Better Energy Supply in the Mobile Home
If you want to be self-sufficient, the question arises as to where the power supply will come from. A solar system is the first choice to become less dependent on the socket.
Virtually nothing works in a motorhome without electricity. Even the gas-powered heating and the absorber refrigerator require electricity to be controlled. Anyone who likes to be away from the nearest socket for a few days fears nothing but a power failure. Solar energy is a very elegant solution to replenish daily electricity consumption partially. Once installed, the solar system on the roof will provide free yields for decades without the user having to lift a finger. And all this without noise, odors, or vibrations. The only disadvantage compared to generators or fuel cells: If the sun is not shining, there is no electricity yield.
The Different Solar Cells at a Glance
Four different types of cells can be used for solar systems. Each cell has very specific advantages and disadvantages.
The dark brown surface is typical of amorphous cells. The amorphous silicon is usually applied as a wafer-thin layer to a flexible carrier film during production. Typical applications are pocket calculators and other small consumers.
+ Simple and relatively cheap to manufacture.
+ Modules are flexible
– Low efficiency (< 10%)
– Lower long-term stability.
Polycrystalline cells can be seen on the polygonal surfaces, which shimmer in shades of blue and are partly reminiscent of ice flowers. They consist of several silicon crystals that have grown into a block and are then cut into slices (wafers).
+ Medium efficiency (approx. 15%).
+ Medium cost.
+ Long lifetime.
– Less efficient in low light.
The monocrystalline cells appear uniformly dark blue to black. They consist of a single silicon crystal cut into thin slices. As with the polycrystalline cells, the individual cells are more or less square.
+ Highest efficiency (> 20%).
+ Also effective in weak light.
+ Long lifetime.
– Higher manufacturing costs.
CIS type cells
Thin black longitudinal stripes are characteristic of modules with CIS cells. They consist of cobalt-indium-diselenide, vapor-deposited onto a carrier foil as a wafer-thin layer. A glass plate protects the cells on both sides.
+ Insensitive to partial shading.
+ Relatively high efficiency (approx. 20%).
+ Efficient in low light.
– Higher module weight.
– Higher manufacturing costs.
First decision: portable or fixed solar system?
If you are thinking about a solar system, first clarify two basic questions about the type of system. Should the solar modules be permanently installed on the vehicle, or is a portable system more suitable? The advantage of the portable solution is that it can be used flexibly. You only take them with you when you need them, and you can move them from one vehicle to the next or use them for the garden shed.
The module can also be placed in the sun while the motorhome is parked in the shade. On the other hand, the permanently installed variant impresses with higher electricity yields and is always ready for use without any effort.
Many options for fixed solar systems
Suppose the tendency is towards a permanently installed system. In that case, the next question is whether classic frame modules should be attached to the roof with brackets or whether flat, flexible panels that are glued directly are better suited. The latter type of installation is mainly used when every centimeter of vehicle height plays a role, or the roof is slightly curved, as is often the case with camper vans. Another argument for these panels is walkability.
On the other hand, frame modules installed on the roof with spacers – similar to houses – usually deliver the highest electricity yield about area and acquisition costs. Rear-ventilated modules have advantages, especially when temperatures are high because they do not heat up as much as panels glued directly to the roof surface. For example, if a module heats up from 20 to 60 degrees, the output drops by around 20 percent.
A power consumption analysis is the best way to determine how much module output should be installed on the motorhome roof. Important for the design is the consideration of whether the solar system should supply the electricity requirement completely and, if possible, in all seasons or whether partial coverage is sufficient. A higher battery capacity can offset a smaller solar system. The calculation provides a value for the required module power in watt peak (Wp). You can then go to the selection of suitable portable solar panels for RV.
Before doing so, however, it must be clarified which areas on the roof are still free for module assembly. Roof hoods, windows, or satellite systems often take up a lot of space so that, under certain circumstances, little is left. A photo of the roof or a sketch, preferably with dimensions, makes further planning much easier. Solar modules are not only available in the typical rectangular format but also in some elongated or square formats. High-quality panels are recommended if there is a lack of space, which, with their higher efficiency, can get more performance out of less space. The comparative value is always the nominal power (Wp). If you put it concerning the module area, you can see which models work particularly efficiently. For such power modules, individual cells with a particularly high level of efficiency are specifically selected, a pane with the higher light transmission is used for the front glazing, or more than the usual 36 cells are used, which, in combination with a special solar charge controller, promise a higher current yield.
Solve the shading problem
One problem that often affects solar harvesting is shading. Just a few leaves on the module or the shadow of a thick branch can be enough to reduce the charging performance significantly. If several of the cells connected in series are affected, the module voltage drops so much that it is no longer sufficient to charge the battery – around 0.5 volts per cell. Two smaller panels are, therefore, often more effective than one large one. If only one module is affected by the shading, the second at least still supplies the full voltage. Modules with 72 instead of 36 cells work similarly; they are practically two modules in one box.
The so-called CIS modules circumvent this problem even more elegantly and effectively. Their cells, made of cobalt indium diselenide, are designed as elongated strips rarely completely obscured by shadow spots. This reduces the charging current slightly, but the voltage remains the same. Using a CIS module can noticeably increase the electricity yield, especially in installation situations where partial shading, for example, from a neighboring satellite antenna, occurs almost regularly during the day. The disadvantage of CIS modules: With the same wattage, they are larger and heavier than classic silicon modules.
Regulators provide conditioned power
In principle, the solar module could be connected directly to the RV battery, but the effective charging capacity would not be satisfactory. That is why most solar systems are equipped with a so-called shunt or series charge controller, which prepares the charging current for the battery. Important: The controller should be adjustable to the battery type – the latest devices already take lithium batteries into account and provide a temperature sensor for the battery. In addition, the controller must match the module output, better still, be more powerful, and be able to retrofit another panel later. Some controllers also have a second charging connection for the starter battery.
There are also so-called MPP charge controllers for solar systems that are supposed to get the last bit of charging power out of them. These work according to the maximum power point tracking method and deliver more yield, particularly when the module and battery voltage are far apart, i.e., especially when the module is cold, and the battery is empty. In practice, however, a noticeable increase in yield can only be expected if you are mostly on the road in cooler regions and are best operating solar modules with 40 cells and, therefore, higher voltage.
Solar panels system with the same module output can be found on the Internet at very different prices. When selecting, you should consider that cheap panels are often not designed for mobile use and that there are also major quality differences in charge controllers and mounting material. It is better to invest a few euros more so that the following applies: once installed, you benefit for a long time.
Tips for assembly
The first and most important step is determining the exact position for mounting the solar module and the roof duct. The roof duct can also be elegantly concealed under the panel with a module mounted with a spacer. However, there should be a hanging cupboard or wardrobe at the appropriate place inside the extension so that the cable can be cleverly routed further down, preferably in a cable duct. It is best to install the solar charge controller near the onboard battery or the electronic block if it has a corresponding input. Before gluing the module, it is best to limit the glued areas with masking tape so that a clean glued joint can be drawn.
The future of the solar cell
Silicon-based monocrystalline solar cells achieve up to 26 percent efficiencies – and are therefore not far from their theoretical maximum of 29 percent. Large leaps in efficiency are, therefore, no longer to be expected here. Researchers have developed so-called concentrator cells, which bundle the light using an optical lens and can even achieve up to 46 percent efficiencies by stacking up to four solar cells made of different semiconductor materials. However, the high manufacturing costs mean that such cells can only be considered for special applications such as satellites.
That is why research is being carried out on solar cells based on other materials, such as those based on organic materials, i.e., hydrocarbons. These organic solar cells are cheap to produce and can also be used in a wide variety of forms, such as transparent film on window panes or processed into clothing. The low efficiency of a maximum of eleven percent and the lack of resistance to environmental influences are still slowing their spread. The same applies to the dye or Grätzel solar cells. As with plants with chlorophyll, dyes such as titanium oxide are used here to absorb the light. Here, too, low production costs are attractive, for example, through printing processes.
However, the greatest potential is attributed to the so-called perovskite solar cells. This class of materials, used for the first time as a solar cell in 2009, has already experienced an increase in efficiency to over 20 percent within 10 years of research, and further progress can be expected. And perovskite cells are inexpensive to manufacture and can be used flexibly. The biggest shortcoming is still the lack of resistance of the cells to solar radiation and moisture. A lot is also expected from combinations of silicon and perovskite cells.