• The solar system
• Choice of module size
• Configuration of the solar system
• Installation
• Thermal expansion
• Solar cells
• Charge regulator

The solar system

A solar system for charging batteries consists of only 3 components:

Solar module + charge regulator + battery.
A photovoltaic solar system converts light to direct current in solar modules. The higher the incident light intensity, the greater the generated power. All solar modules – of whatever type – generate power linearly dependent on the intensity of the light. The nominal output indicated on the module is only realized in case of direct sunlight, clear dark-blue skies and the vertical incidence of light on the module. In case of heavy cloud cover, performance will drop down to 10% to 20% of the nominal output depending on the season.

Power generated by the module is used to charge batteries. Think of a solar system as a normal charger - e.g. alternator, mains charger or power generator – only that the charge current depends on the intensity of the light. In large solar systems, power will be converted to alternating current and fed into the public power grid.

As soon as light falls onto the solar module, power will be generated; i.e. during the day, your battery is permanently charged with a more or less large charge current. In order to prevent the battery from overcharging, a charge regulator should always be connected between battery and module. In literature, there are partly claims made that a charge regulator may be unnecessary under certain conditions. However, from our point of view, we strongly recommend the use of a charge regulator since any damage to the battery due to the lack of a charge regulator will be much more expensive.

In a solar module, many cells are connected in series like a chain. A solar cell supplies approx. 0.5V in the power point at 25°C and 0.4V at 75° cell temperature. In order to fully load a battery, the module needs to give off voltage of at least 15.0V to 16.0V. That's the reason why solar modules for 12V-systems are generally made of 36 to 42 cells.

Solar modules may be series-connected to increase the voltage (take into account the manufacturers' maximum system voltage). In order to increase current output, modules will be connected in parallel.

Choice of module size

In many solar installations, the available space will be decisive for possible module types and sizes. Even only partial shading of a single solar cell may considerably affect the module's power output – accordingly, for the selection of surfaces, any possible shading of modules must be taken into account and prevented by all means. 

In a solar module, the individual solar cells are connected in series – like a chain. If a shadow falls on one solar cell, it will only let as much power pass through as this solar cell itself can generate while being in the shadow. That means, this one shaded solar cell will restrict the current flow of the entire module.
If only a small part of a cell or of several cells is shaded, the power output will be reduced by only this small shaded area. For example, if a line is casting a shadow on the module, the module output will be reduced by approx. 5%.

Especially on sailing boats, several small, independently operating solar modules should rather be installed – instead of a few large solar modules. In practice, the power yield of the smaller modules will be much higher with the same nominal power installed.

Configuration of the solar system

For the configuration of the solar system, several factors need to be harmonized: power consumption, installed capacity, available area for installation, and irradiation depending on season and region. Daily yields of a solar system can here be determined online. 

The data assume horizontal installation without any partial shading. Based on the table, you will be able to determine online the daily yield your solar system will generate. In this respect, it has already been taken into account that the sun will shine more on some days and less on others. The value for yield will present a mean value which can be used in practice. On especially beautiful days, more power will be generated and less in drizzly weather. These fluctuations need to be bridged over by the battery. That's why a sufficiently large dimensioned battery will always be required in connection with solar systems. It is frequently more expedient to enlarge the battery capacity rather than the solar system so that fluctuations in irradiation can be better compensated.

Installation

Solar modules should be firmly installed on the vehicle or boat. Any mobile application can only be recommended with some reservation. It is to be kept in mind that solar cells are silicon crystals and thus so brittle that they are prone to break like an 0.2 mm pane of glass. Local bends, oscillations or point loads may soon result in permanent damage of the cells and thus a permanent loss of output.

Modules with stiff carrier plates or glass modules with frames are stiff enough due to their construction, but they cannot be walked on. Slightly flexible modules like most SunWare modules (Serie-20, Serie-40) must be firmly mounted on a stiff base such as the deck of a boat – modules can then be walked on with deck shoes without any problem. If these modules are to be mounted on the bimini top, a stiffening plate is to be provided behind the module.
Specially designed for mounting on bimini and spray hood, the modules of the TX series have been developed. These modules are enclosed in a textile and can be easily fixed with Tenax elements.

Thermal expansion

For the installation of solar modules, different thermal expansions for different materials are to be taken into account. Although a module actually has no movable parts, there is movement in the module. Different materials have different thermal expansions: Solar cells have very little expansion (factor=2), stainless steel little (factor=12), aluminium has moderate expansion (factor=24); in plastics, expansion is very high (factor=65). This results in the following changes in length at a temperature difference of 80°C and a module length of 1.0 meter:

Solar cells = +0.16mm
Stainless steel = +0.96mm
Aluminium = +1.9mm
Plastics = +5.2mm

This table shows very clearly why solar cells should not be embedded between thicker (greater than 0.5mm) plastic carriers, if at all possible. Because this extremely unfavourable combination of materials will inevitably result in extreme burdens on the cells and the electrical connections, as well as in major delamination forces. Since solar modules are permanently exposed to the weather, there are very frequently high temperature fluctuations in the daily and seasonal rhythm.

Solar cells

Currently, as of 2017, there are four different solar cell types on the market. You will certainly find other types in literature and print media; however, they have no relevant market share or currently still have laboratory status.

Types of solar cells:
amorphous solar cell
multi crystalline solar cell
mono crystalline solar cell
back-contact solar cell

Amorphous solar panels currently have a degree of efficiency of 10% to 12% and have always been described in the last 2 years as the solution for the cost problem of solar cells. Fact is, however, that during this long period of time not even one of the companies had been able to make money on a steady basis with these products; and today, amorphous modules have virtually completely disappeared from the market (exception: pocket calculators and the like)

Multi and mono crystalline solar cells currently have a degree of efficiency between 18% and 21% and can be considered to be equivalent today. Their performance differences are rather manufacturer-specific and process-specific and not crystal-specific and actually negligible in practice. For comparable poly/mono cells, there is a performance difference of only about 1%. This minimal difference of 1% can be easily compensated for by clever installation, e.g. by reducing shading.

With regard to back-contact cells, only Sunpower presented their permanent existence for many years already. These Sunpower cells have a very high efficiency – up to 23% (in ideal cases!). But these extreme efficient cells are very expensive. For the actual production manufacturer use mostly Sunpower cell with an efficiency of 21% - 22%. But in the advertisements the companies use very offten the maximum efficiency values from Sunpower.

Moreover, back-contact cells are considerably more expensive than standard solar cells (price/benefit ratio) and in production, they require a completely different manufacturing process. With back-contact cells, only few module formats can be realized since the cells can only be cut in a few formats.
One advantage specifically of the Sunpower cell is that this cell can be bent much more than conventional solar cells. In case of a cell rupture, only minimal loss of performance will be registered. It is to be noted, however, that curved solar cells have a much lower degree of efficiency due to the different irradiation angles on the cell.

Charge regulator

Every solar system has a charge regulator. The charge regulator has the function to protect the battery against overcharging and exhaustive discharge and to prevent, at night, reverse current flowing from the battery into the modules (although only a few mAs). All other functions of the charge regulator may be, but are not absolutely required.

In the long run, the permanent charge current from the module will overcharge the battery and damage it. Don't make false economies. Solar controllers utilize the specific properties of solar modules and may not be used as charge regulators for other power sources. For instance, solar modules may be short-circuited or lie in the sun with open cable ends without suffering any damage – in complete contrast to a wind generator. 

For the charge regulators currently on the market, 3 different types have prevailed.

• Shunt regulators
• Series regulators
• MPPT regulators

Shunt regulators and series regulators become active when the charge current must be limited. If the battery is fully receptive, the entire current from solar module to battery will simply be passed on. In contrast thereto, an MPPT regulator is permanently connected as a converter between module and battery. There will be no charging without the converter working. Accordingly, a high degree of efficiency is very important for an MPPT regulator and it should be more than 95% in the entire power range.
For shunt regulators, the charge current is reduced such that the module is short-circuited for a short time so that the voltage drops and no charging will be possible. This short-circuiting is effected 50 to 200 times per second depending on the manufacturer. The longer the module will be short circuited, the fewer current flows into the battery. A Schottky diode is installed between battery and module so that there can be no reverse current. However, the Schottky diode results in a permanent voltage loss of 0.6V. These shunt regulators have been proven for a long time and operate very reliably – even over many years. Series regulators have also been used for many years already and proved to be highly reliable. In the regulating process for this regulator type, the module is separated for a short time from the battery (50 to 250 times per second). The charge current may be changed via the length of shutdown. The series regulator can do without the Schottky diode. Active components here take over the reverse current protection. Voltage losses are considerably lower for this regulator (0.0V to 0.3V). The MPPT regulator presents a relatively new and complicated technology which opens up fascinating opportunities. The basic idea is to generate from any random input voltage a defined output voltage with the assistance of the DC/DC converter. If a small voltage is transformed to higher voltages, we talk about a step-up converter. In contrast, there are also step-down converters. As a rule, MPPT regulators can only transform in one direction to still realize an acceptable degree of efficiency. Basically, step-down converters have a higher efficiency than step-up converters. DC/DC converters are high-frequency converters which emit correspondingly strong EMC radiated noise and should not be operated in the vicinity of radio equipment. What exactly is the advantage of such MPPT regulators versus a "normal" regulator? Twelve volt solar modules are designed in terms of the cell number such that ideal charge voltages will result for 12V systems with customary operating temperatures (25°C air, 50°C cell). The MPPT regulator here cannot realize any performance advantage. However, if the modules are operated during the winter season at low temperatures (-10°C air, 20°C cell), the solar module will generate considerably higher voltage than in the summertime. From this higher voltage, a slightly higher charge current can then be generated with the DC/DC converter. MPPT regulators can bring to bear their potential when modules are used which are not designed for 12V systems (house roof modules with 56 cells). For a 12V system, these modules always have an excessively high voltage so that the MPPT regulator can transform this excess voltage to additional charge current. If you shortlist an MPPT regulator, keep an eye on a high degree of efficiency over the entire current range. For example: If the MPPT regulator is rated for 10A and a 5A module is connected, you will now operate the regulator always in a power range from 0 to 50%. With overcast skies, your module supplies 0.5A to 1.0A; i.e. the MPPT regulator must here realize, at 5% to 10% of nominal power, a proper degree of efficiency of more than 90%. In any event, ask for a precise efficiency curve for the regulator. If the MPPT regulator has an efficiency of 90%, it means at the same time that permanently 10% of the module power are lost. The converter must first of all make up this permanent loss through voltage transformation before there will be any profit at all. Sales documentations of MPPT regulators frequently indicate fantastic performance gains of +30% or even +40%. For 12V modules, such performance gains are possible only at the North Pole. So the question remains: Why not use directly a module which is suitable for the application and doing without any elaborate and expensive technology – after all, your system is supposed to work trouble-free for many years.