Solar Energy Systems - Solar Modules - Solar Electric System Design - Solar Power
The heart of a photovoltaic system is the solar module. Many photovoltaic cells are wired together by the manufacturer to produce a solar module. When installed at a site, solar modules are wired together in series to form strings. Strings of modules are connected in parallel to form an array.
Module Types – Rigid flat framed modules are currently most common and most of these are composed of silicon. Silicon cells have atomic structures that are single-crystalline (a.k.a. mono-crystalline), poly-crystalline (a.k.a. multi-crystalline) or amorphous (a.k.a. thin film silicon). Other cell materials used in solar modules are cadmium telluride (CdTe, commonly pronounced “CadTel”) and copper indium diselenide (CIS). Some modules are manufactured using combinations of these materials. An example is a thin film of amorphous silicon deposited onto a substrate of single-crystalline silicon.
In 2005 approximately 90 percent of modules sold in the United States were composed of crystalline silicon, either single-crystalline or poly-crystalline. The market share of crystalline silicon is down from previous years, however, and continues to drop as sales of amorphous silicon, CdTe and CIS modules are growing.
Building Integrated Photovoltaic Products – PV technology has been integrated into roofing tiles, flexible roofing shingles, roofing membranes, adhesive laminates for metal standing-seam roofs, windows, and other building integrated photovoltaic (BIPV) products. BIPV modules are generally more expensive than rigid flat modules, but are anticipated to eventually reduce overall costs of a PV system because of their dual purpose.
Rated Power – Grid-connected residential PV systems use modules with rated power output ranging from 100-300 watts. Modules as small as 10 watts are used for other applications. Rated power is the maximum power the panel can produce with 1,000 watts of sunlight per square meter at a module temperature of 25oC or 77oF in still air. Actual conditions will rarely match rated conditions and so actual power output will almost always be less.
PV System Voltage – Modern systems without batteries are typically wired to provide from 235V to 600V. In battery-based systems, the trend is also toward use of higher array voltages, although many charge controllers still require lower voltages of 12V, 24V or 48V to match the voltage of the battery string.
Using Manufacturer’s Product Information to Compare Modules – Since module costs and efficiencies continue to change as technology and manufacturing methods improve, it is difficult to provide general recommendations that will be true into the future regarding, for example, which type of module is cheapest or the best overall choice. It is best to make comparisons based on current information provided by manufacturers, combined with the specific requirements of your application.
Module Cost per Watt – As a general rule, thin film modules have lower costs than crystalline silicon modules for modules of similar powers.
Module Efficiency (Watts per Area) – Modules with higher efficiency will have a higher ratio of watts to area. The higher the efficiency, the smaller the area (i.e. fewer modules) will be required to achieve the same power output of an array. Installation and racking 9 costs will be less with more efficient modules, but this must be weighed against the higher cost of the modules. Amorphous silicon, thin film CdTe and CIS modules have rated efficiencies that are lower than crystalline silicon modules, but improvements in efficiency continue.
Amorphous Silicon in Cloudy Climates – Of importance to the Pacific Northwest, amorphous silicon modules have higher efficiency than crystalline silicon under overcast conditions. In cloudy weather, all types of amorphous silicon modules tend to perform better than crystalline silicon, with multi-junction (i.e. double- and triple-junction) amorphous silicon modules performing as much as 15 percent better. In Britain, which has a similar climate to ours, multi-junction amorphous silicon modules have been shown to produce more power over the course of the year than crystalline silicon modules.
Poly-crystalline or Single-crystalline Silicon? – The power output of single-crystalline and poly-crystalline modules of the same area is quite similar.5 Both types of crystalline silicon are very durable and have stable power output over time. Therefore, do not be too concerned about the distinction between single-crystalline and poly-crystalline silicon in selecting a module.
Silicon Modules versus Other Module Types? – The power output of CdTe modules has been less stable than silicon modules,6 although improvements are being made. For the time being, as for modules of any type, check manufacturer’s warranties.
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