer guarantees the module will provide at least a certain level of power for the specified period of time. Many warranties are stepped—covering a percentage of minimum peak power output within two different time frames. For example, a common warranty guarantees that the module will produce 90% of its rated power for the first 10 years and 80% for the next 10 years. A 200 W module with a power tolerance of +/-5% means that the module should produce at least 171 W (200 W × 0.95 power tolerance × 0.9) under STC for the first 10 years. For the next 10 years, the module should produce at least 152 W (100 W × 0.95 power tolerance × 0.8). Module replacements are frequently done at a prorated value according to how long the module has been in the field. More manufacturers are now offering linear power warranties, which are represented by a maximum percentage power decrease per year for a set number of years, for example, that module power output shall not decrease by more than approximately 0.7% per year after the initial year of service, for the first 25 years.
At the end module specs are designed to help customers choose the appropriate product for their installations. The most important factors when choosing solar modules should not only the price per watt but the craftsmanship, efficiency and warranties. However, no matter how appealing a module is, it’s crucial how many years the producer was has been on the market and if the company is in good economic and financial standing. Some questions to ask: what are the warranties good for (what does it cover)? Is there an insurance company to backstop the company in case they go under? Do they have a linear warranty? How many years workmanship?
While quality of solar cells is crucial, the quality of connectors (quick connects) and wiring for instance cannot be underestimated since they also have influence on safety and efficiency of the whole system.
Unnecessary repairs due to poor quality of these components might cause unexpected and frustrating delays in power production. From an installers point of view, specs provide crucial information on how to connect modules to the inverter and how to design electrical layouts. Moreover, test conditions under which solar cells were tested are quite often misunderstood. STC testing conditions, while idealistic and rarely fulfilled in the real life conditions provide common ground for testing all solar modules, so different manufacturers can be compared to the same point of reference. To conclude, understanding and getting as much useful information from the specifications will result in better design and overall success of the whole installation. The next article we will discuss the “ins and outs” of micro inverters and power optimizers.