Solar panels, also called photovoltaic (PV) panels, produce DC power from sunlight to charge batteries and provide electrical power. The panels can be used single or in series or parallel arrays.
Many factors will affect output; and panels will rarely generate power equal to their laboratory rating.
As a general rule, expect panels to put out 60% of their rating under absolutely best-case scenarios.
There are several types of PV widely available on the market, and many more in research stages. PV are described by how each panel's cells are constructed.
Mono or single crystal cells are made from complete slices of silicon crystal. Mono cells have rounded edges because they were cut from a single cylindrical crystal.2)
Strained metaphor: if poly cells were it were paneling they would be veneer because the visible surface is made from one piece of material.
Poly cells are made up of smaller pieces of slices. Poly cells are rectangular.4)
If poly cells were paneling they would be OSB, because they are made up of many flat pieces of silicon.
Flexible panel configurations place unusual demands on the materials. Framed panels use extremely durable glass encapsulation6) but this would not work for flex panels. One challenge is to come up with encapsulation that is transparent, strong, and durable.
Originally a clear film called PET was used for the exterior but over time durability issues related to yellowing, increasing opacity, and delamination (“peeling”) were revealed, especially in high temperatures or harsh environments. ETFE is now commonly preferred for exterior encapsulation layers due to greater UV transparency and durability. Flex panels that do not mention ETFE are likely encapsulated in PET.
The other challenge is to make solar cells that can slightly flex.
While the flex panel market has largely gone to poly/mono crystalline, there are obvious issues related to making flexible objects out of brittle silicon crystals. It is not clear why the market went that direction.
Handle with care, and bend as little as possible.
[Note: actual thin-film panels are rarer as of 2018. The market has gone to flexible mono or poly7)]
Thin film or amorphous PV uses photovoltaic material deposited on a substrate rather than silicon crystals.
If thin film cells were were paneling they would be colored plastic veneer because it is produced inexpensively and is least affected by ambient conditions.
While most panels are hard mounted horizontally on the roof of the van, portable folding portable solar panels have dropped in price and have some advantages.
12V panels are not really 12V; they are called that because they charge 12V battery banks (which aren't really 12v either!). Since they are called 12V we say nominal (ie, “named”) 12v. [yes, it's confusing. – frater secessus]
higher voltage panels (24v = 72 cell, 20V = 60 cell) are usually cheaper by the watt than 12V (36 cell)
higher voltage panels are more common on the used market, as people upgrade residential/commercial installations
MPPT controllers do a DC-DC downconversion that is quite efficient. If Vmp isn't required they will move PV voltage away from Vmp to prevent power from ever getting to the CC.
Higher voltage panels can reduce wire costs; amps are cut in half for the same amount of wattage. They are more likely to stay above charging setpoints in poor insolation or high heat.
Higher efficiency doesn't mean the panel makes more power; it means it makes more power from a given area. 100w is 100w, but you might be able to fit a 110w panel high-efficiency panel in the same space as a regular-efficiency panel.
In general, mono has higher efficiency than poly, and poly has higher efficiency than thin film.
The cost of higher efficiency panels typically outstrips the increase power, so unless you are tight for space they typically aren't a good value for money. If you are tight for space and need the power then pony up the cash and enjoy the premium product.
At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, and 32 watts is ultraviolet radiation. – wikipedia
Mono panels can capture about 15-20% of this visible light energy. Poly panels capture 13-16%, and film panels capture 9%.
PV are generally rated by several criteria17)
Note that in this example the Power (W) rating is 190, which is the Vmp x Imp (28.60 x 6.64 = 189.904W).
In real world conditions power output will likely be less than under optimal lab conditions and the Vmp may not be at the voltage given on the label. Vmp will vary due to local conditions like temperature, shade, and sunshine. An MPPT charge controller, if present, will sweep the range of voltages regularly to find Vmp under the existing conditions.
Panels will get closest to their rated output at ambient temperatures around 32f/0f when the panels will be running about 85F. By the time ambient temperatures are 90F panel temps will rise to 145F and power output will drop about 18.45%.18)
To help consumers understand output outside the lab (Standard Test Conditions - STC) some producers also publish specs for conditions that might be more applicable to actual use (Normal Operating Cell Temperature - NOCT). Here is how they differ:19)
|Irradience||1000W / square meter||800W / square meter|
|temperature||cell temp 25C20)||ambient temp 20C|
While NOCT may be useful for gauging normal harvests, STC is used for system component23) sizing because the panels really can make STC power24) in some real world conditions. The system needs to be sized to deal with high-output situations, particularly overly high panel voltages.
A diode is designed to let current flow in one direction…. [it] is the electrical equivalent of a [plumbing] check valve. – Amy@AltE26)
Bypass Diodes are inside the panel junction box, wired parallel to each cell group. It conducts when the cell is shaded and has reverse polarity due to other cells producing. Blocking Diodes are external to the panels. It blocks reverse current from other panels. It must handle the full voltage of system (series panels). – Pappion27)