Solar Panels: What you need to know

[FLYSolartech-GiocoSolutions]

Hi everybody!
I'll take some time of my holidays in order to write this post on some subjects that customers frequently ask me hoping that could help someone here. I think it will be useful to discuss them in a public post so that everyone can benefit from it. Some of you already know me because they have already bought some flexible solar panels from my company, however with this post I'd only like to share my knowledge with you. I feel it as a duty since I always follow this forum in order to learn new technical .. about sailing – a subject in which I don't know that much (I don't think I am a great sailor lol).

Anyway, I don't want to talk about my company, this is only an informative post, so, please, contact me privately for anything related to the company.

It is difficult to summarize highly technical and nuanced issues in few words but I'll try to give you some basics about this theme.

Chapter1
SOLAR PANELS PRODUCTION DIFFERENCES

TRADITIONAL SOLAR PANELS (RIGID ONES)
Traditional solar panels are produced following the scheme below:



SEMI-FLEXIBLE SOLAR PANELS
Semi-flexible solar panels are produced following the scheme below, they can have two or more encapsulating layers (EVA) and different surfaces (PET, PTP, ETFE).




RELEVANT DIFFERENCES:
Semi-flexible solar panels have a different weight (usually 80% less than traditional ones) and they can take a more or less bowed shape, according to the kind of cell and the production process.
NB: Rigid panels are LESS expensive because they are produced in automatic production lines, moreover, since this panels don't have to flex, the materials used are completely different.
However, 90% of rigid panels are not covered by warranty if installed in the marine environment. Only a few producers build them following the standards of "salt mist" test. This means not only that panels will suffer frame and incapsulation damages – since traditional panels have a rigid frame, they only have two layers of encapsulating coating while semi-flexible panels usually have four layers – but also that they are not covered by any warranty.

Corrosion test allows verifying the resistance of components and materials used in high salt mist density areas. Indeed, salt can reduce the resistance both of metal and non-metal parts.level 1 of corrosion test, according to regulation IEC60068-2-52, is used to test products used in marine environments or near the sea and exposed to this conditions for the majority of their working life (e.g. Naval items). Level 1 is usually used as a general corrosion test in the warranty procedures for the components quality.
This test usually lasts 28 days and is made of 4 cycles. Modules are put in a specific room for 7 days. A salt mist solution is nebulized in the room at 33°C and 85% of humidity. This procedure is repeated for times in each test cycle. The accelerated lab test simulates the effects of an environment characterized by high salt concentration, during the entire life of the panel.

Chapter 2
POLYCRYSTALLINE – MONOCRYSTALLINE - SUNPOWER
first of all, I'd like to highlight that SUNPOWER CELLS ARE MONOCRYSTALLINE CELLS. They have the same structure of monocrystalline cells, the only difference is that they have contacts at the back of the cell, allowing it to have a uniform production surface. Since BUSBARs are not present on the cell, it produces more nominal power output at the same amount of surface. For this reason, I will talk about Sunpower as about any other monocrystalline cell.

The main difference between mono and polycrystalline is that the former can usually reach a higher percentage of efficiency (w/m2). However, this gap has been reduced in the last years and is now about 1.5% (between the best mono and the best polycrystalline).
On the other hand, polycrystalline panels are less expensive (the production process is simpler) and usually more flexible.

Which is the best one?
The only answer is that it depends on the situation...
Monocrystalline panels usually produce more instant Ampere when the sun is perpendicular to cells (orientation of crystals), on the contrary, polycrystalline ones will have lower peaks of production when the sun is perpendicular to cells and higher peaks when not perfectly exposed to the sun. Let us omit the description of the reasons (orientation of crystals).

So, can you direct the panel? You'll need a monocrystalline.
Is the panel flat maybe because placed on bimini? Do you have enough space? A polycrystalline will be the right choice.

You don't have much room, you should choose high-efficiency panels as Sunpower ones.


I'll show a simple video, made by our customer, in order to show you what I have explained. The panels are laid out as if they were on Bimini. Results: Sunpower panel: 4.5A; poly panel: 5A. If directed, the result will be the opposite.


https://youtu.be/EYxvz4bLsQk



Chapter3
Differences among semi-flexible solar panels

I'll try to explain the differences among the panels and how this influences the price.

Layers number: semi-flexible panels are made of at least 5 layers. Chinese panels usually use 5 layers, in order to keep prices lower (the structure is similar to that of rigid panels). High-quality panels are usually made of 7 or 9 layers, according to the type of superior and inferior layers.
Cells quality: like all industrial products, also cells have a rating of their qualities. They are divided in:
1. GRADE A: perfect cell
2. GRADE A-: (valid only for mono) the cell is perfect form the technical point of view but as non-uniform color
3. GRADE B: not perfect cell
4. GRADE C: cell with flaws
- BUSBAR type and connections, silver or aluminum
- Production materials, excellent encapsulators, PET or ETFE (this is a long issue if someone is interested I can write a comment on it).


How can we recognize them? It is impossible if you don't have a flash test machine. Anyway, some Chinese panels with Sunpower cells are sold at a price that is equal to the price of purchase of the Grade A Sunpower cells. This can make you understand that the cells used are not of grade A..





PET coating :In our case, it is used as an external coating for flexible panels because it has good mechanical resistance, malleability, and flexibility.


Given the degree and duration of the protection offered by the usage of these materials, it is difficult to suggest indiscriminate use in areas with constant or persistent adverse weather conditions. It is well known that PET has a low thermal resistance in the short term and therefore usage in very hot environments is not recommended.



In addition, PET is unable to withstand chemical attacks caused by the various acid and alkaline compounds dissolved in the water which will, in the long run, cause yellowing and then delamination or breakage of the plastic layer.

The strengths of this type of product thus focus on the flexibility of the panel, its low weight and small dimensions (for both panel and the junction box) and above all in the economical saving due to the use of widely used and easy to fabric plastic polymers.


However, if we look at a field of application such as the nautical one, we realize that these properties, though surely beneficial, are not sufficient to guarantee a high durability of the product.
At sea, exposure to deteriorating agents is practically constant, and in some cases, physical stress caused by a very rough sea conditions can also cause damages to the photovoltaic cells if they do not have adequate protection. For this reason, the best performing panels are built using ETFE.


Concerning the use of ETFE on semi-flexible panels, I suggest you the following link ETFE Based Semi Flexible Solar Panel - Fly Solartech .

Chapter 4
Installation: parallel or in series?
Classic question, simple answer. Parallel when the panel could be potentially covered by shadows. If the panels are compatible the best solution is to install them using Schottky diodes. I found this video (that allows me to avoid a long explanation) that explains briefly why parallel installation is to prefer.


https://youtu.be/1qD3mN8VotQ


Chapter5
PWM or MPPT

MPPT and PWM are the two main types of charge controllers used in the photovoltaic industry for the charging management of the batteries connected to the PV system and during the planning phase of a stand-alone (which means it's not connected to the grid) or on-grid PV system, the right choice of the method of charging and the corresponding controller, is a key component for a smart and conscious setup of your own photovoltaic system.
Our comparison, however, cannot start without the necessary description of the two distinct types of regulators.
Following a chronological order, it makes sense to introduce for first the PWM (acronym of the English term Pulse Width Modulation) technology, introduced on the market when the interest in photovoltaics was addressed solely to off-grid and stand-alone applications.
In this sector, in fact, the "classics" photovoltaic modules features a 36-cell structure with an open circuit voltage equal to about 18 / 20V, designed for charging of the typical 12V batteries even in case of panel overheating conditions as a result of which it can, in fact, occur a decrease in the voltage output by the module itself.
Precisely because the fact that the normal photovoltaic modules deliver current at a voltage which is normally higher than the one at which the storage system operates, the principle of operation of the PWM regulators can be imagined as a switch that operates a rapid series of connections and disconnections between panel and battery in order to protect and control the battery voltage since this is never connected for excessive periods of time to the panel.


Therefore on a practical level, during the charging phase (we assume that the battery is initially discharged) the panel voltage is reduced depending on the closing and opening time of the PWM controller switch so that the panel voltage is the same of the battery. As the battery keeps charging, the charging voltage continues to increase. Finally, when the battery voltage absorption threshold is reached, to protect it from overcharging problems, the regulator switch is constantly open and closed. It is precisely from this mechanism that takes name the PWM class regulators.


This mechanism makes several aspects clear, such as PWM controllers are simple switches and NOT "DC to DC" converters since, to protect the storage system, they are forced to give up part of the additional power that the panel could generate because they can not convert it to amperes at the correct voltage. On the other hand, as the system is brought to the operating standard of the batteries, they undergo less thermal and electrical stress during the charging phase, thus extending life and allowing a virtually permanent state of float (ie maximum charge). It should also be kept in mind that being this the oldest technology, PWM controllers are generally cheaper and more reliable, especially when it comes to the complexity of internal electronics, compared to the MPPT counterparts.
Changing side, we come now to talk about the most recent of the two photovoltaic recharge technologies, introduced when the photovoltaic industry was also beginning to take up on photovoltaic systems connected to the national grid.


Here, however, the panel standard changes and from modules of 30/36 cells it passes to 60 or more cells, with (much) higher open voltages than 30V. In dealing with these panels but always using common 12V batteries, it immediately appears clear that using a PWM controller would waste at least half the power that the panel could deliver from its operating voltage.
It is, therefore, necessary to have a more complex energy collection mechanism that takes into account all the electrical characteristics of the system: from panel to battery.
These features and their correlations can easily be understood by using graphs describing the Ohm law P = V*I on a Cartesian plane Voltage (Vx)/Current (Ay), in which the power of the panel is expressed as the curve shape:

Then the same curve is matched with a second graph placed on a Cartesian plane Power (W)/Voltage (V) always observing the Ohm law P = V*I and deducted by the inverse derivation of the first (of which it represents the slope, that is the graphical representation of the derivative concept), which allows us to understand under what conditions we can obtain as much energy as possible from the panel at a given operating voltage, i.e. The MPP (Maximum Power Point).

The Determination of the Maximum Power Point within this graph occurs between two extremes: the case where the panel does not produce energy since it short circuit (0*I = 0) and the case where there is no load applied to the panel (V*0 = 0). Within this range, you can locate an area underneath the power curve, which reaches its maximum area just when the MPP meets the panel power curve:




Leaving aside further mathematical or geometric considerations on the identification of this point, we can state that the peculiarity of an MPPT regulator lies in its ability to detect the amperage and working voltage of the panel and convert them to the battery amperage and voltage, which makes these appliances real power converters.
In practice, an example of this kind can be helpful: assuming a 3A panel current, with a conventional PWM regulator this current (already calibrated by the controller for a 12V charging system) would be directly transferred to the battery.


An MPPT regulator analyzes the power generated by the panel (P = V x I, as mentioned before), and therefore considers the voltage as well: if we assume that this voltage is at that moment equal to 17V, the power delivered by the panel is 17V x 3A = 51W.


This means that if the battery charge voltage is 13V, considering the maximum power output of 51W, the charge current that will be transmitted to the battery is 51W/13V = 3.9A, which is about 30% more than what can bring the PWM controller.


Another remarkable feature of MPPT technology is its wide inter-compatibility: which means that it is possible to use panels designed to work at high voltages also to charge storage systems that work at significantly lower voltages. This, in turn, leads to another advantage: a lower power loss along the cables. In fact, by using high voltage panels, even considering long cable sizes from the panel to the controller, the loss of power at such high voltages (36V, 48V or more) is irrelevant to what would be on 12V systems.


Given their sophisticated technology, it must be noted that MPPT regulators generally have more advanced charging control functions: they often have systems that, by disconnecting panel and battery, prevent the inversion of the flow of current, a phenomenon that can occur at night when the panel does not produce electricity.


Given all these peculiarities, however, they can not miss some "disadvantages", which do not make the MPPT controller the only possible choice when looking for a charge controller for photovoltaic.
First of all, there is the purely economic factor: all the technology incorporated in the MPPT controllers often makes it double the prices, if not more, than the PWM counterparty with the same dispensable amperage, which is not an indifferent aspect for those who are planning a small-size system and therefore could decide to invest more on the panel than on the controller.

I hope you can be helpful, for any questions I am available