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.
To avoid overcharging,1) a charge controller is placed between the PV and the battery bank.
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 much less than their rated wattage under normal conditions.
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 solar panels10) may have some advantages.
Note that regular framed panels can also be carried as portables. To make storage/placement easier they are usually 100x times however many you need.
Some panels are “half-cut” which means the cells are cut in half and wired to make 2x as many cells. This can result in better harvest in some partial shade conditions with some increase in complexity and expense.
{Secessus reminds us we should avoid shade in the first place}
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.
Panel voltage16) is an important factor when selecting a solar charge controller.
The formula for panel efficiency is simply [ rated kW / square meter ]. A 20% efficient panel will make 200w per square meter under lab conditions, ie, 20% of the lab's 1000w standard. (0.20kW per square meter)
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 higher efficiency panels might be worth the premium.
Framed panels are often warranted to make 80% of their rated output for 25+ years.
There is some evidence that panels with a load connected degrade slower than those stored unconnected (open circuit)
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.
Panel voltage decreases as cell temperature increases. Ramifications:
Example: 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%.19)
The actual amount of drop is dictated by the temperature coefficient, expressed as -0.X%/ºC. In other words, the Vmp will go down by X% per degree celsius of cell temp above 25C. Since
This coeffcient ranges from 0.3% to -0.5%. Crystalline panels average around -0.44% and thin-film lower (in the thirties). The general pattern seems to be that cell chemistries with higher Vmp tend to experience greater heat-related losses. “Mind the [air]gap”.
Let's consider three different panels in 80F ambients:
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:22)
| STC | NOCT | |
|---|---|---|
| Irradience | 1000W / square meter | 800W / square meter |
| temperature | cell temp 25C23) | ambient temp 20C |
| wind speed | n/a | 1m/s |
We can see that with NOCT the sunlight is not as strong, and the panels are assumed to be much warmer24) though some cooling from ambient breezes is present.25)
While NOCT may be useful for gauging normal harvests, STC is used for system component26) sizing because the panels really can make STC power27) in some real world conditions. The system needs to be sized to deal with high-output situations, particularly overly high panel voltages.
NOCT is useful for comparing two panels [that have] the same STC rating. A panel with a higher rated power at NOCT for example, will generally result in a higher performing panel.28)
A diode is designed to let current flow in one direction…. [it] is the electrical equivalent of a [plumbing] check valve. – Amy@AltE30)
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). – Pappion31)
Further reading:
