Solar panels can replenish the energy reserves if the onboard battery runs low. Permanently mounted adhesive modules are a convenient permanent solution for panel vans, but portable systems are more flexible.
If you want to be independent and as self-sufficient as possible in your camper or mobile home, you have thought about a solar panels system for your mobile home. The use of solar energy has been more than attractive not only since electricity costs exploded.
Choosing the right solar system
When designing the individual system, close contact with the provider is recommended. They know their components best and can therefore advise you as needed. Of course, this presupposes that the supplier can also be contacted by the customer – questionable with many of the solar modules from all over the world that are sold off at ridiculous prices on the network.
Even with the established providers in our product overview, there are significant price differences. They can largely be explained by differences in the quality of the material and components – but these are difficult for the customer to identify. In addition, the different wage costs in the various production countries also affect the final price.
A needs analysis of the power guzzlers in your bus is recommended as a first step in selecting the system. This allows the required performance class to be roughly estimated (see below: “Calculation of the service life”).
Rooftop system or portable?
Roof systems are mostly sold as sets with coordinated components. The right charge controller and the necessary accessories, such as connection cables and roof ducts, are included. However, an individual combination is often possible.
The plug-and-play principle generally applies to portable systems – i.e., set up the panel, route the cable inside and plug the connector into a 12-volt socket of the extension, done. In principle, portable systems have no disadvantages compared to permanently installed systems in terms of performance yield with the same performance specification. Your problem lies more in the more complex handling. Even the best solar panel is useless if it remains folded up in the storage space when you are on the go.
Fixed installation: all the advantages and disadvantages
+ No more effort after installation
+ Also works while driving and all year round, also for battery preservation
+ Larger solar yields from several modules can also usually be achieved
– Some installation effort is required, also for cable laying and roof duct
– The vehicle cannot stand in the shade when solar yield is needed
– Horizontal panel alignment is not optimal in winter – except for the pop-up roof
– A little more vehicle height can lead to parking problems
Portable solar panels: all the pros and cons
+ It can be used very flexibly, also alternately in several vehicles and purposes
+ Can be better aligned, especially to the low winter sun
+ The panel can be placed in the sun while the bus is parked in the shade
– Constant effort for assembly and disassembly, especially when using several modules
– Costs luggage space on the way
– No electricity yield while driving
– At risk of theft, should be secured, for example, with a cable lock
The right position in every season
Solar cells convert radiant energy into electrical energy. Semiconductors are required for their production. Silicon is mostly used for this. This is used in three different dosage forms: as amorphous cells with a vapor-deposited layer of silicon on a flexible carrier foil, as polycrystalline cells with silicon crystals grown together to form a block that is sliced into thin slices, and as monocrystalline cells that consist of a single layer of silicon thinly sliced cut silicon crystal.
The latter have meanwhile prevailed for most applications. With well over 20 percent, they have the highest efficiency, work even in weaker light and also impress with their long service life.
The more directly the sun’s rays fall on the cells, the more efficiently they work, which is why the roof surface of a camper van is ideal as an assembly location in summer – there is hardly any space elsewhere anyway. However, in the winter months, when the sun is flatter, the horizontal orientation becomes a disadvantage, which portable, freely adjustable solar modules can at least partially compensate for.
In contrast to large mobile homes, which usually don’t matter, solar modules on camper vans should apply as little as possible. Especially for buses in California format, this would not only have aesthetic disadvantages but could also nullify the advantage of being suitable for parking garages.
With the cells embedded between thin plastic sheets, adhesive modules are now only a few millimeters thick – in contrast to the frame modules used on houses and mobile homes. In addition, they are flexible to a certain extent and can adapt to the roof’s curvature when glued on.
However, this flexibility also has disadvantages: the thin laminate of solar cells and carrier material can lack stability. The actual cells, the so-called wavers, are cast in plastic. Still, in the event of strong forces – for example, the raging storm on the roof of a Marco Polo speeding along at 200 km/h – micro-cracks can still appear in the material and depress the module performance. This is also why most manufacturers recommend full-surface bonding with the roof membrane. However, the disadvantage of this installation is the lack of rear ventilation and, thus, cooling – resulting in a lower electricity yield.
The rated output of the solar system at 25 degrees
However, in conversations with various providers on rear ventilation and temperature, one hears different opinions on this. It is undisputed that solar cells reach their nominal output at 25 degrees, above which the voltage decreases continuously. However, as long as the minimum voltage of 14.4 volts required to charge the onboard battery is reached – which is still the case even when the battery is heated up to 70 degrees – a solar panel does not need any longer to charge the battery. Therefore, higher temperatures have fewer negative effects than are often feared.
Suppose the bus is brooding in the sunny south in the midday heat. In that case, another effect can occur in extreme cases: “Temperatures of up to 100 degrees then occur in the modules, which can lead to delamination,” says Wolfgang Felzen, owner of the Bosswerk company, which sells the Mobil PV and Green Akku brands. Then the panel is destroyed by the heat. Felzen recommends modules with a laminated aluminum layer, which other manufacturers can also find to prevent this.
At 0.8 millimeters, it’s still thin enough for the desired flexibility but provides the necessary stability to mount the modules with at least some rear ventilation. “When installing with the permanently elastic adhesive, we recommend using nuts as spacers. The five millimeters of air between the roof skin and the panel that is achieved in this way enable sufficient heat dissipation,” says Felzen.
The rated power of the mini solar panels is given in watt peak (Wp). It is determined according to the standard at an optimum temperature of 25 degrees and a radiation output of 1000 watts per square meter. This value is only achieved in this country at peak times in midsummer. On average, 800 watts per square meter hit the ground in Germany in summer, which is why you should only expect a yield of 80 percent of the nominal output.
A low-lying sun and, thus, a less favorable angle of incidence further reduce the yield. The higher the quality of the cells, the better they can usually cope with less insolation energy.
The charge controller, which manages the charging current, also plays a role. On the one hand, it must match the module output and the battery type. Its characteristic curve must be switchable accordingly if you want to charge lithium RV batteries instead of lead batteries. So-called MPPT controllers (Maximum PowerPoint Tracking) have prevailed, at least in high-quality systems. They can deliver up to 25 percent more yield under unfavorable conditions such as shadowing and dim light.
Shadows cast on the solar panel can become a problem. Even a few leaves or the shadow of a thick branch can significantly reduce the charging performance because the cells are connected in series to achieve the required charging voltage. If one cell is completely shadowed, the entire row is affected. So-called CIS modules circumvent this problem in that the cells consist of elongated strips that are never completely darkened by individual shadow spots.
However, this type does not exist as flexible modules, nor do modules with twice the number of cells, which also react less sensitively to partial shading. With the standard panels, so-called diode bypasses are used, which bypass shaded cells via redirection. Provider Green Akku has recently added modules from AE Solar to its range, which work with bypass diodes between all cells and react only slightly to partial shading. So far, they have only been available as rigid frame modules.
First, the position of the module and roof duct must be determined. It should be noted that the junction box is usually thicker than the pure module. In the case of curved roofs, positioning them on the module side instead of underneath can save installation height. Attaching the box to the rear edge of a pop-up roof can also save height. The cable is best kept hidden in the interior, for example, on the back of the cabinets. The charge controller is best kept close to the onboard battery. There are even more tips for converting to a self-sufficient mobile home here.
Before gluing the module with permanently elastic glue, the contact surface must be thoroughly cleaned of dirt and grease. It is sufficient if the module is partially glued using “glue sausages.” But then it has to be sealed all around to prevent the ingress of moisture, which could freeze and thus break the bond. In the case of aluminum-reinforced modules, rear-ventilated installation is also possible by inserting spacers into the adhesive bed.
Calculation of the service life
And how much module power would a Marco Polo need to get by for at least two days without a shore power connection?
Requirement: Four hours of use of the auxiliary heating and lighting per day plus a cool box in the Marco Polo:
Lamps (2 x 5 W, 2 x 2 W): 14 W x 4 h = 56 Wh
Compressor refrigerator, 40 liters: 60 W x 20% operating time/h x 24h = 288 Wh
Diesel parking heater: 24 W x 4 h = 96 Wh
Total requirement: 440 Wh, corresponds to approx. 37 Ah
Solar module output: In summer, when the sun is at its highest point around noon, it can achieve up to 120 percent of its nominal output. In winter, when the sun is lower in the sky, it is only around 50 percent. If using the example of Bordeaux in winter, we assume four hours of sunshine around noon and a solar module with 150 Wp, this means for our example:
150 W x 50% x 4 h = 300 Wh, which corresponds to 25 Ah (300 Wh: 12 V = 25 Ah)
Academic service life: Since a lead-acid battery can only deliver around 60 percent of its nominal capacity, the full 80 Ah auxiliary battery has around 48 Ah available for consumption in the evening and is partially recharged the following day when the sun is shining.
Evening 1: 48 Ah – 37 Ah = 11 Ah remaining capacity, evening 2: 11 Ah + 25 Ah = 36 Ah – 37 Ah = 0 Ah remaining capacity
Result: Under the conditions mentioned, the 150 Wp system enables two overnight stays without a shore power connection.
According to our calculation example, a 150 Wp panel would also be sufficient in winter. Let’s hope my parking space neighbor from the Atlantic coast reads that too.
With the right tips, mounting solar panels on rv or installing a fixed or portable solar system on the camper is not difficult. Instead of investing in a new body battery, it is certainly more sustainable to consider a solar system.
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